ESPB AND ESPD REQUIRE A SPECIFIC CHAPERONE FOR PROPER SECRETION FROM ENTEROPATHOGENIC ESCHERICHIA-COLI

Enteropathogenic Escherichia coli uses a type III secretion apparatus to deliver proteins essential for pathogenesis to the host epithelium. Several proteins have been detected in culture supernatants of the pr ototype EPEC strain E2348/69 and three of these, EspA, EspB, and EspD, use type III machinery for export. Here, we report the identification and characterization of CesD, a protein required for proper EspB and EspD secretion. CesD shows sequence homology to chaperone proteins fro m other type III secretion pathways. Based on this, we hypothesize tha t CesD may function as a secretion chaperone in EPEC. A mutation in ce sD abolished EspD secretion into culture supernatants and reduced the amount of secreted EspB, but had little effect on the amount of secret ed EspA. The mutant strain was negative for both FAS and Tir phosphory lation, consistent with the previously described roles for EspB and Es pD in EPEC pathogenesis. CesD was shown to interact with EspD but not EspB or EspA. CesD was detected in the bacterial cytosol, and, surpris ingly, a substantial amount of the protein was also found to be associ ated with the inner membrane. Thus, although CesD has some attributes that are similar to other type III secretion chaperones, its membrane localization separates it from previously described members of this fa mily.