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), fromMethanothermobacter 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 fromM. 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.
细胞膜脂质是一种特征,能够区分古菌和其他生物,其中磷脂是其主要成分之一。本研究聚焦于古菌磷脂合成途径中生成极性脂质如古菌酰基丝氨酸、古菌酰基甘油和古菌酰基肌醇的步骤。仅有来自于Methanothermobacter thermautotrophicus的古菌酰基丝氨酸合成酶(ASS)已被实验鉴定,其他酶尚未完全检查。通过数据库搜索,我们检测到许多古菌假定蛋白,这些蛋白显示出与来自细菌和真核生物的磷酸基醇磷脂转移酶家族成员如磷酸基丝氨酸合成酶(PSS)、磷酸基甘油合成酶(PGS)和磷酸基肌醇合成酶(PIS)的序列相似性。根据序列相似性,将古菌假定蛋白分为两组。第一组成员,包括来自M. thermautotrophicus的ASS,与PSS密切相关。PSS同源物在古菌中的分布与实验鉴定的古菌酰基丝氨酸的分布大致相符,这表明假定蛋白是ASS。我们发现在基因组中,一个开放阅读框(ORF)倾向于与ASS的ORF相邻,并且两个ORF的顺序是保守的。磷酸基丝氨酸脱羧酶与ASS基因旁的ORF产物的序列相似性,以及基因组上下文的保守性,表明ORF编码古菌酰基丝氨酸脱羧酶,可能将古菌酰基丝氨酸转化为古菌酰基乙醇胺。第二组古菌假定蛋白与PGS和PIS相关。将该组成员与PGS和PIS一起进行分子系统发育分析,发现它们在分子系统发育树中形成了两个不同的聚类。每个聚类中成员在古菌中的分布大致对应于实验鉴定的古菌酰基甘油或古菌酰基肌醇的分布。分子系统发育树的模式和与膜组成的对应关系表明,该组中的两个聚类对应于古菌酰基甘油合成酶和古菌酰基肌醇合成酶。本研究未检测到与已知磷脂酰胆碱合成酶序列相似的古菌假定蛋白。