Parker Michelle L, Houston Simon, Wetherell Charmaine, Cameron Caroline E, Boulanger Martin J
Department of Biochemistry & Microbiology, University of Victoria, Victoria, British Columbia, Canada.
PLoS One. 2016 Nov 10;11(11):e0166274. doi: 10.1371/journal.pone.0166274. eCollection 2016.
Treponema pallidum subspecies pallidum is the causative agent of syphilis, a chronic, multistage, systemic infection that remains a major global health concern. The molecular mechanisms underlying T. pallidum pathogenesis are incompletely understood, partially due to the phylogenetic divergence of T. pallidum. One aspect of T. pallidum that differentiates it from conventional Gram-negative bacteria, and is believed to play an important role in pathogenesis, is its unusual cell envelope ultrastructure; in particular, the T. pallidum peptidoglycan layer is chemically distinct, thinner and more distal to the outer membrane. Established functional roles for peptidoglycan include contributing to the structural integrity of the cell envelope and stabilization of the flagellar motor complex, which are typically mediated by the OmpA domain-containing family of proteins. To gain insight into the molecular mechanisms that govern peptidoglycan binding and cell envelope biogenesis in T. pallidum we report here the structural characterization of the putative OmpA-like domain-containing protein, Tp0624. Analysis of the 1.70 Å resolution Tp0624 crystal structure reveals a multi-modular architecture comprised of three distinct domains including a C-terminal divergent OmpA-like domain, which we show is unable to bind the conventional peptidoglycan component diaminopimelic acid, and a previously uncharacterized tandem domain unit. Intriguingly, bioinformatic analysis indicates that the three domains together are found in all orthologs from pathogenic treponemes, but are not observed together in genera outside Treponema. These findings provide the first structural insight into a multi-modular treponemal protein containing an OmpA-like domain and its potential role in peptidoglycan coordination and stabilization of the T. pallidum cell envelope.
梅毒螺旋体苍白亚种是梅毒的病原体，梅毒是一种慢性、多阶段的全身性感染，仍然是全球主要的健康问题。梅毒螺旋体致病的分子机制尚未完全了解，部分原因是梅毒螺旋体的系统发育差异。梅毒螺旋体与传统革兰氏阴性菌不同的一个方面，并且被认为在发病机制中起重要作用的是其不寻常的细胞包膜超微结构；特别是，梅毒螺旋体的肽聚糖层在化学上不同，更薄且距外膜更远。肽聚糖已确定的功能作用包括有助于细胞膜的结构完整性和鞭毛运动复合体的稳定，这通常由含OmpA结构域的蛋白质家族介导。为了深入了解控制梅毒螺旋体中肽聚糖结合和细胞膜生物合成的分子机制，我们在此报告了假定的含OmpA样结构域的蛋白质Tp0624的结构特征。对分辨率为1.70 Å的Tp0624晶体结构的分析揭示了一种多模块结构，由三个不同的结构域组成，包括一个C端不同的OmpA样结构域，我们发现它无法结合传统的肽聚糖成分二氨基庚二酸，以及一个以前未表征的串联结构域单元。有趣的是，生物信息学分析表明，这三个结构域共同存在于致病性密螺旋体的所有直系同源物中，但在密螺旋体属以外的属中未同时观察到。这些发现首次提供了对含有OmpA样结构域的多模块密螺旋体蛋白及其在梅毒螺旋体细胞膜肽聚糖协调和稳定中的潜在作用的结构见解。
