Structural and Functional Basis of the Serine Protease-like Hepatocyte Growth Factor β-Chain in Met Binding and Signaling*

Hepatocyte growth factor (HGF), a plasminogen-related growth factor, is the ligand for Met, a receptor tyrosine kinase implicated in development, tissue regeneration, and invasive tumor growth. HGF acquires signaling activity only upon proteolytic cleavage of single-chain HGF into its α/β heterodimer, similar to zymogen activation of structurally related serine proteases. Although both chains are required for activation, only the α-chain binds Met with high affinity. Recently, we reported that the protease-like HGF β-chain binds to Met with low affinity (Stamos, J., Lazarus, R. A., Yao, X., Kirchhofer, D., and Wiesmann, C. (2004) EMBO J. 23, 2325-2335). Here we demonstrate that the zymogen-like form of HGF β also binds Met, albeit with 14-fold lower affinity than the protease-like form, suggesting optimal interactions result from conformational changes upon cleavage of the single-chain form. Extensive mutagenesis of the HGF β region corresponding to the active site and activation domain of serine proteases showed that 17 of the 38 purified two-chain HGF mutants resulted in impaired cell migration or Met phosphorylation but no loss in Met binding. However, reduced biological activities were well correlated with reduced Met binding of corresponding mutants of HGF β itself in assays eliminating dominant α-chain binding contributions. Moreover, the crystal structure of HGF β determined at 2.53 Å resolution provides a structural context for the mutagenesis data. The functional Met binding site is centered on the “active site region” including “triad” residues Gln⁵³⁴ [c57], Asp⁵⁷⁸ [c102], and Tyr⁶⁷³ [c195] and neighboring “activation domain” residues Val⁶⁹², Pro⁶⁹³, Gly⁶⁹⁴, Arg⁶⁹⁵, and Gly⁶⁹⁶ [c214-c219]. Together they define a region that bears remarkable resemblance to substrate processing regions of serine proteases. Models of HGF-dependent Met receptor activation are discussed.