Báo cáo khoa học: The protein tyrosine phosphatase PTP-Basophil/Basophil-like Interacting proteins and molecular functions doc

REVIEW ARTICLE The protein tyrosine phosphatase PTP-Basophil/Basophil-like Interacting proteins and molecular functions Kai S. Erdmann* Department of Molecular Neurobiochemistry, Ruhr-University Bochum, Germany The protein tyrosine phosphatase PTP-Basophil (PTP-Bas) and its mouse homologue, PTP-Basophil-like (PTP-BL), are high molecular mass protein phosphatases consisting of a number of diverse protein–protein interaction modules. Several splicing variants of these phosphatases are known to exist thus demonstrating the complexity of these molecules. PTP-Bas/BL serves as a central scaffolding protein facilita- ting the assembly of a multiplicity of different proteins mainly via five different PDZ domains. Many of these interacting proteins are implicated in the regulation of the actin cytoskeleton. However, some proteins demonstrate a nuclear function of this protein tyrosine phosphatase. PTP- Bas is involved in the regulation of cell surface expression of the cell death receptor, Fas. Moreover, it is a negative regulator of ephrinB phosphorylation, a receptor playing an important role during development. The phosphorylation status of other proteins such as RIL, IjBa and b-catenin can also be regulated by this phosphatase. Finally, PTP-BL has been shown to be involved in the regulation of cytokinesis, the last step in cell division. Although the precise molecular function of PTP-Bas/BL is still elusive, current data suggest clearly that PTP-Bas/BL belongs to the family of PDZ domain containing proteins involved in the regulation of the cytoskeleton and of intracellular vesicular transport processes. Keywords: cytoskeleton; apoptosis; phosphatase; trafficking; cytokinesis. Introduction Protein phosphorylation is one of the most prominent post- translational modifications regulating the activity, inter- action capability and subcellular localization of proteins. In particular, protein tyrosine phosphorylation has been identified as a major regulator of signal transduction in higher eukaryotes. Protein target phosphorylation is cata- lysed by protein tyrosine kinases and counteracted by the activity of protein tyrosine phosphatases [1]. The family of protein tyrosine phosphatases (PTPs) is divided into two major subtypes: (a) the receptor-like and (b) the nonreceptor subtype. The receptor-like subtype contains one transmembrane domain and one or two phosphatase domains at the C-terminus, which faces the cytosol. The nonreceptor subtype is characterized by the lack of a transmembrane domain and is a cytosolic or membrane-associated phosphatase [2]. The nonreceptor protein tyrosine phosphatases PTP-Bas/ BL [3,4], PTPH1/PTP-MEG [5,6] and PTPD1/PTP-RL10 [7,8] belong to a protein tyrosine phosphatase family characterized by the presence of a Four point one/Ezrin/ Radixin/Moesin (FERM) domain (FERM-PTP family). PTPD1 has been implicated in vesicular trafficking proces- ses due to its interaction with a kinesin motor protein [9] and in the regulation of the Tec tyrosine kinase family [10]. PTPH1, has been suggested to play a role in vesicular fusion processes of the endoplasmatic reticulum [11], in cell cycle regulation [12] and in the regulation of the tumor necrosis factor a-convertase (TACE) [13]. This review will focus on current knowledge of the largest member of this FERM- PTP family, the protein tyrosine phosphatase Basophil/ Basophil-like. The modular structure of the protein tyrosine phosphatase-Basophil/Basophil-like The human homologue of this protein tyrosine phosphatase was discovered by employing a PCR-based strategy to identify new protein tyrosine phosphatases using conserved regions within the catalytic domain. Originally, it was cloned from a basophil cell line, consequently, the new protein tyrosine phosphatase was named PTP-Basophil (PTP-Bas) [3]. Using a similar strategy, it was cloned in parallel from a human glioma cell line and a human breast carcinoma cell line and named PTPL1 and hPTP1e, Correspondence to K. S. Erdmann, Department of Cell Biology, Yale University School of Medicine, New Haven, CT, USA. Fax: + 1 203 737 1762, Tel.: + 1 203 737 4473, E-mail: kai.erdmann@yale.edu Abbreviations: APC, adenomatous polyposis coli protein; BP75, bromodomain containing protein 75; CRIP2, cysteine-rich intestinal protein 2; FERM, Four point one/Ezrin/Radixin/Moesin; GFP, green fluorescence protein; IkBa, inhibitor of kBa; LIM, Lin-11, Isl-1, Mec-3; PARG, PTPL1-associated RhoGAP; PIP2, 1-phosphatidyl- inositol 4,5-biphosphate [PtdIns(4,5)P 2 ]; PRK2, protein kinase C-related kinase 2; PTP, protein tyrosine phosphatase; p75NTR, p75 neurotrophin receptor; RIL, reversion-induced LIM protein; siRNA, small interfering RNA; Trip6, thyroid receptor interacting protein-6; ZRP-1, zyxin related protein-1. *Present address: Department of Cell Biology, Yale University School of Medicine, 06510 New Haven, CT, USA. (Received 2 September 2003, revised 14 October 2003, accepted 23 October 2003) Eur. J. Biochem. 270, 4789–4798 (2003) Ó FEBS 2003 doi:10.1046/j.1432-1033.2003.03895.x respectively [14,15]. Finally, due to its interaction with the human cell surface receptor Fas, it was renamed as FAP-1 (Fas-associated phosphatase-1) [16]. The mouse homologue was cloned by two independent approaches and called RIP andalsoPTP-BL) PTP-basophil-like ) referring to its human homologue (PTP-Bas) [4,17]. To simplify reading and to avoid confusion within this review the human homologue will be consequently assigned PTP-Bas and the mouse homologue as PTP-BL. The protein tyrosine phosphatase PTP-Bas/BL is a highly modular protein of about 2490 amino acids length (mole- cular mass  270 kDa) (Fig. 1). The extreme N-terminus contains a kinase noncatalytic C-lobe (KIND) domain, a protein module identified recently that shows homology to the regulatory C-lobe of protein kinases but that lacks catalytic activity [18]. The function of this domain is currently unknown but a role in mediating protein–protein interactions has been suggested. The KIND domain is followed by a Four-point-one/Ezrin/Radixin/Moesin (FERM) domain. FERM domains are known to play an important role in connecting plasma membrane receptors to the cytoskeleton [19]. Furthermore, PTP-Bas/BL comprises five different PSD-95/Drosophila discs large/Zonula occlu- dens (PDZ) domains [20]. PDZ domains are protein– protein interaction domains playing a fundamental role in the assembly of supramolecular protein complexes [21]. Finally, the protein tyrosine phosphatase domain is located at the extreme C-terminus of the molecule. The complexity of the PTP-Bas/BL molecule is further increased by alternative splicing that is observed