Substrate binding to BamD triggers a conformational change in BamA to control membrane insertion

Area of Science:

• Microbiology
• Molecular Biology
• Biochemistry

Background:

• The β-barrel assembly machine (Bam) complex is essential for inserting integral membrane proteins into the Gram-negative bacterial outer membrane.
• Key components BamA and BamD interact with protein substrates, but their coordination during assembly is poorly understood.

Purpose of the Study:

• To investigate the coordination between BamA and BamD during the assembly of β-barrel proteins.
• To elucidate the mechanism by which the Bam complex facilitates protein folding and membrane integration.

Main Methods:

• A defective β-barrel substrate, LptD, was engineered by altering a conserved residue.
• Assembly kinetics of the defective LptD were analyzed in the presence of the Bam complex.
• Mutations in BamA's extracellular loops were introduced to assess their impact on substrate assembly.

Main Results:

• The engineered LptD substrate was recruited by BamD but showed inefficient membrane integration.
• Modifications in BamA's extracellular loops partially restored the assembly kinetics of the defective substrate.
• This suggests that BamA's ability to engage the substrate is impaired in the defective mutant.

Conclusions:

• Substrate binding to BamD appears to activate BamA for subsequent folding and membrane integration steps.
• Interactions within BamA's extracellular loops are critical for substrate engagement and proper assembly.
• This study provides insights into the coordinated mechanism of the Bam complex in outer membrane protein biogenesis.