Figure 4. Lipid A entry and early transport pathway.

(A) Stereo view of the MsbA structure in the presence of lipid A shows a front membrane portal formed by TM4 and TM6, which is open to the cytoplasmic leaflet of the inner membrane. A row of basic and polar residues (green sticks) on TM4 and TM6 are arrayed to promote entry of the negatively charged lipid A or LPS in combination with a lateral inward motion of TMs as shown in (C and D). Yellow sticks represent an LPS molecule manually modeled at the corresponding positions in 5TV4 and 6BPL, with key residues Arg78 (TM2), Arg148 (TM3), and Arg296 (TM6) (purple sticks) more deeply buried for selective interaction with the two phosphorylated glucosamine units of lipid A at a later stage of transport. A red arrow is drawn to describe a rough path of lipid A entry and halfway through its transport. (B) Electrostatic surface representation of MsbA structures (blue, +10 kT/e; red, −10 kT/e). A decreased opening of the membrane portal is highlighted by the green triangle from the inward open (left) to inward closed (right) conformation. (C) Structure alignment with 5TV4 (dark grey) and 6BPL (light grey). Only a monomer is shown, and the NBDs are omitted for clarity. The inward motion of TM4 and TM5 towards TM6 results in gradual closure of the membrane portal as seen in (B). (D) Cytosolic view of the lateral movements occurring between the current structure (left) and the cryo-EM structure (right) bound with lipid A or LPS. Key lipid A binding residues (Arg78, Arg148 and Arg296) shift closer to the center of the lipid channel, allowing tighter binding and possibly the upward movement of lipid A that bridges the two TMDs.