in the N-terminus between the FERM domain and the first PDZ domain and within the second PDZ domain [3,4,14,16,22]. Additional minor alternative splice products have been described for PTP-Bas predicting C-terminal truncated versions of the PTP-Bas protein [14]. Given the presence of at least seven protein–protein interaction domains including the KIND, FERM and five PDZ domains, PTP-Bas/BL functions most probably as a major scaffolding protein (see below). Expression pattern of PTP-BL RNA in situ hybridization experiments show that the expression pattern of PTP-BL is highly regulated during development [4]. Early in development, PTP-BL is expressed ubiquitously throughout the embryo. At later stages, PTP-BL becomes restricted to epithelial and neur- onal cell lineages, e.g. epithelia of the skin, oesophagus, stomach, nasal cavity, lung, kidney, ureter, and the bladder. Moreover, expression has been observed in the ependymal cell layer, an epithelial cell layer surrounding the ventricles of the brain [4]. A similar expression pattern was observed in transgenic mice used to analyse the distribution of a truncated version of PTP-BL lacking the portion of the protein C-terminal to the first PDZ domain and fused to b-galactosidase [23]. In this in vivo model, additional expression was detected in the pigmented epithelial layer of the eye, in the infundibulum and the anterior lobe of the pituitary. Most interestingly, a timely regulated expression couldbeobservedinperipheralsensoryandsympathetic neurons. With regard to a possible role of PTP-BL under pathological conditions, it was demonstrated that the human homologue PTP-Bas is up regulated in a number of tumour cell lines [24–26]. Most strikingly, PTP-Bas is up regulated in many ovarian tumours [27]. This is of particular interest, as PTP-Bas has been demonstrated to mediate resistance to Fas induced apoptosis (see below) [16]. Given that apoptosis is one of the mechanisms to eliminate transformed cells in a multicellular organism to avoid the onset of cancer, a role of PTP-Bas/BL as a tumour suppressor is conceivable. At subcellular levels, PTP-BL shows an apical localiza- tion in epithelial cells and is accumulated in axons and growth cones of sympathetic and sensory neurons [23,28,29]. Furthermore, PTP-Bas localizes to the Golgi apparatus and to the nucleus [22,30]. Recently, it has been shown that a specific splicing variant of PTP-BL accumu- lates at the centrosome and at the spindle midzone during cell division and is part of the midbody late in cytokinesis [31] (for details see below). Domain specific interactions of PTP-Bas/BL Knowledge about PTP-Bas/BL has increased due to the identification of a number of proteins interacting with one or two of the interaction domains present in PTP-Bas/BL. A summary of these domain specific interactions is given below (see also Fig. 2). The KIND-domain Recently, the KIND domain was identified by its similarity to the C-terminal protein kinase catalytic fold (C-lobe). However, the absence of catalytic and activation loops suggests that this domain is probably noncatalytic [18]. The molecular function of this  200 amino acid new protein module is currently unknown but given that the C-lobe domain serves as a protein/protein interaction domain in protein kinases a similar role for the KIND domain is conceivable. Further experiments are needed to clarify the molecular function of the KIND domain in PTP-Bas/BL. The FERM-domain FERM domains play an important role at the interface between the plasma membrane and the cytoskeleton [19]. These domains have a length of about 300 amino acids and can be divided into three subdomains (A–C) as revealed by X-ray analysis. FERM domains are known to bind to a number of cell surface receptors [19,32]. In addition, binding Fig. 1. Modular structure of PTP-Bas/BL. PTP-Bas/BL is composed of an N-terminal kinase noncatalytic C-lobe (KIND) domain followed by a Four-point-one/Ezrin/Radixin/Moesin (FERM) domain. The core of the protein comprises five different PDZ domains. The protein tyrosine phosphatase (Phos.) domain is located at the C-terminus. The regions of major alternative splicing are indicated by dashed lines and red bars. 4790 K. S. Erdmann (Eur. J. Biochem. 270) Ó FEBS 2003 to 1-phosphatidylinositol 4,5-biphosphate [PtdIns(4,5)P 2 or PIP2) has been demonstrated [33]. Most recently, it has been reported that the FERM domain of PTP-Bas is also able to interact with PIP2 [34]. This interaction has been suggested to be important for membrane localization of PTP-Bas. Moreover, the FERM domain of PTP-BL was described to target the protein to the apical membrane in cultured epithelial cells as well as in vivo [4,23]. Besides binding to PIP2, colocalization as well as cosedimentation with filamentous actin has been demonstrated suggesting an association with the actin cytoskeleton [31]. However, there are currently no proteins known to interact directly with the FERM domain of PTP-Bas/BL. PDZ domains PDZ domains are able to interact selectively with the C-termini of their target proteins [21,35,36]. However, there are also examples of PDZ domain binding to internal sequences [35,37]. Recently, it has been shown that PDZ domains are not only protein–protein interaction domains but can also serve as PIP (PtdInsP) binding modules [38]. PDZ domains have a length of  90 amino acids and adopt an a/b-fold consisting of six b-sheets and two a-helices [39,40]. Currently, only the NMR-structure of one (PDZ2) of the five PDZ domains of PTP-Bas/BL is available [41,42]. Although the overall structure of the PDZ2 domain fits well into the common fold of PDZ domains, it shows an unusual foldback of the loop between b2andb3 to the backbone. This loop has been implicated in the regulation of target recognition of the PDZ2 domain [22,43]. Indeed the NMR- structure of a splice variant of the PDZ2 domain (PDZ2b), with an extended b2/b3 loop, revealed that this foldback can even interfere with protein target binding [44]. There is an increasing number of proteins known to interact with one or more of the PDZ domains of PTP-BL/ PTP-Bas. PDZ1 An interaction of PDZ1 with the bromodomain-containing protein BP75 and with the transcription regulator IjBa was demonstrated [45,46]. Although the function of BP75 is unknown, bromodomains are thought to interact with acetylated lysine residues that are present in histone proteins, thus, hinting at a possible function of BP75 in the nucleus [47]. This is in line with its nuclear localization and the capability of BP75 to translocate the PDZ1 domain of PTP-BL to the nucleus. Interaction of BP75 with PTP- BL needs an intact C-terminus of BP75 although the C-terminus alone is insufficient for binding. Interaction of IjBa with PDZ1 is mediated