Transmembrane domain of surface-exposed outer membrane lipoprotein RcsF is threaded through the lumen of β-barrel proteins

Significance InEscherichia coli , most outer membrane (OM) lipoproteins are thought to be soluble proteins that are simply tethered to the inner leaflet of this membrane by lipid moieties attached to the N terminus. Here we show that lipoprotein RcsF (regulator of capsule synthesis) adopts a transmembrane orientation with the lipidated N terminus on the cell surface and the folded C-terminal domain in the periplasm. The short, unstructured, polar linker domain spans the hydrophobic membrane by passing through the lumen of several different OM β-barrel proteins. This remarkable, interlocked structure is formed by the Bam complex, which folds and inserts all β-barrel proteins in the OM, suggesting that this assembly machine translocates the lipid moieties and then folds the β barrel around the RcsF linker.