Signalling at the plasma membrane 41 Monyer, H. et al. (1992) Science 256, 42

43 44 45

1217-1221 Hutton, M. L., Harvey, R. J, Barnard, E. A. and Darlison, M. G. (1991) FEBS Lett. 292, 111-114 Schuster, C. M. et al. (1991) Science 254, 112-254 Henley, J. M. et al. (1992) Proc. Natl Acad. Sci. USA 89, 4806-4810 Knoflach, F. et al. (1991) FEBS Lett. 293, 191-194

46 Ishihara, T. et al. (1992) Neuron 8, 811-819 47 Tanabe, Y. et al. (1992) Neuron 8, 169-179 48 Vu, T-K. H., Hung, D. T., Wheaton, V. I. and Coughlin, S. R. (1991) Cell 64, 1057-1068 49 Schulz, S., Chinkers, M. and Garbers, D. L. (1989) FASEB J. 3, 2026-2035 50 Brown, A. M. and Birnbaumer, L. (1990) Annu. Rev. Physiol. 52, 218-248 51 Takeshita, T. et al. (1992) Science 257,

379-382 52 Benham, C. D. and Tsien, R. W. (1987) Nature

TIBS 17 - OCTOBER 1992 328, 275-278 53 Bernardi, H., Fosset, M. and Lazdunski, M. (1988) Proc. Natl Acad. Sci. USA 85,

9816-9820 54 Mulle, C., Choquet, D., Korn, H. and Changeux, J-P. (1992) Neuron 8, 135-143 55 Vemino, S. et al. (1992) Neuron 8, 127-134 56 Loosfelt, H. et al. (1992) Proc. Natl Acad. Sci. USA 89, 3765-3769 57 Duggan, M. J., Pollard, S. and Stephenson, F. A. (1991) J. Biol. Chem. 266, 24778-24784

ALL RECEPTOR TYROSINE KINASES

(RTKs) that have been studied phosphorylate themselves on tyrosine (autophosphorylation) in response to ligand binding. Several experiments over the past three years have highlighted an important function of autophosphorylation: individual phosphotyrosine residues of receptors appear to serve as highly selective binding sites that are specific for cytoplasmic signaling molecules. These signaling molecules mediate the pleiotrophic responses of cells to growth factors. Most of the studies have used the platelet-derived growth factor 13-receptor (PDGF[3r) and fibroblast growth factor receptor (FGFr). When activated by a ligand, the intracellular region of the PDGF[3r binds several signaling molecules, including phosphatidylinositol 3-kinase (PtdIns3-kinase), GTPase-activating factor (GAP), phospholipase C~/ (PLCT) and c-Src (for review see Ref. 1). Receptor mutagenesis studies demonstrated that PtdIns3-kinase binds to two distinct sites on the PDGF[3r (Tyr708 and Tyr719 of the murine receptor) 2-4 while GAP binds to a third site (Tyr739) (Fig. 1)4,5. PLC~' binds to Tyr766 of a human FGFr6,7 and to two phosphotyrosines in the carboxy-terminal portion of the PDGFJ3r (Fig. 1; L. R6nstrand, submitted). Determining the structural basis for the specificity of the interaction between RTKs and signaling molecules has established what is likely to be a general principle of how tyrosine-phosphorylated proteins bind regulatory molecules. Phosphotyrosine-containing peptides as short as five amino acids, M. J. Pazin and L. T. Williams are at the Howard Hughes Medical Institute, Program of Excellence in Molecular Biology,and Cardiovascular Research Institute, University of California, San Francisco, CA 94143-0724, USA.

