Tyrosine phosphorylation of platelet protein induced by phorbol ester

The presence of phosphotyrosine in proteins from washed rabbit platelets was demonstrated by electrophoresis and by alkali stability of phosphorylated polypeptides. The phosphotyrosine content increased about twofold by treating the platelets with PMA in spite of the fact that the specific activity of labeled ATP was unchanged. It was shown that phosphorylated proteins with apparent molecular weight of 48k, 53k and 250k were alkali-stable. Although phosphorylation of 48k protein in platelets was stimulated by 1 U thrombin as well as by 100 ng PMA, only the phosphorylated samples from PMA-treated platelets were alkali-stable. The 48k protein from platelets stimulated by PMA showed the fivefold increase of 32P-tyrosine content as compared with the control. Relatively lower increase of 32P-serine or 32P-threonine, 2- and 1.7-fold respectively, was thereby observed. The differences in time-sequence between 48k and 250k protein of total and alkali-stable phosphorylations suggested that tyrosine phosphorylation of 48k protein is the earliest event after the PMA-stimulation.