Section of Microbiology and MRC Centre for Molecular Bacteriology and Infection, Imperial College London, London SW7 2AZ, UK.
Microbiology (Reading). 2013 Sep;159(Pt 9):1868-1877. doi: 10.1099/mic.0.069898-0. Epub 2013 Jul 15.
Teichoic acids (TAs) are important for growth, biofilm formation, adhesion and virulence of Gram-positive bacterial pathogens. The chemical structures of the TAs vary between bacteria, though they typically consist of zwitterionic polymers that are anchored to either the peptidoglycan layer as in the case of wall teichoic acid (WTA) or the cell membrane and named lipoteichoic acid (LTA). The polymers are modified with D-alanines and a lack of this decoration leads to increased susceptibility to cationic antimicrobial peptides. Four proteins, DltA-D, are essential for the incorporation of d-alanines into cell wall polymers and it has been established that DltA transfers D-alanines in the cytoplasm of the cell onto the carrier protein DltC. However, two conflicting models have been proposed for the remainder of the mechanism. Using a cellular protein localization and membrane topology analysis, we show here that DltC does not traverse the membrane and that DltD is anchored to the outside of the cell. These data are in agreement with the originally proposed model for D-alanine incorporation through a process that has been proposed to proceed via a D-alanine undecaprenyl phosphate membrane intermediate. Furthermore, we found that WTA isolated from a Staphylococcus aureus strain lacking LTA contains only a small amount of D-alanine, indicating that LTA has a role, either direct or indirect, in the efficient D-alanine incorporation into WTA in living cells.
磷壁酸（TAs）对于革兰氏阳性细菌病原体的生长、生物膜形成、黏附和毒力非常重要。TAs 的化学结构在细菌之间有所不同，但它们通常由两性离子聚合物组成，这些聚合物要么锚定在肽聚糖层上，如细胞壁磷壁酸（WTA），要么锚定在细胞膜上，称为脂磷壁酸（LTA）。聚合物通过 D-丙氨酸进行修饰，如果缺乏这种修饰，细菌对阳离子抗菌肽的敏感性会增加。DltA-D 这四种蛋白对于将 D-丙氨酸掺入细胞壁聚合物中是必不可少的，并且已经确定 DltA 将细胞质中的 D-丙氨酸转移到载体蛋白 DltC 上。然而，对于其余的机制，已经提出了两种相互矛盾的模型。使用细胞内蛋白质定位和膜拓扑分析，我们在这里表明 DltC 不会穿过膜，并且 DltD 被锚定在细胞的外部。这些数据与最初提出的 D-丙氨酸掺入模型一致，该模型提出了一个通过 D-丙氨酸十一异戊烯磷酸膜中间产物进行的过程。此外，我们发现从缺乏 LTA 的金黄色葡萄球菌株中分离的 WTA 仅含有少量的 D-丙氨酸，这表明 LTA 在活细胞中将 D-丙氨酸有效地掺入 WTA 中具有直接或间接的作用。
