Phosphatidylglycerol Is a Physiologic Activator of Nuclear Protein Kinase C*

Abstract

A major mechanism by which protein kinase C (PKC) function is regulated is through the selective targeting and activation of individual PKC isotypes at distinct subcellular locations. PKC β_II is selectively activated at the nucleus during G₂ phase of cell cycle where it is required for entry into mitosis. Selective nuclear activation of PKC β_II is conferred by molecular determinants within the carboxyl-terminal catalytic domain of the kinase (Walker, S. D., Murray, N. R., Burns, D. J., and Fields, A. P. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 9156–9160). We previously described a lipid-like PKC activator in nuclear membranes, termed nuclear membrane activation factor (NMAF), that potently stimulates PKC β_II activity through interactions involving this domain (Murray, N. R., Burns, D. J., and Fields, A. P. (1994)J. Biol. Chem. 269, 21385–21390). We have now identified NMAF as phosphatidylglycerol (PG), based on several lines of evidence. First, NMAF cofractionates with PG as a single peak of activity through multiple chromatographic separations and exhibits phospholipase sensitivity identical to that of PG. Second, purified PG, but not other phospholipids, exhibits dose-dependent NMAF activity. Third, defined molecular species of PG exhibit different abilities to stimulate PKC β_II activity. 1,2-Dioleoyl-PG possesses significantly higher activity than other PG species, suggesting that both fatty acid side chain composition and the glycerol head group are important determinants for activity. Fourth, in vitro binding studies demonstrate that PG binds to the carboxyl-terminal region of PKC β_II, the same region we previously implicated in NMAF-mediated activation of PKC β_II. Taken together, our results indicate that specific molecular species of nuclear PG function to physiologically regulate PKC β_II activity at the nucleus.