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The enzyme-binding region of human GM2-activator protein Michaela Wendeler 1 , Norbert Werth 1 , Timm Maier 2 , Guenter Schwarzmann 1 , Thomas Kolter 1 , Maike Schoeniger 1 , Daniel Hoffmann 3 , Thorsten Lemm 1 , Wolfram Saenger 2 and Konrad Sandhoff 1 1 Kekule ´ -Institut fu ¨ r Organische Chemie und Biochemie der Universita ¨ t Bonn, Germany 2 Institut fu ¨ r Chemie, Abt. Kristallographie, Freie Universita ¨ t Berlin, Germany 3 Forschungszentrum caesar, Bonn, Germany Keywords GM2-activator; ganglioside degradation; b-hexosaminidase A; lipid transfer; lysosome Correspondence K. Sandhoff, Kekule ´ -Institut fu ¨ r Organische Chemie und Biochemie der Universita ¨ t Bonn, Gerhard-Domagk-Str. 1, D-53121 Bonn, Germany Fax: +49 228 73 7778 Tel: +49 228 73 5346 E-mail: sandhoff@uni-bonn.de (Received 8 November 2005, revised 21 December 2005, accepted 28 December 2005) doi:10.1111/j.1742-4658.2006.05126.x The GM2-activator protein (GM2AP) is an essential cofactor for the lyso- somal degradation of ganglioside GM2 by b-hexosaminidase A (HexA). It mediates the interaction between the water-soluble exohydrolase and its membrane-embedded glycolipid substrate at the lipid–water interface. Functional deficiencies in this protein result in a fatal neurological storage disorder, the AB variant of GM2 gangliosidosis. In order to elucidate this cofactor’s mode of action and identify the surface region of GM2AP responsible for binding to HexA, we designed several variant forms of this protein and evaluated the consequences of these mutations for lipid- and enzyme-binding properties using a variety of biophysical and functional studies. The point mutants D113K, M117V and E123K showed a drastic- ally decreased capacity to stimulate HexA-catalysed GM2 degradation. However, surface plasmon resonance (SPR) spectroscopy showed that the binding of these variants to immobilized lipid bilayers and their ability to solubilize lipids from anionic vesicles were the same as for the wild-type protein. In addition, a fluorescence resonance energy transfer (FRET)- based assay system showed that these variants had the same capacity as wild-type GM2AP for intervesicular lipid transfer from donor to acceptor liposomes. The concentration-dependent effect of these variants on hydro- lysis of the synthetic substrate 4-methylumbelliferyl-2-acetamido-2-deoxy-6- sulfo-b-d-glucopyranoside (MUGS) indicated a weakened association with the enzyme’s a subunit. This identifies the protein region affected by these mutations, the single short a helix of GM2AP, as the major determinant for the interaction with the enzyme. These results further confirm that the function of GM2AP is not restricted to a biological detergent that simply disrupts the membrane structure or lifts the substrate out of the lipid plane. In contrast, our data argue in favour of the critical importance of distinct activator–hexosaminidase interactions for GM2 degradation, and corrobor- ate the view that the activator ⁄ lipid complex represents the true substrate for the degrading enzyme. Abbreviations BMP, bis(monoacylglycero)phosphate; FRET, fluorescence resonance energy transfer; GM1, ganglioside GM1; GM2, ganglioside GM2; GM2AP, GM2-activator protein; GSLs, glycosphingolipids; HexA, b-hexosaminidase A; HexB: b-hexosaminidase B; LUV, large unilamellar vesicle; MUG, 4-methylumbelliferyl-2-acetamido-2-deoxy-b- D-glucopyranoside; MUGS, 4-methylumbelliferyl-2-acetamido-2-deoxy-6-sulfo-b- D-glucopyranoside; NBD, 7-nitrobenz-2-oxa-1,3-diazol-4-yl; PtdCho, phosphatidylcholine; PE, dioleoyl glycero phosphoryl ethanolamine; SAP, sphingolipid activator protein; SPR, surface plasmon resonance. 982 FEBS Journal 273 (2006) 982–991 ª 2006 The Authors Journal compilation ª 2006 FEBS Glycosphingolipids (GSLs) are characteristic compo- nents of eukaryotic plasma membranes. Over the last 20 years, their highly complex degradation pathway in lysosomes and the metabolic diseases associated with inherited defects in this pathway have been intensively investigated [1]. In the case of GSLs with rather short oligosaccharide head-groups of four or fewer sugar resi- dues, these membrane-embedded substrates are not suf- ficiently accessible for the degradation by water-soluble enzymes, and the exohydrolases need the assistance of small glycoprotein cofactors, the sphingolipid activator proteins (SAPs) [2]. For hydrolytic conversion of gan- glioside GM2 catalysed by b-hexosaminidase A (HexA, EC 3.2.1.52), the