Assembly and dynamics of the outer membrane exopolysaccharide transporter PelBC of Pseudomonas aeruginosa

The infamous opportunistic pathogen Pseudomonas aeruginosa enhances its virulence and antibiotic resistance upon formation of durable biofilms. The biofilm stability is mediated by its matrix built of secreted exopolysaccharides, eDNA, and structural proteins. Exopolysaccharides of P. aeruginosa - Pel, Psl and alginate - have the highest biomedical relevance, but the mechanisms behind their synthesis and secretion are poorly understood. Here, we employ cryogenic electron microscopy to resolve the 2.5 [A] structure of the outer membrane complex PelBC for Pel exopolysaccharide, which is uniquely composed of the membrane-embedded {beta}-barrel PelB and the asymmetrical ring of 12 lipoproteins PelC at the periplasmic interface. The assembly captured in the lipid-based nanodisc is stabilized by electrostatic contacts of PelC with the periplasmic loops of PelB and multiple interactions with PelB N-terminal helical domains. Within the membrane, the resolved acyl chains of the PelC lipoproteins are alternated by the tryptophan residues immersed into the lipid leaflet, thus offering a stable anchoring architecture. The highly anionic interior of the PelB {beta}-barrel is sealed by three loops at the extracellular side, where the short Plug-S loop is aligned with the periplasmic helical scaffold, being the potential gating element for the Pel exopolysaccharide tunneling. Molecular dynamic simulations of PelB in native-like membrane environments suggest that Plug-S is sufficiently flexible to open a tunnel, and so serve as a gate. The gating model is further supported by single-channel conductivity measurements, which identify two conductance states of PelB. Via mutational analysis we confirm that Plug-S mediates opening of a narrow tunnel, as required for the controlled exopolysaccharide transport. Our structural and functional analysis of the pathogenicity-relevant complex offer a detailed and comprehensive view on this unique machinery and suggest the route taken by the exopolysaccharide at the final secretion step.