YghG (GspSβ) Is a Novel Pilot Protein Required for Localization of the GspSβ Type II Secretion System Secretin of Enterotoxigenic Escherichia coli

The enterotoxigenic Escherichia coli (ETEC) pathotype, characterized by the prototypical strain H10407, is a leading cause of morbidity and mortality in the developing world. A major virulence factor of ETEC is the type II secretion system (T2SS) responsible for secretion of the diarrheagenic heat-labile enterotoxin (LT). In this study, we have characterized the two type II secretion systems, designated alpha (T2SS(alpha)) and beta (T2SS(beta)), encoded in the H10407 genome and describe the prevalence of both systems in other E. coli pathotypes. Under laboratory conditions, the T2SS(beta) is assembled and functional in the secretion of LT into culture supernatant, whereas the T2SSa is not. Insertional inactivation of the three genes located upstream of gspC(beta) (yghf, pppA, and yghG) in the atypical T2SS(beta) operon revealed that YghJ is not required for assembly of the GspD(beta) secretin or secretion of LT, that PppA is likely the prepilin peptidase required for the function of T2SS(beta), and that YghG is required for assembly of the GspD(beta) secretin and thus function of the T2SS(beta). Mutational and physiological analysis further demonstrated that YghG (redesignated GspS(beta)) is a novel outer membrane pilotin protein that is integral for assembly of the T2SS(beta) by localizing GspD(beta) to the outer membrane, whereupon GspD(beta) forms the macromolecular secretin multimer through which T2SS(beta) substrates are translocated.