374

Growth factor receptors that are tyrosine kinases (RTKs) regulate growth and differentiation of cells in many organisms, including flies, worms, frogs, mice and humans. There has been recent progress in understanding the mechanism by which these receptors transduce signals. Worm and insect studies on RTKs have relied primarily on genetics, while the mammalian studies have employed a combination of molecular genetics and biochemistry. While many RTKs seem to have unique features, there are also many general signal transduction principles that emerge from these studies. In this review, we will focus on common signaling molecules, using RTKs from both vertebrates and invertebrates as examples.

representing RTK sequences at the signaling molecule-binding sites, can selectively block the interaction of signaling molecules with RTKs5. The binding of peptides to signaling molecules is highly specific and occurs at low concentrations of peptide (1-10 Bm), but only when the peptide is phosphorylated on its key Tyr. For example, the peptide YPVPML, where YP represents phosphotyrosine, is a PDGF[~r PtdIns3-kinasebinding site that blocks PtdIns3-kinase binding to PDGF[~r but does not block two other signaling molecules, GAP and PLCy5. YPMAPYDNY, which represents the GAP-binding site on the PDGF[3r, blocks GAP binding and has no effect on the binding of PtdIns3-kinase or PLCy, and SNQEY2"LDLS, representing the FGFr PLCy-binding site, blocks PLC~, binding to FGFr and to PDGFJ3r6,7 (Table I). In separate experiments, the specificity of the peptide interaction was confirmed by mutating the Tyr or Met residues in the PDGF[3r PtdIns3-kinasebinding site, expressing the mutant receptor in fibroblasts and epithelial

cells, and demonstrating that these mutations cause a loss of PDGF-stimulated activation of PtdIns3-kinase 4,5, Motifs homologous to the PDGFJ3r Ptdlns3-kinase-binding site are found in other RTKs, including the macrophage colony stimulating factor receptor (MCSFr). Mutations of the Tyr in this motif cause a loss of PtdIns3-kinase binding to the M-CSFr8. These studies indicate that short sequences flanking receptor phosphotyrosines are major determinants of the remarkable binding specificity of signaling molecules to RTKs.

SH2 domainsand bindingto RTKs The region of the signaling molecules that binds RTKs has been characterized. Hanafusa and co-workers demonstrated that SH2 domains (domains homologous to a non-catalytic region of the c-src proto-oncogene) will bind directly to Tyr-phosphorylated proteins in transformed cells 9. Most of the signaling molecules known to bind RTKs contain SH2 domains. Although at high concentration (achievable in vitro) or in

© 1992, Elsevier Science Publishers, (UK) 0376-5067/92/$05.00

TIBS 17 - OCTOBER 1992

cells that overexpress recombinant proteins, SH2-containing proteins will bind non-specifically to phosphotyrosines in many sequence contexts 1°, the highaffinity interactions that occur between tyrosine-phosphorylated proteins and SH2 domains in vivo are highly specific. Mutations of the Src, Fps and Abl SH2 domains modulate the specificity and activity of these kinases, suggesting that the SH2 domains may be involved in protein-protein interactions (for review see Ref. 11). Mutagenesis studies of the SH2 domain from the c-Abl kinase suggest that the highly conserved FLVRESsequence is required for interaction with phosphotyrosine 12. As described above for the interaction of PtdIns3-kinase and GAP with the PDGFI3r, the specificity is determined by short sequences on the carboxy-terminal side of the receptor phosphotyrosine. One can envision the phosphotyrosine binding directly to the FLVRES sequence with amino acids that flank the phosphotyrosines fitting into a binding pocket on the SH2 domain, which dictates the specificity of the phosphoprotein-SH2 domain interaction. Thus, specific SH2 domains interact with specific phosphotyrosine-containing proteins. Some of the many proteins recently discovered to have SH2 domains have known activities. These include phosphatases, phospholipases, GAP, cytoskeletal-binding proteins and cytoplasmic kinases. Proteins in the crk family, including c-Crk 13, NckTM and Sem-515, consist entirely of SH2 and SH3 domains and may serve to link tyrosinephosphorylated proteins to other intracellular molecules. In the nematode Caenorhabditis elegans, the sem-5 gene encodes a protein consisting of an amino-terminal SH3 domain followed by an SH2 domain and a carboxy-terminal SH3 domain. This protein appears to play an important role in vulval development (see below).

Mapping intracellular signaling cascades The mutation of Tyr residues on the RTKs that are responsible for binding to specific signaling molecules provides an approach for assessing the roles of signaling molecules in cellular responses to growth factors. When the PtdIns3kinase-binding sites of the PDGFI3r are mutated and the mutant receptor is expressed in murine epithelial cells, PDGF-stimulated PtdIns3-kinase activation and mitogenesis are abrogated, even though inositol phosphate accu-

Signalling at

the

plasma m e m b r a n e

(a) i

I I

I

Mitogenesis

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77?

