Optimal functioning of the Lpt bridge depends on a ternary complex between the lipocalin YedD and the LptDE translocon

The outer membrane is an efficient permeability barrier that protects gram-negative bacteria against external assaults, including many antibiotics. The unique permeability features of the outer membrane are due to the presence of lipopolysaccharide (LPS) molecules in its outer leaflet. LPS transport relies on the essential lipopolysaccharide transport (Lpt) pathway, which forms a bridge from the inner to the outer membrane. The LptDE translocon inserts LPS into the outer leaflet. Here, we identify the lipocalin YedD as a component of the translocon. Cryoelectron microscopy of the YedD-LptDE complex reveals that YedD binds LptD at a critical interface between its β-barrel and periplasmic β-taco domain. The YedD-LptDE complex is functionally relevant: under conditions where the connectivity of the β-taco and Lpt bridge is compromised, the absence of YedD decreases cell viability and causes LPS accumulation in the inner membrane. Our findings establish YedD as an Lpt component required for optimal LPS transport.