Role of EscF, a putative needle complex protein, in the type III protein translocation system of enteropathogenic Escherichia coli

Type III secretion systems, designed to deliver effector proteins across th e bacterial cell envelope and the plasma membrane of the target eukaryotic cell, are involved in subversion of eukaryotic cell functions in a variety of human, animal and plant pathogens. In enteropathogenic Escherichia coli (EPEC), several protein substrates for the secretion apparatus were identif ied, including EspA, EspB and EspD. EspA is a structural protein and the ma jor component of a large transiently expressed filamentous surface organell e that forms a direct link between the bacterium and the host cell, whereas EspD and EspB seem to form the mature translocation pore. Recent studies o f the type III secretion systems of Shigella and Salmonella pathogenicity i sland (SPI)-1 revealed the existence of a macromolecular complex that spans both bacterial membranes and consists of a basal structure with two upper and two lower rings and a needle-like projection that extends outwards from the bacterial surface. MxiH (Shigella) and Prgl (Salmonella) are the main components of the needle of the type III secretion complex. A needle-like c omplex has not yet been reported in EPEC. In this study, we investigated Es cF, a protein sharing sequence similarity with MxiH and Prgl. We report tha t EscF is required for type III protein secretion and EspA filament assembl y. Moreover, we show that EscF binds EspA, suggesting that EspA filaments a re an extension of the type III secretion needle complexes in EPEC.