PLC~'

~

Ptdlns hydrolysis Elevated cytoplasmic Ca2+

(b)

Mitogenesis

YP

I

PLCy

~

Ptdlns hydrolysis Elevated cytoplasmic Ca2+

Figure 1 Interactions of (a) PDGF[]r and (b) FGFr with cytoplasmic signaling molecules. Numbers refer to specific phosphotyrosine residues (YP) in the intracellular region of the murine PDGFI~r and human FGFr-1. Each RTK phosphotyrosine-containing region binds a specific signaling molecule. The signaling molecules, which include GTPase activating protein (GAP), phospholipase C-y (PLCy) and phosphatidylinositol kinase (Ptdlns3-kinase) activate the indicated cellular responses. A question mark (?) indicates an unknown phosphotyrosine residue, signaling molecule or cellular response.

mulation, calcium influx and GAP tyrosine phosphorylation are unaffected in this mutant s. In cells that express this mutant receptor, PDGF can not increase the amount of GTP bound by Ras (an indicator of Ras activation). PtdIns3-kinase is, therefore, essential for mitogenesis and may activate Ras in these cells. By contrast, cells expressing the PDGFi3r mutant that lacks the GAPbinding site have normal Ptdlns3-kinase activation, Ras activation and mitogenesis 5. The approach of selectively eliminating a mitogenic pathway by a point mutation of the receptor has been particularly useful in assessing FGF-stimulated processes 6. An FGFr with a point mutation of Tyr766 does not bind PLCT, activate Ptdlns hydrolysis or cause an elevation of cytosolic calcium (Ca2*). Yet

this mutant receptor stimulates mitogenesis in myoblasts to an even greater extent than that of the wild-type receptor. Therefore neither Ptdlns hydrolysis nor Ca2+ mobilization is required for FGFinduced mitogenesis in these cells. It is possible that PLC3'plays a role in attenuating the response to growth factors. The insulin receptor (INr) is an example of a RTK which has PDGF[3r-like as well as alternative signaling pathways. For example, insulin also activates a PtdIns3-kinase. However, most of the PtdIns3-kinase activity does not physically associate with the INr, but instead appears to associate with a substrate, IRS-1 (pp185), of the lNr TM. The IRS-1 protein contains nine repeats of a YXXMX motif (where X is any amino acid), which has been shown to bind PtdIns3-kinase in PDGFH3r.An INr mutant 375

Signalling at the plasma membrane Table I. Tyrosine-phosphorylatedpeptides that bind specifically to signaling molecules and block their interaction with RTKs

