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

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 111; no. 41; pp. E4350 - E4358
• Main Authors: Konovalova, Anna, Perlman, David H., Cowles, Charles E., Silhavy, Thomas J.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 14.10.2014; National Acad Sciences
• Series: PNAS Plus
• Subjects: Amino Acid Sequence; Bacterial Outer Membrane Proteins - chemistry; Bacterial Outer Membrane Proteins - metabolism; Biological Sciences; Cell Membrane - metabolism; Cross-Linking Reagents - metabolism; Escherichia coli; Escherichia coli - metabolism; Escherichia coli Proteins - chemistry; Escherichia coli Proteins - metabolism; hydrophobicity; lipoproteins; Lipoproteins - chemistry; Lipoproteins - metabolism; Models, Biological; Molecular Sequence Data; Mutation - genetics; PNAS Plus; Protein Binding; Protein Folding; Protein Structure, Secondary; Protein Structure, Tertiary; Structure-Activity Relationship; protein folding; membrane biogenesis; signal transduction; envelope stress response
• ISSN: 0027-8424; 1091-6490; 1091-6490
• DOI: 10.1073/pnas.1417138111

Significance In Escherichia 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. RcsF (regulator of capsule synthesis) is an outer membrane (OM) lipoprotein that functions to sense defects such as changes in LPS. However, LPS is found in the outer leaflet, and RcsF was thought to be tethered to the inner leaflet by its lipidated N terminus, raising the question of how it monitors LPS. We show that RcsF has a transmembrane topology with the lipidated N terminus on the cell surface and the C-terminal signaling domain in the periplasm. Strikingly, the short, unstructured, charged transmembrane domain is threaded through the lumen of β-barrel OM proteins where it is protected from the hydrophobic membrane interior. We present evidence that these unusual complexes, which contain one protein inside another, are formed by the Bam complex that assembles all β-barrel proteins in the OM. The ability of the Bam complex to expose lipoproteins at the cell surface underscores the mechanistic versatility of the β-barrel assembly machine.