Sites of in vivo phosphorylation of the T cell tyrosine protein kinase in LSTRA cells and their alteration by tumor-promoting phorbol esters.

The LSTRA cell line contains an elevated level of a tyrosine protein kinase of apparent molecular weight of 56,000 (pp56Tcell). Analysis of the tryptic fragments of this protein labeled in vivo with 32P shows that it contains four sites of tyrosine phosphorylation and one site of serine phosphorylation. Two of the sites of in vivo tyrosine phosphorylation are also labeled in vitro when membranes are incubated with [gamma-32P]ATP. One of the sites that is labeled in vivo and in vitro (site 1) is identical in sequence with the major site of tyrosine phosphorylation in the transforming protein of the Rous sarcoma virus. Analysis of the sites of in vivo phosphorylation in pp56Tcell from LSTRA cells treated with 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA) reveals that this agent induces at least four new sites of serine phosphorylation. Treatment with PMA also causes a selective reduction in the level of tyrosine phosphorylation in site 1. Thus PMA causes new sites of serine phosphorylation in pp56Tcell and reduces the amount of phosphate in one of the sites of tyrosine phosphorylation.