Dimerization of the Pseudomonas aeruginosa translocator chaperone PcrH is required for stability, not function.

Type III secretion systems rely on hydrophobic translocator proteins that form a pore in the host cell membrane to deliver effector proteins into targeted host cells. These translocator proteins are stabilized in the cytoplasm and targeted for export with the help of specific chaperone proteins. In Pseudomonas aeruginosa, the chaperone of the pore-forming translocator proteins is PcrH. Although all translocator chaperones dimerize, the location of the dimerization interface is in dispute. Moreover, it has been reported that interfering with dimerization interferes with chaperone function. However, binding of P. aeruginosa chaperone PcrH to its cognate secretion substrate, PopD, results in dissociation of the PcrH dimer in vitro, arguing that dimerization of PcrH is likely not important for substrate binding or targeting translocators for export. We demonstrate that PcrH dimerization occurs in vivo in P. aeruginosa and used a genetic screen to identify a dimerization mutant of PcrH. The mutant protein is fully functional in that it can both stabilize PopB and PopD in the cytoplasm and promote their export via the type III secretion system. The location of the mutation suggests that the dimerization interface of PcrH mirrors that of the Yersinia homolog SycD and not the dimerization interface that had previously been reported for PcrH based on crystallographic evidence. Finally, we present data that the dimerization mutant of PcrH is less stable than the wild-type protein in P. aeruginosa, suggesting that the function of dimerization is stabilization of PcrH in the absence of its cognate cargo.