The type III secretion chaperone LcrH co‐operates with YopD to establish a negative, regulatory loop for control of Yop synthesis in Yersinia pseudotuberculosis

The enteropathogen Yersinia pseudotuberculosis is a model system used to study the molecular mechanisms by which Gram‐negative pathogens secrete and subsequently translocate antihost effector proteins into target eukaryotic cells by a common type III secretion system (TTSS). In this process, YopD (Yersiniaouter protein D) is essential to establish regulatory control of Yop synthesis and the ensuing translocation process. YopD function depends upon the non‐secreted TTSS chaperone LcrH (low‐calcium response H), which is required for presecretory stabilization of YopD. However, as a new role for TTSS chaperones in virulence gene regulation has been proposed recently, we undertook a detailed analysis of LcrH. A lcrH null mutant constitutively produced Yops, even when this strain was engineered to produce wild‐type levels of YopD. Furthermore, the YopD–LcrH interaction was necessary to regain the negative regulation of virulence associated genes yops). This finding was used to investigate the biological significance of several LcrH mutants with varied YopD binding potential. Mutated LcrH alleles were introduced in trans into a lcrH null mutant to assess their impact on yop regulation and the subsequent translocation of YopE, a Rho‐GTPase activating protein, across the plasma membrane of eukaryotic cells. Two mutants, LcrHK20E, E30G, I31V, M99V, D136G and LcrHE30G lost all regulatory control, even though YopD binding and secretion and the subsequent translocation of YopE was indistinguishable from wild type. Moreover, these regulatory deficient mutants showed a reduced ability to bind YscY in the two‐hybrid assay. Collectively, these findings confirm that LcrH plays an active role in yop regulation that might be mediated via an interaction with the Ysc secretion apparatus. This chaperone–substrate interaction presents an innovative means to establish a regulatory hierarchy in Yersinia infections. It also raises the question as to whether or not LcrH is a true chaperone involved in stabilization and secretion of YopD or a regulatory protein responsible for co‐ordinating synthesis of Yersinia virulence determinants. We suggest that LcrH can exhibit both of these activities.