Drosophila melanogaster is known to have at least four RTKs: Sevenless, Torso, Drosophila EGF Peptidea Source Signaling molecule receptor homolog (DER) YPMDMS Tyr708, PDGF~r Ptdlns3-kinase and Drosophila FGF reY~/PML Tyr719, PDGF~r Ptdlns3-kinase ceptor homolog (DFGFr), YPMAPYDNY Tyr739, PDGF[3r GAP some of which are known SNQEYPLDLS Tyr766, FGFr PLCy to signal through Ras. Sevenless is most similar ayp, phosphotyrosine. Abbreviations: Ptdlns3-kinase, phosphatidylinositol 3-kinase; to the vertebrate RTK GAP, GTPase-activating factor; PLCy, phospholipase C-y. c-Ros, a member of the INr subfamily. It is required to specify cell fate in which Tyr1146 is replaced by Phe in the developing eye and its activity (Y1146F) has reduced autophosphoryl- induces a non-neuronal lens-secreting ation activity and is defective in Tyr cell to develop into a specific photophosphorylation of IRS-1. The mutant receptor cell, R722, Flies carrying loss-ofreceptor is defective in mitogenic sig- function CLOF) alleles of sevenless fail to naling, but not metabolic activation (gly- induce R7 development, but have no cogen synthase activation) ~7. Whether other obvious defects. Elegant genetic PtdIns3-kinase is activated by this or studies have determined that several other autophosphorylation site mutants signal transduction molecules found in has not yet been reported. However, vertebrates function downstream from the result from studies on PDGF[3r that the Sevenless RTK. Four homologs of vertebrate ras and Ptdfns3-kinase activation requires binding to a Tyr-phosphorylated molecule its regulators have been implicated in suggests that Ptdlns3-kinase may not be sevenless signaling (Fig. 2b). It is known activated by the mutant INrs. Thus, a that rasl function is required for proper single point mutation that selectively R7 development; gain-of-function (GOF) inactivates one insulin response (mito- mutants no longer require Sevenless genesis) may disrupt PtdIns3-kinase ac- protein 23. The SOS locus, which encodes a protein homologous to ras activators tivation. called guanine-nucleotide-exchange factors (GNEFs), is also required 23. These Ras and RTKs A variety of experiments have dem- GNEFs, such as CDC25 in Saccharoonstrated a role for Ras GTP-binding myces cerevisiae, activate Ras by proproteins in mammalian RTK signal trans- moting exchange of GDP in Ras with duction. Initial experiments using micro- GTP. In contrast, gapl LOF mutants no injected neutralizing Ras antibodies re- longer require sevenless function, so vealed a requirement for Ras activation they behave like ras GOF mutants 24. for PDGF-induced mitogenesis. More Another putative negative regulator of recently, experiments using dominant- R7 development is the rapl gene prodnegative c-ras alleles have indicated uct. Drosophila rapl, which appears to that Ras plays an important role in epi- antagonize Ras signaling, is homologous dermal growth factor (EGF)-, PDGF- and to vertebrate rapla, a gene whose prodFGF-induced mitogenesislE Dominant- uct suppresses transformation by actinegative c-ras alleles also block FGF- vated ras alleles 25. In GOF rapl mutants, and nerve growth factor (NGF)-induced R7 cells fail to develop, as in rasl LOF neurite outgrowth ~7. Dominant-negative mutants. LOF alleles of SOS, rasl or c-ras alleles block the induction of c-fos rapl have a recessive lethal phenotype, expression by EGF, PDGF, FGF, NGF and suggesting that they function in many insulin ~8J9, suggesting that Ras acts at a Drosophila pathways 23-2s. Cell signaling has been intensively step before gene expression. The protein kinase Raf-1 probably acts at steps studied with genetics in the nematode subsequent to Ras activity, because v-raf C. elegans, a lower metazoan with a nortransformation is not suppressed by mally invariant developmental fate dominant-negative c-ras2°. The PDGF]3r map. The gonadal anchor cell produces results summarized earlier suggest that an inductive signal which causes three Ras is downstream of Ptdlns3-kinase; in- of six vulval precursor ceils to adopt deed, Ptdlns3-kinase has recently been the vulval fate. The development of the found to associate with Ras21. The order- vulva also requires at least two other ing of these events is shown sche- processes, lateral specification among the induced vulval precursor cells and matically in Fig. 2a. 376

TIBS 17 - OCTOBER 1992 an inhibitory signal from the surrounding hypodermal syncitium to the vulval precursor cells. The let-23 gene product is required for induction of vulval precursor ceils, and is proposed to act in the presumptive vulval cells 26, Molecular analysis has revealed that the let-23 gene encodes a RTK of the EGFr family. GOF mutations in let-23 cause a multivulval phenotype and most LOF mutations cause a vulvaless phenotype, suggesting that activation of the let-23 RTK induces vulval precursor ceils. While vulval development is not essential in C. elegans, null mutations in let-23 are lethal, suggesting that let-23 functions in other essential signaling pathways. Another gene essential for vulval precursor cell induction is let-60. GOF mutations cause a multivulval phenotype, while LOF mutations cause a vulvaless phenotype2L Thus, let-60 is a positive regulator of VPC induction. Molecular analysis established that the let-60 product is a nematode homolog of ras. Known GOF mutations in let-60 are similar to mutations which activate mammalian Ras. The let-60 gene is epistatic to let-23, suggesting that it is downstream from let-23 or that it functions in an alternative pathway (Fig. 2c). Like Drosophila rasl, let-60 appears to function as a developmental switch and has properties similar to mammalian ras. The lin-ll gene, encoding a putative transcription factor, is believed to be indirectly regulated by let-60. Null mutations of the C. elegans ras homolog are lethal. Worms that do not possess the inhibitory signal from the hypodermal syncitium (lin-15 mutants) exhibit a multivulval phenotype, which is suppressed by let-23, let-60 or sem-5 LOF alleles ~5. An attractive hypothesis is that sem-5, an SH2-containing protein, binds to a Tyr-phosphorylated protein, perhaps Let-23. Like the leb23 (RTK) gene product, Sem-5 acts upstream of Let-60 (Ras), so it is possible that Sere-5 activates let-60, either directly or through a GNEF. One hypothetical ordering of these signaling molecules is shown in Fig. 2c. Intracellular serine kinases

