Abstract
Cell surface lipoproteins (SLPs) play crucial roles in the pathogenesis and cell biology of gram-negative bacteria, yet the route by which they reach the cell surface has remained unknown except in special cases. Here we show that in Bacteroidota bacteria, a phylum that includes prominent human commensals and anaerobic pathogens, SLP export is carried out by the outer membrane lipopolysaccharide (LPS) translocon. Isolation and structural characterisation of LPS translocon complexes from two Bacteroidota species reveals that they are massively enlarged and elaborated relative to the Escherichia coli paradigm with up to eight additional subunits located within, and on either side of, the outer membrane. We establish a role for the universally conserved core LptD subunit and four of the additional Bacteroidota-specific subunits in SLP export. Expansion of the LPS translocon in the Bacteroidota may allow export of the unusually complex SLPs found in this phylum. Our discoveries raise the possibility that SLP export is a conserved second function of the LPS transport pathway across gram-negative bacteria.