via its ankyrin repeats. IjBa is an inhibitor of the transcription factor NFjB. A complex consisting of both proteins is formed in the cytosol preventing NFjB from entering the nucleus. There are two pathways involved in dissociation of this complex. In the first pathway, IjBa is phosphorylated on Ser32 and 36 resulting in its degradation by the ubiquitin proteasome system. In the second pathway, IjBa is phosphorylated on Tyr42 leading to dissociation without degradation [48,49]. Indeed, IjBa is a substrate for PTP-Bas phosphatase, shown by an increase in tyrosine phosphory- lation of IjBa aftertransfectionofaPTP-Basversion lacking phosphatase activity [46]. Furthermore, a substrate trapping mutant of PTP-Bas was found to bind selectively tyrosine-phosphorylated IjBa [50]. The interaction of IjBa with PTP-Bas provides evidence for a regulation of NFjB dependent transcription via PTP-Bas [46,51]. Fig. 2. PTP-Bas/BL is the central scaffolding component of a supramolecular protein complex. Domain specific interactions of PTP-Bas/BL are indicated (for a detailed description see text). Dashed arrows indicate that an association has been determined but direct interaction was not investigated. The interacting proteins can be divided into three groups. Known or potential regulators of the actin cytoskeleton (red), regulators of the actin and tubulin cytoskeleton (purple) and regulators of gene transcription (green). Human Fas (hFas) does not fit in any of these groups but is a transmembrane receptor regulating apoptosis. APC, Adenomatous polyposis coli protein; BP75, bromodomain containing protein 75; CRIP2, cysteine-rich intestinal protein 2; PARG, PTPL1-associated RhoGAP; PIP2, 1-phosphatidylinositol 4,5-biphosphate, PRK2, protein kinase C-related kinase 2; p75NTR, neurotrophin receptor; RIL, reversion-induced LIM (RIL); Trip6, thyroid receptor interacting protein-6; ZRP-1, zyxin related protein-1. Ó FEBS 2003 The protein tyrosine phosphatase PTP-Bas/BL (Eur. J. Biochem. 270) 4791 PDZ2 A number of interacting proteins were shown to interact with the PDZ2 domain of PTP-Bas/BL. The human cell surface protein Fas was the first protein reported to interact with the PDZ2 domain of PTP-Bas (FAP-1) [16]. However, the corresponding mouse homologue of Fas does not bind to PTP-BL [52] (see below). The neurotro- phin receptor p75 NTR has also been suggested to interact with the PDZ2 domain of PTP-Bas [51]. Additional proteins known to bind to the PDZ2 domain are the LIM domain containing proteins RIL and Trip6/ZRP-1 [37,53,54]. RIL is also able to interact with PDZ4 (see below). Little is known about the molecular functions of RIL and Trip6/ZRP-1 but a regulatory role for the actin cytoskeleton has been suggested for both of them. Recently, Trip6 has been shown to accumulate at focal adhesions and to interact with the adaptor protein p130Cas (Crk associated substrate) implicating this mole- cule in the regulation of cell adhesion [55]. Another protein involved in cell adhesion regulation, the tumour suppressor protein APC (adenomatous polyposis coli protein), interacts selectively with the PDZ2 domain of PTP-BL [22]. Interestingly, it has been demonstrated that APC interacts only with one of the two splice variants of PDZ2 (PDZ2a). The insertion of five amino acids within the b2/b3-loop present in PDZ2b led to a complete abolishment of the interaction with APC [22]. APC is involved in many sporadic and inherited forms of colon carcinoma. One important role of APC is the regulation of the protein b-catenin. b-catenin is involved in cell adhesion processes via the transmembrane receptor cadh- erin and plays a role in the regulation of transcription via the transcription factor LEF/TCF (lymphocyte enhancer binding factor/T-cell factor) [56]. APC also interacts with a recently identified rho exchange factor (Asef) and is involved in the regulation of the tubulin and actin cytoskeleton [57]. PDZ3 The only protein known to interact with the PDZ3 domain of PTP-BL is the protein kinase C-related kinase-2 (PRK2) [58]. PRK2 is a cytosolic serine/threonine kinase regulated by the monomeric G-protein rho. PRK2 is implicated in the modulation of the actin cytoskeleton and based on its similarity to PRK1 a potential role in the regulation of intracellular vesicular trafficking has been proposed [59–61]. Interestingly, the interaction is mediated via an unusual PDZ domain binding motif at the C-terminus of PRK2 (DWC). PDZ4 Proteins known to interact with the PDZ4 domain of PTP- Bas/PTP-BL are the LIM containing proteins RIL (also able to interact with PDZ2, see above) and CRIP2, the rhoGAP protein PARG as well as the class B transmem- brane ephrin receptor (ephrinB) [37,62–64]. The PDZ4 domain shows the highest variability in its target binding motifs. RIL and CRIP2 bind via their LIM domains to the PDZ4 domain whereas ephrinB binds via a C-terminal PDZ domain II binding motif. However, PARG neither contains a LIM domain nor a PDZ domain II binding consensus sequence and the molecular mechanism of binding remains currently unclear. PDZ5 Currently, there are no proteins known to interact with the PDZ5 domain although it was demonstrated that the PDZ5 domain of PTP-BL, like the PDZ2 and PDZ3 domains, is able to interact with PIP2 [38]. Substrates of the PTP-Bas/BL protein tyrosine phosphatase domain Crucial for the understanding of PTP-Bas/BL function is the identification of specific substrates of the protein tyrosine phosphatase domain. Using recombinant gluta- thione-S-transferase fusionproteins of the  230 amino acids comprising phosphatase domain it has been demon- strated that PTP-Bas/BL is a bona fide tyrosine phosphatase [15,22]. Currently, several proteins are known to serve as substrate for the protein tyrosine phosphatase domain of PTP-Bas/BL. Theses proteins are RIL, IjBa and ephrinB, which have been shown to serve as substrates in vivo using transfected cell lines [29,37,46]. Moreover, as mentioned above, IjBa could be coprecipitated using a substrate trapping mutant of the PTP-Bas phosphatase domain [50]. Finally, b-catenin and c-src could be dephosphroylated in vitro by PTP-BL phosphatase [22]. However, the func- tional consequences of PTP-Bas/BL induced dephosphory- lation have not been determined yet. A potential function of PTP-Bas in Fas- mediated apoptosis In spite of the large number of examples of protein interactions, a direct functional involvement of PTP-Bas/ BL in specific pathways is limited. However, a number of reports have described a potential role of PTP-Bas in conferring resistance to Fas-induced cell death [26,65]. Fas is a type-I transmembrane receptor and a member of the tumor necrosis factor-receptor/nerve growth factor recep- tor-family [66,67]. The Fas receptor itself does not