Although oligomerization of receptor tyrosine kinases (RTKs) is necessary for receptor activation and signaling, a quantitative understanding of how oligomerization mediates these critical processes does not exist. We present a comparative thermodynamic analysis of functionally active dimeric and functionally inactive monomeric soluble analogues of the c-MET RTK, which clearly reveal that oligomerization regulates the binding affinity and binding kinetics of the kinase toward ATP and tyrosine-containing peptide substrates. Thermodynamic binding data for oligomeric c-MET were obtained from the dimeric TPR-MET oncoprotein, a functionally active fusion derivative of the c-MET RTK. This naturally occurring oncoprotein contains the cytoplasmic domain of c-MET fused to a coiled coil dimerization domain from the nuclear pore complex. Comparative data were obtained from a soluble monomeric kinase compromising the c-MET cytoplasmic domain (cytoMET). Significantly, under equilibrium binding conditions, the oligomeric phosphorylated kinase snowed a significantly lower dissociation constant (Kd,dimer = 11 μM) for a tyrosine-containing peptide derived from the C-terminal tail of the c-MET RTK when compared to the phosphorylated monomeric kinase cytoMET (Kd,monomer = 140 μM). Surprisingly, equilibrium dissociation constants measured for the kinase and ATP were independent of the oligomerization state of the kinase (∼10 μM). Stopped-flow analysis of peptide substrate binding showed that the association rate constants (k2) differed 2-fold and dissociation rate constants (k-2) differed 10-fold when phosphorylated TPR-MET was compared to phosphorylated cytoMET. ATP binding abrogated the differences in k2 rates observed between the two oligomeric states of the c-MET cytoplasmic domain. These results clearly imply that oligomerization induces important thermodynamic and conformational changes in the substrate binding regions of the c-MET protein and provide quantitative mechanistic insights into the necessary role of oligomerization in RTK activation.
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