Michael Smith Laboratories, University of British Columbia, 2185 East Mall, Vancouver, BC, V6T 1Z4, Canada.
Department of Biochemistry and Molecular Biology, University of British Columbia, 2350 Health Sciences Mall, Vancouver, BC, V6T 1Z3, Canada.
Cell Mol Life Sci. 2019 Nov;76(21):4319-4340. doi: 10.1007/s00018-019-03115-3. Epub 2019 May 6.
The human gut microbiota, which underpins nutrition and systemic health, is compositionally sensitive to the availability of complex carbohydrates in the diet. The Bacteroidetes comprise a dominant phylum in the human gut microbiota whose members thrive on dietary and endogenous glycans by employing a diversity of highly specific, multi-gene polysaccharide utilization loci (PUL), which encode a variety of carbohydrases, transporters, and sensor/regulators. PULs invariably also encode surface glycan-binding proteins (SGBPs) that play a central role in saccharide capture at the outer membrane. Here, we present combined biophysical, structural, and in vivo characterization of the two SGBPs encoded by the Bacteroides ovatus mixed-linkage β-glucan utilization locus (MLGUL), thereby elucidating their key roles in the metabolism of this ubiquitous dietary cereal polysaccharide. In particular, molecular insight gained through several crystallographic complexes of SGBP-A and SGBP-B with oligosaccharides reveals that unique shape complementarity of binding platforms underpins specificity for the kinked MLG backbone vis-à-vis linear β-glucans. Reverse-genetic analysis revealed that both the presence and binding ability of the SusD homolog BoSGBP-A are essential for growth on MLG, whereas the divergent, multi-domain BoSGBP-B is dispensable but may assist in oligosaccharide scavenging from the environment. The synthesis of these data illuminates the critical role SGBPs play in concert with other MLGUL components, reveals new structure-function relationships among SGBPs, and provides fundamental knowledge to inform future (meta)genomic, biochemical, and microbiological analyses of the human gut microbiota.
人类肠道微生物群是营养和全身健康的基础，其组成对饮食中复杂碳水化合物的可用性敏感。拟杆菌门是人类肠道微生物群中的主要门，其成员通过利用多种高度特异性、多基因多糖利用基因座 (PUL) 来利用膳食和内源性糖，这些基因座编码各种糖苷酶、转运蛋白和传感器/调节剂。PUL 总是还编码表面聚糖结合蛋白 (SGBP)，它们在外膜中在糖捕获中发挥核心作用。在这里，我们通过对卵形拟杆菌混合链接 β-葡聚糖利用基因座 (MLGUL) 编码的两个 SGBP 的综合生物物理、结构和体内特征描述，阐明了它们在这种普遍存在的饮食谷物多糖代谢中的关键作用。特别是，通过 SGBP-A 和 SGBP-B 与寡糖的几个晶体复合物获得的分子洞察力揭示了结合平台的独特形状互补性是对扭曲的 MLG 主链相对于线性 β-葡聚糖的特异性的基础。反向遗传分析表明，SusD 同源物 BoSGBP-A 的存在和结合能力对于 MLG 的生长都是必不可少的，而分歧的、多结构域的 BoSGBP-B 是可有可无的，但可能有助于从环境中清除寡糖。这些数据的综合阐明了 SGBP 在与其他 MLGUL 成分协同作用中所起的关键作用，揭示了 SGBP 之间的新结构-功能关系，并为未来（宏）基因组学、生化和微生物学分析人类肠道微生物群提供了基础知识。
