TCR/CD3-triggering causes increased activity of the p50csk tyrosine kinase and engagement of its SH2 domain.

The majority of known protein tyrosine kinases are integral membrane proteins or bound to cellular membranes via a covalently attached myristic acid. An exception is the newly described p50csk tyrosine kinase, which is believed to control the activity of the src-family of protein tyrosine kinases. This small kinase does not contain a myristylation signal or other known membrane attachment motifs, and indeed appears to be cytosolic. Recently, we found that p50csk specifically phosphorylates the negative regulatory Tyr-505 of the T cell-specific src-family kinase p56lck, and thereby suppresses its catalytic activity. Here we show that p50csk is activated in Jurkat T cells within one minute after stimulation of the cells with anti-CD3 MAbs. In parallel with this activation, p50csk formed a stable complex with one major 72 kDa tyrosine phosphorylated protein and minor polypeptides at 90 and 110 kDa. The isolated SH2 domain of p50csk specifically bound the 72 kDa protein in lysates from activated, but not resting, T or B cells. By several criteria, the 72 kDa protein was not a tyrosine kinase itself and it did not react with antibodies to a panel of known proteins of this molecular size. Our results suggest that p50csk is rapidly engaged via its SH2 domain and participates in the earliest events of T cell activation.