Sychantha David, Chapman Robert N, Bamford Natalie C, Boons Geert-Jan, Howell P Lynne, Clarke Anthony J
Department of Molecular and Cellular Biology , University of Guelph , Guelph , ON N1G 2X1 , Canada.
Complex Carbohydrate Research Center , University of Georgia , 315 Riverbend Road , Athens , Georgia 30602 , United States.
Biochemistry. 2018 Apr 3;57(13):1949-1953. doi: 10.1021/acs.biochem.8b00060. Epub 2018 Mar 14.
Bacterial surface (S) layers are paracrystalline arrays of protein assembled on the bacterial cell wall that serve as protective barriers and scaffolds for housekeeping enzymes and virulence factors. The attachment of S-layer proteins to the cell walls of the Bacillus cereus sensu lato, which includes the pathogen Bacillus anthracis, occurs through noncovalent interactions between their S-layer homology domains and secondary cell wall polysaccharides. To promote these interactions, it is presumed that the terminal N-acetylmannosamine (ManNAc) residues of the secondary cell wall polysaccharides must be ketal-pyruvylated. For a few specific S-layer proteins, the O-acetylation of the penultimate N-acetylglucosamine (GlcNAc) is also required. Herein, we present the X-ray crystal structure of the SLH domain of the major surface array protein Sap from B. anthracis in complex with 4,6- O-ketal-pyruvyl-β-ManNAc-(1,4)-β-GlcNAc-(1,6)-α-GlcN. This structure reveals for the first time that the conserved terminal SCWP unit is the direct ligand for the SLH domain. Furthermore, we identify key binding interactions that account for the requirement of 4,6- O-ketal-pyruvyl-ManNAc while revealing the insignificance of the O-acetylation on the GlcNAc residue for recognition by Sap.
细菌表面（S）层是组装在细菌细胞壁上的蛋白质准晶体阵列，可作为保护屏障以及管家酶和毒力因子的支架。S层蛋白与蜡样芽孢杆菌（包括病原体炭疽芽孢杆菌）的细胞壁的附着，是通过其S层同源结构域与次生细胞壁多糖之间的非共价相互作用实现的。据推测，为促进这些相互作用，次生细胞壁多糖的末端N - 乙酰甘露糖胺（ManNAc）残基必须进行缩酮 - 丙酮酸化。对于一些特定的S层蛋白，还需要对倒数第二个N - 乙酰葡糖胺（GlcNAc）进行O - 乙酰化。在此，我们展示了炭疽芽孢杆菌主要表面阵列蛋白Sap的SLH结构域与4,6 - O - 缩酮 - 丙酮酰 - β - ManNAc - (1,4) - β - GlcNAc - (1,6) - α - GlcN复合物的X射线晶体结构。该结构首次揭示了保守的末端次生细胞壁多糖单元是SLH结构域的直接配体。此外，我们确定了关键的结合相互作用，这些相互作用解释了对4,6 - O - 缩酮 - 丙酮酰 - ManNAc的需求，同时揭示了GlcNAc残基上的O - 乙酰化对于Sap识别并不重要。
