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

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.