Structure and action of the binary C2 toxin from Clostridium botulinum

Area of Science:

• Structural biology
• Molecular toxicology
• Bacterial pathogenesis

Background:

• Clostridium botulinum C2 toxin comprises an ADP-ribosylating enzyme (C2-I) and a cell-binding/translocation component (C2-II).
• Understanding the structural basis of C2 toxin's interaction with eukaryotic cells is crucial for developing countermeasures.

Purpose of the Study:

• To determine the high-resolution crystal structures of C2 toxin components C2-I and C2-II.
• To investigate the structural stability of C2-I at varying pH levels.
• To analyze the structural basis for C2-II's interaction with host cells and its prepore formation.

Main Methods:

• X-ray crystallography was employed to determine the structures of C2-I (up to 1.75 Å) and C2-II (lower resolution).
• Structural analysis included comparisons at different pH values (3.0, 6.1 for C2-I; 4.3, 6.0 for C2-II).
• A model of the C2-IIa prepore was constructed based on homology with anthrax toxin.

Main Results:

• C2-I crystal structures revealed stability across a pH range of 3.0 to 6.1, indicating no significant unfolding.
• C2-II structure showed minimal conformational differences between pH 4.3 and 6.0, with a potentially mobile C-terminal domain.
• The C2-IIa prepore model highlighted a high density of asparagine residues lining the pore.

Conclusions:

• The structural data provide insights into the stability and conformational behavior of C2 toxin components.
• The findings contribute to understanding the mechanism of C2 toxin entry into target cells.
• Further research is needed to explain observed activity differences between C2-I and actin isoforms.