contain catalytic activity but is able to recruit a Ôdeath signalling complexÕ after activation by the Fas-ligand [68]. PTP-Bas binds via its PDZ2 domain to the extreme C-terminus of human Fas, which is known to exert a negative regulatory effect on Fas signalling [69,70]. Indeed, it was shown that Jurkat T leukemia cells, which do not express endogenous PTP-Bas, were rendered more resistant to Fas-induced apoptosis after overexpression of PTP-Bas [16]. Further evidence for a role of PTP-Bas in regulating Fas signalling was established by injection experiments using a tripeptide derived from the extreme C-terminus of human Fas. Injection of this tripeptide into a colon cancer cell line or into thyrocytes restored sensitivity to Fas-induced cell death [71,72]. In addition, a correlation of PTP-Bas expression and sensitivity to Fas induced apoptosis has been observed in a number of different tumour cell lines [16,65,73]. It was also suggested that hepatomablastoma cells avoid apop- tosis from coexpressing Fas and Fas-ligand by expressing 4792 K. S. Erdmann (Eur. J. Biochem. 270) Ó FEBS 2003 PTP-BAS as a negative regulator of Fas signalling [73]. An up regulation of PTP-BAS has been observed in ovarian cancers and ovarian cancer cell lines show a correlation of Fas-induced cell death resistance and PTP-BAS expression [27]. PTP-Bas has also been implicated in the escape of HTLV1-infected T cells from Fas-mediated immune sur- veillance and finally, down regulation of PTP-Bas has been suggested to underlie the increased apoptotic death of hematopoitic cells in myelodysplastic syndrome [24,74]. However, the interaction between Fas and PTP-Bas is evolutionary not conserved, thus, mouse Fas does not interact with PTP-BL, the mouse homologue of PTP-Bas. Moreover, PTP-BL was not able to confer resistance or to decrease sensitivity to human Fas induced cell death, although PTP-BL is able to interact with human Fas via its PDZ2 and PDZ4 domain [52]. There are also reports unable to identify any correlation between sensitivity to Fas-induced cell death and expression of PTP-Bas [75]. Moreover, there is evidence that PTP-Bas can even exert a pro-apoptotic effect in human breast cancer cells, which is associated with an early inhibition of the insulin receptor substrate-1/PtdIns 3-kinase pathway [76]. The mechanism of how PTP-Bas is able to confer resistance to Fas-induced apoptosis, at least in some cell lines, is currently unclear, although regulation of tyrosine phosphorylation of Fas has been suggested [16]. Recently however, two reports point to a possible role of PTP-BAS in the regulation of Fas cell surface expression. PTP-Bas is able to regulate cell surface expression of Fas As mentioned above, a number of cell lines are resistant to Fas-induced cell death. A similar observation has been made for several pancreatic adenocarcinoma cell lines [65]. To elucidate a possible mechanism for PTP-Bas mediated resistance to Fas-induced apoptosis, Ungefroren et al. analysed the subcellular distribution of PTP-Bas and Fas in Panc89 cells upon stimulation with Fas-ligand [30]. Unstimulated cells showed limited colocalization of PTP- Bas and Fas. However, upon treatment with Fas-ligand, colocalization of PTP-Bas and Fas was increased signifi- cantly. Strong colocalization was then observed at the Golgi apparatus and at peripheral vesicular structures. This increase in colocalization to intracellular compartments was accompanied by a strong decrease in Fas surface expression. However, this accumulation to intracellular stores was not observed in Capan-1 cells, a pancreatic adenocarcinoma cell line lacking PTP-Bas expression and being sensitive to Fas induced apoptosis. Based on the spatial temporal relationship of PTP-Bas and Fas the authors suggested an interfering role of PTP-Bas with the translocation of Fas from intracellular stores to the plasma membrane. These results were recently confirmed and extended analysing the expression of PTP-Bas and Fas in melanoma cell lines [77]. Several melanoma cell lines show a correla- tion between PTP-Bas expression and reduced cell surface expression of a transfected Fas-GFP fusion protein. More- over, expression of PTP-Bas in a melanoma cell line lacking endogenous PTP-Bas (FEMX) redistributed Fas-GFP from the cell surface to intracellular pools. Similar results were obtained analysing the subcellular distribution of endo- genous Fas. The inhibitory effect of PTP-Bas on cell surface expression of Fas was dependent on the presence of the PDZ2 and protein tyrosine phosphatase domain of PTP- Bas. Moreover, an intact C-terminus of Fas was crucial for the interfering role of PTP-Bas in Fas trafficking. In summary, both reports provide strong evidence that PTP-Bas is able to act as a negative regulator of Fas cell surface expression giving an explanation for its inhibitory role in Fas-induced cell death. In principle, there are two major possibilities of how PTP-Bas could down regulate cell surface expression of Fas (Fig. 3A). Firstly, PTP-Bas could increase the trafficking of Fas from the cell surface to intracellular pools, disturbing the equilibrium of endocytotic and secretory events leading to a net decrease of Fas cell surface expression. Secondly, the recycling of Fas as well as the transport from intracellular stores to the cell surface could be affected. Both possibilities are not exclusive and given the modular complexity of PTP-Bas, an involvement in both processes has to be considered. Taken together, PTP-Bas joins the increasing number of PDZ-containing proteins involved in intracellular trafficking processes [78,79]. However, further experiments are needed to eluci- date the precise function of PTP-Bas in receptor trafficking. PTP-BL is a negative regulator of ephrinB phosphorylation – the switch model of ephrinB signalling As described above, the ephrinB receptor interacts with the fourth PDZ domain of PTP–Bas. This interaction has been confirmed for mouse ephrinB and PTP-BL [29,64]. EphrinB is a type-I transmembrane receptor with no obvious catalytic activity in its cytoplasmic domain. However, ephrinB is part of a dual receptor system consisting of the ephrins and the erythropoetin-producing human hepatocel- lular derived (eph)-receptors. Eph-receptors belong to the large family of receptor tyrosine kinases. The ephrin/eph-receptor system regulates an amazing variety of developmental processes, including cell migration, angiogenesis, segmentation and compartment boundary formation as well as synaptogenesis and axon guidance [80– 82]. The ephrinB/eph-receptor system is able to induce a bi-directional signalling involving src kinase family-depend- ent tyrosine phosphorylation of conserved tyrosine residues at the cytoplasmic domain of ephrinB. Tyrosine–phosphory- lated ephrinB then engages the SH2/SH3 adaptor molecule, GRB4, leading to further downstream signalling [83]. All ephrinB molecules contain a class II PDZ domain binding motif, –YXV, at their C-terminus [64]. This motif hasbeenshowntobecrucialfortheinteractionwiththe fourth PDZ domain of PTP-Bas/BL (see above). PTP-BL is able to dephosphorylate ephrinB in vitro and overexpression of PTP-BL in HeLa cells stably transfected with ephrinB led to a significant decrease in ephrinB tyrosine phosphoryla- tion. This interfering effect of PTP-BL on tyrosine phos- phorylation of ephrinB was dependent on a catalytically active phosphatase domain [29]. After stimulation with eph- receptor bodies, PTP-BL is recruited to ephrinB with delayed kinetics showing a tight correlation with the kinetics of ephrinB dephosphorylation. This led to the proposal of a switch model of ephrinB signalling (Fig. 3C) After binding Ó FEBS 2003 The protein tyrosine phosphatase PTP-Bas/BL (Eur. J. Biochem. 