Conformational Plasticity of LptC Regulates Lipopolysaccharide Transport by the LptB2FGC Complex

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 LptC_TMH is tightly associated with the transporter in the apo state. Upon LPS or ATP binding, LptC_TMH 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.