Speaker:  George M. Carman

Department of Food Science and the Rutgers Center for Lipid Research, Rutgers University, New Brunswick, New Jersey 08901

Abstract:

Phosphatidic acid (PA) phosphatase (PAP) is an evolutionarily conserved Mg²⁺-dependent enzyme that plays a key role in lipid homeostasis by controlling the cellular levels of its substrate, PA, and its product, diacylglycerol.  These lipids are essential intermediates for the synthesis of triacylglycerol and membrane phospholipids.  They also function in phospholipid synthesis gene expression, lipid droplet formation, and vesicular trafficking.  The importance of PAP to lipid homeostasis and cell physiology is exemplified in yeast, mouse, and human by a host of cellular defects and lipid-based diseases associated with loss or overexpression of enzyme function.  PAP function is largely controlled by its cellular location, which is mediated by phosphorylation and dephosphorylation.  Multiple phosphorylations sequester PAP in the cytosol and protect it from proteasomal degradation.  The endoplasmic reticulum-associated PAP phosphatase complex recruits and dephosphorylates PAP allowing the enzyme to associate with and dephosphorylate its membrane-bound substrate PA.  PAP contains domains/regions that include the N-LIP and haloacid dehalogenase (HAD)-like catalytic domains, N-terminal amphipathic helix for membrane binding, C-terminal acidic tail for PAP phosphatase interaction, and a conserved tryptophan within the WRDPLVDID domain required for enzyme function.  Through bioinformatics, molecular genetics, and biochemical approaches, we identified a novel RP (regulation of phosphorylation) domain that regulates the phosphorylation state of PAP.  We showed that the DRP mutation results in a 57% reduction in the endogenous phosphorylation of the enzyme, an increase in membrane association and PAP activity, but reduced cellular abundance.  This work not only identifies a novel regulatory domain within PAP but emphasizes the importance of the phosphorylation-based regulation of PAP abundance, location, and function in lipid synthesis. That disruption in the regulation of PAP function through alterations in phosphorylation leads to broader disruptions in lipid synthesis and cellular growth raises the suggestion that the RP domain may represent a possible therapeutic target for inhibiting growth of pathogenic fungi for which the domain is well conserved.