Outer membrane protein biogenesis in Gram-negative bacteria

Abstract

Gram-negative bacteria contain a double membrane which serves for both protection and for providing nutrients for viability. The outermost of these membranes is called the outer membrane (OM), and it contains a host of fully integrated membrane proteins which serve essential functions for the cell, including nutrient uptake, cell adhesion, cell signalling and waste export. For pathogenic strains, many of these outer membrane proteins (OMPs) also serve as virulence factors for nutrient scavenging and evasion of host defence mechanisms. OMPs are unique membrane proteins in that they have a β-barrel fold and can range in size from 8 to 26 strands, yet can still serve many different functions for the cell. Despite their essential roles in cell survival and virulence, the exact mechanism for the biogenesis of these OMPs into the OM has remained largely unknown. However, the past decade has witnessed significant progress towards unravelling the pathways and mechanisms necessary for moulding a nascent polypeptide into a functional OMP within the OM. Here, we will review some of these recent discoveries that have advanced our understanding of the biogenesis of OMPs in Gram-negative bacteria, starting with synthesis in the cytoplasm to folding and insertion into the OM.

Keywords: insertase; lateral gate; outer membrane; protein folding; β-barrel assembly machinery complex; β-barrel membrane protein.

Figure 1.

The OMP journey to the outer membrane. Nascent OMPs (maroon) are synthesized in the bacterial cytoplasm and delivered by the SecA/SecB chaperones (purple) or by the ribosome (red) to the Sec translocase (grey) for passage through the IM. Once in the periplasm, nascent OMPs can be delivered to the BAM complex by the periplasmic chaperones SurA (slate) or Skp (blue). Any OMPs which misfold during their journey will be degraded by the protease DegP (light blue) to prevent aggregation.

Figure 2.

Structures of the major periplasmic chaperones involved in OMP biogenesis. (a) The overall structure of full-length SurA alone and in (b), the PPIase domain bound to two different length peptides. (c) The structure of prefoldin with three of the six monomers coloured. The structure of Skp with each monomer coloured as for prefoldin (d) and the electrostatic surface potential calculated by ABPS (e). (f) The trimeric building block of DegP binds to another trimer to form an inactive hexamer (g, left) in the absence of substrate. Upon substrate addition, the trimers rearrange and form a larger cage-like structures, such as a 12-mer (g, right).

Figure 3.

Structures of the components of the BAM complex. (a) The crystal structure of full-length BamA from Neisseria gonorrhoeae. (b) The crystal structure of BamB from E. coli. (c) The crystal structure of the two domains of BamC from E. coli. (d) The crystal structure of BamD from E. coli. (e) The NMR structure of BamE from E. coli.