Activation-dependent phosphorylation of endogeneous protein kinase c substrates in quiescent human T lymphocytes

Activation of quiescent human T lymphocytes with phorbol ester, synthetic diacylglycerol, or an antibody specific for the antigen receptor associated CD3 antigen resulted in a rapid phosphorylation of a Mr 80,000 (termed 80K) and a Mr 19,000 (termed 19K) cellular protein. The 80K (pI 4.4-5.1) protein was evidently analogous to a previously described 80K protein, which is a putative in vivo substrate for the Ca²⁺-activated phospholipid-dependent protein kinase (PK.C), while the identity of the 19K protein is unknown. We present evidences that the 19K protein is variably phosphorylated on serine residues and that phosphorylation involves activation of the PK.C pathway. The biological significance of these phosphorylation events was suggested both by the ligand specificity and the correlation with subsequent induction of cellular proliferation.