GM2-activator protein is required [3,4]. Three different isoforms of human lysosomal b-hexo- saminidases are known: b-hexosaminidase A (HexA), the heterodimer of the noncovalently linked a chain and b chain, and the homodimeric isoenzymes b-hex- osaminidase B (HexB, bb) and b-hexosaminidase S (aa). Despite the a and b subunits being 60% identical in their primary structure and functionally very similar, they show distinct specificity [5]. Only HexA is able to degrade the physiologically most important substrate, ganglioside GM2, at significant rates in the presence of the GM2 activator. Defects in any of the three genes encoding the polypeptides involved in GM2 degrada- tion, the a and b subunit of HexA or the GM2AP, result in the accumulation of nondegraded glycolipids within the lysosomal compartment and the develop- ment of severe neurodegenerative storage diseases known as GM2-gangliosidoses [4,6,7]. In its mature form present in the lysosomes, the GM2 activator is a small glycoprotein of 162 amino acids [8]. It is known to bind avidly to a variety of ani- onic lipids in vitro [9], to be able to extract several gly- cosphingolipid monomers from micelles or liposomes and to transport them as soluble 1 : 1 complexes between donor and acceptor membranes [10]. The membrane activity of GM2AP was found to depend critically on several physiological parameters, most notably a lateral pressure of the lipid bilayer below 15–25 mNÆm )1 [11] and the presence of anionic lipids, in particular bis(monoacylglycero)phosphate (BMP) [12]. Recently, a novel function of GM2AP in the con- text of glycolipid antigen presentation via CD1d was identified [13]. For the function of GM2AP in the lysosomal GM2 hydrolysis, the term ‘liftase’ was coined: it recognizes the lipid substrate within the membrane plane and lifts it out of the lipid bilayer, thereby presenting it to the water-soluble enzyme for degradation [14]. In addition, it has been suggested that GM2AP modifies the conformation of the trisaccharide unit of ganglioside GM2, thus facilitating the enzymatic hydrolysis of the terminal N-acetyl-d-galactosamine (GalNAc) residue [15]. Additional protein–protein interactions between GM2AP and HexA were implied in the catalysis of GM2 degradation [16,17]. Several lines of evidence indicate that GM2AP interacts with the a subunit of HexA, but the presence of the b subunit enhances binding [5,16,18,19]. The crystal structure of mature GM2AP expressed in Escherichia coli [20], as well as that of lipid complexes of GM2AP [21,22], revealed a novel protein fold, denoted b cup, which consists of an eight-stranded antiparallel b-pleated sheet forming a spacious hydrophobic cavity, as well as several surface loops and a single short a helix. The dimensions of the central pocket are such that it can accommodate the ceramide tail of GM2. Using photoaffinity labelling, we were able to establish that the most flexible of the surface loops, the chain segment V153–L163 constitutes the part of the activator protein that directly interacts with the ganglioside substrate [23]. (The amino acid numbering used here refers to the complete, prepro, form of GM2AP, including residues 1–31 which are removed by proteolytic processing. Please note that Protein Data Bank entry 1G13 describing the crystal structure of mature GM2AP expressed in E. coli, and also Wright et al. [21], differ from this nomenclature; they assign the number 1 to S32, which represents the N-terminus of mature GM2AP.) This interaction might be crucial for stabilization of the position of the glycoli- pid within the spacious cavity, thereby ensuring the correct orientation of the tetrasaccharide head-group with respect to the degrading enzyme’s active site. The goal of this study was to elucidate the interac- tion between cofactor and enzyme and to develop a comprehensive model for the GM2 degradation pro- ceeding at a phase frontier on intralysosomal mem- branes. Previously, attempts to identify the lipid- and enzyme-binding region of GM2AP were hampered because assay systems for GM2AP, which measure its stimulatory potential for the degradation of GM2 by HexA, reflect simultaneously both the interaction of GM2AP with GM2 and the interaction of the GM2AP ⁄ GM2 complex with the enzyme. To delineate in detail the separate effect of the introduced muta- tions on the lipid- and the enzyme-binding of this cofactor, we combined a variety of functional and bio- physical analyses. Crystallographic analysis of the homodimeric iso- enzyme HexB [24,25] and subsequent comparative modelling studies of heterodimeric HexA [25] suggested that the dimer interface of the enzyme forms the dock- ing site for the GM2AP ⁄ GM2 complex. Furthermore, M. Wendeler et al. Enzyme-binding region of GM2-activator protein FEBS Journal 273 (2006) 982–991 ª 2006 The Authors Journal compilation ª 2006 FEBS 983 based on the crystal structures of the enzyme [24,25] and of GM2AP [20–22], computational modelling and theoretical docking studies identified the single short a helix of GM2AP as the region most likely to be in direct contact with the enzyme. We therefore expressed a series of site-directed mutants of GM2AP with amino acid exchanges in this region of the protein. Only vari- ants with conformations identical to the wild-type (WT) GM2AP, as judged by CD spectroscopy, were considered. To study the stimulatory potential of GM2AP variants for ganglioside GM2 degradation by HexA, conventional micellar assays, as well as a deter- gent-free, liposomal assay system [12], were employed. The influence on the hydrolysis of the synthetic sub- strate 4-methylumbelliferyl-2-acetamido-2-deoxy-6-sulfo- b-d-glucopyranoside (MUGS), which is degraded by the a subunit of HexA and which hydrolysis is inhibited by GM2AP [5], allowed probing the variant activator’s binding to the enzyme. To study the membrane activity and lipid-binding properties, SPR spectroscopy with immobilized lipid bilayers was performed. Finally, a recently developed assay based on fluorescence reson- ance energy transfer (FRET) [26] enabled us to observe in real-time the ability for intervesicular lipid transfer from donor to acceptor vesicles. This allowed identification of the a-helical region of GM2AP as the major determinant in interacting with the enzyme. Our data further confirm conclusively the critical importance of distinct protein–protein interac- tions in GM2 degradation and corroborate the view that the activator ⁄ glycolipid complex represents the true substrate for the degrading enzyme. Results To identify the protein region of GM2AP that inter- acts with HexA in the lysosomal degradation of gan- glioside GM2, we performed site-directed mutagenesis of GM2AP and evaluated the biophysical and func- tional properties of the variant proteins. Initial homology modelling of HexA based on the recently published crystal structure of human HexB [25], and subsequent theoretical docking studies of the GM2AP structure [20,21] suggested that the a helix of GM2AP plays a major role in binding to the enzyme. In this helix, comprising amino acids F111 to P120, every fourth residue points in the same direction and the side chains of D113 and M117 were found to be oriented away from the GM2AP core towards the putative interface with the enzyme. The first residue following the GM2AP helix, which likewise points towards this suggested interaction region, was E123. The long and charged side chain of glutamate is known to be particularly well suited for protein–pro- tein interactions. In contrast, the residues between the end of the helix and E123 (P120, T121, G122) most likely serve a structural role only, allowing the loop to adopt a bent conformation. We therefore introduced the following mutations in this region of the protein: D113K, D113A, D113Y, M117V, E123K, E123A and E123Y. All variant pro- tein forms were readily produced in the insect cell⁄ baculovirus expression vector system (BEVS) and puri- fied to homogeneity by cation exchange and sub- sequent Ni-NTA chromatography. Protein yields of GM2AP mutants were in the range 4–7 mg purified protein per litre of expression supernatant, compared with 7–9 mg of the WT protein. To monitor any possible disturbances in the protein fold upon amino acid exchange, we subjected all protein variants to UV circular dichroism (CD) spectro- scopy (Fig. 1). Although secondary structure pre- dictions suggested identical conformations for the above-described GM2AP variants, it was found that the point mutants D113A, D113Y, E123A and E123Y showed altered CD spectra. In contrast, the mutations M117V, D113K and E123K resulted in CD spectra identical to that of the WT protein. Only these vari- ants of GM2AP were examined in all further studies. To assess the potential of the variant GM2AP forms for the stimulation of HexA catalysed hydrolysis of GM2, two established assays, involving either micellar or liposomal glycolipid substrate, were performed. As shown in Fig. 2, the mutant forms of GM2AP exam- ined here showed a drastically decreased