The injectisome is the structural core of the membrane-embedded type III secretion system (T3SS), and bacterial effector proteins are secreted through this complex into the host. Radics et al. used cryo-electron tomography to visualize individual Salmonella enterica serovar Typhimurium T3SSs loaded with GFP-fused substrates that were transported into the injectisome but that subsequently became trapped and blocked the secretion of other substrates. Structural studies of substrate-trapped complexes showed that transport occurs in a polarized manner, with the amino-terminal domain of the substrate entering the complex first. Characterization of the secretion path within the injectisome suggested that T3SS effectors are transported in an unfolded state.
References
Radics, J., Königsmaier, L. & Marlovits, T. C. Structure of a pathogenic type 3 secretion system in action. Nature Struct. Mol. Biol. http://dx.doi.org/10.1038/nsmb.2722 (2013)
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Du Toit, A. Caught in the act. Nat Rev Microbiol 12, 75 (2014). https://doi.org/10.1038/nrmicro3212
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DOI: https://doi.org/10.1038/nrmicro3212