Structural Basis of Ca(2+)-Dependent Self-Processing Activity of Repeat-in-Toxin Proteins

The posttranslational Ca(2+)-dependent “clip-and-link” activity of large repeat-in-toxin (RTX) proteins starts by Ca(2+)-dependent structural rearrangement of a highly conserved self-processing module (SPM). Subsequently, an internal aspartate-proline (Asp-Pro) peptide bond at the N-terminal end of SPM breaks, and the liberated C-terminal aspartyl residue can react with a free ε-amino group of an adjacent lysine residue to form a new isopeptide bond. Here, we report a solution structure of the calcium-loaded SPM (Ca-SPM) derived from the FrpC protein of Neisseria meningitidis. The Ca-SPM structure defines a unique protein architecture and provides structural insight into the autocatalytic cleavage of the Asp-Pro peptide bond through a “twisted-amide” activation. Furthermore, in-frame deletion of the SPM domain from the ApxIVA protein of Actinobacillus pleuropneumoniae attenuated the virulence of this porcine pathogen in a pig respiratory challenge model. We hypothesize that the Ca(2+)-dependent clip-and-link activity represents an unconventional strategy for Gram-negative pathogens to adhere to the host target cell surface.