Secondary-structure switch regulates the substrate binding of a YopJ family acetyltransferase.

The Yersinia outer protein J (YopJ) family effectors are widely deployed through the type III secretion system by both plant and animal pathogens. As non-canonical acetyltransferases, the enzymatic activities of YopJ family effectors are allosterically activated by the eukaryote-specific ligand inositol hexaphosphate (InsP6). However, the underpinning molecular mechanism remains undefined. Here we present the crystal structure of apo-PopP2, a YopJ family member secreted by the plant pathogen Ralstonia solanacearum. Structural comparison of apo-PopP2 with the InsP6-bound PopP2 reveals a substantial conformational readjustment centered in the substrate-binding site. Combining biochemical and computational analyses, we further identify a mechanism by which the association of InsP6 with PopP2 induces an α-helix-to-β-strand transition in the catalytic core, resulting in stabilization of the substrate recognition helix in the target protein binding site. Together, our study uncovers the molecular basis governing InsP6-mediated allosteric regulation of YopJ family acetyltransferases and further expands the paradigm of fold-switching proteins. The Yersinia outer protein J (YopJ) family of effectors, which are present in many plant and animal pathogens are non-canonical acetyltransferases that are activated by the eukaryote-specific cofactor inositol hexaphosphate (InsP6). Here, the authors combine X-ray crystallography, biochemical and functional analyses to characterise the structure and activation mechanism of the YopJ family effector PopP2 from the plant pathogen Ralstonia solanacearum and observe that InsP6 binding induces major conformational changes in PopP2 with a helix-to-strand fold-switching in its catalytic core.