Cryo-EM structures of the translocational binary toxin complex CDTa-bound CDTb-pore from Clostridioides difficile

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• Some bacteria utilize binary toxin translocation systems to deliver enzymes into host cells via endocytosis.
• Clostridioides difficile, a clinically significant bacterium, possesses such a system involving CDTa and CDTb proteins, which are crucial therapeutic targets.

Purpose of the Study:

• To determine the cryo-electron microscopy (cryo-EM) structure of the CDTa-bound CDTb-pore complex.
• To elucidate the structural dynamics and conformational changes involved in toxin translocation.
• To provide insights for drug design against hypervirulent C. difficile.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was employed to determine the structure of the CDTa-CDTb-pore complex.
• 3D variability analysis was utilized to study the dynamic movements of the CDTa subunit.

Main Results:

• The cryo-EM structure revealed that CDTa binding induces partial unfolding and tilting of the first CDTa α-helix.
• An NSS-loop within the CDTb-pore was observed in both 'in' and 'out' conformations, suggesting a role in substrate translocation.
• 3D variability analysis showed dynamic transitions of CDTa between folded and unfolded states.

Conclusions:

• The study provides the first reported structure of the CDTb-pore bound to CDTa, detailing key conformational changes.
• The observed structural dynamics offer a mechanistic understanding of toxin translocation by C. difficile.
• These findings are valuable for the rational design of therapeutics targeting hypervirulent C. difficile strains.