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Koga, Yosuke (2)
Daiyasu, Hiromi (1)
Kuma, Kei-Ichi (1)
Matsuno, Koji (1)
Nishihara, Masateru (1)
Ohga, Mami (1)
Stetter, Karl O. (1)
Toh, Hiroyuki (1)
Yokoi, Toshiro (1)
Year of Publication
A study of archaeal enzymes involved in polar lipid synthesis linking amino acid sequence information, genomic contexts and lipid composition
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.
archaetidylcholine; archaetidylglycerol; archaetidylinositol; archaetidylserine; genomic context; phylogenetic tree
Structural analysis by reductive cleavage with LiAlH4 of an allyl ether choline-phospholipid, archaetidylcholine, from the hyperthermophilic methanoarchaeon Methanopyrus kandleri
Stetter, Karl O.
A choline-containing phospholipid (PL-4) in Methanopyrus kandleri cells was identified as archaetidylcholine, which has been described by Sprott et al. (1997). The PL-4 consisted of a variety of molecular species differing in hydrocarbon composition. Most of the PL-4 was acid-labile because of its allyl ether bond. The identity of PL-4 was confirmed by thin-layer chromatography (TLC) followed by positive staining with Dragendorff-reagent and fast-atom bombardment–mass spectrometry. A new method of LiAlH4 hydrogenolysis was developed to cleave allyl ether bonds and recover the corresponding hydrocarbons. We confirmed the validity of the LiAlH4 method in a study of the model compound synthetic unsaturated archaetidic acid (2,3-di-O-geranylgeranyl-sn-glycerol-1-phosphate). Saturated ether bonds were not cleaved by the LiAlH4 method. The hydrocarbons formed following LiAlH4 hydrogenolysis of PL-4 were identified by gas–liquid chromatography and mass spectrometry. Four kinds of hydrocarbons with one to four double bonds were detected: 47% of the hydrocarbons had four double bonds; 11% had three double bonds; 14% had two double bonds; 7% had one double bond; and 6% were saturated species. The molecular species composition of PL-4 was also estimated based on acid lability: 77% of the molecular species had two acid-labile hydrocarbons; 11% had one acid-labile and one acid-stable hydrocarbon; and 11% had two acid-stable hydrocarbons. To our knowledge, this is the first report of a specific chemical degradation method for the structural analysis of allyl ether phospholipid in archaea.
acid-labile phospholipid; LiAlH4 hydrogenolysis; molecular species composition; unsaturated isoprenoid
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