Context-dependent protein folding of a virulence peptide in the bacterial and host environments: structure of an SycH–YopH chaperone–effector complex

Yersinia pestis injects numerous bacterial proteins into host cells through an organic nanomachine called the type 3 secretion system. One such substrate is the tyrosine phosphatase YopH, which requires an interaction with a cognate chaperone in order to be effectively injected. Here, the first crystal structure of a SycH–YopH complex is reported, determined to 1.9 Å resolution. The structure reveals the presence of (i) a nonglobular polypeptide in YopH, (ii) a so-­called β-motif in YopH and (iii) a conserved hydrophobic patch in SycH that recognizes the β-motif. Biochemical studies establish that the β-motif is critical to the stability of this complex. Finally, since previous work has shown that the N-­terminal portion of YopH adopts a globular fold that is functional in the host cell, aspects of how this polypeptide adopts radically different folds in the host and in the bacterial environments are analysed.