Bacteroidetes are abundant members of the human microbiota, utilizing a myriad of diet- and host-derived glycans in the distal gut¹. Glycan uptake across the bacterial outer membrane of these bacteria is mediated by SusCD protein complexes, comprising a membrane-embedded barrel and a lipoprotein lid, which is thought to open and close to facilitate substrate binding and transport. However, surface-exposed glycan-binding proteins and glycoside hydrolases also play critical roles in the capture, processing and transport of large glycan chains. The interactions between these components in the outer membrane are poorly understood, despite being crucial for nutrient acquisition by our colonic microbiota. Here we show that for both the levan and dextran utilization systems of Bacteroides thetaiotaomicron, the additional outer membrane components assemble on the core SusCD transporter, forming stable glycan-utilizing machines that we term utilisomes. Single-particle cryogenic electron microscopy structures in the absence and presence of substrate reveal concerted conformational changes that demonstrate the mechanism of substrate capture, and rationalize the role of each component in the utilisome.

拟杆菌是人类微生物群中丰富的成员，利用远端肠道中的无数饮食和宿主衍生的聚糖¹。这些细菌的细菌外膜的聚糖摄取是由 SusCD 蛋白复合物介导的，该复合物包括膜嵌入的桶和脂蛋白盖，其打开和关闭被认为有利于底物结合和运输。然而，表面暴露的聚糖结合蛋白和糖苷水解酶在大聚糖链的捕获、加工和运输中也发挥着关键作用。尽管外膜中这些成分之间的相互作用对于我们结肠微生物群获取营养至关重要，但人们对它们之间的相互作用知之甚少。在这里我们表明，对于果聚糖和葡聚糖利用系统Bacteroides thetaiotaomicron，额外的外膜成分组装在核心 SusCD 转运蛋白上，形成稳定的聚糖利用机器，我们称之为利用体。在不存在和存在底物的情况下，单颗粒低温电子显微镜结构揭示了一致的构象变化，证明了底物捕获的机制，并合理化了利用体中每个成分的作用。