ExoU and VipD-like proteins; homologus to patatin, cPLA2, and iPLA2
ExoU, a 74-kDa enzyme, is a potent virulence factor of Pseudomonas aeruginosa. One of the pathogenic mechanisms of P. aeruginosa is to induce cytotoxicity by the injection of effector proteins (e.g. ExoU) using the type III secretion (T3S) system. ExoU is homologus to patatin and also has the conserved catalytic residues of mammalian calcium-independent (iPLA2) and cytosolic (cPLA2) PLA2. In vitro, ExoU cytotoxity is blocked by the inhibitor of cytosolic and Ca2-independent phospholipase A2 (cPLA2 and iPLA2) enzymes, suggesting that phospholipase A2 inhibitors may represent a novel mode of treatment for acute P. aeruginosa infections. ExoU requires eukaryotic superoxide dismutase as a cofactor and cleaves phosphatidylcholine and phosphatidylethanolamine in vitro. VipD, a 69-kDa cytosolic protein, belongs to the members of Legionella pneumophila family and is homologus to ExoU from Pseudomonas. Even though VipD shows high sequence similarity with several functional regions of ExoU (e.g. oxyanion hole, active site serine, active site aspartate), it has been shown to have no phospholipase activity. This family includes ExoU from Pseudomonas aeruginosa and VipD of Legionella pneumophila.