Phosphatidylcholine-hydrolysing phospholipase D (PLD) isoforms are activated by ADP-ribosylation factors (ARFs). PLD produces phosphatidic acid from phosphatidylcholine, which may be essential for the formation of certain types of transport vesicles or may be constitutive vesicular transport to signal transduction pathways. PC-hydrolysing PLD is a homolog of cardiolipin synthase, phosphatidylserine synthase, bacterial PLDs, and viral proteins. Each of these appears to possess a domain duplication which is apparent by the presence of two motifs containing well-conserved histidine, lysine, and/or asparagine residues which may contribute to the active site. aspartic acid. An E. coli endonuclease (nuc) and similar proteins appear to be PLD homologs but possess only one of these motifs. The profile contained here represents only the putative active site regions, since an accurate multiple alignment of the repeat units has not been achieved.