Uncoupling ligand-dependent and -independent mechanisms for mitogen-activated protein kinase activation by the murine Ron receptor tyrosine kinase.

Receptor tyrosine kinases (RTKs) activate downstream signaling through cognate growth factor receptor-induced dimerization and autophosphorylation. Overexpression of RTKs can lead to constitutive activation due to increased dimerization in the absence of ligand, and downstream signals are presumed to be the same as the ligand-induced signals. We have shown that the murine Ron (mRon) receptor tyrosine kinase exhibits constitutive activation of the MAP kinase pathway that is independent of the two docking site tyrosines, whereas activation of this pathway in response to ligand (macrophage-stimulating protein) is abolished in the absence of these tyrosines. Furthermore, we identified three tyrosines (Tyr-1175, Tyr-1265, and Tyr-1294) within the kinase domain that play critical but overlapping roles in controlling constitutive Erk activation by mRon. Phenylalanine mutations at these three tyrosines results in a receptor that fails to constitutively activate the Erk pathway but retains the ability to induce Erk phosphorylation in response to ligand stimulation. The ability of mRon to activate the MAP kinase pathway is dependent on c-Src activity, and we have shown that c-Src co-immunoprecipitates with mRon. c-Src fails to interact with mRon when the three tyrosines required for MAP kinase activation are mutated, whereas the presence of any one of these tyrosines alone restores Erk phosphorylation and recruitment of c-Src. Thus, the ligand-dependent and -independent activity of mRon can be uncoupled through the alteration of selective sets of tyrosines.