Outer membrane lipoprotein biogenesis: Lol is not the end

Abstract

Bacterial lipoproteins are lipid-anchored proteins that contain acyl groups covalently attached to the N-terminal cysteine residue of the mature protein. Lipoproteins are synthesized in precursor form with an N-terminal signal sequence (SS) that targets translocation across the cytoplasmic or inner membrane (IM). Lipid modification and SS processing take place at the periplasmic face of the IM. Outer membrane (OM) lipoproteins take the localization of lipoproteins (Lol) export pathway, which ends with the insertion of the N-terminal lipid moiety into the inner leaflet of the OM. For many lipoproteins, the biogenesis pathway ends here. We provide examples of lipoproteins that adopt complex topologies in the OM that include transmembrane and surface-exposed domains. Biogenesis of such lipoproteins requires additional steps beyond the Lol pathway. In at least one case, lipoprotein sequences reach the cell surface by being threaded through the lumen of a beta-barrel protein in an assembly reaction that requires the heteropentomeric Bam complex. The inability to predict surface exposure reinforces the importance of experimental verification of lipoprotein topology and we will discuss some of the methods used to study OM protein topology.

Keywords: Bam complex; RcsF; localization of lipoproteins pathway; protein topology; surface-exposed protein.

Figure 1.

Lipoprotein maturation and export pathway. Lipoprotein (green) is synthesized in the cytoplasm with the N-terminal SS (red) which targets it for translocation across the IM by the Sec or Tat translocon. The lipoprotein remains anchored in the IM by its SS and Lgt adds a diacylglyceryl moiety to the Cys residue. LspA cleaves the SS and Lnt adds another acyl chain to the newly formed N-terminus. The lipoprotein is then recognized by IM LolCDE complex which powers extraction of the lipoprotein from the IM using the energy of ATP. The lipoprotein is released to the periplasm in a complex with the chaperone LolA. LolA delivers lipoprotein to the OM acceptor protein LolB which inserts it in the inner leaflet of the OM. Empty LolA returns to LolCDE and is recycled. (Online version in colour.)

Figure 2.

Examples of topologies of OM lipoproteins in E. coli. OM lipoproteins (green) were assumed to adopt a typical topology with the protein domain in the periplasm and lipid anchored in the inner leaflet of the OM. By contrast, the topology of experimentally studied lipoproteins is often more complex. Lipoproteins can be assembled in interlocked complexes with beta-barrel proteins (blue) like RcsF/OMP and LptE/LptD, or they can form transmembrane multimeric channels like CsgG and Wza. Assembly of the lipoproteins at the OM often results in surface exposure of parts of the protein (RcsF, CsgG and Wza). In the case of RcsF, the lipid moiety anchors the protein in the outer leaflet of OM. (Online version in colour.)