Critical Review Phospholipid Signalling Through Phospholipase D and Phosphatidic Acid Rosanna Cazzolli 1 , Anne N. Shemon 2 , Michelle Q. Fang 1 and William E. Hughes 1,3 1 Phospholipid Biology Group, The Garvan Institute of Medical Research, Sydney, New South Wales, Australia 2 Department of Medicine, University of Sydney, Nepean Hospital, New South Wales, Australia 3 Department of Medicine, University of New South Wales, St. Vincent’s Hospital, New South Wales, Australia Summary Phospholipase D (PLD) hydrolyzes the phosphodiester bond of the predominant membrane phospholipid, phosphatidylcholine producing phosphatidic acid and free choline. This activity can participate in signal transduction pathways and impact on vesicle trafficking for secretion and endocytosis, as well as receptor signalling. Phospholipids can regulate PLD activity directly, through specific intermolecular interactions, or indirectly, through their effect on the localization or activity of PLD’s protein effectors. This short review highlights these various phospholipid inputs into the regulation of PLD activity and also reviews potential roles for PLD-generated phosphatidic acid, particularly a mechanism by which the phospholipid may participate in the process of vesicular trafficking. IUBMB Life, 58: 457–461, 2006 Keywords Phospholipase D; phosphatidic acid; protein kinase C; ARF; Rho; membrane curvature; vesicle trafficking. INTRODUCTION In the late 1970s, work by Yasutomi Nishizuka helped trigger a new understanding in cellular signal transduction which still holds a pivotal place in our understanding of lipid signalling (1, 2). This work identified the role of the membrane glycerophospholipid, phosphatidylinositol (PtdIns) and its hydrolytic product, diacylglycerol (DAG), in activating the serine-threonine kinase, protein kinase C (PKC). Nearly 30 years on, it is clear that this ‘classical’ PtdIns-DAG-PKC cascade is only one arm of numerous inter-related lipid signalling cascades within the cellular ‘lipidome’ (3). The lipidome consists of more than 1000 identified cellular lipids, of which phosphatidic acid (PtdOH) is one member (3). PtdOH has long been studied because it is a source of DAG but, recently it has become clear that this glycerophospholipid has more to offer than a potential alternate PKC activation pathway. PtdOH can be formed by the addition of phosphate to DAG, through the action of DAG kinases or by the action of lysophosphatidic acid acyltransferases on lysophosphatidic acid (Fig. 1). However, recently much interest has focused on the enzyme phospho- lipase D (PLD) which hydrolyses the relatively abundant phospholipid phosphatidylcholine (PtdCho) to also produce PtdOH (4 – 7). Studies on PLD have established that the enzyme itself is regulated by a series of lipid signalling cascades and recent studies discussed below have come closer to revealing a mechanism for the function of PtdOH. PHOSPHOLIPASE D IS AT THE CENTRE OF A PHOSPHOLIPID SIGNALLING NETWORK Cellular PLD activity is tightly regulated via a number of mechanisms. These include direct control by membrane phospholipids, and direct inputs from a number of protein effectors. Some Ras-like small GTPases and PKCs, for example, have been demonstrated regulate PLD’s activity via specific intermolecular interactions. However, phospholipids can also indirectly regulate PLD, since PKC and GTPase regulation of PLD can be highly dependent on the phospho- lipid environment. Phosphatidylinositol Polyphosphates Phospholipids of the phosphatidylinositol polyphosphate (PtdIns) class are essential cofactors for PLD. Derivatives of this class of phospholipid have been demonstrated to interact directly with PLD. Phosphatidylinositol(4,5)bisphosphate (PtdIns(4,5)P 2 ) recognizes sequences towards the middle of PLD1 (8) whereas phosphatidylinositol(3,4,5)triphosphate (PtdIns(3,4,5)P 3 ) interacts with the amino-terminal Phox homology (PX) domain (Fig. 2) (9). These phospholipid interactions are required for enzyme activity from PLDs identified in mammals, plants and protozoa. The PtdIns Received 13 June 2006; accepted 20 June 2006 Address correspondence to: William E. Hughes, Phospholipid Biology Group and ‘‘Pieter Huveneers’’ Molecular Imaging Facility, The Garvan Institute of Medical Research, 384 Victoria Street, Sydney, NSW 2010, Australia. E-mail: w.hughes@garvan.org.au IUBMB Life, 58(8): 457 – 461, August 2006 ISSN 1521-6543 print/ISSN 1521-6551 online Ó 2006 IUBMB DOI: 10.1080/15216540600871142