Figure 7

Schematic representation of the secretion of the unfolded MIIA domain and its Ca-dependent folding and catalytic activity in the host cell. Two Ca²⁺-binding sites (white filled circles) detected in this study lie at yet unidentified positions. At low Ca²⁺ concentration in the pathogen, the binding sites are unoccupied and the MIIA domain unstructured for efficient secretion by T3SS. At elevated Ca²⁺ concentrations in the host, two Ca²⁺ ions bind to the MIIA domain (dissociation constant K_d < 1.6 µM) and induce hydrophobic collapse with clustering of intrinsic tryptophan residues (green filled circles) into the interior of the folded protein. Thereby, autocatalytic activity cleaves the GDPH site (red filled oval) leading to release of the fragments S_C and S_N which promote biological function.