BtuB TonB-dependent transporters and BtuG surface lipoproteins form stable complexes for vitamin B12 uptake in gut Bacteroides.

Vitamin B12 (cobalamin) is the most complex vitamin and essential for many human gut microbes. However, cobalamin is synthesised only by a limited number of bacteria, making many gut microbes dependent on scavenging to meet their cobalamin requirements. Since bacterial densities in the gut are extremely high, competition for cobalamin is severe, making it a keystone micronutrient that shapes human gut microbial communities. Contrasting with Enterobacteria like Escherichia coli which only have one outer membrane (OM) transporter dedicated to B12 uptake (BtuB), members of the dominant genus Bacteroides often encode several vitamin B12 OM transporters together with a conserved array of surface-exposed B12-binding lipoproteins. Here we show, via X-ray crystallography, cryogenic electron microscopy (cryoEM) and molecular dynamics (MD) simulations, that the BtuB1 and BtuB2 transporters from the prominent human gut bacterium Bacteroides thetaiotaomicron form stable complexes with the surface-exposed lipoproteins BtuG1 and BtuG2. The lipoproteins cap the external surface of their cognate BtuB transporter and, when open, capture B12 via electrostatic attraction. After B12 capture, the BtuG lid closes, with concomitant transfer of the vitamin to the BtuB transporter and subsequent transport. We propose that TonB-dependent, lipoprotein-assisted small molecule uptake is a general feature of Bacteroides spp. that is important for the success of this genus in colonising the human gut. ### Competing Interest Statement The authors have declared no competing interest.