Mandlik Anjali, Das Asis, Ton-That Hung
Department of Molecular, Microbial, and Structural Biology, University of Connecticut Health Center, 263 Farmington Avenue, Farmington, CT 06030, USA.
Proc Natl Acad Sci U S A. 2008 Sep 16;105(37):14147-52. doi: 10.1073/pnas.0806350105. Epub 2008 Sep 8.
Cell surface pili in gram-positive bacteria orchestrate the colonization of host tissues, evasion of immunity, and the development of biofilms. Recent work revealed that pilus assembly is a biphasic process wherein pilus polymerization is catalyzed by a pilus-specific sortase followed by cell wall anchoring of the pilus that is promoted by the housekeeping sortase. Here, we present molecular genetic and biochemical studies of a heterotrimeric pilus in Corynebacterium diphtheriae, uncovering the molecular switch that terminates pilus polymerization in favor of cell wall anchoring. The prototype pilus contains a major pilin (SpaA) forming the shaft, a tip pilin (SpaC), and another minor pilin (SpaB). Cells lacking SpaB form pilus fibers, but they are largely secreted in the medium, a phenotype also observed when cells lack the housekeeping sortase. Furthermore, the average pilus length is greatly increased in the absence of SpaB. Remarkably, a SpaB mutant that lacks the cell wall sorting signal but contains a critical lysine residue is incorporated in the pilus. However, the resulting pili fail to anchor to the cell wall. We propose that a specific minor pilin acts as the terminal subunit in pilus assembly. Cell wall anchoring ensues when the pilus polymer assembled on the pilus-specific sortase is transferred to the minor pilin presented by the housekeeping sortase via lysine-mediated transpeptidation.
革兰氏阳性菌的细胞表面菌毛参与宿主组织的定殖、免疫逃避以及生物膜的形成。最近的研究表明，菌毛组装是一个双相过程，其中菌毛特异性分选酶催化菌毛聚合，随后由管家分选酶促进菌毛的细胞壁锚定。在这里，我们对白喉棒状杆菌中的一种异三聚体菌毛进行了分子遗传学和生物化学研究，揭示了终止菌毛聚合以利于细胞壁锚定的分子开关。原型菌毛包含形成菌毛杆的主要菌毛蛋白（SpaA）、尖端菌毛蛋白（SpaC）和另一种次要菌毛蛋白（SpaB）。缺乏SpaB的细胞形成菌毛纤维，但它们大多分泌到培养基中，当细胞缺乏管家分选酶时也观察到这种表型。此外，在没有SpaB的情况下，菌毛的平均长度大大增加。值得注意的是，一种缺乏细胞壁分选信号但含有关键赖氨酸残基的SpaB突变体被整合到菌毛中。然而，产生的菌毛无法锚定到细胞壁上。我们提出一种特定的次要菌毛蛋白在菌毛组装中作为末端亚基。当在菌毛特异性分选酶上组装的菌毛聚合物通过赖氨酸介导的转肽作用转移到管家分选酶呈现的次要菌毛蛋白上时，就会发生细胞壁锚定。
