Structural basis of BAM-mediated outer membrane β-barrel protein assembly

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• Outer membrane proteins (OMPs) form essential channels in Gram-negative bacteria, mitochondria, and chloroplasts.
• All OMPs share a conserved beta-barrel structure, suggesting a common evolutionary origin and folding mechanism.
• While models exist for the initiation of OMP folding by the bacterial beta-barrel assembly machinery (BAM), the complete assembly mechanism remains largely unknown.

Purpose of the Study:

• To elucidate the late-stage mechanisms of OMP assembly by the BAM complex.
• To characterize the conformational dynamics of BAM during OMP substrate insertion.
• To identify critical residues in BAM and OMP substrates involved in the assembly process.

Main Methods:

• Determined intermediate structures of the BAM complex assembling an OMP substrate (EspP).
• Utilized molecular dynamics simulations to support observed conformational changes.
• Performed mutagenic in vitro and in vivo assembly assays to probe functional residues.

Main Results:

• Captured sequential conformational dynamics of BAM during the late stages of OMP assembly.
• Identified specific functional residues in BamA and EspP crucial for barrel hybridization, closure, and release.
• Provided structural evidence for the step-wise progression of OMP assembly.

Conclusions:

• The study offers novel insights into the conserved mechanism of OMP assembly.
• BAM undergoes dynamic conformational changes to facilitate OMP insertion.
• Specific protein-protein interactions are vital for the successful completion of OMP biogenesis.