32.4 How Are Receptor Signals Transduced? 1033 More than 170 calcium-modulated proteins are known. All possess a characteris- tic peptide domain consisting of a short ␣-helix, a loop of 12 amino acids, and a sec- ond ␣-helix (Figure 32.33). Robert Kretsinger at the University of Virginia initially discovered this pattern in parvalbumin, a protein first identified in the carp fish and later in neurons possessing a high firing rate and a high oxidative metabolism. Kretsinger lettered the six helices of parvalbumin A through F. He noticed that the E and F helices, joined by a loop, resembled the thumb and forefinger of a right hand and named this structure the EF hand, a name in common use today to iden- tify the helix-loop-helix motif in calcium-binding proteins. In the EF hand, Ca 2ϩ is coordinated by six carboxyl oxygens contributed by a glutamate and three aspar- tates, by a carbonyl oxygen from a peptide bond, and by the oxygen of a coordi- nated water molecule. The EF hand was subsequently identified in calmodulin, tro- ponin C, and calbindin-9K. Most of the known EF-hand proteins possess two or more (as many as eight) EF-hand domains, usually arranged so that two EF-hand do- mains may directly contact each other. Calmodulin Target Proteins Possess a Basic Amphiphilic Helix The conformations of EF-hand proteins change dramatically upon binding of Ca 2ϩ ions. This change promotes binding of the EF-hand protein with its target pro- tein(s). For example, calmodulin (CaM), a 148-residue protein found in many cell types, modulates the activities of a large number of target proteins, including Ca 2ϩ -ATPases, protein kinases, phosphodiesterases, and NAD ϩ kinase. CaM binds to these and to many other proteins with extremely high affinities (K D values typically in the high picomolar to low nanomolar range). All CaM target proteins possess a basic amphiphilic alpha helix (a Baa helix), to which CaM binds specifically and with high affinity. Viewed end-on, in the so-called helical wheel representation (Fig- ure 32.34), a Baa helix has mostly hydrophobic residues on one face; basic residues are collected on the opposite face. However, the Baa helices of CaM target proteins, although conforming to the model, show extreme variability in sequence. How does CaM, itself a highly conserved protein, accommodate such variety of sequence and structure? Each globular domain consists of a large hydrophobic surface flanked by regions of highly negative electrostatic potential—a surface suitable for interacting with a Baa helix. The long central helix joining the two globular regions behaves as Lys Lys Lys Lys Lys Lys Leu Leu Leu Leu Leu Trp Leu Leu 1 2 3 4 5 6 7 8 9 10 11 12 13 14 (a) His Lys Thr Leu Thr Ser Phe Met Val Ala Val Trp Ala Arg 1 2 3 4 5 6 7 8 9 10 11 12 13 14 (b) FIGURE 32.34 Helical wheel representations of (a) a model calmodulin-binding peptide, Ac-WKKLLKLLKKLLKL- CONH 2 , and (b) the calmodulin-binding domain of spectrin. Positively charged and polar residues are indi- cated in green, and hydrophobic residues are orange. (Adapted from O’Neil, K., and DeGrado,W., 1990. How calmodulin binds its targets: Sequence independent recognition of amphiphilic ␣-helices. Trends in Biochemical Sciences 15:59–64.) 1034 Chapter 32 The Reception and Transmission of Extracellular Information a long, flexible tether. When the target protein is bound, the two globular domains fold together (Figure 32.33b). The flexible nature of the tethering helix allows the two globular domains to adjust their orientation synergistically for maximal binding of the target protein or peptide. 32.5 How Do Effectors Convert the Signals to Actions in the Cell? Transduction of the hormonal signal leads to activation of effectors—usually pro- tein kinases and protein phosphatases—that elicit a variety of actions that regulate discrete cellular functions. Of the thousands of mammalian kinases and phos- phatases, the structures and functions of a few are representative. A DEEPER LOOK Mitogen-Activated Protein Kinases and Phosphorelay Systems In multicellular organisms, many physiological processes, includ- ing mitosis, gene expression, metabolism, and programmed death of cells, are regulated by a family of mitogen-activated protein kinases (MAPKs). (A