A host-specific virulence protein of Erwinia herbicola pv. gypsophilae is translocated into human epithelial cells by the type III secretion system of enteropathogenic Escherichia coli

HsvG is a virulence factor that determines the host specificity of Erwinia herbicola pathovars gypsophilae and betae on gypsophila. We used the calmodulin adenylate cyclase reporter (CyaA) to demonstrate that HsvG is secreted and translocated into HeLa cells by the type III secretion system (TTSS) of the enteropathogenic Escherichia coli (EPEC). A fusion of HsvG-CyaA containing 271 amino acids of the N-terminus of HsvG were introduced into a wild-type EPEC, espB mutant deficient in translocation and an escV mutant deficient in secretion. A significant secretion was detected in EPEC/HsvG-CyaA and its espB mutant, but not with the escV mutant. Translocation was only observed with the wildtype EPEC, and not with the other two mutants. To localize the secretion and translocation signals of HsvG, fusions containing 39, 11 and 3 amino acids of the N-terminus of HsvG were constructed and expressed in EPEC. A fusion containing the first 39 N-terminal amino acids of HsvG was secreted and translocated at significant level (31-35%) as compared to the original fusion. In contrast, fusions containing the 3 and 11 amino acids failed to be secreted and translocated.