Assembly of multi-component supramolecular machines is fundamental to biology, yet in most cases, assembly pathways and their control are poorly understood. An example is the type III secretion machine, which mediates the transfer of bacterial virulence proteins into host cells1. A central component of this nanomachine is the needle complex or injectisome, an organelle associated with the bacterial envelope that is composed of a multi-ring base, an inner rod, and a protruding needle2. Assembly of this organelle proceeds in sequential steps that require the reprogramming of the secretion machine. Here we provide evidence that, in Salmonella typhimurium, completion of the assembly of the inner rod determines the size of the needle substructure. Assembly of the inner rod, which is regulated by the InvJ protein, triggers conformational changes on the cytoplasmic side of the injectisome, reprogramming the secretion apparatus to stop secretion of the needle protein.
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We thank members of the Galán laboratory for critical reviews of this manuscript. We are also grateful to the Yale School of Medicine for the support of the Cryo Electron Microscopy Core Facility. Molecular graphic images were produced using the Chimera package from the Computer Graphics Laboratory, University of California, San Francisco (supported by the NIH). This work was supported by grants from the National Institutes of Health (to V.M.U. and J.E.G.).
The three-dimensional density maps have been deposited into the EBI-MSD EMD database under the accession code EMD-1214. Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
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Marlovits, T., Kubori, T., Lara-Tejero, M. et al. Assembly of the inner rod determines needle length in the type III secretion injectisome. Nature 441, 637–640 (2006). https://doi.org/10.1038/nature04822
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