Effects of heterologous kinase domains on growth factor receptor specificity

The kinase domains of receptor tyrosine kinases (RTKs) are highly conserved, yet they are able to discriminate among potential substrates to selectively activate downstream signaling pathways. In this study, we tested the importance of catalytic domain specificity by creating two series of chimeric RTKs. In one set, the kinase domain of insulin-like growth factor I receptor (IGF1R) was replaced by the kinase domains from insulin receptor (IR), macrophage stimulating protein 1 receptor/Ron (Ron) or Src. In the other set of chimeras, the kinase domain of epidermal growth factor receptor (EGFR) was similarly replaced by the kinase domains of IR, Ron, or Src. We expressed the wild-type and chimeric forms of the receptors in mammalian cells. For some signaling events, such as recognition of IRS1, the identity of the tyrosine kinase catalytic domain did not appear to be crucial. In contrast, recognition of some sites, such as the C-terminal autophosphorylation sites on EGFR, did depend on the identity of the kinase domain. Our data also showed that ligand dependence was lost when the native kinase domains were replaced by Src, suggesting that the identity of the kinase domains could be important for proper receptor regulation. Overall, the results are consistent with the idea that the fidelity of RTK signaling depends on co-localization and targeting with substrates, as well as on the intrinsic specificity of the kinase domain.