The Enterococcus faecalis MSCRAMM ACE binds its ligand by the Collagen Hug model

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• Enterococcus faecalis possesses a collagen-binding microbial surface components recognizing adhesive matrix molecules (MSCRAMM) called ACE.
• ACE is crucial for bacterial adhesion to host tissues, particularly collagen.
• Understanding ACE's structure and collagen-binding mechanism is vital for developing anti-adhesion strategies.

Purpose of the Study:

• To determine the crystal structure of the Enterococcus faecalis ACE ligand-binding segment.
• To elucidate the mechanism by which ACE binds to collagen.
• To provide biochemical evidence supporting the proposed collagen-binding model.

Main Methods:

• X-ray crystallography was used to determine the three-dimensional structure of the ACE ligand-binding segment.
• Site-directed mutagenesis was employed to investigate the role of specific residues in collagen binding.
• Biochemical assays were performed to assess collagen binding affinity and the effect of structural modifications.

Main Results:

• The crystal structure revealed that ACE consists of two IgG-like fold subdomains (N(1) and N(2)) with a putative collagen-binding surface at their interface.
• Mutagenesis studies showed that altering residues in the ACE N(2) subdomain's collagen-binding trench significantly impacted collagen binding.
• A closed ACE conformation, stabilized by a disulfide bond, demonstrated a complete loss of collagen-binding capability.

Conclusions:

• ACE likely binds collagen via a mechanism analogous to the 'Collagen Hug' observed for Staphylococcus aureus CNA.
• Specific residues within the ACE N(2) extension are critical for stabilizing the MSCRAMM-ligand complex.
• The findings provide the first biochemical evidence supporting the Collagen Hug model for ACE-collagen interactions.