Most growth factors stimulate a group of intrace]lular protein kinases that includes Rafl kinase, MAP kinase and protein kinase C. Activation of Rafl kinase appears to be essential for serum-induced proliferation of fibroblasts. Rafl kinase may also be involved

TIBS 17 - OCTOBER 1992 in activating specific genes, such as c-fos 2s. Although the Rafl kinase can be directly activated by RTKs in vitro 29, it is likely that in vivo there are intermediate molecules that mediate the activation of Rafl. Dominant-negative ras alleles can block growth-factor activation of Rafl kinase and MAP kinase 3°, placing Ras upstream of both kinases. The Rafl protein has an amino-terminal regulatory domain that appears to suppress the activity of the kinase domain. Relief of this suppression may involve modification of the regulatory domain or a non-covalent interaction of this domain with an activator. Recently, an essential role for Rafl kinase in the mesoderm-inducing effects of FGF in Xenopus has been demonstrated by using dominant-negative mutants of Rafl (A. MacNicol, T. Muslin and L. T. Williams, unpublished). Whether the dominant-negative mutant inhibits Rafl activators or competes for substrates has not been established. Members of the family of MAP kinases are activated by most RTKs and are strong candidates as substrates of Raf kinases. Mitogen activation of MAP kinases has recently been reviewed 31. A review of protein kinase C family members and their potential roles in mitogenesis is included in this issue. Drosophila raf [D-raf, or I(1) polehole] and torso RTK genes were both isolated from a screen for mutants in embryonic differentiation. Both genes are required for correct induction of other genes that specify the anterior and posterior termini of the developing embryo 32. The torso gene product is present throughout the embryo and activated only at the termini by a spatially localized ligand. LOF alleles of torso prevent formation of the anterior-most and posterior-most structures. LOF alleles of D-raf closely mimic these features, while GOF mutations of D-raf have similar phenotypes to torso GOF mutations. Furthermore, D-raf mutations are epistatic to torso, suggesting a model in which the D-raf protein kinase is activated, directly or indirectly, by the Torso RTK. One gene which is farther downstream is tailless, whose product encodes a putative transcription factor in the steroid hormone receptor family. Thus, D-raf may function to regulate gene expression in higher metazoans. Recently, Rasl and SOS have been demonstrated to play a role in terminal specification (H. Doyle and J. M. Bishop, submitted). By analogy to mammalian systems, they probably function between

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Figure 2 Signaling cascades initiated by RTKs. Stimulatory pathways (~) and inhibitory pathways (4) are indicated. Confirmed and proposed pathways are indicated by solid and dashed lines, respectively. (a) In vertebrate systems, phosphatidylinositol 3-kinase (Ptdlns3kinase) appears to act upstream of Ras and may also interact with pathways mediated by Rafl and MAP kinases. Recently identified homologues of Sem-5 may also regulate Ras. (b) In Drosophila photoreceptor development an RTK (Sevenless) regulates a guanine nueleotide exchange protein (SOS) which, in turn, regulates Rasl. (c) In C. elegans an RTK (Let-23) appears to act through an SH2-containing protein (Sere-5) to regulate a Ras-like protein (Let-60) in the pathway that controls vulval development. (d) Development of the anterior and posterior termini of Drosophila is regulated by a series of genes that encode an RTK (Torso), and an SH2 protein that is a tyrosine phosphatase (Corkscrew, not shown), a guanine nucleotide-exchange protein (SOS), a homologue of Ras and a homologue of Raf-1 (D-Raf).

