The complex that inserts lipopolysaccharide into the bacterial outer membrane forms a two-protein plug-and-barrel

Area of Science:

• Bacterial cell envelope biogenesis
• Membrane protein structure and function
• Lipopolysaccharide transport

Background:

• Gram-negative bacteria possess an asymmetric outer membrane (OM) with lipopolysaccharide (LPS) in the outer leaflet.
• LPS is crucial for bacterial viability, acting as a barrier against toxic molecules.
• The transport of LPS from the cytosol to the cell surface involves crossing multiple cellular compartments.

Purpose of the Study:

• To elucidate the structural mechanism of lipopolysaccharide (LPS) translocation across the Gram-negative bacterial outer membrane (OM).
• To investigate the interaction between the LptD and LptE proteins, essential for LPS assembly in the OM.
• To understand how the LptD/E complex facilitates LPS insertion into the outer leaflet of the OM.

Main Methods:

• In vitro and in vivo structural and biochemical analyses of the LptD/E complex.
• Investigating the interaction interface between LptE and LptD.
• Assessing the impact of disrupting the LptD/E interaction site on LptD biogenesis.

Main Results:

• LptE localizes within the LptD β-barrel, forming a plug-and-barrel architecture.
• LptD and LptE associate through an extensive interface, including a specific interaction within the LptD lumen.
• Disruption of this interaction site impairs LptD biogenesis.

Conclusions:

• The LptD/E complex exhibits an unprecedented two-protein plug-and-barrel structure.
• This architecture provides a mechanism for inserting LPS from the periplasm into the OM outer leaflet.
• The LptD/E complex is essential for establishing the asymmetric lipid bilayer of the Gram-negative OM.