Structural basis for outer membrane lipopolysaccharide insertion

Abstract

Lipopolysaccharide (LPS) is essential for most Gram-negative bacteria and has crucial roles in protection of the bacteria from harsh environments and toxic compounds, including antibiotics. Seven LPS transport proteins (that is, LptA-LptG) form a trans-envelope protein complex responsible for the transport of LPS from the inner membrane to the outer membrane, the mechanism for which is poorly understood. Here we report the first crystal structure of the unique integral membrane LPS translocon LptD-LptE complex. LptD forms a novel 26-stranded β-barrel, which is to our knowledge the largest β-barrel reported so far. LptE adopts a roll-like structure located inside the barrel of LptD to form an unprecedented two-protein 'barrel and plug' architecture. The structure, molecular dynamics simulations and functional assays suggest that the hydrophilic O-antigen and the core oligosaccharide of the LPS may pass through the barrel and the lipid A of the LPS may be inserted into the outer leaflet of the outer membrane through a lateral opening between strands β1 and β26 of LptD. These findings not only help us to understand important aspects of bacterial outer membrane biogenesis, but also have significant potential for the development of novel drugs against multi-drug resistant pathogenic bacteria.

Synthesis and antimicrobial activity against Pseudomonas aeruginosa of macrocyclic β-hairpin peptidomimetic antibiotics containing N-methylated amino acids

“…Computational, mutagenesis and photo-crosslinking studies suggest that the lipid chains in LPS move along the hydrophobic cavity in the jelly-roll domain and exit into the outer leaflet of the OM through a transient lateral opening created between ß-strands-1 and -26 in the LptD ß-barrel domain. [9][10][11] A similar architecture is expected for the LptD/E complex from P. aeruginosa, except that sequence comparisons show that an additional ≈90-residue domain of unknown structure and function is inserted close to the N-terminus of the ß-jelly roll. This insert domain is a distinguishing feature of LptD in Pseudomonas spp.…”

Lipopolysaccharide rolls out the barrel

“…Many of the cellular processes by which LPS is assembled and transported to the outer membrane are essential for survival, and thus are targets for the development of new antibiotics. In two papers published on Nature's website today, Dong et al 1 and Qiao et al 2 present structural studies that disclose a remarkable molecular mechanism by which LPS arrives at its final destination in the outer membrane.…”

Autotransporter proteins of Uropathogenic Escherichia coli: regulation, characterisation and the role of periplasmic proteins in their expression

“…However, LptE differs from BamB in structure, size (BamB: 40KDa, LptE: 20KDa), and location in relation to the central protein component. While BamB is known to interact with the periplasmic domains of BamA, LptE is situated within the central channel of LptD (Dong et al, 2014). In fact, the overall structure of LptE resembles a similar topology to that of the lipoprotein of BamE (Kim et al, 2011b).…”

“…This suggests crucial interactions between Skp and the lipid membrane are important for outer membrane biogenesis. LPS is known to spontaneously insert into lipid membranes from aqueous solutions (Alam and Yamazaki, 2011 (Dong et al, 2014). Indeed, the lateral gate , Noinaj et al, 2013a conformational change simulated for BamA was also suggested for LptD (Dong et al, 2014).…”

“…LPS is known to spontaneously insert into lipid membranes from aqueous solutions (Alam and Yamazaki, 2011 (Dong et al, 2014). Indeed, the lateral gate , Noinaj et al, 2013a conformational change simulated for BamA was also suggested for LptD (Dong et al, 2014). The BAM exists as a BamAB and BamACDE protein complex (Sklar et al, 2007a, Malinverni et al, 2006, Hagan et al, 2010, Jansen et al, 2015a.…”