Abstract
Diacylglycerol kinase (DGK) converts diacylglycerol (DG) to phosphatidic acid (PA). The roles of both DG and PA as lipid second messengers have recently been attracting much attention. DGK is thus thought to be one of the key enzymes involved in the regulation of the signal transduction process through controlling the intracellular DG and PA levels. In the first part of this review, we describe the diversity of DGK isozymes. A DGK (α isozyme) with an apparent molecular weight of 80,000 was the first to be purified from the cytosol of pig brain in 1983. From immunological and biochemical studies as well as enzyme purification, it soon became clear that DGK represented a group of several isozymes, but the major breakthrough emerged from cDNA cloning of ever increasing DGK isoforms. It is now recognized that, similar to other signaling molecules such as protein kinase C and phosphatidylinositol-specific phospholipase C, DGK represents a large gene family of isozymes differing remarkably in their structures, the modes of tissue expression and enzymological properties. In the second part, we show that three DGK isozymes (α, β and γ) possess EF-hand structures with intrinsic properties different from each other with respect to affinities for Ca^<2+> and Ca^<2+>-induced conformational changes. It is quite likely that the DGK zinc fingers do not serve as DG-binding sites, in contrast to those present in other proteins such as protein kinase C and n-chimaerin. In the third part, the mechanisms of DGK activation by Ca^<2+>, sphingosine, enzyme phosphorylation and intracellular translocation of DGK are described. In the last part, we describe the biological roles of DGKα in IL-2-mediated lymphocyte proliferation. Studies using DGK inhibitors, substrate analogues and DGKα-transfected fibloblasts suggest that DGK participates in the regulation of PKC action by metabolizing DG.
