Regulation of the p59fyn protein tyrosine kinase by the CD45 phosphotyrosine phosphatase

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Abstract
Triggering of the T cell antigen receptor/CD3 (TcR/CD3) complex leads to rapid tyrosine phosphorylation of regulatory proteins that participate in initiating T cell activation and proliferation. This signal transduction event requires the presence of the TcR/CD3‐associated protein tyrosine kinase p59fyn. There is also evidence that the CD45 phosphotyrosine phosphatase is involved in TcR/CD3 signalling. We show here by capping experiments using double indirect immunofluorescence techniques that the receptor phosphotyrosine phosphatase CD45 and the intracellular protein tyrosine kinase p59fyn specifically co‐distribute in functional T lymphocytes. Furthermore, we provide evidence that isolated p59fyn is a substrate for CD45 as indicated by the rapid dephosphorylation of the regulatory Tyr531 of p59fyn by CD45. This dephosphorylation is accompanied by a severalfold increase in the catalytic activity of p59fyn as measured by its autophosphorylation and phosphorylation of an exogenous substrate. We also demonstrate that CD45‐mediated dephosphorylation and activation of p59fyn apparently occurs at a slow basal rate in resting T cells. This represents the first identification of a physiologic regulator of p59fyn and implies a mechanism for the role of CD45 in TcR/CD3 signal transduction.