270) 4793 to the cognate eph-receptor, ephrinB becomes phosphoryl- ated at conserved tyrosine residues leading to the recruit- ment of SH2 domains containing proteins like GRB4. This is followed by binding to PTP-BL and subsequent dephos- phorylation of ephrinB, switching off the phosphotyrosine dependent signalling and replacing it with a PDZ depending signalling [29]. PTP-Bas/PTP-BL is involved in the regulation of cytokinesis Besides being involved in Fas trafficking and ephrinB signalling, a regulatory function of PTP-BL in cytokinesis has been suggested (Fig. 3B) [31]. Cytokinesis is the last step of mitosis, dividing the cell into two parts after chromosome segregation. In mammalian cells, an actin–myosin ring is established beneath the plasma membrane accomplishing the division of the cytoplasm by contraction. The spindle midzone is formed in late anaphase by bundles of inter- digitating microtubules and plays an important role in the regulation of cytokinesis [84]. A detailed localization study of PTP-Bas in HeLa cells revealed that its localization is highly regulated during cell division. PTP-Bas accumulates at the spindle midzone during anaphase and becomes part of the midbody at the end of cytokinesis [31]. Moreover, PTP-Bas is highly enriched at centrosomes. A domain specific localization analysis of the mouse homologue PTP- BL showed that PTP-BL is targeted to the midbody and to the centrosome by a specific splice variant of the N-terminus characterized by an insertion of 182 amino acids. Addition- ally, it was demonstrated that the FERM domain of PTP-BL is associated with the actin-based contractile ring and can be cosedimented with filamentous actin (F-actin), whereas the N-terminus can be cosedimented with micro- tubules. Elevating the expression level of wildtype PTP-BL or expression of PTP-BL with an inactive tyrosine phos- phatase domain interfered with the contraction of the contractile ring. The defects in contractile ring contraction led to a significant increase of multinucleate cells suggesting a regulatory role of PTP-BL in cytokinesis. Currently, the Fig. 3. Involvement of PTP-Bas/BL in different biological systems has been demonstrated. (A) PTP-Bas regulates the cell surface expression of the human Fas receptor. There are at least two possibilities of how PTP-Bas can affect cell surface expression of Fas. PTP-Bas could regulate the transport of Fas from the Golgi apparatus (G) to the cell surface ÔaÕ. Alternatively, PTP-Bas could increase the transport of Fas from the cell surface to endosomal compartments (EC) ÔbÕ or from endosomal compartments to the Golgi apparatus ÔcÕ.N,nucleus;Ly,lysosome.(B)PTP-Bas/BLis involved in the regulation of cytokinesis. During anaphase and cytokinesis PTP-Bas/BL accumulates at the spindle midzone and contractile ring. PTP-Bas/BL associates via its FERM domain with F-actin of the contractile ring and via its N-terminus with the midzone microtubules. Overexpression of PTP-BL leads to multinucleate cells. (C) PTP-BL is involved in the regulation of ephrinB signalling. The eph/ephrinB receptor system evokes bi-directional signalling. After binding, eph-receptors are tyrosine phosphorylated (autophosphorylation) and ephrinB is phos- phorylated by the src kinase family. Phosphotyrosine signalling of ephrinB is started via binding to SH2 containing proteins. PTP-BL is recruited to ephrinB with a delayed kinetic leading to its dephosphorylation and replacing the SH2 dependent signalling by a PDZ domain dependent signalling (switch-model of ephrinB signalling). 4794 K. S. Erdmann (Eur. J. Biochem. 270) Ó FEBS 2003 precise function of PTP-BL in cytokinesis is not known, however, signal transduction via the small G-protein rho plays an important role in contractile ring assembly and in the regulation of its contraction during cytokinesis [85]. PTP-Bas/BL has been shown to interact with the rho GTPase activating protein PARG and the rho regulated protein kinase, PRK2, implicating PTP-BL in a rho signaling pathway probably relevant to cytokinesis. In additon, PTP-BL is able to interact with F-actin and with microtubules, suggesting a role in connecting the contractile ring with the spindle midzone microtubules thereby coordi- nating cleavage furrow ingression with microtubule dynam- ics. Finally, given the role of PTP-Bas in trafficking of Fas (see above), PTP-BL could be important for targeted vesicle transport during cleavage furrow ingression, which has been demonstrated to be important for the final step of cyto- kinesis [86]. Concluding remarks PTP-Bas/BL is an exceptionally large protein tyrosine phosphatase comprising a number of protein–protein interaction domains. As summarized above, many proteins have been identified as interacting with one or two of the five PDZ domains of PTP-Bas/BL. Moreover, PTP-BL is associated with F-actin and microtubules and binding to PIP2 has been demonstrated. Thus, PTP-BL/Bas serves clearly as a major scaffolding protein of a supramolecular protein complex. However, in spite of the large number of interacting proteins a clear cut function has not been assigned to PTP-Bas/BL. Given the modular complexity of PTP-Bas/BL and the number of different splicing variants it is reasonable to assume that this protein tyrosine phospha- tase is involved in a number of different physiological processes. This is already reflected in the diversity of biological systems where an involvement of PTP-BL/PTP- Bas has already been defined (Fas trafficking, ephrinB signaling and regulation of cytokinesis). Moreover, there is evidence that PTP-Bas/BL is also part of a protein complex within the nucleus. A challenge for the future will be to elucidate whether there is a common mechanism underlying the involvement of PTP-Bas/BL in these different systems. Focusing on splice variant specific interactions as well as identifying major substrates for the protein tyrosine phos- phatase will lead to a better understanding of this fascin- ating molecule. Moreover, mice knockout technology and the recently developed siRNA methodology will certainly contribute in identifying the precise molecular functions of PTP-Bas/BL. Acknowledgements This work was supported by a grant from the Deutsche Forschungsg- emeinschaft, SFB 452. References 1. Hunter, T. (1995) Protein kinases and phosphatases: the yin and yang of protein phosphorylation and signaling. Cell 80, 225–236. 