Signaling-induced association of a tyrosine-phosphorylated 36-kDa protein with p50csk

The protein-tyrosine kinase, p50csk, is thought to participate in the regulation of signal transduction pathways by catalyzing the phosphorylation of the Src-related protein-tyrosine kinases on a negative regulatory tyrosine residue located near the COOH terminus. To study possible mechanisms by which the activity of p50csk might be regulated, we searched for p50csk-interacting proteins in human erythroleukemia cells. We found that in response to the treatment of cells with pervanadate, a potent inhibitor of protein tyrosine phosphatases, or to the cross-linking of Fc gamma RIIA receptors, p50csk becomes tightly associated with a 36-kDa protein (p36). This association is dependent on the tyrosine phosphorylation of p36 and involves its interaction with the SH2 domain of p50csk.p36 can be phosphorylated in vitro by p50csk or by a full-length GST-Csk fusion protein expressed in Escherichia coli. Tyrosine-phosphorylated p36 is found exclusively in the particulate membrane fraction of the cell. Conditions that induce the formation of the p50csk.p36 complex promote the appearance of p50csk in the particulate fraction. These data suggest that the association between p50csk and p36 serves to translocate the normally cytosolic p50csk to the membrane, where it presumably interacts with its physiologically relevant substrates.