Mechanism of phorbol ester activation of calcium-activated, phospholipid-dependent protein kinase.

Tumor-promoting phorbol esters activated a calcium-activated, phospholipid-dependent protein kinase by direct complexation with this protein kinase and phospholipid in the presence of divalent cations and this complexation was identical to association of kinase/receptor activity with cell membranes. Treatment of isolated mouse spleen lymphocytes with phorbol ester tumor promoters resulted in a rapid shift in the subcellular localization of both the phorbol ester receptor and a calcium-activated, phospholipid-dependent protein kinase activity. Both activities shifted from almost entirely soluble to largely membrane-associated, which is consistent with a single protein possessing both activities. Activation of partially purified kinase/receptor activity by phorbol ester or calcium alone or in combination occurred in parallel to the formation of a complex between the kinase/receptor and phospholipid. Magnesium also was important both for complex formation and for activation of the protein kinase. Although phorbol ester did not appear to affect the affinity of the kinase/receptor for phospholipid, it did increase the extent of formation of a stable complex between the receptor and the phospholipid. These observations support the hypothesis that the cell membrane is the locus of action of both the phorbol esters and the calcium-activated, phospholipid-dependent protein kinase activity.