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

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.