ability to stimulate the hydrolysis of GM2 by HexA. In the mi- cellar system (Fig. 2A), the point mutant M117V dis- played 25–26%, the variant D113K 4–5% and E123K 3–4% of the WT capacity to stimulate the enzymatic degradation of GM2. In the liposomal system (Fig. 2B), which we introduced to mimic more closely the in vivo reaction conditions on the surface of intralysosomal vesicles and membrane structures [12], these results were confirmed and the differences between WT protein and mutants were even more pronounced. In this system, the variant M117V showed 7–8% of the WT activity, whereas the mutants D113K and E123K displayed almost a complete loss of activity. To assess the membrane activity and lipid-binding behaviour of the GM2AP variants, their interaction with immobilized lipid bilayers was measured using SPR spectroscopy in a Biacore instrument (Fig. 3). All mutant protein forms showed binding to the same extent as the WT protein. In the presence of the ani- onic lipid BMP, which is known to occur in inner membranes of the acidic compartments and which Enzyme-binding region of GM2-activator protein M. Wendeler et al. 984 FEBS Journal 273 (2006) 982–991 ª 2006 The Authors Journal compilation ª 2006 FEBS stimulates membrane activity of GM2AP [12], all mutants showed a comparable pronounced decrease in the SPR signal during the subsequent injection of pro- tein-free buffer. This response has been interpreted as the solubilization of lipids from the surface of immobi- Fig. 2. Stimulation of HexA-catalysed ganglioside GM2 degradation by WT and variant GM2AP. (A) Substrate hydrolysis measured in a micellar assay system. (B) GM2 degradation measured in a lipo- somal assay system with LUVs containing 50 mol% PtdCho, 20 mol% Chol, 10 mol% GM2 and 20 mol% BMP. Values repre- sent means of duplicate measurements; deviations observed were < 5%. Fig. 3. Interaction of WT and variant GM2AP with immobilized vesi- cles measured by SPR spectroscopy. Negatively charged LUVs containing 20 mol% Chol, 50 mol% PtdCho, 10 mol% GM2 and 20 mol% BMP, were immobilized on a Pioneer L1 sensorchip. GM2AP (2 l M) was injected at a flow rate of 20 lLÆmin )1 in 50 mM sodium citrate buffer, pH 4.2, for 180 s, followed by the injection of protein-free buffer. The measurement started with the protein injection. A B C Fig. 1. UV circular dichroism (CD) spectra of wild-type (WT) GM2AP and variant proteins examined in this study. (A) WT GM2AP and mutant M117V. (B) WT GM2AP and D113 mutants. (C) WT GM2AP and E123 mutants. Spectra were obtained at 10 °C, at a protein concentration of 0.5 mgÆmL )1 in 15 mM sodium phosphate, pH 7.0, 100 m M NaCl. M. Wendeler et al. Enzyme-binding region of GM2-activator protein FEBS Journal 273 (2006) 982–991 ª 2006 The Authors Journal compilation ª 2006 FEBS 985 lized membrane structures via the action of bound GM2AP [12], either by directly extracting lipids or by destabilizing the bilayers with a concomitant loss of lipids. The point mutants examined here were found to interact with immobilized lipid bilayers in the same way as WT GM2AP. To study directly the lipid-transfer properties of GM2AP, we recently introduced a novel FRET-based assay system [26]. A new class of fluorescent glyco- sphingolipid analogue, either 2-(N-(7-nitrobenz-2-oxa- 1,3-diazol-4-yl)) (NBD)–GM1 or 2-NBD–GM2, serves as FRET-donor for rhodamine–dioleoyl glycero phos- phoryl ethanolamine (PE) as acceptor which is initially localized within the same model membrane and thereby quenches NBD fluorescence. It has been shown previ- ously that GM1 and GM2 are transferred almost equally well between vesicles by GM2AP [10]. These novel fluorescent gangliosides are easily incorporated into model membranes, where they exhibit an extremely low ‘off rate’ and are recognized and degraded by HexA in the presence of GM2AP, similar to their natural ana- logue. Upon addition of catalytic amounts of GM2AP to donor vesicles containing NBD–gangliosides, the activator-mediated glycolipid transfer from donor to acceptor liposomes increases the NBD fluorescence, permitting continuous real-time monitoring of GM2AP transfer activity. As shown in Fig. 4, the point mutants examined here showed the same ability as WT GM2AP to transfer NBD–GM1 from donor to acceptor vesicles. The initial rate of NBD transfer and the capacity observed under equilibrium conditions are identical. It has previously been shown that, in the absence of ganglioside GM2, the GM2 activator protein inhibits hydrolysis of the synthetic fluorogenic