The Src and Csk families of tyrosine protein kinases in hemopoietic cells.

There is increasing evidence that the Src family of cytoplasmic tyrosine protein kinases is involved in the signal transduction of antigen receptor- and Fc receptor-mediated cellular activation. This function relates at least in part to the ability of Src-related enzymes to phosphorylate conserved tyrosine-based motifs in the cytoplasmic domains of the antigen and Fc receptors. The catalytic function of Src-like products is repressed by phosphorylation of a conserved carboxy-terminal tyrosine residue, which is mediated by another cellular tyrosine protein kinase, p50csk. Based on this property, it is postulated that Csk is a potent negative regulator of antigen and Fc receptor signalling. The balance between the actions of Src-related kinases and the p50csk is likely a major determinant of immune responsiveness.