RESEARCH HIGHLIGHTS Nature Reviews Microbiology | AOP, published online 30 June 2014; doi:10.1038/nrmicro3311

B AC T E R I A L P H Y S I O LO GY

LptD–LptE form the only known twoprotein ‘barrel and plug’ complex

Lipopolysaccharide (LPS) is the main component of the outer membrane of Gram-negative bacteria, and it has an essential role in protection against harsh environments and toxic compounds, including antibiotics. However, how LPS is transported across the outer membrane to its final location has been unclear. The three components of LPS — core oligosaccharide, lipid A and O antigen — are assembled at the periplasmic side of the inner membrane. From there, LPS is transported to the outer membrane by seven LPS transport proteins, which are known as LptA to LptG. The LptD–LptE complex is responsible for the final step in this process — the translocation of LPS across the outer membrane. Dong et al. and Qiao et al. both solved the structure of the LptD–LptE complex; Dong et al. solved that of Salmonella enterica subsp. enterica serovar Typhimurium to 2.86 Å resolution and Qiao et al. solved that of Shigella flexneri to 2.4 Å resolution. Both papers show that LptD forms the largest β-barrel that has ever been reported, which comprises 26 antiparallel β-strands that form a pore in the outer membrane. The pore is completely closed by the extracellular loops of LptD and by LptE, which adopts a roll-like structure that plugs the LptD β-barrel and extends into the periplasm. Together, LptD– LptE form the only known twoprotein ‘barrel and plug’ complex. The structures also elucidate how the LptD–LptE complex might

function. Two proline residues that are located in the first two β-strands of LptD weaken the β-barrel structure, which creates a lateral opening that Qiao et al. termed the LPS ‘exit portal’. In addition, LptD has four cysteine residues — two in a periplasmic flexible loop and two in the amino-terminal domain — which are perfectly positioned to form disulphide bonds that could result in a conformational change that is capable of opening the LptD pore and its lateral opening. Qiao et al. analysed the LptD–LptE complex by SDS–PAGE under reducing and non-reducing conditions; this validated the establishment of disulphide bridges, as well as their contribution to the stability of the protein complex. Dong et al. showed that mutations in LptD that prevent lateral opening are lethal, which reinforces the idea that this feature of LptD is required for LPS translocation. Finally, both groups analysed the role of LptE in LPS translocation. Dong et al. speculated that the hydrophobic residues of LptE may be involved in binding to lipid A of LPS, whereas Qiao et al. analysed the charge distribution at the surface of LptE and suggested that it might bind to the hydrophilic head groups of LPS. However, both groups agreed that the hydrophilic parts of LPS travel through the pore. These data suggest a new model for the mechanism of LPS transport, in which the formation of disulphide bridges between LptD residues opens

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Putting it out there

the β-barrel pore and triggers the lateral opening of the exit gate. The hydrophobic portion of LPS can then enter the lipidic membrane through the lateral opening, while the hydrophilic portion travels through the pore, which results in proper insertion of LPS into the outer membrane. As LPS is essential for bacterial viability, these findings also establish the LptD–LptE complex as a potential drug target in pathogenic Gram-negative bacteria. Cláudio Nunes-Alves ORIGINAL RESEARCH PAPERS Dong, H. et al. Structural basis for outer membrane lipopolysaccharide insertion. Nature http://dx.doi. org/10.1038/nature13464 (2014) | Qiao, S. et al. Structural basis for lipopolysaccharide insertion in the bacterial outer membrane. Nature http://dx. doi.org/10.1038/nature13484 (2014)

VOLUME 12 | AUGUST 2014 © 2014 Macmillan Publishers Limited. All rights reserved

Bacterial physiology: Putting it out there.

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