Bacterial Outer Membrane Proteins Are Targeted to the Bam Complex by Two Parallel Mechanisms

Area of Science:

• Microbiology
• Molecular Biology
• Protein Folding and Assembly

Background:

• Gram-negative bacteria outer membrane proteins (OMPs) feature a β barrel structure for membrane integration.
• A conserved C-terminal β signal motif in OMPs has an unclear function.
• Periplasmic chaperones are thought to assist OMP biogenesis, but their specific roles are not fully understood.

Purpose of the Study:

• To investigate the function of the β signal motif in OMP assembly.
• To elucidate the roles of periplasmic chaperones SurA and Skp in OMP targeting and quality control.

Main Methods:

• Site-directed mutagenesis of the β signal motif in model Escherichia coli OMPs.
• Analysis of OMP assembly efficiency and degradation in wild-type and mutant strains lacking specific chaperones (SurA, Skp).
• Assessment of OMP affinity for the β barrel Assembly Machinery (Bam) complex.

Main Results:

• Mutations in the β signal motif reduced OMP affinity for the Bam complex, delaying assembly and causing partial periplasmic degradation.
• Absence of SurA exacerbated the effects of β signal mutations, leading to complete degradation; absence of Skp suppressed these effects and enhanced assembly.
• These findings indicate parallel targeting pathways involving the β signal/SurA and distinct roles for SurA and Skp.

Conclusions:

• The β signal motif acts as a cis-acting peptide targeting OMPs to the Bam complex, working in parallel with the trans-acting SurA chaperone.
• Periplasmic chaperones SurA and Skp possess specialized, non-redundant functions in OMP biogenesis and quality control.
• This study challenges the notion of chaperone redundancy and highlights distinct roles in OMP targeting and degradation pathways.