mitogen is any agent that induces cell divi- sion, that is, mitosis.) MAPKs phosphorylate specific serines and threonines of target protein substrates, and these phosphorylation events function as switches to turn on or off the activity of the sub- strate proteins. These “substrates” may be other protein kinases, phospholipases, transcription factors, and cytoskeletal proteins. Protein phosphatases reverse the process, removing the phos- phates that were added by MAPKs. MAPKs are part of a phosphorelay system composed of three kinases that are activated in sequence (see accompanying figure). In such systems, the MAPK itself is phosphorylated by a MAPK kinase (denoted MKK), which is itself phosphorylated by a MAPK kinase kinase (denoted MKKK). MKKKs have distinct domains and motifs that respond to different cellular stimuli, and they have other domains that recognize specific MKKs. The same kinds of specificities regulate the action of MKKs. These specificities are accounted for in the classification of four subfamilies of MAPKs: One group is that of the extracellular signal-regulated kinases, notably ERK1 and ERK2; another is the c-Jun-amino-terminal kinases, including JNK, JNK1, and JNK2; a third group depends on the ERK5 kinase; and the fourth group involves the p38 ki- nases, including p38␣, p38, p38␥, and p38␦. There are un- doubtedly other MAPK families yet to be discovered. As shown in the figure, these phosphorelay systems link a variety of stimuli to substrates that affect many cellular functions. From Johnson, G. L., and Lapadat, R., 2002. Mitogen-activated protein ki- nase pathways mediated by ERK, JNK, and p38 protein kinases. Science 298: 1911–1912. Oxidative stress Src MEKK2 MKK5 ERK5 MEF2 IL-1 TRAF6- TAB1/2 TAK1 MKK6 p38 MNK1 Integrin Rac1 MEKK1 MKK4 JNK1 c-Jun Growth factor RasGTP c-Raf1 MKK1 ERK1 p90RSK Stimulus Activator MKKK MKK MAPK Substrate ᮣ MAPK phosphorelay systems. The left column is a general model. The four columns to the right show the four known MAPK phosphorelay families. 32.5 How Do Effectors Convert the Signals to Actions in the Cell? 1035 Protein Kinase A Is a Paradigm of Kinases Most protein kinases share a common catalytic core first characterized in protein ki- nase A (PKA), the enzyme that phosphorylates phosphorylase kinase (see Figure 15.9 and Figure 15.17). The active site of the catalytic subunit of PKA in a ternary complex with MnAMP–PNP and a pseudosubstrate inhibitor peptide, as shown in Figure 32.35, includes a glycine-rich -strand that acts as a flap over the triphosphate moiety of the bound nucleotide. A conserved residue, Asp 166 , is the catalytic base that deprotonates the Ser/Thr-OH during phosphorylation, and Lys 168 stabilizes the tran- sition state of the reaction. Three Glu residues on the enzyme are involved in recog- nition of the pseudosubstrate inhibitor peptide. Protein Kinase C Is a Family of Isozymes The enzymes called protein kinase C are actually a family of similar enzymes— isozymes—that encompass three subclasses. The “conventional PKCs,” ␣, I, II, and ␥, are regulated by Ca 2ϩ , diacylglycerol (DAG), and phosphatidylserine (PS). Because Ca 2ϩ levels increase in the cell in response to IP 3 , the activation of conven- tional PKCs depends on both of the second messengers released by the hydrolysis of PIP 2 . The “novel PKCs,” ␦, ⑀, , and , are Ca 2ϩ -independent but are regulated by DAG and PS. PKCs , , and are termed “atypical” and are activated by PS alone. These various cofactor requirements are imparted by subdomains represented in the conventional PKC polypeptide sequence. Conventional PKCs are comprised (Figure 32.36) of four conserved domains (C1–C4) and five variable regions (V1–V5). Domain C1 is a pseudosubstrate sequence that regulates the kinase by intra- steric control (see page 461), C2 is a Ca 2ϩ -binding domain, C3 is the ATP-binding do- main, and C4 binds peptide substrates. FIGURE 32.35 The structure of the catalytic subunit of PKA in a ternary complex with MnAMP–PNP and a pseudosubstrate inhibitor peptide (pink). A glycine-rich -strand acts as a flap over the triphosphate moiety of the bound nucleotide.The glycine-rich flap that covers the ATP-binding site is shown in white, AMP–PNP is bound in the ATP site, Asp 166 is shown