Extracellular Vesicle-Linked Vitamin B12 Acquisition via Novel Binding Proteins in Bacteroides thetaiotaomicron

Abstract

The acquisition and utilization of vitamin B12 (cobalamin) are essential for the metabolic functions of many gut bacteria, including Bacteroides thetaiotaomicron, which relies on external sources of cobalamin due to its inability to synthesize it de novo. To scavenge cobalamin efficiently, B. thetaiotaomicron employs a sophisticated cobamide uptake system comprising multiple operons encoding outer membrane binding proteins and transporters. This study identifies and characterizes several novel cobalamin-binding proteins and elucidates their roles in cobamide uptake and delivery via bacterial extracellular vesicles (BEVs), highlighting their siderophore-like function for competitive nutrient acquisition. We demonstrate that BtuJ1 and BtuJ2, members of the IPR027828 protein family, bind cobalamin with distinct structural and kinetic properties and are key components of BEVs responsible for cobamide capture and transfer to cells. Structural analyses reveal a conserved augmented {beta}-jellyroll architecture in these proteins, with tyrosine residues playing a central role in stabilizing cobalamin binding. Comparative proteomics of BEVs and cells under cobamide starvation underscore the selective enrichment of BtuJ proteins in BEVs, suggesting a specialized mechanism for nutrient acquisition. Additionally, we identify another novel B12-binding protein, BtuK1. We further establish BtuL as a critical player in early BEV release and propose a theoretical model in which BEVs function similarly to siderophores, scavenging cobalamin in the environment and delivering it to cells via specific receptors. This study provides new insights into the interplay between BEV-mediated transport, cobamide uptake, and the metabolic strategies employed by gut bacteria to thrive in nutrient-limited environments.