Reversible palmitoylation of the protein-tyrosine kinase p56lck.

The myristoylated protein-tyrosine kinase, p56lck, is expressed predominantly in T cells where it is believed to play a role in T cell activation. We observed a 56-kDa protein that became metabolically labeled in intact T lymphoid cells that were incubated with either 3Hmyristate or 3Hpalmitate. This protein was identified as p56lck based on its specific immunoprecipitation with polyclonal antisera to p56lck, by induction of a shift in its electrophoretic mobility following treatment of cells with 12-O-tetradecanoylphorbol-13-acetate and by co-chromatography with p56lck on protamine-agarose. Characterization of the two acylation events revealed that, in contrast to the p56lck-associated radioactivity from 3Hmyristate-labeled cells, the p56lck-associated radioactivity from 3Hpalmitate-labeled cells was susceptible to cleavage by neutral hydroxylamine and was not blocked by inhibitors of protein synthesis. Pulse-chase analyses revealed that the labeling of p56lck with 3Hpalmitate, but not 3Hmyristate, was reversible. The presence of covalently attached palmitate on p56lck from 3Hpalmitate-labeled cells was verified by thin-layer chromatography following acid hydrolysis of the acylated protein. 2-Hydroxymyristate, which is metabolically activated to form a potent inhibitor of protein myristoylation, specifically inhibited the acylation of p56lck with 3Hmyristate without affecting its labeling with 3Hpalmitate. These studies indicate that p56lck is both a cotranslationally myristoylated and post-translationally palmitoylated protein.