Structural basis of lipopolysaccharide extraction by the LptB₂FGC complex

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• Lipopolysaccharide (LPS) is crucial for Gram-negative bacterial outer membrane integrity and antibiotic resistance.
• The lipopolysaccharide transport (Lpt) system, comprising LptA-G proteins, facilitates LPS translocation from the inner to the outer membrane.
• The ATP-binding cassette transporter LptB2FG, associated with LptC, is key for extracting LPS from the inner membrane, but its mechanism and LptC's role are unclear.

Purpose of the Study:

• To elucidate the structural mechanism of the LptB2FG-LptC complex (LptB2FGC) in lipopolysaccharide transport.
• To understand the role of LptC in regulating the ATP-binding cassette transporter LptB2FG.
• To reveal the detailed interactions between the LptB2FGC complex and lipopolysaccharide.

Main Methods:

• Single-particle cryo-electron microscopy (cryo-EM) was employed to determine the structures of LptB2FG and LptB2FGC.
• Structures were resolved in both nucleotide-free and vanadate-trapped states to capture different functional conformations.
• Detailed analysis of the cryo-EM structures identified bound lipopolysaccharide and protein-lipid interactions.

Main Results:

• High-resolution cryo-EM structures of LptB2FG and LptB2FGC were obtained, visualizing bound lipopolysaccharide.
• The structures reveal detailed side-chain interactions between the transporter and lipopolysaccharide, explaining capture and extrusion mechanisms.
• LptC was found to insert its transmembrane helix within LptB2FG, a novel regulatory mechanism for ABC transporters, coupling LPS transport to conformational changes.

Conclusions:

• The LptB2FGC structure provides unprecedented insight into the mechanism of lipopolysaccharide transport across the bacterial inner membrane.
• LptC plays a critical regulatory role by modulating LptB2FG activity and potentially coordinating periplasmic Lpt protein interactions.
• Understanding this complex is vital for developing novel strategies to combat antibiotic resistance in Gram-negative bacteria.