The Bam machine: a molecular cooper

Abstract

The bacterial outer membrane (OM) is an exceptional biological structure with a unique composition that contributes significantly to the resiliency of Gram-negative bacteria. Since all OM components are synthesized in the cytosol, the cell must efficiently transport OM-specific lipids and proteins across the cell envelope and stably integrate them into a growing membrane. In this review, we discuss the challenges associated with these processes and detail the elegant solutions that cells have evolved to address the topological problem of OM biogenesis. Special attention will be paid to the Bam machine, a highly conserved multiprotein complex that facilitates OM β-barrel folding. This article is part of a Special Issue entitled: Protein Folding in Membranes.

Figure 1

The general β-barrel fold. These E. coli OMPs exhibit characteristic typical of bacterial OM β-barrels, including short periplasmic turns, long extracellular loops (some of which fold back into the barrel lumen in the case of LamB, and an even number of β-strands. Note that OmpG (top, PDB ID: 2X9K) is monomeric, whereas LamB (bottom, PDB ID: 1AF6) forms stable trimers in the OM.

Figure 2

Figure 2A. The alternating (dyad repeat) pattern for hydrophobic residues in TM strands 8–10 of the E. coli OM beta-barrel protein LamB (PDB ID: 1AF6). Non-polar, membrane-exposed residues are highlighted in yellow, and the aromatic side chains forming part of the aromatic girdle (see Fig. 2B) are highlighted in cyan. Figure 2B. Residues defining the “aromatic girdle”, shown in cyan on this structure of the LamB monomer from E. coli (PDB ID: 1AF6), demarcate the membrane boundaries.

Figure 2

Figure 2A. The alternating (dyad repeat) pattern for hydrophobic residues in TM strands 8–10 of the E. coli OM beta-barrel protein LamB (PDB ID: 1AF6). Non-polar, membrane-exposed residues are highlighted in yellow, and the aromatic side chains forming part of the aromatic girdle (see Fig. 2B) are highlighted in cyan. Figure 2B. Residues defining the “aromatic girdle”, shown in cyan on this structure of the LamB monomer from E. coli (PDB ID: 1AF6), demarcate the membrane boundaries.

Figure 3

OM biogenesis in E. coli. All components of the bacterial OM are synthesized in the cytoplasm and translocated across the IM into the periplasmic space. Once in the periplasm, each OM constituent traverses the envelope and integrates into the OM via a dedicated transport system (shown above). The Lpt components form a transenvelope protein bridge that shields the lipid A moiety from the aqueous environment and funnels LPS through LptDE to the cell surface. OM β-barrels (OMPs) cross the IM through the Sec translocon and associate with periplasmic chaperones that target OMPs to the Bam machine for assembly. OM lipoproteins are extracted from the IM in an ATP-dependent fashion and transferred to an OM receptor (LolB) via a periplasmic carrier protein (LolA). Phospholipid transport is not shown, as the mechanism by which they are trafficked to the OM is not known.

Figure 4

The periplasmic domain of BamA exhibits conformation flexibility about the hinge between POTRA domains 2 and 3. Two independently structures are superimposed at POTRA 3. The yellow structure (PDB ID: 2QDF) represents the “bent” conformation, and the purple conformation (PDB ID: 3EFC, 3OG5) represents the “extended” conformation.

Figure 5

The solution structure of BamB (PDB ID: 3P1L). Residues shown previously to be involved in the BamA-BamB interaction are highlighted in magenta (Vuong et al 2008).

Figure 6

The solution structures of E. coli BamCDE. The BamD (PDB ID: 2YHC) and BamE (PDB ID: 2KM7) structures are oriented with the N-termini pointing toward the top of the page. The structurally homologous helix grip domains of BamC (PDB ID: 2LAE, 2LAF) are shown on the left, with the extreme C-terminal domain at bottom. A dashed white line indicates the unresolved helix linking the two domains.