2. Andersen, J.N., Mortensen, O.H., Peters, G.H., Drake, P.G., Iversen, L.F., Olsen, O.H., Jansen, P.G., Andersen, H.S., Tonks, N.K. & Moller, N.P. (2001) Structural and evolutionary relationships among protein tyrosine phosphatase domains. Mol. Cell Biol. 21, 7117–7136. 3. Maekawa, K., Imagawa, N., Nagamatsu, M. & Harada, S. (1994) Molecular cloning of a novel protein-tyrosine phosphatase con- taining a membrane-binding domain and GLGF repeats. FEBS Lett. 337, 200–206. 4. Hendriks, W., Schepens, J., Bachner, D., Rijss, J., Zeeuwen, P., Zechner,U.,Hameister,H.&Wieringa,B.(1995)Molecular cloning of a mouse epithelial protein-tyrosine phosphatase with similarities to submembranous proteins. J. Cell Biochem. 59, 418–430. 5. Gu, M.X., York, J.D., Warshawsky, I. & Majerus, P.W. (1991) Identification, cloning, and expression of a cytosolic mega- karyocyte protein-tyrosine-phosphatase with sequence homology to cytoskeletal protein 4.1. Proc. Natl Acad. Sci. USA 88, 5867– 5871. 6. Yang, Q. & Tonks, N.K. (1991) Isolation of a cDNA clone encoding a human protein-tyrosine phosphatase with homology to the cytoskeletal-associated proteins band 4.1, ezrin, and talin. Proc.NatlAcad.Sci.USA88, 5949–5953. 7. Moller, N.P., Moller, K.B., Lammers, R., Kharitonenkov, A., Sures, I. & Ullrich, A. (1994) Src kinase associates with a member of a distinct subfamily of protein-tyrosine phosphatases containing an ezrin-like domain. Proc. Natl Acad. Sci. USA 91, 7477–7481. 8. Higashitsuji, H., Arii, S., Furutani, M., Imamura, M., Kaneko, Y., Takenawa, J., Nakayama, H. & Fujita, J. (1995) Enhanced expression of multiple protein tyrosine phosphatases in the regenerating mouse liver: isolation of PTP-RL10, a novel cyto- plasmic-type phosphatase with sequence homology to cytoskeletal protein 4.1. Oncogene 10, 407–414. 9. Dorner, C., Ciossek, T., Muller, S., Moller, P.H., Ullrich, A. & Lammers, R. (1998) Characterization of KIF1C, a new kinesin- like protein involved in vesicle transport from the Golgi apparatus to the endoplasmic reticulum. J. Biol. Chem. 273, 20267–20275. 10. Jui,H.Y.,Tseng,R.J.,Wen,X.,Fang,H.I.,Huang,L.M.,Chen, K.Y., Kung, H.J., Ann., D.K. & Shih, H.M. (2000) Protein-tyro- sine phosphatase D1, a potential regulator and effector for Tec family kinases. J. Biol. Chem. 275, 41124–41132. 11. Lavoie, C., Chevet, E., Roy, L., Tonks, N.K., Fazel, A., Posner, B.I., Paiement, J. & Bergeron, J.J. (2000) Tyrosine phosphoryla- tion of p97 regulates transitional endoplasmic reticulum assembly in vitro. Proc. Natl Acad. Sci. USA 97, 13637–13642. 12. Zhang, S.H., Liu, J., Kobayashi, R. & Tonks, N.K. (1999) Iden- tification of the cell cycle regulator VCP (p97/CDC48) as a sub- strate of the band 4.1-related protein-tyrosine phosphatase PTPH1. J. Biol. Chem. 274, 17806–17812. 13. Zheng, Y., Schlondorff, J. & Blobel, C.P. (2002) Evidence for regulation of the tumor necrosis factor alpha-convertase (TACE) by protein-tyrosine phosphatase PTPH1. J. Biol. Chem. 277, 42463–42470. 14. Banville, D., Ahmad, S., Stocco, R. & Shen, S.H. (1994) A novel protein-tyrosine phosphatase with homology to both the cyto- skeletal proteins of the band 4.1 family and junction-associated guanylate kinases. J. Biol. Chem. 269, 22320–22327. 15. Saras, J., Claesson-Welsh, L., Heldin, C.H. & Gonez, L.J. (1994) Cloning and characterization of PTPL1, a protein tyrosine phos- phatase with similarities to cytoskeletal-associated proteins. J. Biol. Chem. 269, 24082–24089. 16. Sato, T., Irie, S., Kitada, S. & Reed, J.C. (1995) FAP-1: a protein tyrosine phosphatase that associates with Fas. Science 268, 411–415. 17. Chida,D.,Kume,T.,Mukouyama,Y.,Tabata,S.,Nomura,N., Thomas, M.L., Watanabe, T. & Oishi, M. (1995) Characterization of a protein tyrosine phosphatase (RIP) expressed at a very early stage of differentiation in both mouse erythroleukemia and embryonal carcinoma cells. FEBS Lett. 358, 233–239. Ó FEBS 2003 The protein tyrosine phosphatase PTP-Bas/BL (Eur. J. Biochem. 270) 4795 18. Ciccarelli, F.D., Bork, P. & Kerkhoff, E. (2003) The KIND module: a putative signalling domain evolved from the C lobe of the protein kinase fold. Trends Biochem. Sci. 28, 349–352. 19. Chishti, A.H., Kim, A.C., Marfatia, S.M., Lutchman, M., Hanspal, M., Jindal, H., Liu, S.C., Low, P.S., Rouleau, G.A., Mohandas, N., Chasis, J.A., Conboy, J.G., Gascard, P., Taka- kuwa, Y., Huang, S.C., Benz, E.J. Jr, Bretscher, A., Fehon, R.G., Gusella, J.F., Ramesh, V., Solomon, F., Marchesi, V.T., Tsukita, S., Tsukita, S. & Hoover, K.B. (1998) The FERM domain: a unique module involved in the linkage of cytoplasmic proteins to the membrane. Trends Biochem. Sci. 23, 281–282. 20. Kennedy, M.B. (1995) Origin of PDZ (DHR, GLGF) domains. Trends Biochem. Sci. 20, 350. 21. Sheng, M. & Sala, C. (2001) PDZ domains and the organization of supramolecular complexes. Annu. Rev. Neurosci. 24, 1–29. 22. Erdmann, K.S., Kuhlmann, J., Lessmann, V., Herrmann, L., Eulenburg, V., Muller, O. & Heumann, R. (2000) The Adeno- matous Polyposis Coli-protein (APC) interacts with the protein tyrosine phosphatase PTP-BL via an alternatively spliced PDZ domain. Oncogene 19, 3894–3901. 23. Thomas, T., Voss, A.K. & Gruss, P. (1998) Distribution of a murine protein tyrosine phosphatase BL-beta-galactosidase fusion protein suggests a role in neurite outgrowth. Dev. Dyn. 212, 250–257. 24. Arai, M., Kannagi, M., Matsuoka, M., Sato, T., Yamamoto, N. & Fujii, M. (1998) Expression of FAP-1 (Fas-associated phospha- tase) and resistance to Fas-mediated apoptosis in T cell lines derived from human T cell leukemia virus type 1-associated myelopathy/tropical spastic paraparesis patients. AIDS Res. Hum. Retroviruses 14, 261–267. 25. Lee, S.H., Shin, M.S., Lee, H.S., Bae, J.H., Lee, H.K., Kim, H.S., Kim,S.Y.,Jang,J.J.,Joo,M.,Kang,Y.K.,Park,W.S.,Park,J.Y., Oh,R.R.,Han,S.Y.,Lee,J.H.,Kim,S.H.,Lee,J.Y.&Yoo,N.J. (2001) Expression of Fas and Fas-related molecules in human hepatocellular carcinoma. Hum. Pathol. 32, 250–256. 26. Li, Y., Kanki, H., Hachiya, T., Ohyama, T., Irie, S., Tang, G., Mukai, J. & Sato, T. (2000) Negative regulation of Fas-mediated apoptosis by FAP-1 in human cancer cells. Int. J. Cancer 87, 473–479. 