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 , 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 (T2SSα) and beta (T2SSβ), encoded in the genome and describe the prevalence of both systems in other E. coli pathotypes. Under laboratory conditions, the T2SSβ is assembled and functional in the secretion of LT into culture supernatant, whereas the T2SSα is not. Insertional inactivation of the three genes located upstream of gspCβ (yghJ, pppA, and yghG) in the atypical T2SSβ operon revealed that YghJ is not required for assembly of the GspDβ secretin or secretion of LT, that PppA is likely the prepilin peptidase required for the function of T2SSβ, and that YghG is required for assembly of the GspDβ secretin and thus function of the T2SSβ. Mutational and physiological analysis further demonstrated that YghG (redesignated GspSβ) is a novel outer membrane pilotin protein that is integral for assembly of the T2SSβ by localizing GspDβ to the outer membrane, whereupon GspDβ forms the macromolecular secretin multimer through which T2SSβ substrates are translocated.