Identification and characterization of the functional LolB ortholog in Bacteroides

Abstract

Bacteroidota are prolific members of the human gut microbiota, influencing overall health through the degradation of various polysaccharides. To aid in this process, these bacteria deploy cell surface lipoproteins, proteins anchored to the membrane by a lipidated N-terminal cysteine. Despite their importance, lipoprotein synthesis and transport in Bacteroidota is not well defined, particularly how lipoproteins reach the outer membrane and cell surface. In Escherichia coli, lipoproteins are inserted into the outer membrane by LolB, the final step of the Lol pathway for localizing lipoproteins, previously thought to be confined to {gamma}- and {beta}-proteobacteria. Herein, we report a structural ortholog of LolB in Bacteroides that rescues the lethal phenotype associated with LolB depletion in E. coli. We demonstrate this orthologs LolB-like activity through mutagenesis studies and a lipoprotein insertion activity assay. We find that the gene cannot be deleted from Bacteroides, but that depletion does not significantly impact growth on polysaccharides, nor the surface localization of lipoproteins involved in starch degradation. Nonetheless, depletion significantly alters the composition of many proteins in both the inner and outer membranes, while others remain unchanged. Altogether, our findings contribute to elucidating the lipoprotein transport pathway in Bacteroides and how it impacts cell physiology. ImportanceBacteroidota are abundant members of the human gut microbiota that influence health and disease. These bacteria deploy numerous cell surface lipoproteins that mediate their interactions with the host and play key roles in cell physiology. However, their mechanism(s) of lipoprotein transport is understudied. Here, we used a genetic screen to identify orthologs to E. coli LolB, the protein responsible for lipoprotein insertion into the outer membrane. Our screen revealed a structural ortholog to LolB in Bacteroides that performs this function. We show that when LolB is depleted, lipoproteins still reach the cell surface, even though the overall protein composition of the membrane is significantly altered. Our results broaden the understanding of both lipoprotein transport and Bacteroidota physiology.