Little is known about the large ectodomain of MET, the product of the c-met protooncogene and receptor for hepatocyte growth factor͞scatter factor (HGF͞SF). Here, we establish by deletion mutagenesis that the HGF͞SF and heparin-binding sites of MET are contained within a large N-terminal domain spanning the ␣-chain (amino acids 25-307) and the first 212 amino acids of the ␤-chain (amino acids 308 -519). Neither the cystine-rich domain (amino acids 520 -561) nor the C-terminal half of MET (amino acids 562-932) bind HGF͞SF or heparin directly. The MET ectodomain, which behaves as a rod-shaped monomer with a large Stokes radius in solution, binds HGF͞SF in the absence or presence of heparin, and forms a stable HGF͞SF-heparin-MET complex with 1:1:1 stoichiometry. We also show that the ligand-binding domain adopts a ␤-propeller fold, which is similar to the N-terminal domain of ␣V integrin, and that the C-terminal half contains four Ig domains (amino acids 563-654, 657-738, 742-836, and 839 -924) of the unusual structural E set, which could be modeled on bacterial enzymes. Our studies provide 3D models and a functional map of the MET ectodomain. They have broad implications for structurefunction of the MET receptor and the related semaphorin and plexin proteins.Ig domain ͉ sema domain ͉ integrin ␣-chain ͉ hidden Markov models ͉ semaphorins R eceptor tyrosine kinases (RTKs) mediate intercellular signals, which are essential for the development and maintenance of the cells of multicellular organisms. The minimal domain structure of RTKs consists of an extracellular ligandbinding domain, a single transmembrane helix, and a cytoplasmic kinase domain. This minimal structure, however, is very rare, and, typically, the extracellular moiety of RTKs, the ectodomain, consists of complex and distinctive domain sets, which enable classification of the RTKs in different families (1).There is a strong preference for certain domains to occur in the ectodomain of RTKs. The fibronectin type-3 (FN-3) domain, for example, is present as two copies in the large Eph receptor family, three copies in the insulin and IGF-1 receptors, and at least seven copies in the rod outer segment receptor (1). Cystine-rich domains of variable length are also commonly found in RTKs.A large number of RTKs contain Ig domains and the ectodomain of certain families consists solely of Ig domains: the fibroblast growth factor (FGF) receptors contain two or three, depending on RNA splicing, the platelet-derived growth factor (PDGF), colony-stimulating factor 1 (CSF1), KIT, and FLT kinase͞serine-threonine kinase 1 (FLK2͞STK1) receptors contain five, and the FMS-like (FLT1), FLK1, FLT4, and cholecystokinin 4 (CCK4) receptors contain seven (1). Ig domains can also be present in combination with FN-3, cystine-rich, or other domains (1). Interestingly, most Ig domains present in RTKs and cell adhesion molecules belong to a distinct structural set known as the I set, with architecture intermediate between the V and C1 sets (2).MET, the RTK encoded by the c-met protoon...

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