A NOVEL FAMILY OF PHOSPHOLIPASE-D HOMOLOGS THAT INCLUDES PHOSPHOLIPIDSYNTHASES AND PUTATIVE ENDONUCLEASES - IDENTIFICATION OF DUPLICATED REPEATS AND POTENTIAL ACTIVE-SITE RESIDUES

Phosphatidylcholine-specific phospholipase D (PLD) enzymes catalyze hy drolysis of phospholipid phosphodiester bonds, and also transphosphati dylation of phospholipids to acceptor alcohols. Bacterial and plant PL D enzymes have not been shown previously to be homologues or to be hom ologous to any other protein. Here we show, using sequence analysis me thods, that bacterial and plant PLDs show significant sequence similar ities both to each other, and to two other classes of phospholipid-spe cific enzymes, bacterial cardiolipin synthases, and eukaryotic and bac terial phosphatidylserine synthases, indicating that these enzymes for m an homologous family. This family is suggested also to include two P oxviridae proteins of unknown function (p37K and protein K4), a bacter ial endonuclease (nuc), an Escherichia coli putative protein (o338) co ntaining an N-terminal domain showing similarities with helicase motif s V and VI, and a Synechocystis sp. putative protein with a C-terminal domain likely to possess a DNA-binding function, Surprisingly, four r egions of sequence similarity that occur once in nuc and o338, appear twice in all other homologues, indicating that the latter molecules ar e bi-lobed, having evolved from an ancestor or ancestors that underwen t a gene duplication and fusion event. It is suggested that, for each of these enzymes, conserved histidine, lysine, aspartic acid, and/or a sparagine residues may be involved in a two-step ping-pong mechanism i nvolving an enzyme-substrate intermediate.