substrate MUGS by HexA [5]. This negatively charged substrate can only be degraded by the a subunit of the enzyme, and the inhibitory effect of GM2AP has been inter- preted as being indicative of a physical association between GM2AP and the enzyme which blocks this subunit’s active site. As shown in Fig. 5, all variant forms of GM2AP examined show concentration- dependent inhibition of MUGS degradation. However, their inhibitory effect is less pronounced than that of WT GM2AP. The presence of GM2AP and of all point mutants has no effect on the degradation of the neutral, unsulfated substrate MUG (data not shown). Apparently, all GM2AP variants studied here retain the ability to interact specifically with the a subunit of the enzyme. However, the strength of their interaction with HexA is weaker than for the WT protein, suggest- ing that the introduced mutations did indeed affect the binding interface to the enzyme. Discussion The GM2 activator protein is a small glycoprotein cofactor essential for the lysosomal degradation of ganglioside GM2 by HexA [3]. It mediates the interac- tion between the water-soluble enzyme and the mem- brane-embedded glycolipid substrate. Our goal was to map the protein region on GM2AP responsible for binding to the enzyme and to further elucidate the activator’s mode of action at the lipid–water interface. The capacity of the variant proteins examined to sti- mulate HexA-catalysed GM2 degradation was meas- ured in a micellar and a liposomal assay system. In order to separate the influence of the mutations on the Fig. 4. Increase in NBD fluorescence through GM2AP-mediated NBD–GM1 transfer measured in a FRET-based liposomal assay sys- tem as a function of time. The assay volume of 400 lL contained 8 nmol donor liposomes (20 mol% Chol, 56 mol% PC, 20 mol% phosphatidic acid, 2 mol% NBD-GM1, 2 mol% rhodamine-PE) and 40 nmol acceptor liposomes (20 mol% Chol, 60 mol% PtdCho, 20 mol% phosphatidic acid) and 37.5 n M WT or variant GM2AP. Values are means of duplicate measurements, deviations observed were always < 5%. Fig. 5. Competition experiment. Degradation of the synthetic fluo- rogenic substrate MUGS by HexA is inhibited in the presence of GM2AP in a concentration-dependent manner, suggesting that HexA and GM2AP form a complex which sterically blocks the act- ive site of the HexA a subunit. In the presence of GM2AP variants, MUGS degradation is reduced to a lesser extent, indicating that the physical association of HexA with these mutant forms is weaker. Enzyme-binding region of GM2-activator protein M. Wendeler et al. 986 FEBS Journal 273 (2006) 982–991 ª 2006 The Authors Journal compilation ª 2006 FEBS lipid- and enzyme-binding properties, we tested the membrane activity of GM2AP using SPR spectroscopy and probed the lipid-transfer properties in a FRET- based assay system. The ability to bind specifically the a subunit of HexA could be assessed by studying the competition of GM2AP and its variants with the hydrolysis of the synthetic fluorogenic hexosaminidase substrate MUGS. Our data clearly indicate that mutations introduced into the a-helical region of GM2AP drastically decreased the ability of the activator to stimulate HexA-catalysed degradation of GM2. However, the membrane activity of these variants, as well as their lipid-binding and transfer ability, was identical to that of the WT protein. All variants could still bind speci- fically to the a subunit of HexA, but their association with the enzyme was weaker than for WT GM2AP. This clearly identifies the region affected by these mutations, the single a helix of GM2AP, as the major surface epitope interacting with the enzyme. That the mutants D113K and E123K exhibit the most drastically reduced stimulatory activity for GM2 degradation and show the weakest binding to HexA might be attributed to the reversal of local charge resulting from the exchange of acidic amino acids to lysine. In addition to possible electrostatic repulsion, all salt bridges and hydrogen bonds which are possible between positively charged residues on HexA and amino acids D113 and E123 in WT GM2AP can no longer be formed. Figure 6 shows the position of the examined point mutations at the interface between activator and HexA, as a result of theoretical docking studies based on the known crystal structures of GM2AP [20–22] and HexB [24,25]. Our results are consistent with the location of the ceramide tail of GM2 as identified by crystallo- graphic analysis of lipid complexes of GM2AP [21], which positions the