Two major tyrosine protein kinases of resting human T lymphocytes are down-regulated following mitotic stimulation

Human lymphocyte tyrosine protein kinases (TPKs) have been analyzed by gel-filtration chromatography. The major TPK species with activity towards an exogenous tyrosine-containing peptide had molecular masses of 70–100 kDa (TPK I) and 35–40 kDa (TPK II). TPKs I and II were distinct from the well-characterized autophosphorylating lymphoid cell TPK, pp56lck [(1983) J. Biol. Chem. 258, 10738–107421]. Both TPK I and TPK II were down-regulated following mitogenic stimulation of lymphocytes with phytohae-magglutinin. By contrast, pp56lck remained clearly detectable in stimulated lymphocytes. We suggest that TPKs I and II may play a role in the regulation of the lymphocyte cell cycle.