torso and D-raf. A proposed signal transduction pathway is shown in Fig. 2d. The combined approach of genetics and biochemistry has yielded great insight into RTK signaling. Several general features have emerged from these studies. Many signaling molecules appear to have SH2 domains or interact with SH2 proteins. SH2 domains can target proteins by binding phosphotyrosine-containing proteins with high affinity. Ras, which has long been known as a central molecule in mammalian systems, has now been demonstrated to mediate the action of RTKs. The immediate challenge is to determine how SH2 proteins regulate Ras proteins,

intracellu]ar kinases and transcription factors.

Acknowledgements We thank colleagues for sharing unpublished data, and R. Duan, S. Demo and J. Escobedo for critical reading of the manuscript. The authors acknowledge the support of the NIH (R01 H]32898) (L. T. W.) and National Institutes of Health Program of Excellence in Molecular Biology (HL-43821) (L. T. W.).

References Due to TIBS policy of short reference lists, it has not been possible to cite all significant contributions, but most can be traced from those listed.

377

S i g n a l l i n g at the p l a s m a m e m b r a n e 1 Kypta, R. M., Goldgerg,Y, Ulug, E. T. and Courneidge, S. A. (1990) Cell 62, 481-492 2 Coughlin,S. R., Escobedo,J. A. and Williams, L. T. (1989) Science 243, 1191-1194 3 Escobedo,J. A. et al. (1991) Mol. Cell. Biol. 11, 1125-1132 4 Kazlauskas,A., Kashishian,A., Cooper, J. A. and Valius, M. (1992) Mol. Cell. Biol. 12, 2534-2544 5 Fantl,W. J. et al. (1992) Cell 69, 413-423 6 Peters, K. G. et al. Nature (in press) 7 Mohammadi, M. et al. (1991) MoL Cell. Biol. 11, 5068-5078 8 Reedijk, M. et al. (1992) EMBO J. 11, 1365-1372 9 Matsuda, M., Mayer, B. J. and Hanafusa, H. (1991) Mol. Cell. Biol. 11, 1607-1613 10 Anderson, D. et al. (1990) Science 250, 435-441 11 Hirai, H. and Varmus, H. E. (1990) Genes Dev.

CYTOKINES, a large family of protein mediators, regulate proliferation, differentiation and functions of various lineages of cellsL Each cytokine interacts with different types of cells and exhibits pleiotropic functions depending on the target cell. Conversely, a single cell often responds to more than one cytokine. Thus, cytokine-producing cells and target ceils form complex cellular networks within the immune system or the hemopoietic systemL Since a single cell responds to more than one cytokine, synergism or interference between those cytokines may occur either at their receptors or intracellular signal transduction pathways. It has also been noticed that distinct cytokines sometimes show similar or identical function on the same cell 1. What is the basis for the interaction among distinct cytokines? Why is there functional redundancy among distinct cytokines? In this review we describe two cytokine receptor systems that provide an explanation to these questions. IL-3, IL-5 and GM-CSF receptors Development of hemopoietic cells from a multi-potential hemopoietic stem cell is regulated by a sequential action of a number of cytokines 1,2.Interleukin 3 (IL-3) and granulocyte-macrophage colony stimulating factor (GM-CSF) stimulate multi-potential hemopoietic cells as A. Miyajima, T. Hara and T. Kitamura are at the Department of Molecular Biology, DNAX Research Institute of Molecular and Cellular Biology, 901 California Avenue, Palo Alto, CA 94304, USA. 378

4, 2342-2352 12 Mayer, B. J., Jackson, P. K., Van Etten, R. A. and Baltimore, D. (1992) Mol. Cell. Biol. 12,

