Secretion signal recognition by YscN, the Yersinia type III secretion ATPase.
Yersinia type III machines secrete protein substrates across the bacterial envelope. Secretion signals of some substrates have been identified; however, the mechanisms whereby these signals interact with type III machines are not known. Here we show that fusion of YopR, an early secretion substrate, to the N terminus of dihydrofolate reductase (DHFR) or other tightly folded proteins generates impassable hybrids that cannot travel the type III pathway. YopR hybrids capture YscN, the ATPase that provides energy for type III transport reactions, in the bacterial cytoplasm. Eleven N-terminal residues function as the YopR secretion signal, which is required for both binding to YscN and blocking the type III pathway. When expressed during type III machine assembly, YopR-DHFR blocks all secretion. Delayed expression of YopR-DHFR, when yersiniae have already engaged the type III pathway, blocks secretion of early (YscP) but not of late (effector Yops) substrates. These observations support a model whereby type III machines are programmed to secrete a sequence of proteins that can be disrupted when an impassable early substrate interacts with the YscN ATPase and blocks the transport of late substrates.