Calcium-Induced Folding and Stabilization of the Intrinsically Disordered RTX Domain of the CyaA Toxin

The adenylate cyclase toxin (CyaA) is one of the major virulence factors of Bordetella pertussis the causative agent of whooping cough Its C terminal region the receptor binding domain (RD) contains similar to 40 calcium binding Repeat in ToXin (RTX) motifs which are characteristic of many virulence factors of pathogenic bacteria We previously showed that RD is intrinsically disordered in the absence of calcium and acquires its functional three dimensional structure upon calcium binding To gain further insight into the physicochemical properties of RD we characterized its calcium induced conformational and stability changes by combining spectroscopic approaches We show that RD in the absence of calcium adopts premolten globule conformations due in part to the strong internal electrostatic repulsions between the negative charges of the aspartate rich polypeptide sequence Accordingly sodium is able to screen these electrostatic repulsions allowing a partial compaction of the polypeptide whereas calcium triggers a strong compaction as well as the acquisition of secondary and tertiary structures in a highly cooperative manner The differential sensitivity of the calcium loaded state to guanidinium and urea induced denaturations provides further evidence that electrostatic interactions play a critical role in the folding and stability of RD These results provide new insights into the folding/function relationship of the RTX motifs