Lipopolysaccharide transport to the cell surface: periplasmic transport and assembly into the outer membrane

Abstract

Gram-negative bacteria possess an outer membrane (OM) containing lipopolysaccharide (LPS). Proper assembly of the OM not only prevents certain antibiotics from entering the cell, but also allows others to be pumped out. To assemble this barrier, the seven-protein lipopolysaccharide transport (Lpt) system extracts LPS from the outer leaflet of the inner membrane (IM), transports it across the periplasm and inserts it selectively into the outer leaflet of the OM. As LPS is important, if not essential, in most Gram-negative bacteria, the LPS biosynthesis and biogenesis pathways are attractive targets in the development of new classes of antibiotics. The accompanying paper (Simpson BW, May JM, Sherman DJ, Kahne D, Ruiz N. 2015 Phil. Trans. R. Soc. B 370, 20150029. (doi:10.1098/rstb.2015.0029)) reviewed the biosynthesis of LPS and its extraction from the IM. This paper will trace its journey across the periplasm and insertion into the OM.

Keywords: beta-barrel; envelope biogenesis; lipopolysaccharide transport; lipoprotein; outer membrane.

Figure 1.

LPS is transported across the periplasm via a transenvelope bridge. (a) Current model for LPS transport. The number of LptA monomers in the bridge is unknown. (b) LptA [7] and the periplasmic domains of LptC [8] and LptD [9] form a filament. Residues highlighted are sites that cross-link between the bridge components [10]. (c) LptA, LptC and the N-terminal domain of LptD all exhibit a beta-jellyroll fold. Highlighted sites in LptA and LptC cross-link to LPS [11]. (PDB accession numbers: 2R1A, 3MY2, 4Q35.)

Figure 2.

Current model for the assembly pathway of the LptDE OM translocon. Several unfolded LptD intermediates exist with varying oxidation states of the four cysteine residues [46]. In the non-functional OM translocon, a disulfide bond exists between C₃₁ and C₁₇₃. In the functional OM translocon, two disulfide bonds are present (Cys₃₁–Cys₇₂₄ and Cys₁₇₃–Cys₇₂₅). Note that LptE is required for disulfide rearrangement.

Figure 3.

Structure of the plug-and-barrel complex of LptD and LptE [9]. Disulfide bonds connect the N-terminal domain of LptD to its C-terminal domain. The barrel is thought to open during LPS insertion between beta-strands 1 and 26 (highlighted). (PDB accession number: 4Q35).