tetrasaccharide head-group of the substrate in direct vicinity of the enzyme’s active site. Among patients with variant AB of GM2 gangliosi- dosis, whose severe neurological storage of ganglioside GM2 results from the functional deficiency of GM2AP, no case has been found in which the muta- tion occurred directly within the a-helical region of GM2AP. However, the biochemical consequence of one fatal mutation, the amino acid exchange C138R, was thought to be mainly a disrupted interaction with the enzyme [17], and this might well be affected by altered orientation of the helix. The crystal structure of GM2AP [20,21] and the known disulfide bond connec- tivity [27] indicate that the disulfide bond formed by C138 with C112 forms a ‘clamp’ that structurally restrains the activator’s helical domain. It is therefore most likely that the mutation C138R distorts the orien- tation of the helix and compromises the activator’s ability to interact with the enzyme. In addition to mapping the activator’s binding inter- face with the enzyme, our data confirm the critical importance of distinct protein–protein interactions in lysosomal GM2 degradation. The activator’s role is not limited to that of a biological detergent that simply disrupts the membrane structure. In contrast, the acti- vator ⁄ glycolipid complex represents the true Michaelis substrate for the degrading enzyme. Experimental procedures Materials For insect cell culture, serum-free IPL-41 medium was pur- chased from Invitrogen Life Technologies (Carlsbad, CA). The baculovirus transfer vector pAcMP3, linearized baculo- virus DNA (BaculoGold), the transfection kit and the insect cell lines were from Pharmingen, obtained via BD Bio- sciences (Erembodegem, Belgium). Phosphatidylcholine (egg yolk, PtdCho) and cholesterol (Chol) were purchased from Sigma (Taufkirchen, Germany). BMP was obtained from Avanti Polar Lipids (Alabaster, AL), Lichroprep RP18 was Fig. 6. Model for the interface in the ternary complex of GM2AP, GM2 and HexA resulting from theoretical docking studies based on the known crystal structures of GM2AP [20–22] and HexB [25]. In the GM2AP structure b sheets are blue, the single a helix is green, and regions exhibiting conformational differences in free and ligand- bound form are red. The position of the bound ligand GM2 follows the model presented in Fig. 4 of Wright et al. [21], as far as the lipid tail is concerned. Regarding the tetrasaccharide head-group, however, we consider a more ‘strained’ orientation as presented here, more likely. The picture was generated using MOLSCRIPT [30]. M. Wendeler et al. Enzyme-binding region of GM2-activator protein FEBS Journal 273 (2006) 982–991 ª 2006 The Authors Journal compilation ª 2006 FEBS 987 from Merck (Darmstadt, Germany). The Pioneer L1 Chip was purchased from Biacore (Uppsala, Sweden). All other chemicals and solvents were of analytical grade or the high- est purity available. Protein expression and mutagenesis Recombinant GM2AP was expressed in BEVS as described previously [28]. Site-directed mutagenesis using the vector pAcMP3-GM2APHis 6 [28] as a template was performed using the QuikChange site-directed mutagenesis kit from Stratagene (La Jolla, CA). Synthetic oligonucleotides were obtained from Sigma and Invitrogen Life Technologies. Mutations encoding the following amino acid exchanges were introduced into the cDNA of GM2AP: D113K, D113A, D113Y, M117V, E123K, E123A, E123Y. All expression vectors encoded a hexahistidine tag at the C-ter- minus of GM2AP. The resulting constructs encoding point mutants of GM2AP were fully sequenced using an ABI 310 sequencer and the BigDye cycle sequencing kit, both from Applied Biosystems (Foster City, CA). Recombinant bacu- loviruses were then generated by cotransfection of Sf9 cells with the respective transfer vector and linearized BaculoGold viral DNA according to the manufacturer’s protocol. Pure viral stocks were obtained using the end-point dilution method. For protein expression, Sf9 cells grown in serum-free IPL-41 to a cell density of 1.5 · 10 6 cellsÆmL )1 were infec- ted with a viral stock at a multiplicity of infection of 5. The medium was harvested 96 h after infection. Protein purification Proteins were purified, with modification, following the method described by Wendeler et al. [28]. Briefly, the super- natant was first subjected to perfusion chromatography on a cation exchange resin (Poros HS) using a BIOCADSprint System HPLC workstation (Applied Biosystems). Fractions containing recombinant