SRC autophosphorylation blocks its inactivation by CSK

Src protein tyrosine kinase plays important roles in cellular signal transduction and its activation is associated with many types of human cancer. Activation of Src in tumor cells is not caused by mutations or over-expression, but mainly the result of increased specific activity. These findings suggest that elevation of Srr activity in human tumor cells is due to a disruption of its regulation. The cur rent model for regulating Src is tyrosine phosphorylation near its C-terminus hy Csk, which down-regulates Src activity. In characterizing this inactivation, we observed that it requires a stoichiometric Csk:Src ratio to achieve a reasonable level of Src inactivation and the inactivation is time-independent. Examination of this reaction revealed that this inefficiency is the result of Src autophosphorylation, which blocks its inactivation by Csk. When autophosphorylation is kept to a minimum or prevented by inclusion of a specific protein tyrosine phosphatase or specific mutation, Src can be effectively inactivated by a sub stoichiometric amount of Csk. These findings indicate that the level of Src autophosphorylation plays a predominant role in its activation. We will also present evidence that protein tyrosine phosphatase IB selectively dephospho rylates the autophosphorylation site and allows Srr to be inactivated by Csk phosphorylation. These results provide a better understanding of Src activation in cells that paradoxically also contain high levels of Csk activity.