title: A study of archaeal enzymes involved in polar lipid synthesis linking amino acid sequence information, genomic contexts and lipid composition.
abstract: Cellular membrane lipids, of which phospholipids are the major constituents, form one of the characteristic features that distinguish Archaea from other organisms. In this study, we focused on the steps in archaeal phospholipid synthetic pathways that generate polar lipids such as archaetidylserine, archaetidylglycerol, and archaetidylinositol. Only archaetidylserine synthase (ASS), from Methanothermobacter thermautotrophicus, has been experimentally identified. Other enzymes have not been fully examined. Through database searching, we detected many archaeal hypothetical proteins that show sequence similarity to members of the CDP alcohol phosphatidyltransferase family, such as phosphatidylserine synthase (PSS), phosphatidylglycerol synthase (PGS) and phosphatidylinositol synthase (PIS) derived from Bacteria and Eukarya. The archaeal hypothetical proteins were classified into two groups, based on the sequence similarity. Members of the first group, including ASS from M. thermautotrophicus, were closely related to PSS. The rough agreement between PSS homologue distribution within Archaea and the experimentally identified distribution of archaetidylserine suggested that the hypothetical proteins are ASSs. We found that an open reading frame (ORF) tends to be adjacent to that of ASS in the genome, and that the order of the two ORFs is conserved. The sequence similarity of phosphatidylserine decarboxylase to the product of the ORF next to the ASS gene, together with the genomic context conservation, suggests that the ORF encodes archaetidylserine decarboxylase, which may transform archaetidylserine to archaetidylethanolamine. The second group of archaeal hypothetical proteins was related to PGS and PIS. The members of this group were subjected to molecular phylogenetic analysis, together with PGSs and PISs and it was found that they formed two distinct clusters in the molecular phylogenetic tree. The distribution of members of each cluster within Archaea roughly corresponded to the experimentally identified distribution of archaetidylglycerol or archaetidylinositol. The molecular phylogenetic tree patterns and the correspondence to the membrane compositions suggest that the two clusters in this group correspond to archaetidylglycerol synthases and archaetidylinositol synthases. No archaeal hypothetical protein with sequence similarity to known phosphatidylcholine synthases was detected in this study.
authors.affiliation: Bioinformatics Center, Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan.
authors.forename: Hiromi
authors.initials: H
authors.lastname: Daiyasu
authors.forename: Kei-Ichi
authors.initials: K
authors.lastname: Kuma
authors.forename: Toshiro
authors.initials: T
authors.lastname: Yokoi
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Morii
authors.forename: Yosuke
authors.initials: Y
authors.lastname: Koga
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Toh
mesh_terms: D001105:Archaea / Q000201:enzymology* / Q000235:genetics / Q000378:metabolism; D019843:Archaeal Proteins / Q000145:classification / Q000235:genetics; D019766:Genes, Archaeal; D016366:Open Reading Frames; D010743:Phospholipids / Q000096:biosynthesis*; D010770:Phosphotransferases / Q000145:classification / Q000235:genetics / Q000378:metabolism; D013045:Species Specificity
chemical_list: D019843:Archaeal Proteins; D010743:Phospholipids; D010770:Phosphotransferases