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

摘要

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_α) and beta (T2SS_β), encoded in the H10407 genome and describe the prevalence of both systems in other E. coli pathotypes. Under laboratory conditions, the T2SS_β is assembled and functional in the secretion of LT into culture supernatant, whereas the T2SS_α is not. Insertional inactivation of the three genes located upstream of gspC_β (yghJ, pppA, and yghG) in the atypical T2SS_β operon revealed that YghJ is not required for assembly of the GspD_β secretin or secretion of LT, that PppA is likely the prepilin peptidase required for the function of T2SS_β, and that YghG is required for assembly of the GspD_β secretin and thus function of the T2SS_β. Mutational and physiological analysis further demonstrated that YghG (redesignated GspS_β) is a novel outer membrane pilotin protein that is integral for assembly of the T2SS_β by localizing GspD_β to the outer membrane, whereupon GspD_β forms the macromolecular secretin multimer through which T2SS_β substrates are translocated.