The Type III Secretion Pathway in Pathogenic Bacteria

Abstract

The findings that Yersinia spp. release Yop proteins into the culture medium even though these proteins do not contain a classical N-terminal signal sequence (Heesemann et al., 1986; Michiels et al., 1990) and that their secretion requires a large set of “accessory” proteins (Michiels et al., 1991), together with the observation that proteins required for the virulence of other bacteria exhibit striking sequence similarities with proteins required for Yop secretion (Andrews and Maurelli, 1992; Gough et al., 1992), led to the characterization of what is now designated the Type III secretion (TTS) pathway (van Gijsegem et al., 1993). This secretion pathway is present in numerous Gram-negative bacteria that are pathogenic for humans, animals, or plants. The pathway is required for pathogenicity of these bacteria. Some characteristic features of this secretion pathway include (1), the nature of the components and the structure of the secretion apparatus; (2), the regulation of the activity of the secretion apparatus by external signals; (3), the storage of the “secreted” proteins in the bacterial cytoplasm prior to their secretion; (4), the regulation of gene transcription by the activity of the secretion apparatus; and (5), the insertion of some proteins into the membrane of eukaryotic target cells and the injection of other proteins from the bacterial cytoplasm into the host cell cytoplasm, which is the real purpose of this secretion pathway. It should be emphasized that not all the aforementioned features have been demonstrated for, and might not even be common to all bacteriak that have been proposed to use the TTS pathway.