Protein secretion pathways in the phylum Bacteroidetes

类杆菌属的成员是迄今为止人类肠道微生物群中最丰富的革兰氏阴性细菌。最近，我们用质谱仪检测了实验室条件下生长的类杆菌属中最常见的成员之一--脆弱杆菌的外膜蛋白质组和分泌组。虽然我们没有发现任何新的分泌途径，但我们发现这种生物使用的分泌策略与大肠杆菌等变形杆菌非常不同。脆弱芽孢杆菌缺乏在变形杆菌中广泛存在的许多途径(例如，II型、III型和IV型途径)，但同时产生多种I型分泌系统。脆弱芽孢杆菌与变形杆菌也有很大的不同，因为它产生大量暴露在细胞表面的脂蛋白。I型途径的底物和脂蛋白通过OM运输的机制尚不清楚。有趣的是，分泌组中发现的许多蛋白质与Genbank数据库中的蛋白质缺乏显著的同源性，推测它们具有新的功能。

Members of the genus Bacteroides are by far the most abundant Gram-negative bacteria in the human gut microbiome. Recently we used mass spectrometry to examine the outer membrane (OM) proteome and secretome of one of the most commonly studied members of the Bacteroides genus, B. fragilis, grown under laboratory conditions. Although we did not identify any novel secretion pathways, we found that this organism uses a very different range of secretion strategies than Proteobacteria such as E. coli. B. fragilis lacks many of the pathways that are widespread among the Protebacteria (e.g., the type II, III and IV pathways) but produces multiple type I secretion systems simultaneously. B. fragilis also differs dramatically from Proteobacteria in that it produces a large number of lipoproteins that are exposed on the cell surface. The substrates of the type I pathways and the mechanism by which lipoproteins are transported across the OM are unknown. Interestingly, many of the proteins found in the secretome lack significant homology to proteins that are in the Genbank database and presumably have novel functions.