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 ∼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.
Unite de Biochimie des Interactions Macromoléculaires, Institut Pasteur, Centre National de la Recherche Scientifique URA 2185, Paris, France. alexandre.chenal@pasteur.fr
Alexandre
A
Chenal
Johanna C
JC
Karst
Ana Cristina
AC
Sotomayor Pérez
Anna Katarzyna
AK
Wozniak
Bruno
B
Baron
Patrick
P
England
Daniel
D
Ladant
D037361:Adenylate Cyclase Toxin / Q000737:chemistry* / Q000378:metabolism*; D001886:Bordetella pertussis / Q000378:metabolism*; D002118:Calcium / Q000494:pharmacology*; D002942:Circular Dichroism; D005453:Fluorescence; D019791:Guanidine / Q000494:pharmacology; D008958:Models, Molecular; D017510:Protein Folding / Q000187:drug effects*; D055550:Protein Stability / Q000187:drug effects; D017434:Protein Structure, Tertiary; D012965:Sodium Chloride / Q000494:pharmacology; D017550:Spectroscopy, Fourier Transform Infrared; D014364:Tryptophan / Q000378:metabolism; D014508:Urea / Q000494:pharmacology
D037361:Adenylate Cyclase Toxin; D012965:Sodium Chloride; D014364:Tryptophan; D014508:Urea; D019791:Guanidine; D002118:Calcium