Deng Dong Mei, Urch Jonathan E, ten Cate Jacob M, Rao Vincenzo A, van Aalten Daan M F, Crielaard Wim
Department of Cariology Endodontology Pedodontology, ACTA, Louwesweg 1, 1066 EA Amsterdam, The Netherlands.
J Bacteriol. 2009 Jan;191(1):394-402. doi: 10.1128/JB.00838-08. Epub 2008 Oct 31.
The genome sequence of the oral pathogen Streptococcus mutans predicts the presence of two putative polysaccharide deacetylases. The first, designated PgdA in this paper, shows homology to the catalytic domains of peptidoglycan deacetylases from Streptococcus pneumoniae and Listeria monocytogenes, which are both thought to be involved in the bacterial defense mechanism against human mucosal lysozyme and are part of the CAZY family 4 carbohydrate esterases. S. mutans cells in which the pgdA gene was deleted displayed a different colony texture and a slightly increased cell surface hydrophobicity and yet did not become hypersensitive to lysozyme as shown previously for S. pneumoniae. To understand this apparent lack of activity, the high-resolution X-ray structure of S. mutans PgdA was determined; it showed the typical carbohydrate esterase 4 fold, with metal bound in a His-His-Asp triad. Analysis of the protein surface showed that an extended groove lined with aromatic residues is orientated toward the active-site residues. The protein exhibited metal-dependent de-N-acetylase activity toward a hexamer of N-acetylglucosamine. No activity was observed toward shorter chitooligosaccharides or a synthetic peptidoglycan tetrasaccharide. In agreement with the lysozyme data this would suggest that S. mutans PgdA does not act on peptidoglycan but on an as-yet-unidentified polysaccharide within the bacterial cell surface. Strikingly, the pgdA-knockout strain showed a significant increase in aggregation/agglutination by salivary agglutinin, in agreement with this gene acting as a deacetylase of a cell surface glycan.
口腔病原体变形链球菌的基因组序列预测存在两种假定的多糖脱乙酰酶。第一种,在本文中命名为PgdA,与肺炎链球菌和单核细胞增生李斯特菌的肽聚糖脱乙酰酶的催化结构域具有同源性,这两种酶都被认为参与了细菌对人粘膜溶菌酶的防御机制,并且是碳水化合物活性酶家族4碳水化合物酯酶的一部分。缺失pgdA基因的变形链球菌细胞表现出不同的菌落质地,细胞表面疏水性略有增加,但不像肺炎链球菌那样对溶菌酶变得高度敏感。为了解这种明显的活性缺失,我们测定了变形链球菌PgdA的高分辨率X射线结构;它显示出典型的碳水化合物酯酶4折叠结构,金属结合在一个组氨酸-组氨酸-天冬氨酸三联体中。对蛋白质表面的分析表明,一条由芳香族残基排列的延伸凹槽朝向活性位点残基。该蛋白质对N-乙酰葡糖胺六聚体表现出金属依赖性脱N-乙酰酶活性。对较短的壳寡糖或合成肽聚糖四糖未观察到活性。与溶菌酶数据一致,这表明变形链球菌PgdA不作用于肽聚糖,而是作用于细菌细胞表面一种尚未鉴定的多糖。引人注目的是,pgdA基因敲除菌株显示唾液凝集素介导的聚集/凝集显著增加,这与该基因作为细胞表面聚糖的脱乙酰酶的作用一致。