GM2AP were combined and then further purified by immobilized metal ion affinity chroma- tography on Ni-NTA-agarose (Qiagen, Hilden, Germany). The eluted proteins were analysed by electrophoresis on a 12.5% tricine-SDS ⁄ polyacrylamide gel and visualized by silver staining. The polyclonal antibody raised against WT GM2AP [29] recognized all GM2AP mutants. Amino acid exchanges in the recombinant proteins were confirmed by ESI-TOF-MS. Micellar GM2AP assay In a micellar in vitro system, the activity of recombinant GM2AP was tested by measuring the stimulation of hex- osaminidase A-catalysed hydrolysis of [ 3 H]GM2 tritium- labelled in its terminal GalNAc moiety [3]. In 40 lLof 100 mm sodium citrate buffer containing 2.5 lg BSA, 10 nmol [ 3 H]GM2 were incubated with 80 mU HexA (puri- fied from post mortem human liver) in the presence of 3 lg recombinant GM2AP for 1 h. Reactions were stopped by adding 40 lL methanol. Liberated [ 3 H]GalNAc was isola- ted using self-packed RP18 cartridges: 0.5 mL LiChroprep RP18 (Merck) was applied to a glass Pasteur pipette, pre- viously stuffed with a small amount of glass wool. The material was equilibrated by washing subsequently with 2 · 1 mL chloroform ⁄ methanol 1 : 1 (v ⁄ v), 2 · 1mL methanol and 2 · 1 mL chlorofom ⁄ methanol ⁄ 0.1 m KCl 3 : 47 : 48 (v ⁄ v ⁄ v). The assay solution was then applied and the flow-through collected. Soluble [ 3 H]GalNAc was then eluted with 2 · 1 mL water, and the eluate combined with the flow-through and 10 mL scintillation liquid. Radioactivity in the effluents was measured in a scintilla- tion counter (Packard). One activator unit is defined as the amount of GM2AP that stimulates the degradation of 1 nmol GM2 per minute and enzyme unit. Liposomal assay systems To mimic more closely the reaction conditions encountered on intralysosomal vesicles and membrane structures, the degradation of membrane-bound ganglioside GM2 by hex- osaminidase and GM2AP was studied in a detergent-free, liposomal assay system as described previously [12]. Vesicle prepapration Large unilamellar vesicles (LUVs) of 100 nm were prepared by the following procedure. Appropriate aliquots of the lipid solutions PtdCho (50 mm, toluol ⁄ ethanol 2 : 1 v ⁄ v), BMP (10 mm, chloroform ⁄ methanol 1 : 1 v ⁄ v), Chol (25.6 mm, chloroform ⁄ methanol 2 : 1 v ⁄ v) and GM2 trit- ium-labelled in its terminal GalNAc moiety (0.5 mm, tolu- ol ⁄ ethanol 1 : 1 v ⁄ v) were mixed and dried in a stream of nitrogen. The lipid mixture was dissolved to a total lipid concentration of 2 mm in sodium citrate buffer (50 mm, pH 4.2) and freeze–thawed 10 times in liquid nitrogen to ensure solute equilibration between trapped and bulk solutions. Unilamellar vesicles were prepared by passage through two polycarbonate filters (pore size, 100 nm; Aves- tin) mounted in tandem in a mini-extruder (Liposo-Fast; Avestin, Ottawa, Canada) a total of 19 times. Liposomal assay The standard incubation mixture using GM2 as substrate contained the following components in a final volume of 50 lL: BSA (50 lgÆmL )1 ), sodium citrate buffer (pH 4.2, 50 mm), unilamellar liposomes (total lipid concentration: 1mm), GM2AP (2 lm) and HexA (25 mU). Liposomes had the following composition: [ 3 H]GM2 (10 mol%, Enzyme-binding region of GM2-activator protein M. Wendeler et al. 988 FEBS Journal 273 (2006) 982–991 ª 2006 The Authors Journal compilation ª 2006 FEBS 1.8 CiÆmol )1 ), Chol (20 mol%), PtdCho (50 mol%) and BMP (20 mol%). The standard incubation conditions were 37 °C for 30 min, and the enzyme assays were stopped by the addition of 50 lL methanol. Terminated enzyme assays were loaded onto a reverse-phase column (RP18, 1 mL) equilibrated with a solution of chloroform ⁄ methanol ⁄ 0.1 m KCl (3 : 48 : 47, v ⁄ v ⁄ v). The column was eluted with 2 mL of the same solvent, and the radioactivity in the effluents was measured in a scintillation counter. Determination of HexA activity with the fluorogenic substrates 4-methylumbelliferyl- 2-acetamido-2-deoxy-b- D-glucopyranoside and its sulfated derivative MUGS The activity of HexA towards the synthetic substrates 4-methylumbell iferyl-2-acetamido-2-deoxy-b-d-glucopyrano- side (MUG) and its sulfated derivative MUGS was meas- ured essentially as described previously [5]. For routine activity measurements, MUG was used as substrate under the following standard conditions. In a volume of 40 lL, 10 mm citrate buffer, pH 4.2, 2 mm substrate, 6 lg BSA and an appropriate amount of HexA were incubated at 37 °C for 30 min. One enzyme unit is defined