27. Meinhold-Heerlein, I., Stenner-Liewen, F., Liewen, H., Kitada, S., Krajewska, M., Krajewski, S., Zapata, J.M., Monks, A., Scudiero, D.A., Bauknecht, T. & Reed, J.C. (2001) Expression and potential role of Fas-associated phosphatase-1 in ovarian cancer. Am. J. Pathol. 158, 1335–1344. 28. Cuppen, E., Wijers, M., Schepens, J., Fransen, J., Wieringa, B. & Hendriks, W. (1999) A FERM domain governs apical confine- ment of PTP-BL in epithelial cells. J. Cell Sci. 112, 3299–3308. 29. Palmer, A., Zimmer, M., Erdmann, K.S., Eulenburg, V., Porthin, A.,Heumann,R.,Deutsch,U.&Klein,R.(2002)EphrinB phosphorylation and reverse signaling: regulation by Src kinases and PTP-BL phosphatase. Mol. Cell 9, 725–737. 30. Ungefroren, H., Kruse, M.L., Trauzold, A., Roeschmann, S., Roeder,C.,Arlt,A.,Henne-Bruns,D.&Kalthoff,H.(2001) FAP-1 in pancreatic cancer cells: functional and mechanistic stu- dies on its inhibitory role in CD95-mediated apoptosis. J. Cell Sci. 114, 2735–2746. 31. Herrmann, L., Dittmar, T. & Erdmann, K.S. (2003) The Protein Tyrosine Phosphatase PTP-BL Associates with the Midbody and Is Involved in the Regulation of Cytokinesis. Mol. Biol. Cell 14, 230–240. 32. Pearson, M.A., Reczek, D., Bretscher, A. & Karplus, P.A. (2000) Structure of the ERM protein moesin reveals the FERM domain fold masked by an extended actin binding tail domain. Cell 101, 259–270. 33. Hamada, K., Shimizu, T., Matsui, T., Tsukita, S. & Hakoshima, T. (2000) Structural basis of the membrane-targeting and unmasking mechanisms of the radixin FERM domain. EMBO J. 19, 4449–4462. 34. Bompard, G., Martin, M., Roy, C., Vignon, F. & Freiss, G. (2003) Membrane targeting of protein tyrosine phosphatase PTPL1 through its FERM domain via binding to phosphatidylinositol 4,5-biphosphate. J. Cell Sci. 116, 2519–2530. 35. Hillier, B.J., Christopherson, K.S., Prehoda, K.E., Bredt, D.S. & Lim, W.A. (1999) Unexpected modes of PDZ domain scaffolding revealed by structure of nNOS-syntrophin complex. Science 284, 812–815. 36. Kornau, H.C., Schenker, L.T., Kennedy, M.B. & Seeburg, P.H. (1995) Domain interaction between NMDA receptor subunits and the postsynaptic density protein PSD-95. Science 269, 1737–1740. 37. Cuppen, E., Gerrits, H., Pepers, B., Wieringa, B. & Hendriks, W. (1998)PDZmotifsinPTP-BLandRILbindtointernalprotein segments in the LIM domain protein RIL. Mol. Biol. Cell 9, 671–683. 38. Zimmermann, P., Meerschaert, K., Reekmans, G., Leenaerts, I., Small, J.V., Vandekerckhove, J., David, G. & Gettemans, J. (2002) PIP (2)-PDZ domain binding controls the association of syntenin with the plasma membrane. Mol. Cell 9, 1215–1225. 39. Morais Cabral, J.H., Petosa, C., Sutcliffe, M.J., Raza, S., Byron, O., Poy, F., Marfatia, S.M., Chishti, A.H. & Lidding- ton, R.C. (1996) Crystal structure of a PDZ domain. Nature 382, 649–652. 40.Doyle,D.A.,Lee,A.,Lewis,J.,Kim,E.,Sheng,M.& MacKinnon, R. (1996) Crystal structures of a complexed and peptide-free membrane protein-binding domain: molecular basis of peptide recognition by PDZ. Cell 85, 1067–1076. 41. Kozlov, G., Gehring, K. & Ekiel, I. (2000) Solution structure of the PDZ2 domain from human phosphatase hPTP1E and its interactions with C-terminal peptides from the Fas receptor. Biochemistry 39, 2572–2580. 42. Walma, T., Spronk, C.A., Tessari, M., Aelen, J., Schepens, J., Hendriks, W. & Vuister, G.W. (2002) Structure, dynamics and binding characteristics of the second PDZ domain of PTP-BL. J. Mol. Biol. 316, 1101–1110. 43. Kozlov, G., Banville, D., Gehring, K. & Ekiel, I. (2002) Solution structure of the PDZ2 domain from cytosolic human phosphatase hPTP1E complexed with a peptide reveals contribution of the beta2-beta3 loop to PDZ domain–ligand interactions. J. Mol. Biol. 320, 813–820. 44. Kachel,N.,Erdmann,K.S.,Kremer,W.,Wolff,P.,Gronwald, W., Heumann, R. & Kalbitzer, H.R. (2003) Structure determin- ation and ligand interactions of the PDZ2b domain of PTP-Bas (hPTP1E): Splicing-induced modulation of ligand specificity. J. Mol. Biol. 334, 143–155. 45. Cuppen, E., van Ham, M., Pepers, B., Wieringa, B. & Hendriks, W. (1999) Identification and molecular characterization of BP75, a novel bromodomain-containing protein. FEBS Lett. 459, 291–298. 46. Maekawa,K.,Imagawa,N.,Naito,A.,Harada,S.,Yoshie,O.& Takagi, S. (1999) Association of protein-tyrosine phosphatase PTP-BAS with the transcription-factor-inhibitory protein Ikappa- Balpha through interaction between the PDZ1 domain and ankyrin repeats. Biochem. J. 337, 179–184. 47. Horn, P.J. & Peterson, C.L. (2001) The bromodomain: a regulator of ATP-dependent chromatin remodeling? Front Biosci. 6, D1019–D1023. 48. Imbert, V., Rupec, R.A., Livolsi, A., Pahl, H.L., Traenckner, E.B., Mueller-Dieckmann, C., Farahifar, D., Rossi, B., Auberger, P., Baeuerle, P.A. & Peyron, J.F. (1996) Tyrosine phosphorylation of I kappa B-alpha activates NF-kappa B without proteolytic degradation of I kappa B-alpha. Cell 86, 787–798. 49. Baeuerle, P.A. & Baltimore, D. (1996) NF-kappa B: ten years after. Cell 87, 13–20. 4796 K. S. Erdmann (Eur. J. Biochem. 270) Ó FEBS 2003 50. Nakai, Y., Irie, S. & Sato, T.A. (2000) Identification of Ikappa- Balpha as a substrate of Fas-associated phosphatase-1. Eur. J. Biochem. 267, 7170–7175. 51. Irie, S., Hachiya, T., Rabizadeh, S., Maruyama, W., Mukai, J., Li, Y., Reed, J.C., Bredesen, D.E. & Sato, T.A. (1999) Functional interaction of Fas-associated phosphatase-1 (FAP-1) with p75 (NTR) and their effect on NF-kappaB activation. FEBS Lett. 460, 191–198. 52. Cuppen, E., Nagata, S., Wieringa, B. & Hendriks, W. (1997) No evidence for involvement of mouse protein-tyrosine phosphatase- BAS-like Fas-associated phosphatase-1 in Fas-mediated apopto- sis. J. Biol. Chem. 272, 30215–30220. 53. Cuppen, E., van Ham, M., Wansink, D.G., de Leeuw, A., Wier- inga, B. & Hendriks, W. (2000) The zyxin-related protein TRIP6 interacts with PDZ motifs in the adaptor protein RIL and the protein tyrosine phosphatase PTP-BL. Eur. J. Cell Biol. 79, 283–293. 54. Murthy, K.K., Clark, K., Fortin, Y., Shen, S.H. & Banville, D. (1999) ZRP-1, a zyxin-related protein, interacts with the second PDZ domain of the cytosolic protein tyrosine phosphatase hPTP1E. J. Biol. Chem. 274, 20679–20687. 55. Yi, J., Kloeker, S., Jensen, C.C., Bockholt, S., Honda, H., Hirai, H. & Beckerle, M.C. (2002) Members of the Zyxin family of LIM proteins interact with members of the p130Cas family of signal transducers. J. Biol. Chem. 277, 9580–9589. 56. Behrens,J.,vonKries,J.P.,Kuhl,M.,Bruhn,L.,Wedlich,D., Grosschedl, R. & Birchmeier, W. (1996) Functional interaction of beta-catenin with the transcription factor LEF-1. Nature 382, 638–642. 57. Kawasaki, Y., Sato, R. & Akiyama, T. (2003) Mutated APC and Asef are involved in the migration of colorectal tumour cells. Nat. Cell Biol. 5, 211–215. 