Ubiquitin-Yop hybrids as probes for post-translational transport by the Yersinia type III secretion pathway

A solvent-exposed patch in chaperone-bound YopE is required for translocation by the type III secretion system

Most effector proteins of bacterial type III secretion (T3S) systems require chaperone proteins for translocation into host cells. Such effectors are bound by chaperones in a conserved and characteristic manner, with the chaperone-binding (Cb) region of the effector wound around the chaperone in a highly extended conformation. This conformation has been suggested to serve as a translocation signal in promoting the association between the chaperone-effector complex and a bacterial component required for translocation. We sought to test a prediction of this model by identifying a potential association site for the Yersinia pseudotuberculosis chaperone-effector pair SycE-YopE. We identified a set of residues in the YopE Cb region that are required for translocation but are dispensable for expression, SycE binding, secretion into the extrabacterial milieu, and stability in mammalian cells. These residues form a solvent-exposed patch on the surface of the chaperone-bound Cb region, and thus their effect on translocation is consistent with the structure of the chaperone-bound Cb region serving as a signal for translocation.

Impassable YscP substrates and their impact on the Yersinia enterocolitica type III secretion pathway

Yersinia type III machines secrete protein substrates across the bacterial envelope and, following assembly of their secretion needles, transport effector Yops into host cells. According to their destination during type III secretion, early, middle, and late secretion substrates can be distinguished; however, the signals and mechanisms whereby these proteins are recognized and transported by the secretion machine are not understood. Here, we examine several hybrids between secretion substrates and the impassable reporter protein glutathione S-transferase (GST). YscP-GST and YopR-GST blocked type III secretion; however, YscF-, YopD-, YopN-, and LcrV-GST did not. Unlike YopR-GST, which can block type III machines only during their assembly, expression of YscP-GST led to an immediate and complete block of all secretion. The secretion signal of YscP was mapped to its first 10 codons or amino acids; however, YscP(Delta 2-15)-GST, lacking this secretion signal, imposed a partial blockade. YscP-GST copurified with the type III ATPase complex (YscN, YscL, and YscQ) and with YscO, suggesting that the association of specific machine components with the impassable substrate may cause the block in type III secretion.

Yersinia outer proteins: Yops

The pathogenic bacteria Yersinia spp. contain a virulence plasmid that encodes a type III secretion system and effectors. During infection, four of the effectors target the actin cytoskeleton, crippling the phagocytic machinery in the infected cell. The remaining two effectors dampen the innate immune response by targeting important signalling pathways. Although the biochemical activity for each of these effectors is known, the mechanisms involved in their ordered secretion and delivery remain elusive.