Templated folding of the RTX domain of the bacterial toxin adenylate cyclase revealed by single molecule force spectroscopy

The RTX (repeats-in-toxin) domain of the bacterial toxin adenylate cyclase (CyaA) contains five RTX blocks (RTX-i to RTX-v) and its folding is essential for CyaA’s functions. It was shown that the C-terminal capping structure of RTX-v is critical for the whole RTX to fold. However, it is unknown how the folding signal transmits within the RTX domain. Here we use optical tweezers to investigate the interplay between the folding of RTX-iv and RTX-v. Our results show that RTX-iv alone is disordered, but folds into a Ca(2+)-loaded-β-roll structure in the presence of a folded RTX-v. Folding trajectories of RTX-iv-v reveal that the folding of RTX-iv is strictly conditional upon the folding of RTX-v, suggesting that the folding of RTX-iv is templated by RTX-v. This templating effect allows RTX-iv to fold rapidly, and provides significant mutual stabilization. Our study reveals a possible mechanism for transmitting the folding signal within the RTX domain.