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Evidence that treatment of platelets with phorbol ester causes proteolytic activation of Ca2+-activated, phospholipid-dependent protein kinase.

Incubation of human platelets with the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA) caused the accumulation of a protein kinase in the particulate fraction which was not dependent on Ca2+ and phosphatidylserine (Ptd-Ser). The Ca2+/Ptd-Ser-independent kinase eluted from DEAE-cellulose at a NaCl concentration of 0.18-0.22 M compared with 0.08 - 0.16 M for Ca2+/Ptd-Ser-dependent protein kinase (C-kinase). Formation of the Ca2+/Ptd-Ser-independent kinase in 12-O-tetradecanoylphorbol-13-acetate-treated platelets was blocked by leupeptin, an inhibitor of Ca2+-dependent neutral proteases. The Ca2+/Ptd-Ser-independent kinase and C-kinase both catalysed the same pattern of phosphorylation of smooth muscle myosin light chains and histone H1 as detected by one-dimensional or two-dimensional peptide mapping after tryptic digestion. The phosphorylation sites were different from those obtained with myosin light chain kinase or cAMP-dependent kinase. The Ca2+/Ptd-Ser-independent kinase and C-kinase had Mr values of about 50 000 and 77 000 respectively as determined by sucrose-gradient centrifugation. It was concluded that 12-O-tetradecanoylphorbol-13-acetate induces the proteolytic cleavage of C-kinase to a Ca2+/Ptd-Ser-independent form.