609-618 13 Mayer, B. J., Hamaguchi, M. and Hanafusa, H. (1988) Nature 332, 272-275 14 Lehmann,J. M., Reithmuller,G. and Johnson,J. (1990) Nucleic Acids Res. 18, 1048 15 Clark, S. G., Stern, M. J. and Horvitz, H. R. (1992) Nature 356, 340-344 16 Sun, X. J. et al. (1991) Nature 352, 73-77 17 Wilden, P. A. et al. (1990) Proc. Natl Acad. Sci. USA 87, 3358-3362 18 Cai, H., Szeberenyi,J. and Cooper, G. M. (1990) Mol. Cell. Biol. 10, 5314-5323 19 Szeberenyi,J., Cai, H. and Cooper, G. M. (1990) Mol. Cell. Biol. 10, 5324-5332 20 Feig, L. A. and Cooper, G. M. (1988) MoI. Cell. Biol. 8, 3235-3243 21 Sjolander, A., Yamamoto, K., Huber, B. E. and

TIBS 17 - O C T O B E R 1 9 9 2 Lapetina, E. G. (1991) Prec. Natl Acad. Sci. USA 88, 7908-7912 22 Rubin,G. M. (1991) Trends Genet. 7 , 3 7 2 - 3 7 7 23 Simon, M. A. et al. (1991) Cell 67,701-716 24 Gaul, U., Mardon, G. and Rubin,G. M. (1992) Cell 68, 1007-1019 25 Hariharan, I. K., Carthew, R. W. and Rubin, G. M. (1991) Cell 67,717-722 26 Sternberg, P. W. and Horvitz,H. R. (1991) Trends Genet. 7, 366-371 27 Han, M. and Sternberg, P. W. (1990) Cell 63, 921-931 28 Rapp, U. R. (1991) Oncogene 6,495-500 29 Morrison, D. K. et al. (1989) Cell 58, 649-657 30 Wood, K. W., Sarnecki, C., Roberts, T. M. and Blenis, J. (1992) Cell 68, 1041-1050 31 Pelech, S. L. and Sanghera,J. S. (1992) Trends Biochem. Sci. 17,233-238 32 Ambrosio, L., Mahowald,A. P. and Perrimon, N. (1989) Nature 342, 288-291

Common subunits of cytokine receptors and the functional redundancy of cytokines

Several d i s t i n c t cytokines often exhibit similar biological activities. The findings t h a t high-affinity receptors for a group of cytokines with similar function share a c o m m o n s u b u n i t with a critical role in signal t r a n s d u c t i o n have provided a molecular basis for the functional redundancy of cytokines, Since the c o m m o n subunit, t o g e t h e r with d i s t i n c t cytokinespecific receptor subunits, form high-affinity receptors, binding of one cytokine to its high-affinity receptor can be c o m p e t e d for by other cytokines in the same group.

well as progenitors committed to vari- compete with each other in binding to ous lineages. In contrast, granulocyte their receptors 4. This reciprocal inhicolony stimulating factor (G-CSF), mono- bition in receptor binding (referred to cyte colony stimu]ating factor (M-CSF), as cross-competition), which occurs erythropoietin (Epo) and interleukin 5 even at 4°C, is observed for high-affinity (IL-5) are more lineage-specific. Al- receptors in human hemopoietic cells, though IL-3, GM-CSF and IL-5 show no but not in mouse ceils. There is another significant sequence homology at the notable difference between the human amino acid level, they exhibit a very and mouse receptors, i.e. the mouse similar biological activity on eosino- IL-3 receptor (mIL-3R) clearly has two phils and basophilsL Protein tyrosine distinct binding sites with high and low phosphorylation patterns induced by affinities5 but the human IL-3 receptor the three factors are remarkably similar (hIL-3R) has only a high-affinity site 6. and they induce activation of the GTPExpression cloning of cDNAs for binding protein Ras and phosphoryla- receptor subunits and reconstitution of tion of the protein kinase Raf 3. In ad- high-affinity receptors have provided a dition, IL-3 and GM-CSF display a num- molecular explanation for these obserber of similar biological activities on a vations 3. The high-affinity receptors for wide variety of ceils (Table I). IL-3, IL-5 and GM-CSF are composed of Moreover, IL-3, GM-CSF and IL-5 often two distinct subunits, c¢ and ]3 (Fig. 1). © 1992,ElsevierSciencePublishers,(UK) 0376-5067/92/$05.00