Translocation path of a substrate protein through its Omp85 transporter

Area of Science:

• Microbiology
• Structural Biology
• Protein Transport

Background:

• TpsB proteins, members of the Omp85 superfamily, are crucial for protein translocation across Gram-negative bacterial outer membranes.
• These transporters feature N-terminal POTRA domains and a C-terminal transmembrane β-barrel structure.

Purpose of the Study:

• To elucidate the in vivo secretion pathway of Bordetella pertussis filamentous haemagglutinin (FHA), a substrate of the TpsB transporter FhaC.
• To map the interactions between FHA and FhaC during protein translocation.

Main Methods:

• Utilized site-specific crosslinking to track the in vivo secretion path of FHA.
• Analyzed interactions between the FHA secretion domain and various regions of FhaC, including POTRA domains, extracellular loops, and the β-barrel.

Main Results:

• Identified specific interactions between the FHA conserved secretion domain and FhaC's POTRA domains, extracellular loops, and β-barrel surface.
• Revealed a funnel-like translocation pathway where flexible FHA enters the channel and exits along a specific four-stranded β-sheet of the FhaC barrel.
• Demonstrated that this β-sheet guides FHA translocation and folding through discrete steps.

Conclusions:

• The Omp85 β-barrel functions as a channel facilitating the translocation of substrate proteins across the bacterial outer membrane.
• The study provides a detailed map of protein-substrate interactions within the FhaC transporter, offering insights into bacterial secretion mechanisms.