General N-and O-Linked Glycosylation of Lipoproteins in Mycoplasmas and Role of Exogenous Oligosaccharide

The lack of a cell wall, flagella, fimbria, and other extracellular appendages and the possession of only a single membrane render the mycoplasmas structurally simplistic and ideal model organisms for the study of glycoconjugates. Most species have genomes of about 800 kb and code for few proteins predicted to have a role in glycobiology. The murine pathogens Mycoplasma arthritidis and Mycoplasma pulmonis have only a single gene annotated as coding for a glycosyltransferase but synthesize glycolipid, polysaccharide and glycoproteins. Previously, it was shown that M. arthritidis glycosylated surface lipoproteins through O-linkage. In the current study, O-linked glycoproteins were similarly found in M. pulmonis and both species of mycoplasma were found to also possess N-linked glycans at residues of asparagine and glutamine. Protein glycosylation occurred at numerous sites on surface-exposed lipoproteins with no apparent amino acid sequence specificity. The lipoproteins of Mycoplasma pneumoniae also are glycosylated. Glycosylation was dependent on the glycosidic linkages from host oligosaccharides. As far as we are aware, N-linked glycoproteins have not been previously described in Gram-positive bacteria, the organisms to which the mycoplasmas are phylogenetically related. The findings indicate that the mycoplasma cell surface is heavily glycosylated with implications for the modulation of mycoplasma-host interactions.支原体（Mycoplasma）缺乏细胞壁、鞭毛、菌毛及其他胞外附属结构，且仅拥有单层细胞膜，这使其结构极为简化，成为研究糖缀合物（glycoconjugate）的理想模式生物。多数支原体物种的基因组大小约为800 kb，编码的预测参与糖生物学功能的蛋白质数量极少。作为鼠类病原体的关节炎支原体（Mycoplasma arthritidis）与肺支原体（Mycoplasma pulmonis）仅拥有一个被注释为编码糖基转移酶（glycosyltransferase）的基因，却能够合成糖脂、多糖与糖蛋白。此前已有研究表明，关节炎支原体通过O-连接方式对其表面脂蛋白进行糖基化修饰。本研究中，我们同样在肺支原体中发现了O-连接糖蛋白，且两种支原体均在天冬酰胺与谷氨酰胺残基上存在N-连接聚糖（glycan）。表面暴露脂蛋白的多个位点均可发生蛋白质糖基化，且未表现出明显的氨基酸序列特异性。肺炎支原体（Mycoplasma pneumoniae）的脂蛋白同样存在糖基化修饰。糖基化修饰依赖于宿主寡糖（oligosaccharide）所形成的糖苷键。据我们所知，此前从未在系统发育上与支原体亲缘关系相近的革兰氏阳性（Gram-positive）细菌中报道过N-连接糖蛋白。本研究结果表明，支原体细胞表面存在广泛的糖基化修饰，这对调控支原体与宿主的相互作用具有重要意义。