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

Yersinia enterocolitica uses type III secretion to transport Yop proteins into the cytoplasm of host cells. Previous work generated hypotheses for both co- and post-translational transport mechanisms in the Yersinia type III pathway. Here, we used ubiquitin (Ub) and UBP1, the Ub-specific protease, to examine whether Yops can be secreted when synthesized prior to recognition by the type III machinery. Fusion of Ub to the N-terminus of Yops blocked substrate recognition and secretion of hybrids generated with YopE, YopQ or YopR. UBP1 removed Ub from the N-terminus of these hybrids and allowed YopE, YopQ or YopR cleavage products to enter the secretion pathway. Following the release of Ub, Yersinia type III machines also transported the YopE cleavage product into the cytosol of tissue culture cells. Minimal secretion signals were also examined with the Ub/UBP1 system and some, but not all, of these signals promoted type III secretion even after polypeptides had been freed from Ub. These results suggest that recognition and secretion of Yop substrates by the type III machinery can occur by a post-translational mechanism.