RTX calcium binding motifs are intrinsically disordered in the absence of calcium: implication for protein secretion

The Repeat in Toxin (RTX) motif is a tandemly repeated calcium-binding nonapeptide sequence present in proteins that are secreted by the type I secretion system (T1SS) of Gram-negative bacteria. Here, we have characterized the structural and hydrodynamic properties of the RTX Repeat Domain (RD) of the CyaA toxin from Bordetella pertussis. This 701-amino acid long domain contains about 40 RTX motifs. We showed that, in the absence of calcium, RD was natively disordered, weakly stable, and highly hydrated. Calcium binding induced compaction and dehydration of RD, along with the formation of stable secondary and tertiary structures. The calcium-induced conformational switch between unfolded conformations of apo-RD and stable structures of holo-RD is likely to be a key property for the biological function of the CyaA toxin: in the low calcium environment of the bacterial cytosol, the intrinsically disordered character of the protein may facilitate its secretion through the secretion machinery. In the extracellular medium, calcium binding can then trigger the folding of the polypeptide into its functional state. The intrinsic disorder of RTX-containing proteins in the absence of calcium may thus be directly involved in the efficient secretion of proteins through T1SS.