Lipids mediate supramolecular outer membrane protein assembly in bacteria

Area of Science:

• Microbiology
• Structural Biology
• Biophysics

Background:

• Gram-negative bacteria possess a unique outer membrane (OM) essential for their survival.
• This OM is primarily composed of β-barrel outer membrane proteins (OMPs) organized into functional supramolecular assemblies.
• The precise mechanisms governing OMP assembly and clustering within the OM remain largely unelucidated.

Purpose of the Study:

• To investigate the in vivo mechanisms underlying the formation of OMP supramolecular assemblies in the Gram-negative bacterial OM.
• To identify the fundamental units and molecular interactions driving OMP clustering.

Main Methods:

• Utilized photoactivatable cross-linking within the Escherichia coli OM.
• Integrated computational simulations with biochemical and biophysical analyses.
• Characterized the role of lipids and specific OMPs in assembly formation.

Main Results:

• Discovered that asymmetric lipid shells surrounding OMPs mediate interactions with neighboring proteins.
• Identified abundant porins (OmpF, OmpC) as central hubs for OMP assembly.
• Demonstrated that low-abundance monomeric β-barrels, like TonB-dependent transporters, pack against these porin hubs.
• Revealed OMP-lipid-OMP complexes as the fundamental building blocks of OMP assemblies.

Conclusions:

• OMP assemblies are formed through OMP-lipid-OMP complexes, acting as the basic unit of supramolecular organization.
• These assemblies span the entire bacterial cell surface, integrating OM multifunctionality with stability and impermeability.
• The findings provide a mechanistic understanding of how the Gram-negative outer membrane achieves its structural integrity and functional diversity.