Summary
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 PM A-stimulation.
Keywords
Platelet - Protein phosphorylation - Tyrosine kinase - Phorbol ester
