Substrate recognition by the bacterial type II secretion system: more than a simple interaction.

Area of Science:

• Microbiology
• Molecular Biology
• Biochemistry

Background:

• The Type II secretion system (T2SS) is crucial for translocating proteins across the outer membrane of Gram-negative bacteria.
• The molecular mechanisms and specific targeting signals for exoprotein recruitment by T2SS remain largely unknown.
• Understanding T2SS exoprotein recruitment is vital for studying bacterial pathogenesis and developing novel antimicrobial strategies.

Purpose of the Study:

• To elucidate the molecular basis of exoprotein recruitment by the Type II secretion system (T2SS).
• To identify the specific targeting motifs responsible for exoprotein interaction with T2SS components.
• To investigate the synergistic interactions governing exoprotein secretion.

Main Methods:

• Bacterial two-hybrid assays to detect protein-protein interactions.
• Surface plasmon resonance (SPR) for quantifying binding affinities.
• In vivo site-specific photo-cross-linking to map interaction interfaces.
• Functional analyses and in silico approaches to validate findings.

Main Results:

• The fibronectin-like Fn3 domain of exoprotein PelI interacts with periplasmic domains of T2SS components GspD and GspC.
• A 9-residue loop on PelI was identified as a specific secretion signal interacting with the GspC PDZ domain.
• Synergistic interactions involving GspD and GspC modulate exoprotein binding and secretion.
• Equivalent secretion motifs were found in other exoproteins, suggesting a conserved mechanism.

Conclusions:

• A specific exoprotein loop region acts as a secretion signal for T2SS-mediated translocation.
• Exoprotein recruitment by T2SS involves a cascade of synergistic protein-protein interactions.
• This study proposes a general mechanism for exoprotein targeting and recruitment by the T2SS in Gram-negative bacteria.