Activation and regulation of protein kinase C enzymes.

Protein Kinase C (PKC) has been a principal regulatory enzyme whose function has been intensely investigated in the past decade. The primary features of this family of enzymes includes phosphorylation of serine and threonine residues located on basic proteins and peptide in a manner that is stimulated by calcium, phospholipid, and either diacylglycerol or phorbol esters. An additional intriguing feature of the enzymes is its ability to form two membrane-associated states, one of which is calcium dependent and reversible and the second is an irreversible complex which has the characteristics of an intrinsic membrane protein. Formation of the irreversible membrane-bound form is greatly facilitated by calcium and the tumor-promoting phorbol esters but does not appear to include covalent changes in the PKC structure. The intrinsic membrane-bound form is a very different enzyme in that its activity is no longer dependent on the other cofactors. It is proposed that formation of the irreversible membrane-bound form may be a mechanism for generating long-term cell regulation events where transient cell signals and second messengers induce long-term changes in the distribution of an enzyme in the cell. This property may be common to a number of regulatory proteins that are known to be distributed between the cytosol and membrane-fractions in the cell. Unfortunately, many problems have confronted study of PKC mechanism using the in vitro assay. This assay involves aggregation of the substrate, phospholipid, and enzyme to form a discontinuous mixture. Such a complex system prevents straightforward interpretation of enzyme kinetic data. Although many compounds affect the in vitro activity of PKC, most appear to accomplish this by relatively uninteresting mechanisms such as interference with the aggregation process. While some highly potent inhibitors undoubtedly interact directly with PKC, they also inhibit other enzymes and there are no entirely specific inhibitors of PKC known. Speculation on the possible roles of PKC in cell regulation are abundant and exciting. However, delineation of the regulatory roles of PKC may require another decade of intense effort.