in beige, and Lys 168 is shown below in white (pdb id ϭ 1ATP). Conventional ␣ I II ␥ pseudosubstrate hinge N C1A C1B C2 C3 C4 C N CNovel ␦ ⑀ NC Atypical FIGURE 32.36 The primary structures of the PKC isozymes. Conserved domains C1–C4 are indicated. Variable regions are shown as simple lines. Go to CengageNOW at www.cengage.com/login and click BiochemistryInteractive to explore the structure and function of protein kinase C. 1036 Chapter 32 The Reception and Transmission of Extracellular Information PKC phosphorylates serine and threonine residues on a wide range of protein substrates. A role for protein kinase C in cellular growth and division is demon- strated through its strong activation by phorbol esters (Figure 32.37). These com- pounds, from the seeds of Croton tiglium, are tumor promoters—agents that do not themselves cause tumorigenesis but that potentiate the effects of carcinogens. The phorbol esters mimic DAG, bind to the regulatory pseudosubstrate domain of the enzyme, and activate protein kinase C. Protein Tyrosine Kinase pp60 c-src Is Regulated by Phosphorylation/Dephosphorylation The structure of protein tyrosine kinase pp60 c-src (see Figure 32.20) consists of an N-terminal “unique domain,” an SH2 domain, an SH3 domain, and a kinase do- main that includes a small lobe comprised mainly of a twisted -sheet and a large lobe that is predominantly ␣-helical (Figure 32.38; see also Section 32.2). Phospho- rylation of Tyr 527 in the SH2 domain inhibits tyrosine kinase activity by drawing an “activation loop” into the active site, blocking ATP and/or substrate binding. De- phosphorylation of Tyr 527 induces a conformation change that removes the activa- tion loop from the active site, permitting autophosphorylation of Tyr 416 , which stim- ulates tyrosine kinase activity. Protein Tyrosine Phosphatase SHP-2 Is a Nonreceptor Tyrosine Phosphatase The human phosphatase SHP-2 is a cytosolic nonreceptor tyrosine phosphatase. It comprises two SH2 domains, a catalytic phosphatase domain, and a C-terminal do- main. The SH2 domains enable the enzyme to bind to its target substrates, and they also regulate the phosphatase activity. The catalytic domain of SHP-2 consists of nine ␣-helices and a ten-stranded mixed -sheet that wraps around one of the helices (Fig- ure 32.39). The other eight helices pack together on the opposite side of the -sheet. The N-terminal SH2 domain regulates phosphatase activity by binding to the phosphatase domain and directly blocking the active site. When a target peptide containing a phosphotyrosine group binds to the SH2 domain, a conformation CH 2 OH 12-O-Tetradecanoylphorbol-13-acetate H 3 C O 1 2 3 OH 4 10 56 7 8 9 H 11 12 13 14 H 3 C 18 O O 15 CH 3 CH 3 17 16 CCH 3 O CCh 3 (CH 2 ) 12 O OH FIGURE 32.37 The structure of a phorbol ester. Long- chain fatty acids predominate at the 12-position, whereas acetate is usually found at the 13-position. Small lobe Large lobe C helix G-rich loop Activation loop (A-loop helix) Cat. loop Linker Tail SH2 SH3 Tyr 416 pTyr 527 FIGURE 32.38 A ribbon diagram showing the struc- ture of protein tyrosine kinase pp60 c-src with bound AMP–PNP.(Image kindly provided by Stephen C. Harrison.) 32.6 How Are Signaling Pathways Organized and Integrated? 1037 change causes this domain to dissociate from the catalytic domain, exposing the active site and allowing peptide substrate to bind. Binding of phosphotyrosine- containing peptide to the second SH2 domain provides additional activation. Target peptides with two phosphotyrosines (one to bind to each SH2 domain) provide maximal activation of the phosphatase activity. 32.6 How Are Signaling Pathways Organized and Integrated? All signaling pathways are organized in time and space in the cell, they are care- fully regulated, and they are integrated with one another. Remarkably, these com- plex features of signaling depend on the simple concepts already covered in this chapter: PIDs (see Figures 32.5 and 32.6) modulate and control the association of signaling molecules with one another, often in large signalsomes; signaling mole- cules are switched on and off by covalent modifications such as phosphorylations; and signaling often involves amplification and cooperative effects. The multifac- eted behavior of GPCRs