Smad1 Recognition and Activation by the ALK1 Group of Transforming Growth Factor-β Family Receptors

Abstract

Two structural elements, the L45 loop on the kinase domain of the transforming growth factor-β (TGF-β) family type I receptors and the L3 loop on the MH2 domain of Smad proteins, determine the specificity of the interactions between these receptors and Smad proteins. The L45 sequence of the TGF-β type I receptor (TβR-I) specifies Smad2 interaction, whereas the related L45 sequence of the bone morphogenetic protein (BMP) type I receptor (BMPR-I) specifies Smad1 interactions. Here we report that members of a third receptor group, which includes ALK1 and ALK2 from vertebrates and Saxophone from Drosophila, specifically phosphorylate and activate Smad1 even though the L45 sequence of this group is very divergent from that of BMPR-I. We investigated the structural elements that determine the specific recognition of Smad1 by ALK1 and ALK2. In addition to the receptor L45 loop and the Smad1 L3 loop, the specificity of this recognition requires the α-helix 1 of Smad1. The α-helix 1 is a conserved structural element located in the vicinity of the L3 loop on the surface of the Smad MH2 domain. Thus, Smad1 recognizes two distinct groups of receptors, the BMPR-I group and the ALK1 group, through different L45 sequences on the receptor kinase domain and a differential use of two surface structures on the Smad1 MH2 domain. Two structural elements, the L45 loop on the kinase domain of the transforming growth factor-β (TGF-β) family type I receptors and the L3 loop on the MH2 domain of Smad proteins, determine the specificity of the interactions between these receptors and Smad proteins. The L45 sequence of the TGF-β type I receptor (TβR-I) specifies Smad2 interaction, whereas the related L45 sequence of the bone morphogenetic protein (BMP) type I receptor (BMPR-I) specifies Smad1 interactions. Here we report that members of a third receptor group, which includes ALK1 and ALK2 from vertebrates and Saxophone from Drosophila, specifically phosphorylate and activate Smad1 even though the L45 sequence of this group is very divergent from that of BMPR-I. We investigated the structural elements that determine the specific recognition of Smad1 by ALK1 and ALK2. In addition to the receptor L45 loop and the Smad1 L3 loop, the specificity of this recognition requires the α-helix 1 of Smad1. The α-helix 1 is a conserved structural element located in the vicinity of the L3 loop on the surface of the Smad MH2 domain. Thus, Smad1 recognizes two distinct groups of receptors, the BMPR-I group and the ALK1 group, through different L45 sequences on the receptor kinase domain and a differential use of two surface structures on the Smad1 MH2 domain. The Smad family of proteins play a central role in signal transduction by the transforming growth factor-β (TGF-β) 1The abbreviations used are: TGF-β, transforming growth factor β; BMP, bone morphogenetic protein. family (1Attisano L. Wrana J.L. Curr. Opin. Cell Biol. 1998; 10: 188-194Crossref PubMed Scopus (177) Google Scholar, 2Heldin C.-H. Miyazono K. ten Dijke P. Nature. 1997; 390: 465-471Crossref PubMed Scopus (3358) Google Scholar, 3Massagué J. Annu. Rev. Biochem. 1998; 67: 753-791Crossref PubMed Scopus (3999) Google Scholar). Smad proteins are directly phosphorylated by type I TGF-β family receptors, an event that induces their accumulation in the nucleus where they activate transcription of specific genes. Smad proteins act as signal transducers for different members of the TGF-β family, including TGF-β itself, the activins, and the bone morphogenetic proteins (BMPs). The type I receptors for TGF-β and activin, which are known as TβR-I and ActR-IB, respectively, signal via Smad2 and its close homolog Smad3. The BMP type I receptors BMPR-IA and BMPR-IB signal via Smad1 and possibly its close homologs, Smad5 and Smad8. Upon receptor-mediated phosphorylation, and on their way to the nucleus, all these Smad proteins associate with Smad4, a member of a separate subclass that is required for the formation of transcriptional complexes. The Drosophila orthologs of BMP (Dpp), BMPR-I (Thickveins), Smad1 (Mad), and Smad4 (Medea) are functionally linked in a similar fashion (4Das P. Maduzia L. Wang H. Finelli A. Cho S.H. Smith M. Padgett R. Development. 