as the amount of hexosaminida se that g enerates 1 lmolÆmin )1 of 4-methylumbell iferone. MUGS was used in competition experiments designed to study the association of GM2AP with the a subunit of HexA. In a volume of 40 lL, 10 mm citrate buffer, pH 4.2, 2 · 10 )5 m substrate, 6 lg BSA and an appropriate amount of HexA were incubated at 37 °C for 30 min in the absence or presence of various amounts of GM2AP (up to 10 lg). Reactions were stopped by the addition of 5 vol. of a 0.2 m glycine, 0.2 m Na 2 CO 3 solution and the generated 4-methyl- umbelliferone determined by measuring the fluorescence at 440 nm after excitation at 365 nm. UV CD spectroscopy CD spectroscopy was used to monitor potential distur- bances of the protein fold upon introduction of the individ- ual mutations. CD spectra in the range of 250 to 200 nm were acquired on a JASCO J600 spectropolarimeter with four accumulations in a 0.1 cm cuvette at 10 °C for solu- tions of 0.5 mgÆmL )1 of the mutant proteins in 15 mm sodium phosphate, 100 mm NaCl, pH 7. Data processing and evaluation was carried out with jasco software as recommended by the manufacturer. SPR spectroscopy Biomolecular interaction analyses (BIA, SPR spectroscopy) were carried out at 25 °C with a Bialite instrument (Bia- core) on a Pioneer L1 Chip. For SPR spectroscopy, lipo- somes were prepared as described above, but diluted to a lipid concentration of 0.5 mm in NaCl ⁄ P i (10 mm phos- phate, 140 mm NaCl, 10 mm KCl pH 7.4). These liposomes contained unlabelled GM2. First, LUVs (with a total lipid concentration of 0.5 mm), diluted in NaCl ⁄ P i , were immobilized on the sensorchip by two injections at a flow rate of 5 lL Æ min )1 (first injection 60 lL, second injection 20 lL) as described previously [12]. This resulted in a RU shift of 5000–7000 RU. Ten micro- litres of NaOH (25 mm) was then injected at 100 lLÆmin )1 to remove multilamellar structures and to stabilize the base- line, resulting in a RU shift of ~ 20–50 RU. Sixty micro- litres of GM2AP (1 or 2 lm) in running buffer (50 mm sodium citrate buffer, pH 4.2) were injected into the flow cells at a rate of 20 lLÆmin )1 with a dissociation time of 180 s. Measurements started with the injection of GM2AP. FRET-based assay for GM2AP The fluorescent analogue 2-NBD–GM1 was synthesized as described previously [26]. Large unilamellar donor vesicles were prepared with the following composition: PtdCho (56 mol%), Chol (20 mol%), phosphatidic acid (20 mol%), 2-NBD–GM1 (2 mol%) and rhodamine–PE (2 mol%). Acceptor vesicles comprised PtdCho (60 mol%), Chol (20 mol%) and phosphatidic acid (20 mol%). The final donor vesicle concentration in the assay mixture was 20 nmolÆmL )1 , the concentration of acceptor vesicles was 100 nmolÆmL )1 in a total volume of 400 lLin50mm citrate buffer, pH 4.2. The transfer of NBD–GM1 was started by the addition of GM2AP to a final concentration of 37.5 nm (WT or point mutants). Fluorescence measurements were performed at 28 °C using a quartz cuvette in a Shimadzu RF5000 instrument (Kyoto, Japan) with an excitation wave- length of 480 nm and an emission wavelength of 522 nm. Without the addition of GM2AP, no increase in fluorescence was observed over a period of 2 h. For each experimental time point over the interval ranging from 0.5 to 30 min, the instrument’s shutter was opened for only 3–4 s, resulting in a total of maximal 52 s illumination of NBD–GM2. Under these conditions, photobleaching was negligible. Acknowledgements We thank Dr Christina Schuette, Luebeck, for provi- ding purified HexA, and Dr Joerg Hoernschemeyer, Loerrach, for performing ESI-TOF-MS. This study was supported by the Deutsche Forschungsgemeinsc- haft, SFB 284 and SA-257 ⁄ 21-1. References 1 Kolter T & Sandhoff K (1999) Sphingolipids – their metabolic pathways and the pathobiochemistry of M. Wendeler et al. 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Biochem J 292, 571–576. 30 Kraulis PJ (1991) MOLSCRIPT: a program to produce both detailed and schematic plots of protein structures. J Appl Crystallogr 24, 946–950. M. Wendeler et al. Enzyme-binding region of GM2-activator protein FEBS Journal 273 (2006) 982–991 ª 2006 The Authors Journal compilation ª 2006 FEBS 991 . interface of the enzyme forms the dock- ing site for the GM2AP ⁄ GM2 complex. Furthermore, M. Wendeler et al. Enzyme-binding region of GM2-activator protein FEBS. that of the WT protein. Only these vari- ants of GM2AP were examined in all further studies. To assess the potential of the variant GM2AP forms for the