serves as a paradigm for the organization and integration of signaling pathways and is the focus of this section. GPCRs Can Signal Through G-Protein–Independent Pathways The classic GPCR signaling pathway involves coupling to heterotrimeric G proteins, but GPCRs can interact with other effector molecules as well. The cellular src kinase (page 1023) can be activated directly by the ß 2 -adrenergic receptor (Figure 32.40), leading to activation of a MAPK pathway (see A Deeper Look box, page 1034). Ac- tivated src phosphorylates Tyr 317 on the Shc adaptor protein, promoting binding by an SH2 domain of Grb2. Grb2 in turn binds to and activates Sos1, which activates Ras. Ras then activates a kinase cascade (Figure 32.40). The Janus protein kinase (JAK) and its associated transducers and activators of transcription (STAT) constitute the JAK/STAT signaling pathway (Figure 32.41). For example, Jana Stankova and her colleagues have shown that binding of the platelet-activating factor (see Figure 8.8b) to the platelet-activating factor receptor (a GPCR) directly stimulates phosphorylation of tyk2, a Janus kinase. The activa- tions of src (described earlier) and tyk2 raise the intriguing possibility that GPCR action may represent an alternate means of activating these signaling pathways (Fig- ures 32.40 and 32.41). PTP N-SH2 C-SH2 FIGURE 32.39 A ribbon diagram showing the structure of protein tyrosine phosphatase SHP-2 in its auto- inhibited, closed conformation.The N- and C-terminal SH2 domains are orange and purple, respectively. The catalytic domain is blue (pdb id ϭ 2SHP). GPCR Src kinase Grb2 SH2 SH2 P Tyr Shc P Raf I (MAPKKK)(MAPKK) MEK1 ERK1 (MAPK) Ras P P P P Sos Tyr 317 FIGURE 32.40 GPCRs can initiate cellular signaling path- ways without involvement of G proteins. Under certain conditions, binding to GPCRs can induce autophospho- rylation of the src kinase. Activated src can then bind Shc and phosphorylate Tyr 317 , which promotes binding of Grb2 and initiation of a Ras-dependent kinase cascade. 1038 Chapter 32 The Reception and Transmission of Extracellular Information G-Protein Signaling Is Modulated by RGS/GAPs The signal-transducing effects of G proteins persist as long as the bound GTP is not hydrolyzed. However, G proteins, such as Ras p21 and G s␣ , are themselves weak GTPases. For example, Ras p21 hydrolyzes GTP with a rate constant of only 0.02 min Ϫ1 . If Ras p21 and G s␣ were efficient enzymes, the GTP-bound state would be short-lived and G-protein–mediated signaling would be ineffective. How can G proteins be switched off if they are inherently poor GTPases? The an- swer is provided by regulators of G-protein signaling (RGS), which act as GTPase- activating proteins (GAPs). RGS/GAPs elicit dramatic increases in GTPase activity STAT P STAT STAT P STAT P STAT P P Tyk2 Altered gene expression Transphosphorylation DNA P Tyk2 P PP PP P EPO EPO receptor JAK P Platelet-activating factor receptor oligomer Tyk2Tyk2 STAT STAT STAT dimer Nucleus STAT FIGURE 32.41 Platelet-activating factor receptor (a GPCR) can trigger autophosphorylation of the tyk2 kinase (a Janus protein kinase).Tyk2-induced phosphorylation of STAT essentially mimics the activation of the JAK/STAT signaling pathway by hormones such as erythropoietin (EPO). 32.6 How Are Signaling Pathways Organized and Integrated? 1039 when bound to G proteins. For example, RGS/GAPs increase the GTPase activity of Ras p21 by a factor of 100,000 and accelerate G s␣ -catalyzed GTP hydrolysis nearly 100-fold. RGS/GAPs induce conformation changes in the switch domains of their G protein targets and stabilize the transition state of the GTPase reaction (Figure 32.42). Interestingly, however, RGS proteins are more than GAPs. All RGS proteins con- tain several other signaling modules in addition to the 120-residue RGS module (Figure 32.43). These additional modules enable RGS proteins to bind to a variety of signaling proteins, to behave as effector molecules themselves, and to act as scaf- folding proteins in the formation of signalsome complexes. RGS proteins interact directly, for example, with adenylyl cyclase, phospholipase C-, cGMP phospho- diesterase, guanylyl cyclase, Ca 2ϩ channels, and potassium channels (Figure 32.44). These RGS-mediated interactions