1998; 125: 1519-1528Crossref PubMed Google Scholar, 5Hudson J. Podos S. Keith K. Simpson S. Fergusson E. Development. 1998; 125: 1407-1420Crossref PubMed Google Scholar, 6Wisotzkey R. Mehra A. Sutherland D. Dobens L. Liu X. Dohrmann C. Attisano L. Raftery L. Development. 1998; 125: 1433-1445Crossref PubMed Google Scholar). The maintenance of specificity in this system requires that each member of the type I receptor family be able to discriminate among different groups of Smad proteins. The specific interaction between Smad proteins and type I receptors is determined by two structural elements, namely, the L3 loop in the carboxyl-terminal domain (or MH2 domain) of Smad proteins and the L45 loop in the kinase domain of the receptors (7Lo R.S. Chen Y.G. Shi Y.G. Pavletich N. Massagué J. EMBO J. 1998; 17: 996-1005Crossref PubMed Scopus (208) Google Scholar, 8Chen Y.G. Hata A. Lo R.S. Wotton D. Shi Y. Pavletich N. Massagué J. Genes Dev. 1998; 12: 2144-2152Crossref PubMed Scopus (305) Google Scholar). Both elements consist of a short amino acid sequence that is highly conserved among Smad proteins or receptors of similar specificity but differs on a few critical residues between functionally distinct Smad proteins or receptors. Thus, the L45 loop sequence of the TβR-I/ActR-IB group (which also includes the orphan receptors ALK7, XTrR-I and Atr-I) is compatible with the L3 loop sequence common to Smad2 and -3, allowing functional interactions between these receptors and Smad proteins. A similar relationship exists between the L45 loop of the BMPR-I group (which also includes Thickveins) and the L3 loop of Smad1, -5, -8, and Mad (see Figs. 1 A and 4 A for L45 and L3 sequences).Figure 4A, α-helix 1 and L3 sequences of receptor-regulated Smad proteins. Structural elements (arrows, β-strands; rounded boxes, α-helices) correspond to the Smad4 MH2 domain (40Shi Y. Hata A. Lo R.S. Massagué J. Pavletich N.P. Nature. 1997; 388: 87-93Crossref PubMed Scopus (378) Google Scholar). Subtype-specific residues areboxed. The corresponding sequences of Smad4 is also shown. Numbering of the last residue in each sequence corresponds to the Smad species not in the parentheses. B, a close-up view of the Smad4 MH2 structure showing the L3 loop (yellow) with subtype-specific residues (red) and the α-helix 1 (purple) with subtype-specific residues (green). The insert shows a frontal view of the location of the L3 loop and helix 1 of each MH2 monomer in the crystallographic trimer.View Large Image Figure ViewerDownload Hi-res image Download (PPT) An important question left open by previous studies is about the Smad specificity of a third group of type I receptors. This group includes ALK1, ALK2, and Drosophila Saxophone. Saxophone is essential for dorsal closure of the Drosophila embryo and is believed to mediate Dpp signals along with Thickveins (9Brummel T. Twombly V. Marques G. Wrana J.L. Newfeld S. Attisano L. Massagué J. O'Connor M.B. Gelbart W.M. Cell. 1994; 78: 251-261Abstract Full Text PDF PubMed Scopus (264) Google Scholar, 10Nellen D. Affolter M. Basler K. Cell. 1994; 78: 225-237Abstract Full Text PDF PubMed Scopus (237) Google Scholar, 11Penton A. 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Hereditary hemorrhagic telangiectasia is an autosomal dominant disorder characterized by epithelial vascular dysplasia with a high propensity to hemorrhage in the nasal and gastrointestinal mucosa. ALK2 (also known as ActR-I or Tsk7L) is a broadly expressed receptor that can bind BMPs, activin, and, under certain conditions, TGF-β in vitro (1Attisano L. Wrana J.L. Curr. Opin. Cell Biol. 1998; 10: 188-194Crossref PubMed Scopus (177) Google Scholar, 2Heldin C.