58. Gross, C., Heumann, R. & Erdmann, K.S. (2001) The protein kinase C-related kinase PRK2 interacts with the protein tyrosine phosphatase PTP-BL via a novel PDZ domain binding motif. FEBS Lett. 496, 101–104. 59. Vincent, S. & Settleman, J. (1997) The PRK2 kinase is a potential effector target of both Rho and Rac GTPases and regulates actin cytoskeletal organization. Mol. Cell Biol. 17, 2247–2256. 60. Mellor, H., Flynn, P., Nobes, C.D., Hall, A. & Parker, P.J. (1998) PRK1 is targeted to endosomes by the small GTPase, RhoB. J. Biol. Chem. 273, 4811–4814. 61. Gampel, A., Parker, P.J. & Mellor, H. (1999) Regulation of epi- dermal growth factor receptor traffic by the small GTPase rhoB. Curr. Biol. 9, 955–958. 62. vanHam,M.,Croes,H.,Schepens,J.,Fransen,J.,Wieringa,B.& Hendriks, W. (2003) Cloning and characterization of mCRIP2, a mouse LIM-only protein that interacts with PDZ domain IV of PTP-BL. Genes Cells 8, 631–644. 63. Saras, J., Franzen, P., Aspenstrom, P., Hellman, U., Gonez, L.J. & Heldin, C.H. (1997) A novel GTPase-activating protein for Rho interacts with a PDZ domain of the protein-tyrosine phosphatase PTPL1. J. Biol. Chem. 272, 24333–24338. 64. Lin, D., Gish, G.D., Songyang, Z. & Pawson, T. (1999) The carboxyl terminus of B class ephrins constitutes a PDZ domain binding motif. J. Biol. Chem. 274, 3726–3733. 65. Ungefroren, H., Voss, M., Jansen, M., Roeder, C., Henne-Bruns, D., Kremer, B. & Kalthoff, H. (1998) Human pancreatic adeno- carcinomas express Fas and Fas ligand yet are resistant to Fas- mediated apoptosis. Cancer Res. 58, 1741–1749. 66. Itoh, N., Yonehara, S., Ishii, A., Yonehara, M., Mizushima, S., Sameshima, M., Hase, A., Seto, Y. & Nagata, S. (1991) The polypeptide encoded by the cDNA for human cell surface antigen Fas can mediate apoptosis. Cell 66, 233–243. 67. Oehm, A., Behrmann, I., Falk, W., Pawlita, M., Maier, G., Klas, C.,Li-Weber,M.,Richards,S.,Dhein,J.&Trauth,B.C.(1992) Purification and molecular cloning of the APO-1 cell surface antigen, a member of the tumor necrosis factor/nerve growth factor receptor superfamily. Sequence identity with the Fas anti- gen. J. Biol. Chem. 267, 10709–10715. 68. Peter, M.E., Krammer, P.H. (2003) The CD95 (APO-1/Fas) DISC and beyond. Cell Death. Differ. 10, 26–35. 69. Itoh, N. & Nagata, S. (1993) A novel protein domain required for apoptosis. Mutational analysis of human Fas antigen. J. Biol. Chem. 268, 10932–10937. 70.Saras,J.,Engstrom,U.,Gonez,L.J.&Heldin,C.H.(1997) Characterization of the interactions between PDZ domains of the protein-tyrosine phosphatase PTPL1 and the carboxyl-terminal tail of Fas. J. Biol. Chem. 272, 20979–20981. 71. Yanagisawa, J., Takahashi, M., Kanki, H., Yano-Yanagisawa, H.,Tazunoki,T.,Sawa,E.,Nishitoba,T.,Kamishohara,M., Kobayashi, E., Kataoka, S. & Sato, T. (1997) The molecular interaction of Fas and FAP-1. A tripeptide blocker of human Fas interaction with FAP-1 promotes Fas-induced apoptosis. J. Biol. Chem. 272, 8539–8545. 72. Myc,A.,Arscott,P.L.,Bretz,J.D.,Thompson,N.W.&Baker, J.R. Jr (1999) Characterization of FAP-1 expression and function in thyroid follicular cells. Endocrinology 140, 5431–5434. 73. Lee, S.H., Shin, M.S., Lee, J.Y., Park, W.S., Kim, S.Y., Jang, J.J., Dong, S.M., Na, E.Y., Kim, C.S., Kim, S.H. & Yoo, N.J. (1999) In vivo expression of soluble Fas and FAP-1: possible mechanisms of Fas resistance in human hepatoblastomas. J. Pathol. 188, 207–212. 74. Mundle, S.D., Mativi, B.Y., Bagai, K., Feldman, G., Cheema, P., Gautam, U., Reza, S., Cartlidge, J.D., Venugopal, P., Shetty, V., Gregory,S.A.,Robin,E.,Rifkin,S.,Shah,R.&Raza,A.(1999) Spontaneous down-regulation of Fas-associated phosphatase-1 may contribute to excessive apoptosis in myelodysplastic mar- rows. Int. J. Hematol. 70, 83–90. 75. Komada, Y., Inaba, H., Zhou, Y.W., Zhang, X.L., Tanaka, S., Azuma, E. & Sakurai, M. (1997) mRNA expression of Fas receptor (CD95)-associated proteins (Fas-associated phosphatase- 1/FAP-1, Fas-associating protein with death domain/FADD, and receptor-interacting protein/RIP) in human leukaemia/lymphoma cell lines. Br. J. Haematol. 99, 325–330. 76. Bompard, G., Puech, C., Prebois, C., Vignon, F. & Freiss, G. (2002) Protein-tyrosine phosphatase PTPL1/FAP-1 triggers apoptosis in human breast cancer cells. J. Biol. Chem. 277, 47861– 47869. 77. Ivanov, V.N., Lopez, B.P., Maulit, G., Sato, T.A., Sassoon, D. & Ronai, Z. (2003) FAP)1 association with Fas (Apo-1) inhibits Fas expression on the cell surface. Mol. Cell Biol. 23, 3623–3635. 78. Cao, T.T., Deacon, H.W., Reczek, D., Bretscher, A. & von Zas- trow, M. (1999) A kinase-regulated PDZ–domain interaction controls endocytic sorting of the beta2-adrenergic receptor. Nature 401, 286–290. 79. Swiatecka-Urban, A., Duhaime, M., Coutermarsh, B., Karlson, K.H., Collawn, J., Milewski, M., Cutting, G.R., Guggino, W.B., Langford, G. & Stanton, B.A. (2002) PDZ domain interaction controls the endocytic recycling of the cystic fibrosis transmembrane conductance regulator. J. Biol. Chem. 277, 40099– 40105. 80. Grunwald, I.C., Korte, M., Wolfer, D., Wilkinson, G.A., Uns- icker, K., Lipp, H.P., Bonhoeffer, T. & Klein, R. (2001) Kinase- independent requirement of EphB2 receptors in hippocampal synaptic plasticity. Neuron 32, 1027–1040. 81. Wilkinson, D.G. (2001) Multiple roles of EPH receptors and ephrins in neural development. Nat. Rev. Neurosci. 2, 155–164. 82. Kullander, K. & Klein, R. (2002) Mechanisms and functions of Eph and ephrin signalling. Nat. Rev. Mol. Cell Biol. 3, 475–486. 83. Cowan, C.A. & Henkemeyer, M. (2001) The SH2/SH3 adaptor Grb4 transduces B-ephrin reverse signals. Nature 413, 174–179. Ó FEBS 2003 The protein tyrosine phosphatase PTP-Bas/BL (Eur. J. Biochem. 270) 4797 84. Glotzer, M. (2001) Animal cell cytokinesis. Annu. Rev. Cell Dev. Biol. 17, 351–386. 85. Prokopenko, S.N., Saint, R. & Bellen, H.J. (2000) Untying the Gordian knot of cytokinesis. Role of small G proteins and their regulators. J. Cell Biol. 148, 843–848. 86. Skop, A.R., Bergmann, D., Mohler, W.A. & White, J.G. (2001) Completion of cytokinesis in C. elegans requires a brefeldin A-sensitive membrane accumulation at the cleavage furrow apex. Curr. Biol. 11, 735–746. 4798 K. S. Erdmann (Eur. J. Biochem. 270) Ó FEBS 2003 . REVIEW ARTICLE The protein tyrosine phosphatase PTP-Basophil/Basophil-like Interacting proteins and molecular functions Kai S. Erdmann* Department of Molecular Neurobiochemistry,. kinases and counteracted by the activity of protein tyrosine phosphatases [1]. The family of protein tyrosine phosphatases (PTPs) is divided into two major subtypes: (a) the receptor-like and (b) the. role in the assembly of supramolecular protein complexes [21]. Finally, the protein tyrosine phosphatase domain is located at the extreme C-terminus of the molecule. The complexity of the PTP-Bas/BL