enable “cross-talk” between many signaling pathways. GPCR Desensitization Leads to New Signaling Pathways Activation of GPCRs by hormones and other extracellular signals leads to hetero- trimeric G protein binding, which triggers a variety of intracellular signals as shown. Importantly, the activated GPCR also binds two other classes of molecules: a family ANIMATED FIGURE 32.42 (a) A fragment of an RGS/GAP (brown) bound to Ras p21 (blue). GAPs increase the GTPase activity of Ras p21 by a factor of 100,000 (pdb id ϭ 1WQ1). (b) An RGS/GAP (pink) bound to G i␣ (blue). GDP is shown in yellow (pdb id ϭ 2IK8). See this figure ani- mated at www.cengage.com/login. (a) (b) PH PDZ PTB Kinase RGS RGS PDZ PEST RGS PP2A -Cat DIX RBD GoLoco GSKRGS GGLDEP RGS RGS G/GRK D/R12 B/R4 E/RA C/R7 B/R4 Subfamily ␣-Helix Acidic FIGURE 32.43 The RGS proteins (mammalian cells contain more than 30) are classified in subfamilies. Representative members of six of those subfamilies are shown. In addition to the RGS module of 121 residues, each RGS protein contains other motifs and modules that define its functionality. Many of those shown are discussed elsewhere in Chapter 32. (Adapted from Bansal, G., Druey, K.M., and Xie, Z., 2007. R4 RGS proteins: Regulation of G-protein signaling and beyond. Pharmacology and Therapeutics 116:473–495.) 1040 Chapter 32 The Reception and Transmission of Extracellular Information C/R7 B/R4 B/R4 D/R12 PDZ R9AP P PTB PTB Rhodopsin RGS? RGS Phospholipase C- Retinal guanylyl cyclase cGMP Phospho- diesterase N-type Ca 2ϩ channel GPCR GPCR GPCR GIRK channel GPCR Adenylyl cyclase ␥ ␦ ␣ 2 ␣ a  ␣ ␥ ␥ C1 C2 ␥   ␥  ␥   5  ␣t ␥ ␣q ␣ i/o ␥  ␣o ␣s FIGURE 32.44 Several interactions between RGS proteins and G-protein effectors. RGS proteins interact directly with adenylyl cyclase, phospholipase C-, cGMP phosphodiesterase, and retinyl guanylyl cyclase, as well as with the potassium channel known as GIRK and certain calcium channels. (Adapted from Abramow-Newerly, M., Roy, A. A., Nunn, C., and Chidiac, P., 2006. RGS proteins have a signaling complex: Interactions between RGS proteins and GPCRs, effectors, and auxiliary proteins. Cellular Signaling 18:579–591.) A DEEPER LOOK Whimsical Names for Proteins and Genes The study of cell signaling and the identification of hundreds of new signaling proteins provided an unprecedented creative opportunity for cell biologists and geneticists in the naming of these proteins. In the early days of molecular biology, such names were typically arcane abbreviations and acronyms. One such case is the family of 14-3-3 proteins, named for the migration patterns of these proteins on DEAE-cellulose chromatography and starch-gel electrophoresis. In the 1970s, a few creative scientists suggested whimsical names for newly discovered genes, such as sevenless, named in reference to R7, one of the eight photoreceptor cells in the compound eye of Drosophila, the common fruit fly. What began as a trickle became a torrent of whimsical names for proteins and genes. Sevenless was fol- lowed by bride of sevenless (boss, a ligand of sevenless), and son of sevenless (sos, first isolated in a genetic screen of the sevenless receptor tyrosine kinase pathway in Drosophila). The hedgehog (hh) genes, including sonic hedgehog (Shh), play critical roles in the devel- opment and patterning of vertebrate embryonic tissues but were named for a popular video game. The accompanying table lists a few notable examples of whim- sically named genes and gene products, many of which were first identified in Drosophila. Name Role or Function Armadillo Plakoglobin ϭ -catenin Bag of marbles Novel protein involved in oogenesis and spermatogenesis Bullwinkle Oocyte protein Cactus Signaling protein—IkB homolog Cheap date Alcohol sensitivity Chickadee Profilin homolog—regulation of actin cytoskeleton Corkscrew A protein tyrosine phosphatase Dachshund Novel nuclear protein of unknown function Dishevelled Novel cytoplasmic protein in the wingless pathway Dunce A cAMP phosphodiesterase Hopscotch A Janus family tyrosine kinase Naked A segment polarity gene Reaper A death-domain protein functioning in apoptosis RutabagaA Ca 2ϩ /calmodulin-dependent protein kinase Shark A tyrosine kinase SH2-nonreceptor Yak Literally “yet another kinase” 32.6 How Are Signaling Pathways Organized and Integrated? 