A mechanism of activation of c-MET receptor tyrosine kinase
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c-MET receptor tyrosine kinase-mediated signaling governs numerous important cellular responses including cellular proliferation, differentiation, migration and apoptosis. Deregulation of these signals result in malignant behaviors, often leading to cancers. While the identity of the many signaling molecules that are activated following hepatocyte-growth factor (HGF)-induced activation of c-MET had been established, little was known about the mechanism of activation of c-MET. From a therapeutic perspective, it is necessary to understand the detailed molecular mechanisms regulating c-MET activation to selectively target these molecules. c-MET, in presence of its cognate ligand, is oligomerized, and is autophosphorylated on specific tyrosines on its cytoplasmic domain. The phosphorylated tyrosines in specific sub-domains of c-MET cytoplasmic region perform specific functions including increase in catalytic activity and recruitment of effector molecules. Classically, it has been believed that the sole role of ligand-induced oligomerization was to autophosphorylate the receptor, thereby switching the receptor’s kinase activity on. However, in light of a recent body of evidence suggesting that certain RTKs are kinase active on cell surface in absence of ligand-induced oligomerization, we hypothesized that oligomerization could be important for other aspects of RTK activation. Using c-MET as our model system, we investigated the role of oligomerization, irrespective of its role in autophosphorylation, in regulating c-MET activation. Previous studies from our laboratory have conclusively shown that oligomerization increases c-MET’s substrate binding affinity and substrate phosphorylation kcat. The work presented here addresses the role of oligomerization in regulating c-MET’s susceptibility to dephosphorylation, another important regulator of c-MET activation. The biochemical parameters measured for c-MET are used to build a unified kinetic model for c-MET activation. The model building and its subsequent validation using cell culture experiments are described here. Furthermore, the model is probed using parameter sensitivity analyses to understand how oligomerization-induced changes in the kinetic, thermodynamic and dephosphorylation properties of c-MET work synergistically to selectively induce specific signaling from the dimeric and not the monomeric receptor. Using these data, we propose an alternative feed-forward model for c-MET activation mechanism differs from the traditional view of the RTK activation.