Structural Basis for the Binding Specificity of Human Recepteur d'Origine Nantais (RON) Receptor Tyrosine Kinase to Macrophage-stimulating Protein

Background: RON and MET receptors bind their ligands MSP and HGF selectively and activate different signaling pathways. Results: Crystallographic and analytical ultracentrifugation studies provide important information about RON-MSP interaction. Conclusion: RON-MSP and MET-HGF exhibit 2:2 complex stoichiometry, but differences within the respective interfaces explain the strict ligand-receptor specificity. Significance: Signaling pathways must be exquisitely regulated with no cross-reactivity between related systems.Recepteur d'origine nantais (RON) receptor tyrosine kinase and its ligand, serum macrophage-stimulating protein (MSP), play important roles in inflammation, cell growth, migration, and epithelial to mesenchymal transition during tumor development. The binding of mature MSP (disulfide-linked - and -chains) to RON ectodomain modulates receptor dimerization, followed by autophosphorylation of tyrosines in the cytoplasmic receptor kinase domains. Receptor recognition is mediated by binding of MSP -chain (MSP) to the RON Sema. Here we report the structure of RON Sema-PSI-IPT1 (SPI1) domains in complex with MSP at 3.0 resolution. The MSP serine protease-like -barrel uses the degenerate serine protease active site to recognize blades 2, 3, and 4 of the -propeller fold of RON Sema. Despite the sequence homology between RON and MET receptor tyrosine kinase and between MSP and hepatocyte growth factor, it is well established that there is no cross-reactivity between the two receptor-ligand systems. Comparison of the structure of RON SPI1 in complex with MSP and that of MET receptor tyrosine kinase Sema-PSI in complex with hepatocyte growth factor -chain reveals the receptor-ligand selectivity determinants. Analytical ultracentrifugation studies of the SPI1-MSP interaction confirm the formation of a 1:1 complex. SPI1 and MSP also associate primarily as a 1:1 complex with a binding affinity similar to that of SPI1-MSP. In addition, the SPI1-MSP ultracentrifuge studies reveal a low abundance 2:2 complex with approximate to 10-fold lower binding affinity compared with the 1:1 species. These results support the hypothesis that the -chain of MSP mediates RON dimerization.