The Cbl proto-oncogene product negatively regulates the Src-family tyrosine kinase Fyn by enhancing its degradation

Fyn is a prototype Src-family tyrosine kinase that plays specific roles in neural development, keratinocyte differentiation, and lymphocyte activation , as well as roles redundant with other Src-family kinases. Similar to othe r Src-family kinases, efficient regulation of Fyn is achieved through intra molecular binding of its SH3 and SH2 domains to conserved regulatory region s. We have investigated the possibility that the tyrosine kinase regulatory protein Cbl provides a complementary mechanism of Fyn regulation. We show that Cbl overexpression in 293T embryonic kidney and Jurkat T-lymphocyte ce lls led to a dramatic reduction in the active pool of Fyn; this was seen as a reduction in Fyn autophosphorylation, reduced phosphorylation of in vivo substrates, and inhibition of transcription from a Src-family kinase respo nse element linked to a luciferase reporter. Importantly, a Fyn mutant (Fyn Y528F) relieved of intramolecular repression was still negatively regulated by Cbl. The Cbl-dependent negative regulation of Fyn did not appear to be mediated by inhibition of Fyn kinase activity but was correlated with enhan ced protein turnover. Consistent with such a mechanism, elevated levels of Fyn protein were observed in cell lines derived from Cbl(-/-) mice compared to those in wild-type controls. The effects of Cbl on Fyn were not observe d when the 70ZCbl mutant protein was analyzed. Taken together, these observ ations implicate Cbl as a component in the negative regulation of Fyn and p otentially other Src-family kinases, especially following kinase activation . These results also suggest that protein degradation may be a general mech anism for Cbl-mediated negative regulation of activated tyrosine kinases.