-H. Miyazono K. ten Dijke P. Nature. 1997; 390: 465-471Crossref PubMed Scopus (3358) Google Scholar, 3Massagué J. Annu. Rev. Biochem. 1998; 67: 753-791Crossref PubMed Scopus (3999) Google Scholar, 13Attisano L. Cárcamo J. Ventura F. Weis F.M.B. Massagué J. Wrana J.L. Cell. 1993; 75: 671-680Abstract Full Text PDF PubMed Scopus (604) Google Scholar, 15Ebner R. Chen R.-H. Lawler S. Zioncheck T. Derynck R. Science. 1993; 262: 900-902Crossref PubMed Scopus (208) Google Scholar, 16ten Dijke P. Yamashita H. Ichijo H. Franzén P. Laiho M. Miyazono K. Heldin C.-H. Science. 1994; 264: 101-104Crossref PubMed Scopus (512) Google Scholar, 17ten Dijke P. Yamashita H. Sampath T.K. Reddi A.H. Estevez M. Riddle D.L. Ichijo H. Heldin C.-H. Miyazono K. J. Biol. Chem. 1994; 269: 16985-16988Abstract Full Text PDF PubMed Google Scholar, 18Liu F. Ventura F. Doody J. Massagué J. Mol. Cell. Biol. 1995; 15: 3479-3486Crossref PubMed Scopus (522) Google Scholar, 19Rosenzweig B.L. Imamura T. Okadome T. Cox G.N. Yamashita H. ten Dijke P. Heldin C.-H. Miyazono K. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 7632-7636Crossref PubMed Scopus (478) Google Scholar). However, ALK2 does not mediate activin responses like those mediated by ActR-IB (20Cárcamo J. Weis F.M.B. Ventura F. Wieser R. Wrana J.L. Attisano L. Massagué J. Mol. Cell. Biol. 1994; 14: 3810-3821Crossref PubMed Google Scholar). In Xenopus, the activity of ALK2 (21Armes N.A. Smith J.C. Development. 1997; 124: 3797-3804Crossref PubMed Google Scholar) is akin to that of BMPR-I (22Graff J.M. Bansal A. Melton D.A. Cell. 1996; 85: 479-487Abstract Full Text Full Text PDF PubMed Scopus (400) Google Scholar, 23Suzuki A. Chang C. Yingling J.M. Wang X.F. Hemmati-Brivanlou A. Dev. Biol. 1997; 184: 402-405Crossref PubMed Scopus (141) Google Scholar), because both receptors signal ventral mesoderm induction, and defects in ventral mesoderm formation caused by a dominant-negative ALK2 construct can be rescued by overexpression of Smad8 (24Chen Y. Bhushan A. Vale W. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 12938-12943Crossref PubMed Scopus (138) Google Scholar). In this study, we have investigated the Smad specificity of this third group of type I receptors. We were intrigued by the fact that the L45 sequence of these receptors is very different from that of the TβR-I group or the BMPR-I group (see Fig. 1 A). Nonetheless, we observed that ALK1 and ALK2, like BMPR-I, recognize and activate Smad1. This was paradoxical because TβR-I, whose L45 sequence is much closer to that of BMPR-I, does not recognize Smad1. Insights from the crystal structure of the Smad MH2 domain allowed us to address this paradox. R1B/L17, COS-1, and HepG2 cells were maintained as described previously (8Chen Y.G. Hata A. Lo R.S. Wotton D. Shi Y. Pavletich N. Massagué J. Genes Dev. 1998; 12: 2144-2152Crossref PubMed Scopus (305) Google Scholar, 25Chen Y.G. Liu F. Massagué J. EMBO J. 1997; 16: 3866-3876Crossref PubMed Scopus (287) Google Scholar). Mouse embryonic P19 cells were maintained in Dulbecco's modified Eagle's medium containing 10% fetal bovine serum and 2 mm glucose. Mutagenesis of Smad proteins and receptors was performed by polymerase chain reaction using appropriate oligonucleotides, and verified by DNA sequencing. TβR-I(LA) contains a replacement of the L45 sequence ADNKDNGTW with the sequence SDMTSRHSS. Smad2(H1-1) contains the mutations A323S, V325I, and M327N, and Smad2(HL1) contains the mutations A323S, V325I, M327N, R427H, and T430D. Other constructs have been reported previously (7Lo R.S. Chen Y.G. Shi Y.G. Pavletich N. Massagué J. EMBO J. 1998; 17: 996-1005Crossref PubMed Scopus (208) Google Scholar, 8Chen Y.G. Hata A. Lo R.S. Wotton D. Shi Y. Pavletich N. Massagué J. Genes Dev. 1998; 12: 2144-2152Crossref PubMed Scopus (305) Google Scholar). Activation of the 3TP-luciferase reporter (20Cárcamo J. Weis F.M.B. Ventura F. Wieser R. Wrana J.L. Attisano L. Massagué J. Mol. Cell. Biol. 1994; 14: 3810-3821Crossref PubMed Google Scholar) and Mix2 A3-luciferase reporter (26Chen X. Rubock M.J. Whitman M. Nature. 