Protein secretion pathways in the phylum Bacteroidetes

拟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。我们使用质谱法检测了在实验室条件下生长的拟杆菌属中最常研究的成员之一脆弱杆菌的外膜（OM）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变形杆菌（如大肠杆菌）非常不同。脆弱芽孢杆菌缺乏许多在变形杆菌中广泛存在的途径（例如，II型，III型和IV型途径），但同时产生多个I型分泌系统。I型途径的底物是未知的，不能根据与变形杆菌中I型底物的蛋白的同源性轻易预测。易碎芽孢杆菌与变形杆菌也有很大的不同，因为它会产生大量暴露在细胞表面的脂蛋白。我们目前正在使用多种方法来研究脂蛋白通过OM运输的机制，并确定指定特定脂蛋白用于出口的靶向信号。有趣的是，在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，可能具有新的功能。

Members of the genus Bacteroides are by far the most abundant Gram-negative bacteria in the human gut microbiome. 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. The substrates of the type I pathways are unknown and cannot be easily predicted based on homology to proteins that are type I substrates in Proteobacteria. B. fragilis also differs dramatically from Proteobacteria in that it produces a large number of lipoproteins that are exposed on the cell surface. We are currently using a variety of methods to investigate the mechanism(s) by which lipoproteins are transported across the OM and identifying targeting signals that earmark specific lipoproteins for export. 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.