Limited proteolysis of protein kinase C subspecies by calcium-dependent neutral protease (calpain).

Limited proteolysis of three distinct subspecies of protein kinase C (Ca2+/phospholipid-dependent enzyme, PKC), types I (gamma), II (beta I and beta II), and III (alpha), with Ca2+-dependent neutral proteases I and II (calpains I and II) was studied. All forms of PKC (82 kDa) were converted to two major fragments: a 45-49-kDa catalytic fragment and a 36-kDa regulatory fragment. The cleavage of these PKC subspecies by calpain I (active in the micromolar range Ca2+/ion concentration), and, to a lesser extent, by calpain II (active in the millimolar range), was enhanced by the simultaneous presence of phospholipid and diacylglycerol or phorbol ester, suggesting that the activated form of PKC is the preferred target for proteolysis. Analysis of the NH2-terminal sequence of the resulting catalytic fragments indicated that both calpains I and II cleave at one or two specific sites in the third variable region (V3) of each PKC molecule. Under comparable conditions with calpains I and II, the relative rates of cleavage of types I, II, and III PKC were approximately 100:16:2 and 100:48:23, respectively. The results imply that within the cell various PKC subspecies may be cleaved at different rates under different physiological conditions.