Structure of CARDS toxin, a unique ADP-ribosylating and vacuolating cytotoxin from Mycoplasma pneumoniae

Area of Science:

• Microbiology
• Structural Biology
• Pathogenesis

Background:

• Mycoplasma pneumoniae (Mp) causes respiratory illnesses like tracheobronchitis and walking pneumonia.
• Mp infections are associated with asthma and reactive airway diseases.
• The bacterium produces Community-Acquired Respiratory Distress Syndrome Toxin (CARDS TX), a virulence factor with mono-ADP ribosyltransferase (mART) and vacuolating activities.

Purpose of the Study:

• To determine the structure of the CARDS TX virulence factor.
• To elucidate the mechanisms by which CARDS TX mediates pathogenicity in airway epithelial cells.
• To identify potential therapeutic targets for Mp-associated respiratory diseases.

Main Methods:

• X-ray crystallography was used to determine the three-dimensional structure of CARDS TX.
• Lipid-binding assays were performed to identify specific membrane lipids targeted by CARDS TX.
• Cell surface binding and internalization assays were conducted to map functional domains of the toxin.

Main Results:

• CARDS TX possesses a unique triangular architecture, distinct from other bacterial ADP-ribosylating toxins, with an N-terminal mART domain and C-terminal tandem β-trefoil domains.
• CARDS TX specifically binds to phosphatidylcholine and sphingomyelin lipids on the cell surface.
• The C-terminal β-trefoil domain is responsible for both cell surface binding and internalization of the toxin.

Conclusions:

• The distinct structure of CARDS TX contributes to its unique pathogenic mechanisms in Mycoplasma pneumoniae infections.
• Targeting the lipid-binding and internalization functions of the C-terminal domain may offer a novel therapeutic strategy.
• This research enhances understanding of Mp pathogenicity and provides a basis for developing treatments for Mp-associated airway diseases.