Structural Basis for the Binding Specificity of Human RON Receptor Tyrosine Kinase to Macrophage Stimulating Protein [Molecular Biophysics]

September 5th, 2014 by Chao, K. L., Gorlatova, N. V., Eisenstein, E., Herzberg, O.

Recepteur d'Origine Nantais (RON1) 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 MSPαβ (disulfide-linked α and β 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 β-chain (MSPβ) to the RON Sema domain. Here we report the structure of RON Sema-PSI-IPT1 (SPI1) domains in complex with MSPβ at 3.0 Å resolution. The MSPβ serine protease-like β-barrel uses the degenerate serine protease active site to recognize blades 2, 3, and 4 of the β-propeller fold of RON Sema. Despite the sequence homology between RON and MET and between MSP and HGF, 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 MSPβ and that of MET Sema-PSI in complex with HGFβ reveals the receptor-ligand selectivity determinants. Analytical ultracentrifugation studies of the SPI1/MSPβ interaction confirm the formation of a 1:1 complex. SPI1 and MSPαβ also associate primarily as a 1:1 complex with a similar binding affinity to that of SPI1/MSPβ. In addition, the SPI1/MSPαβ ultracentrifuge studies reveals a low abundance 2:2 complex with ~10-fold lower binding affinity compared with the 1:1 species. These results support the hypothesis that the α-chain of MSPαβ mediates RON dimerization.