Yersinia enterocolitica type III secretion - On the role of SycE in targeting YopE into HeLa cells

Yersinia enterocolitica inject toxic proteins (effector Yops) into the cyto sol of eukaryotic cells by a mechanism requiring the type III machinery. Pr evious work mapped a signal sufficient for the targeting of fused reporter proteins to amino acids 1-100 of YopE. Targeting requires the binding of Sy cE to YopE residues 15-100 in the bacterial cytoplasm. We asked whether Syc E functions only to stabilize YopE in the bacterial cytoplasm, or whether t he secretion chaperone itself contributes to substrate recognition by the t ype III machinery. Fusions of glutathione S-transferase to either the N or C terminus of SycE resulted in hybrid proteins that bound YopE but prevente d targeting of the export substrate into HeLa cells. As compared with wild- type SycE, glutathione S-transferase-SycE bound and stabilized YopE in the bacterial cytoplasm but failed to release the polypeptide for export by the type LII machinery. Thus, it appears that SycE functions to deliver YopE t o the type III secretion machinery. A model is presented that accounts for substrate recognition of effector Yops, a group of proteins that do not sha re amino acid sequence or physical similarities.