Identification of a LolB-like protein in Porphyromonas gingivalis reveals selective LolA–LolB pairing

Abstract

Abstract The lipoprotein transport (Lol) system is essential for outer membrane biogenesis in Gram-negative bacteria, yet its composition and organization vary markedly across bacterial phyla. While lipoprotein transport via the Lol pathway has been extensively characterized in Escherichia coli , its components in the Bacteroidota phylum remain poorly understood. Porphyromonas gingivalis , a major periodontal pathogen has long been thought to lack the outer membrane lipoprotein insertase LolB, leaving the mechanism of lipoprotein insertion unclear. Here, we have identified and characterized a LolB-like protein in P. gingivalis (LolB-PG). We determined its crystal structure at 2.1 Å resolution and revealed a conserved LolB fold but with an enlarged and more accessible lipid-binding cleft compared to proteobacterial homologs. Biophysical analyses demonstrate that LolB-PG selectively interacts with the cognate periplasmic chaperone LolA but not with the paralog LolA3, indicating a conserved yet specific LolA–LolB pairing. Deletion of the gene encoding LolB - PG did not affect bacterial growth or the assembly, localization, or formation of type-V fimbriae—which are polymerized from lipoproteins— suggesting the existence of alternative lipoprotein trafficking routes in P. gingivalis . Together, our findings reveal that Bacteroidota encode a functional LolB-like protein and highlight diversification of lipoprotein transport pathways beyond well-studied γ-proteobacteria.