Fig. 6

Mechanistic model for LPS transporter LptB₂FG. LPS transporter LptB₂FG may extract and transport LPS through a cycle of ATP-free state (resting stage), ATP-binding state, and ATP hydrolyzed state. The dimeric LptB molecules move closer to bind ATP when ATP enters into the active site of LptB (step 1). This triggers the conformational changes of TM5F-1G, which fully opens the lateral gate TM5F-1G of the transporter to extract the lipid A of LPS from the IM into the internal hydrophobic cavity (lateral opening). The lipid A is then pushed to the periplasmic domain of LptF when ATP is hydrolyzed, while the lateral gate TM5F-1G may be closed and the lateral gate TM1F-5G may be fully open to extract LPS and finally the LPS may be pushed to the periplasmic domain of LptG (step 2). The highly conserved hydrophobic residues and positively charged residues in the cavity may be involved in LPS extraction and transport. Next, ADP is released and LptB₂FG returns to the ATP-free state (step 3) and the cycle repeats