Nambu Takayuki, Inagaki Yuji, Kutsukake Kazuhiro
Department of Bioscience, Nagahama Institute of Bioscience and Technology, 1266 Tamura, Nagahama, Shiga 526-0829, Japan.
Genes Genet Syst. 2006 Dec;81(6):381-9. doi: 10.1266/ggs.81.381.
Bacterial flagellar rod structure is built across the peptidoglycan (PG) layer. A Salmonella enterica flagellar protein FlgJ is believed to consist of two functional domains, the N-terminal half acting as a scaffold or cap essential for rod assembly and the C-terminal half acting as a PG hydrolase (PGase) that makes a hole in the PG layer to facilitate rod penetration. In this study, molecular data analyses were conducted on FlgJ data sets sampled from a variety of bacterial species, and three types of FlgJ homologs were identified: (i) "canonical dual-domain" type found in beta- and gamma-proteobacteria that has a domain for one of the PGases, acetylmuramidase (Acm), at the C terminus, (ii) "non-canonical dual-domain" type found in the genus Desulfovibrio (delta-proteobacteria) that bears a domain for another PGase, M23/M37-family peptidase (Pep), at the C terminus and (iii) "single-domain" type found in phylogenetically diverged lineages that lacks the Acm or Pep domain. FlgJ phylogeny, together with the domain architecture, suggested that the single-domain type was the original form of FlgJ and the canonical dual-domain type had evolved from the single-domain type by fusion of the Acm domain to its C terminus in the common ancestor of beta- and gamma-proteobacteria. The non-canonical dual-domain type may have been formed by fusion of the Pep domain to the single-domain type in the ancestor of Desulfovibrio. In some lineages of gamma-proteobacteria, the Acm domain appeared to be lost secondarily from the dual-domain type FlgJ to yield again a single-domain type one. To rationalize the underlying mechanism that gave rise to the two different types of dual-domain FlgJ homologs, we propose a model assuming the lineage-specific co-option of flagellum-specific PGase from diverged housekeeping PGases in bacteria.
细菌鞭毛杆结构跨越肽聚糖（PG）层构建。肠炎沙门氏菌鞭毛蛋白FlgJ被认为由两个功能域组成，N端的一半作为鞭毛杆组装所必需的支架或帽，C端的一半作为PG水解酶（PGase），在PG层上形成一个孔以促进鞭毛杆穿透。在本研究中，对从多种细菌物种中采样的FlgJ数据集进行了分子数据分析，鉴定出三种类型的FlgJ同源物：（i）在β-和γ-变形杆菌中发现的“典型双结构域”类型，其C端具有一种PGase（乙酰胞壁酸酶，Acm）的结构域；（ii）在脱硫弧菌属（δ-变形杆菌）中发现的“非典型双结构域”类型，其C端具有另一种PGase（M23/M37家族肽酶，Pep）的结构域；（iii）在系统发育上分化的谱系中发现的“单结构域”类型，其缺乏Acm或Pep结构域。FlgJ系统发育以及结构域结构表明，单结构域类型是FlgJ的原始形式，典型双结构域类型是通过在β-和γ-变形杆菌的共同祖先中将Acm结构域融合到其C端而从单结构域类型进化而来。非典型双结构域类型可能是通过在脱硫弧菌的祖先中将Pep结构域融合到单结构域类型而形成的。在γ-变形杆菌的一些谱系中，Acm结构域似乎从双结构域类型的FlgJ中再次丢失，从而再次产生单结构域类型。为了阐明产生两种不同类型双结构域FlgJ同源物的潜在机制，我们提出了一个模型，假设细菌中从分化的管家PGases中进行鞭毛特异性PGase的谱系特异性选择。
