Exopolysaccharide export complex PelBC of Pseudomonas aeruginosa attenuates the dynamics of the surrounding outer membrane

The opportunistic pathogen Pseudomonas aeruginosa ensures its survival by forming mechanically and chemically resistant biofilms, with cationic exopolysaccharide Pel as an abundant constituent of the structural matrix. Despite its biomedical relevance, the mechanisms of Pel synthesis and secretion via the trans-envelope protein machinery are not understood. Here, we examine the structure of the outer membrane export complex PelBC embedded in synthetic nanodiscs and polymer-extracted particles. Both environments preserve the unique architecture of the complex, where the beta-barrel PelB is capped with the dodecameric ring of PelC lipoproteins. Cryogenic electron microscopy shows that the polymer-extracted PelB beta-barrel is tightly associated with phospholipids and lipid A molecules, and the membrane-facing PelC ring may stabilize lipids of the periplasmic leaflet in defined positions. All-atom molecular dynamics simulation of PelBC in the asymmetric outer membrane of P. aeruginosa corroborate the structural findings and visualize how the essential C-terminal helix of PelC forms multiple electrostatic contacts with the periplasmic leaflet of the outer membrane. Those interactions reduce the lateral mobility of the lipids, stabilize the position of the ring at the interface and may guide folding and assembly of the polysaccharide export machinery.