The various pilus assembly pathways described in the previous sections all rely on components of the sec machinery for the translocation of their respective pilus subunits across the inner membrane. Two new types of pili that are assembled by a sec-independent pathway known as type III secretion have been identified. The type III secretion system is encoded by numerous gramnegative pathogens and enables these bacteria to secrete and inject pathogenic effector molecules into the cytosol of host eukaryotic cells. About 20 gene products, most of which are inner-membrane proteins, make up the type III secretion system. The components mediating type III secretion are conserved in pathogens as diverse as Yersinia and Erwinia, but the secreted effector proteins vary significantly among species. The type III secretion apparatus, which appears to span the periplasmic space, resembles the basal body of a flagellum connected to a straight rod that extends across the outer membrane. Interestingly, all type III secretion systems encode some components with homologies to proteins involved in flagellar assembly. The secretion of proteins by the type III system is an ATP-dependent process that involves no distinct periplasmic intermediates. Type III-secreted proteins of EPEC and the plant pathogen Pseudomonas syringae have recently been shown to assemble into piluslike structures.
EPEC encodes four proteins, EspA, EspB, EspD, and Tir, that are secreted by a type III pathway. These proteins facilitate intimate contact between the pathogen and host intestinal cells and are required for the formation of specific (attaching and effacing) lesions. Knutton and colleagues (1998) showed that one of these proteins, EspA, can assemble into 7- to 8-nm thick peritrichously expressed pilus-like fibers that are organized into ~50-nm-wide bundles that extend up to 2 ^m from the bacterial surface (Fig. 38.1F). These fibers appear to be made up of only EspA molecules. Interestingly, EspA shares substantial sequence identity with a flagellin from Y. enterolitica. During the infection process, the EspA fibers appear to mediate contact between EPEC and the host-cell surface prior to the establishment of more intimate bacterial attachment. The EspA fibers seem to assist the translocation of EspB effector molecules into host cells, where they can subvert host signal-transduction pathways.
In P. syringae and other plant pathogens, hypersensitive response and pathogenicity (hrp) genes control the ability of these bacteria to cause disease in susceptible plants and to elicit the hypersensitive response in resistant plants. The hypersensitive response is a phenomenon characterized by rapid localized host-cell death at the site of infection that appears to limit the spread of a pathogen in an infected plant. A subset of the hrp genes, recently renamed hrc genes, encode components of a type III secretion system. In 1997, Roine and co-workers showed that one of the proteins, HrpA, secreted by the Hrp type III secretion system is assembled into 6- to 8-nm-wide, peritrichously expressed pili. It was proposed that these pili, known as Hrp pili, are involved in mediating bacteria-plant interactions in the intercellular spaces of the host plant. In addition, Hrp pili may assist the delivery of effector proteins into host-plant cells. The exact nature and functions of the Hrp pili of P. syringae and the EspA-containing pili of EPEC remain to be elucidated.
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