Gut Commensal Bacteroidetes Encode a Novel Class of Vitamin B-12-Binding Proteins

The gut microbiome is a complex microbial community with important impacts on human health. One of the major groups within the gut microbiome, the Bacteroidetes, rely on their ability to capture vitamin B-12 and related molecules for fitness in the gut. Human gut commensal Bacteroidetes rely on multiple transport systems to acquire vitamin B-12 and related cobamides for fitness in the gut. In addition to a set of conserved transport proteins, these systems also include a diverse repertoire of additional proteins with unknown function. Here, we report the function and structural characterization of one of these proteins, BtuH, which binds vitamin B-12 directly via a C-terminal globular domain that has no known structural homologs. This protein is required for efficient B-12 transport and competitive fitness in the gut, demonstrating that members of the heterogeneous suite of accessory proteins encoded in Bacteroides cobamide transport system loci can play key roles in vitamin acquisition. IMPORTANCE The gut microbiome is a complex microbial community with important impacts on human health. One of the major groups within the gut microbiome, the Bacteroidetes, rely on their ability to capture vitamin B-12 and related molecules for fitness in the gut. Unlike well-studied model organisms, gut Bacteroidetes genomes often include multiple vitamin B-12 transport systems with a heterogeneous set of components. The role, if any, of these components was unknown. Here, we identify new proteins that play key roles in vitamin B-12 capture in these organisms. Notably, these proteins are associated with some B-12 transport systems and not others (even in the same bacterial strain), suggesting that these systems may assemble into functionally distinct machines to capture vitamin B-12 and related molecules.