A NEW-PROTEIN KINASE-C, NPKC-ETA', AND NPKC-THETA ARE EXPRESSED IN HUMAN PLATELETS - INVOLVEMENT OF NPKC-ETA' AND NPKC-THETA IN SIGNAL TRANSDUCTION STIMULATED BY PAF

We have detected in human platelets two protein kinase C isozymes that have not been reported previously. Using an anti-nPKCθ antibody and Western blotting, we calculated the molecular weight of platelet nPKCθ as 79K. This molecular weight is identical to that described for nPKCθ in skeletal muscle and in COS cells transfected with the nPKCθ-cDNA. Using an anti-nPKCη antibody, we determined the molecular weight of an immunoreactive protein, which we called nPKCη′, to be 95K. This molecular weight is higher than that of nPKCη found in lung and skin tissue of 82K and 78K, and it is higher than nPKCη of COS cells transfected with the nPKCη-cDNA expression plasmid.Together with previous reports, these findings make the total number of PKC isozymes in human platelets equal to six. These are the PKC isozymes: α, β, δ and ζ, which have been previously described, and η′ and θ which we describe here. To assess the functionality of these new PKC isoforms, we stimulated platelets with PAF. We found a 200% and 175% increase in the levels of membrane-bound nPKCη′ and nPKCθ, respectively, in human platelets stimulated by PAF. A concomitant decrease in the level of these isoforms in the cytoplasm was observed. This PAF-induced translocation was time-dependent, and it reached its peak after a 1 minute incubation of human platelets with PAF for nPKCθ and 30 seconds for nPKCη′. © 1993 Academic Press.