Abstract
The type III secretion system (T3SS) is a protein delivery system which is involved in a wide spectrum of interactions, from mutualism to pathogenesis, between Gram negative bacteria and various eukaryotes, including plants, fungi, protozoa and mammals. Various phylogenetic families of the T3SS have been described, including the Salmonella Pathogenicity Island 1 family (SPI-1). The SPI-1 T3SS was initially associated with the virulence of enteric pathogens, but is actually found in a diverse array of bacterial species, where it can play roles in processes as different as symbiotic interactions with insects and colonization of plants. We review the multiple roles of the SPI-1 T3SS and discuss both how these discoveries are changing our perception of the SPI-1 family and what impacts this has on our understanding of the specialization of the T3SS in general.
Highlights
Non-flagellar Type III secretion systems (NF-T3SSs) are macromolecular complexes, apparently derived from exaptation of the flagella for the delivery of bacterial effectors into eukaryotic cells (Abby and Rocha, 2012)
Multiple phylogenetic analyses based on proteins involved in the assembly of the transmembrane export apparatus have split the NF-T3SSs into seven distinct families: Salmonella Pathogenicity Island 1 family (SPI-1), SPI-2, Hrp1, Hrp2, Ysc, Rhizobiales, and Chlamydiales (Pallen et al, 2005; Troisfontaines and Cornelis, 2005; Barret et al, 2013b)
From initial characterization and genomic distribution, the Ysc, Chlamydiales, SPI-1, and SPI-2 families were associated with animal–bacterial interactions while the Rhizobiales, Hrp1, and Hrp2 families were associated with plant–bacterial interactions
Summary
Non-flagellar Type III secretion systems (NF-T3SSs) are macromolecular complexes, apparently derived from exaptation of the flagella for the delivery of bacterial effectors into eukaryotic cells (Abby and Rocha, 2012). As well as host-range, phylogenetic groups differed in their extracellular appendages, with plant-associated families having long flexible pili, while animal pathogens have short rigid needles, which in some cases (e.g., SPI-2) can be appended by a filamentous sheath (Chakravortty et al, 2005) The core components are highly conserved between each family, which probably contributes to the phenomenon of promiscuous secretion; that is, there are multiple reports of effector secretion via non-cognate T3SS families, including effectors which are normally used during the infection of animals being heterologously expressed and secreted via phytopathogenic T3SSs and vice versa (Anderson et al, 1999; Subtil et al, 2001).
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