1996; 383: 691-696Crossref PubMed Scopus (634) Google Scholar) by receptors were analyzed in R1B/L17 cells as described previously (8Chen Y.G. Hata A. Lo R.S. Wotton D. Shi Y. Pavletich N. Massagué J. Genes Dev. 1998; 12: 2144-2152Crossref PubMed Scopus (305) Google Scholar, 25Chen Y.G. Liu F. Massagué J. EMBO J. 1997; 16: 3866-3876Crossref PubMed Scopus (287) Google Scholar). To measure the activity of a XVent2-luciferase reporter (27Candia A.F. Watabe T. Hawley S.H. Onichtchouck D. Zhang Y. Derynck R. Niehrs C. Cho K.W. Development. 1997; 124: 4467-4480Crossref PubMed Google Scholar), P19 cells were transfected with this construct, and the constitutively active forms of type I receptors, using Lipofectin (Life Technologies, Inc.) according to manufacturer's instructions. Luciferase activity was measured 40 h after transfection. Metabolic labeling of transfected cells with [32P]orthophosphate or [35S]Met/Cys was performed as described previously (8Chen Y.G. Hata A. Lo R.S. Wotton D. Shi Y. Pavletich N. Massagué J. Genes Dev. 1998; 12: 2144-2152Crossref PubMed Scopus (305) Google Scholar, 25Chen Y.G. Liu F. Massagué J. EMBO J. 1997; 16: 3866-3876Crossref PubMed Scopus (287) Google Scholar). Immunoprecipitates with monoclonal anti-Flag M2 antibody (IBI, East Kodak) were visualized by SDS-polyacrylamide gel electrophoresis followed by autoradiography. Subcellular localization of Smad proteins was examined in HepG2 cells transfected with N-terminally Flag-tagged Smad proteins and the constitutively active receptors. Immunofluorescence was carried as described previously (8Chen Y.G. Hata A. Lo R.S. Wotton D. Shi Y. Pavletich N. Massagué J. Genes Dev. 1998; 12: 2144-2152Crossref PubMed Scopus (305) Google Scholar). cells were The of cells with or is in the To the specificity of ALK1 and ALK2, we used transcriptional that discriminate between and the for ALK1 and ALK2 have not been we ALK1 and ALK2 constructs and containing a to in the residue of the domain which type I receptors in a R. Wrana J.L. Massagué J. EMBO J. 1995; 14: PubMed Scopus Google Scholar, L. Wrana J.L. E. Massagué J. Mol. Cell. Biol. 1996; 16: PubMed Scopus (287) Google Scholar, T. S. M. O'Connor M.B. Attisano L. Wrana J.L. Cell. 1996; 85: Full Text Full Text PDF PubMed Scopus Google Scholar). The corresponding forms of the BMP type I receptors, and activate with the reporter construct P19 cells 1 contains a from the of (27Candia A.F. Watabe T. Hawley S.H. Onichtchouck D. Zhang Y. Derynck R. Niehrs C. Cho K.W. Development. 1997; 124: 4467-4480Crossref PubMed Google Scholar) and can be via Smad1 but not Smad2 (8Chen Y.G. Hata A. Lo R.S. Wotton D. Shi Y. Pavletich N. Massagué J. Genes Dev. 1998; 12: 2144-2152Crossref PubMed Scopus (305) Google Scholar). and were also able to activate whereas the constitutively active TGF-β receptor construct, We similar using two and contains elements from and (20Cárcamo J. Weis F.M.B. Ventura F. Wieser R. Wrana J.L. Attisano L. Massagué J. Mol. Cell. Biol. 1994; 14: 3810-3821Crossref PubMed Google Scholar). contains of the activin element from the (26Chen X. Rubock M.J. Whitman M. Nature. 1996; 383: 691-696Crossref PubMed Scopus (634) Google Scholar). Both are by TGF-β or activin signals mediated by Smad2 M. S. R. Attisano L. Wrana J.L. Cell. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar, F. C. Massagué J. Genes Dev. 1997; PubMed Scopus Google Scholar, A. Imamura T. S. M. A. E. K. J. Heldin Miyazono K. ten Dijke P. EMBO J. 1997; 16: PubMed Scopus Google Scholar) or Y. Derynck R. Nature. 1996; 383: PubMed Scopus Google Scholar, X. Y. E. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: PubMed Scopus Google Scholar, Y. Vale W. Proc. Natl. Acad. Sci. U. S. A. 1996; PubMed Scopus Google Scholar). Liu and J. on Both were by the TGF-β receptor and the activin receptor in transfected R1B/L17 cells and and like to activate and and Thus, the of these reporter constructs that the specificity of ALK1 and ALK2 is similar to that of the BMP type I receptors BMPR-IA and the that ALK1 and ALK2 be able to phosphorylate and activate Smad1 but not To this receptor constructs were with N-terminally Flag-tagged Smad1 or Smad2 R1B/L17 and Smad was determined by anti-Flag from the The of Smad1 and Smad2 observed in the of receptor activity in these cells left is caused by on to the receptor-mediated M. Doody J. Massagué J. Nature. 1997; PubMed Scopus Google Scholar). J. and J. and the of Smad1 but not that of which is similar to the of and to the of 2 A). were with ALK1 and ALK2 as specific of Smad1 but not To that or specifically Smad1, we determined the of these constructs to accumulation of Smad proteins. with the of or accumulation of but not which is similar to the of and the of these conditions, localization of proteins was observed in a of the The of this be because of in the of the receptors to activate the Smad proteins. In TGF-β induces a of Smad2 and the We these using a Smad1 construct that contains to mutations in the sequence M. Liu F. Hata A. Doody J. Massagué J. Genes Dev. 1997; PubMed Scopus Google Scholar). are the phosphorylated by BMPR-I, and their to receptor-mediated and of Smad1 by BMP M. Liu F. Hata A. Doody J. Massagué J. Genes Dev. 1997; PubMed Scopus Google Scholar). The to in the nucleus with or not ALK1 and ALK2 can the of BMP type I receptors to specifically phosphorylate and activate Smad1, to transcriptional The sequence of the L45 loop in the kinase domain of TβR-I TGF-β activity Derynck R. EMBO J. 1997; 16: PubMed Scopus Google Scholar). the L45 loop of TβR-I and BMPR-I specifies the of Smad proteins by these receptors in the (8Chen Y.G. Hata A. Lo R.S. Wotton D. Shi Y. Pavletich N. Massagué J. Genes Dev. 1998; 12: 2144-2152Crossref PubMed Scopus (305) Google Scholar). of residues in this sequence are conserved between TβR-I and BMPR-I (see Fig. 1 A). Thus, the residues are for the different Smad specificity of these two receptors. TβR-I and BMPR-I constructs with these residues a in their to recognize and activate Smad1 and Smad2 (8Chen Y.G. Hata A. Lo R.S. Wotton D. Shi Y. Pavletich N. Massagué J. Genes Dev. 1998; 12: 2144-2152Crossref PubMed Scopus (305) Google Scholar). The L45 loop is conserved between ALK1 and ALK2 but is very divergent between these receptors and TβR-I or BMPR-I (see Fig. 1 A). the Smad specificity of ALK1 and ALK2 is similar to that of BMPR-I receptors, we investigated this specificity is determined by the L45 sequence of ALK1 and ALK2 their of to the BMPR-I L45 To this we a TβR-I containing the L45 sequence which corresponds to the ALK2 L45 The specificity of this construct was with those of the type TβR-I and the previously described containing the L45 sequence of BMPR-I (8Chen Y.G. Hata A. Lo R.S. Wotton D. Shi Y. Pavletich N. Massagué J. Genes Dev. 1998; 12: 2144-2152Crossref PubMed Scopus (305) Google Scholar). TβR-I(LA) was similar to and BMPR-I in its to accumulation of Smad1 and its of of and that the of the BMPR-I L45 sequence to an interaction with Smad1 is by the very divergent L45 sequence of ALK1 and but not by the related L45 sequence of In of we Smad1 recognize ALK1 and and BMPR-I by different The of Smad1 and Smad2 to recognize BMPR-I and TβR-I, respectively, is determined by the sequence of the L3 loop (7Lo R.S. Chen Y.G. Shi Y.G. Pavletich N. Massagué J. EMBO J. 1998; 17: 996-1005Crossref PubMed Scopus (208) Google Scholar). from the crystal structure of the Smad4 MH2 the L3 loop from the surface of this domain (40Shi Y. Hata A. Lo R.S. Massagué J. Pavletich N.P. Nature. 