Nature Structural & Molecular Biology (2017) doi:10.1038/nsmb.3399, Received 11 January 2017, Accepted 14 March 2017, Published online 10 April 2017
Structural basis for lipopolysaccharide extraction by ABC transporter LptB2FG
Qingshan Luo, Xu Yang, Shan Yu, Huigang Shi, Kun Wang, Le Xiao, Guangyu Zhu, Chuanqi Sun, Tingting Li, Dianfan Li, Xinzheng Zhang, Min Zhou & Yihua Huang
After biosynthesis, bacterial lipopolysaccharides (LPS) are transiently anchored to the outer leaflet of the inner membrane (IM). The ATP-binding cassette (ABC) transporter LptB2FG extracts LPS molecules from the IM and transports them to the outer membrane. Here we report the crystal structure of nucleotide-free LptB2FG from Pseudomonas aeruginosa. The structure reveals that lipopolysaccharide transport proteins LptF and LptG each contain a transmembrane domain (TMD), a periplasmic β-jellyroll-like domain and a coupling helix that interacts with LptB on the cytoplasmic side. The LptF and LptG TMDs form a large outward-facing V-shaped cavity in the IM. Mutational analyses suggest that LPS may enter the central cavity laterally, via the interface of the TMD domains of LptF and LptG, and is expelled into the β-jellyroll-like domains upon ATP binding and hydrolysis by LptB. These studies suggest a mechanism for LPS extraction by LptB2FG that is distinct from those of classical ABC transporters that transport substrates across the IM.