Molecular characterization of the type 2 phosphatidic acid phosphatase

Phosphatidic acid phosphatase (PAP) converts phosphatidic acid to diacylgly cerol, thus regulating the de novo synthesis of glycerolipids and also sign al transduction mediated by phospholipase D. We initially succeeded in the cDNA cloning of the mouse 35 kDa PAP bound to plasma membranes (type 2 enzy me). This work subsequently led us to the identification of two human PAP i sozymes designated 2a and 2b. A third human PAP isozyme (2c) has also been described. The cloned enzymes are, in common, N-glycosylated and possess si x transmembrane domains. The transmembrane dispositions of these enzymes ar e predicted and the catalytic sites are tentatively located in the 2nd and 3rd extracellular loops, thus suggesting that the type 2 PAPs may act as ec to-enzymes dephosphorylating exogenous substrates. Furthermore, the type 2 PAPs have been proposed to belong to a novel phosphatase superfamily consis ting of a number of soluble and membrane-bound enzymes. In vitro enzyme ass ays show that the type 2 PAPs can dephosphorylate lyso-phosphatidate, ceram ide-l-phosphate, sphingosine-l-phosphate and diacylglycerol pyrophosphate. Although the physiological implications of such a broad substrate specifici ty need to be further investigated, the type 2 PAPs appear to metabolize a wide range of lipid mediators derived from both glycero-and sphingolipids. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.