1997; 388: 87-93Crossref PubMed Scopus (378) Google Scholar). The sequence of the L3 loop is the and but differs two critical between these two A and these two residues between Smad1 and Smad2 their to with specific receptors (7Lo R.S. Chen Y.G. Shi Y.G. Pavletich N. Massagué J. EMBO J. 1998; 17: 996-1005Crossref PubMed Scopus (208) Google Scholar). Thus, a Smad2 containing the two residues in the L3 loop is by BMPR-I (7Lo R.S. Chen Y.G. Shi Y.G. Pavletich N. Massagué J. EMBO J. 1998; 17: 996-1005Crossref PubMed Scopus (208) Google Scholar) A). this we determined that and are of accumulation of even though they accumulation of Smad1 A). that the L3 sequence is not to Smad1 recognition by ALK1 or ALK2. We Smad MH2 for containing subtype-specific that residues that be different between Smad1 and but conserved between Smad1, -5, -8, and Mad or between Smad2 and The the contains residues (7Lo R.S. Chen Y.G. Shi Y.G. Pavletich N. Massagué J. EMBO J. 1998; 17: 996-1005Crossref PubMed Scopus (208) Google Scholar). However, of these mutations along with the L3 loop not the of or to accumulation of the construct with subtype-specific residues is a sequence corresponding to α-helix which is a surface structure in Smad4 (40Shi Y. Hata A. Lo R.S. Massagué J. Pavletich N.P. Nature. 1997; 388: 87-93Crossref PubMed Scopus (378) Google Scholar). The corresponding sequence is conserved between Smad1 and Smad2 for two residues that are subtype-specific 4 A). residues in Smad4 and are by of the α-helix and are to in the vicinity of the L3 loop (40Shi Y. Hata A. Lo R.S. Massagué J. Pavletich N.P. Nature. 1997; 388: 87-93Crossref PubMed Scopus (378) Google Scholar) 4 of their and we investigated these residues play a role as of the interaction of Smad1 and ALK1 and A Smad2 containing these two residues from Smad1 not to or in the accumulation A). However, a Smad2 construct containing the two α-helix 1 residues and the two L3 loop residues from Smad1 a similar of accumulation with or A). a critical role of α-helix 1 in Smad1 recognition by ALK1 and ALK2. To that both α-helix 1 and the L3 loop are required for Smad1 recognition by ALK1 and type and Smad2 constructs were transfected HepG2 cells with the constitutively active forms of receptors, and of Smad2 was with the of of the of and but not type whereas of and of Smad2(HL1) these we that two groups of receptors in vertebrates and possibly Drosophila, the BMPR-I group and the ALK1 A similar was by using a different BMP reporter M. S.J. M. Wrana J.L. J. Biol. Chem. 1998; Full Text Full Text PDF PubMed Scopus Google Scholar). the shows that the recognition and of Smad1 by ALK1 and ALK2 is by the L45 loop on the receptors and the L3 loop with the α-helix 1 on Smad1 is that Saxophone recognizes Mad in a similar fashion because the L45 sequences of Saxophone and the L3 sequence and α-helix 1 sequence of Mad are very similar or to those of their in is important to that the structural elements and previously (7Lo R.S. Chen Y.G. Shi Y.G. Pavletich N. Massagué J. EMBO J. 1998; 17: 996-1005Crossref PubMed Scopus (208) Google Scholar, 8Chen Y.G. Hata A. Lo R.S. Wotton D. Shi Y. Pavletich N. Massagué J. Genes Dev. 1998; 12: 2144-2152Crossref PubMed Scopus (305) Google Scholar), a role in the specificity of the recognition between type I receptors and Smad proteins, be of the between these proteins. the of receptor and Smad protein the of be important for the of receptors and Smad proteins. The BMPR-I group and the ALK1 group recognize Smad1 through related but different which different L45 sequences on the receptor kinase and a differential use of two surface structures on the Smad1 MH2 domain. Smad1 recognition and by these two be different in that be by the In a receptor the as an of the Smad1 or both receptors activate Smad1 in or with different We Shi and Pavletich for and for the structure We Niehrs for the for BMP, and S. Lo for the