Conformational Plasticity of LptC Regulates Lipopolysaccharide Transport by the LptB

The outer membrane of Gram-negative bacteria is coated with lipopolysaccharide (LPS). The Lpt system generates membrane asymmetry by transporting LPS from the inner to the outer membrane. Transport begins with the LptB2FGC complex, where the ABC transporter LptB2FG associates with LptC to extract LPS. LPS is then passed via LptA to the LptDE translocon. While LptB2FGC structures suggest an extrusion mechanism, the role of LptC remains unclear. Here, we reconstituted the complex in vitro from purified LptB2FG and LptC, and demonstrate that LptC stabilizes the complex and modulates ATPase activity. Using differential isotope labeling and solid-state NMR including dynamic nuclear polarization, we observed that the LptC transmembrane helix LptCTMH is tightly associated with the transporter in the apo state. Upon LPS or ATP binding, LptCTMH becomes more flexible and samples a distinct conformational space which favors cavity collapse and substrate-coupled ATPase activity. Our data support a model in which LptC acts as a mechanical transducer linking transport and energy consumption.