1041 of protein kinases known as known as GPCR kinases (GRKs) and a family of adap- tor and scaffolding proteins known as -arrestins. On one level, these two protein families work together to desensitize the GPCRs, “arresting” the G-protein activation by GPCRs. On another level, the GRKs and -arrestins act together as a molecular switch, directing GPCRs to a distinctly different role in cell signaling. GRK phos- phorylation of several sites on the C-terminal sequence of the GPCR promotes bind- ing of -arrestin (Figure 32.45) to form a signalsome assembly. Binding of -arrestin has two effects: The GPCR is no longer able to interact with and activate G pro- teins, and the GPCR is targeted for clathrin-mediated endocytosis (Figure 32.45). Following formation of the endosome, the GPCR–-arrestin signalsome enters an arrestin signaling mode, recruiting a variety of catalytically active proteins, such as components of the Raf-MEK-ERK signaling cascade. Arrestin-mediated signaling continues until the GPCR returns to the plasma membrane, where it waits for a new extracellular signal. It is important to appreciate that G-protein signaling oc- curs within seconds of extracellular GPCR activation and typically lasts for 10 min- utes or less, whereas signaling by the GPCR–-arrestin complex becomes maximal around 10 minutes and typically persists for 30 minutes. Receptor Responses Can Be Coordinated by Transactivation Hormone and signaling receptors and their pathways do not operate in isolation. Rather, one or more of the second messengers and effectors of a given pathway can P P P E H Signal initiation Clathrin-coated pit Early endosome G-protein– signaling mode Arrestin- signaling mode Clathrin-coated vesicle  ␣ GTP ␥ H H H H H GRK -Arr P P  AP-2 ␥ P P Dyn Dyn -Arr Raf-1 ERK1/2 MEK -Arr Raf-1 ERK1/2 MEK -Arr Raf-1 ERK1/2 MEK P PP P P P PP P P P PP P P P P FIGURE 32.45 GPCRs have two signaling modes. Signal initiation by ligand binding first activates the G-protein– mediated signaling pathways at the plasma membrane.At the same time, G-protein receptor kinases (GRKs) begin to phosphorylate the receptor, creating high-affinity binding sites for arrestin. Arrestin binding uncouples the receptor from G proteins and targets the receptor for endocytosis.The GPCR–arrestin complex serves as a sig- nalsome scaffold, recruiting catalytically active proteins such as components of the Raf-MEK-ERK cascade.This ini- tiates a distinct set of signals from the endosome-bound signalsome. Endosomes are eventually processed and degraded, returning the GPCRs to the plasma membrane. (Adapted from Figure 1 of Gesty-Palmer, D., and Luttrell,L. M., 2008. Heptahelical terpsichory.Who calls the tune? Journal of Receptors and Signal Transduction 28:39–58.) 1042 Chapter 32 The Reception and Transmission of Extracellular Information often activate (or inhibit) another pathway. For example, binding of insulin-like growth factor 1 (IGF-1) to its own RTK signals enhanced synthesis of a peptide known as RANTES. RANTES is secreted into the extracellular medium, where it binds to a GPCR known as the CCR5 receptor (Figure 32.46a). As an extracellular signal, RANTES initiates a signaling pathway that induces cell migration, a part of (a) Transcriptional regulation of GPCR ligand synthesis Cell migration  IGF-1 IGF-1R Cytosol Nucleus ␣ i ␥ P P P P CCR5 RANTES RANTES RANTES mRNA RANTES promotor (b) Ras Shc/Grb2/Sos P13-K PKC/Ca 2+ Ros MEK ERK Raf Survival MMP Pro HB-EGF HB-EGF EGFR Akt P Src G q /PLC ␣ 1 -AR FIGURE 32.46 (a) An example of transactivation of cell signaling pathways. Binding of IGF-1 to RTK (in human breast cancer cells) initiates a signaling pathway that leads to an enhanced synthesis of RANTES (a peptide). RANTES is secreted into the extracellular medium, where it can bind a GPCR known as CCR5. CCR5 activation triggers G-protein–mediated signaling pathways. (b) Transactivation that is the reverse of that in panel a. Activation of the ␣ 1 -adrenergic receptor activates phospholipase C-, which triggers metalloproteinase (MMP) activation. MPP cleaves a precursor of heparin-binding EGF-like growth factor (HB-EGF). Binding of HB-EGF to the EGF receptor initiates an RTK signaling pathway.