BtuJ1, a novel surface-exposed B12-binding protein in Bacteroidetes, functions as an extracellular vitamin reservoir that enhances fitness.

The acquisition of vitamin B12 and related cobamides is a key determinant for the fitness of Bacteroidetes in the gut. Depending on the species, this uptake process relies on one to four transport systems centered on conserved core outer membrane (OM) complexes composed of the TonB-dependent transporter BtuB and the surface-exposed lipoprotein BtuG. Additionally, the surface-exposed lipoprotein BtuH, although not tightly associated with the BtuBG complex, contributes to cobamide uptake and provides a fitness advantage. Here, we report the functional and structural characterization of BtuJ1 from Bacteroides thetaiotaomicron (B. theta), a novel B12-binding lipoprotein. Under limiting B12 conditions, BtuJ1 is the most abundant component among the three B12-transport systems encoded by B. theta. BtuJ1 is surface exposed and binds vitamin B12 and cobinamide (an intermediate in B12 biosynthesis) with low nM affinity, conferring a fitness advantage in B12-limited environments. In vitro B12 transfer experiments suggest a role for BtuJ1 as an extracellular reservoir for B12, expanding the functionalities of the diverse group of accessory OM proteins employed by Bacteroides to scavenge this essential cofactor in the competitive environment of the human gut. IMPORTANCEUnderstanding how key molecules support bacterial colonization of the human GI tract is essential to rationalize the structure of the complex microbial community inhabiting the distal gut. The Bacteroidetes are one of the dominant phyla in this environment. Given that most Bacteroidetes cannot make vitamin B12 but depend on it for growth, the fitness of many species likely depends on the acquisition of vitamin B12. Unlike the classical model bacterium Escherichia coli, which encodes a single OM B12 transporter, the genomes of Bacteroidetes often encode multiple uptake systems comprising a heterogeneous repertoire of surface-exposed lipoproteins. These proteins may assist OM assemblies for vitamin B12 transport that provide fitness advantages in vivo.