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Proc Natl Acad Sci U S A. 2016 Oct 11;113(41):11585-11590. Epub 2016 Sep 28.

A ring-shaped conduit connects the mother cell and forespore during sporulation in Bacillus subtilis.

Author information

1
Department of Microbiology and Immunobiology, Harvard Medical School, Boston MA 02115.
2
Institut de Biologie Structurale (IBS), Université Grenoble Alpes-Commissariat à l'Energie Atomique et aux Energies Alternatives-CNRS, 38044 Grenoble, France.
3
Institute for Physical Chemistry II, Ruhr-Universität Bochum, 44780 Bochum, Germany.
4
Department of Microbiology and Immunobiology, Harvard Medical School, Boston MA 02115; cecile.morlot@ibs.fr rudner@hms.harvard.edu.
5
Institut de Biologie Structurale (IBS), Université Grenoble Alpes-Commissariat à l'Energie Atomique et aux Energies Alternatives-CNRS, 38044 Grenoble, France; cecile.morlot@ibs.fr rudner@hms.harvard.edu.

Abstract

During spore formation in Bacillus subtilis a transenvelope complex is assembled across the double membrane that separates the mother cell and forespore. This complex (called the "A-Q complex") is required to maintain forespore development and is composed of proteins with remote homology to components of type II, III, and IV secretion systems found in Gram-negative bacteria. Here, we show that one of these proteins, SpoIIIAG, which has remote homology to ring-forming proteins found in type III secretion systems, assembles into an oligomeric ring in the periplasmic-like space between the two membranes. Three-dimensional reconstruction of images generated by cryo-electron microscopy indicates that the SpoIIIAG ring has a cup-and-saucer architecture with a 6-nm central pore. Structural modeling of SpoIIIAG generated a 24-member ring with dimensions similar to those of the EM-derived saucer. Point mutations in the predicted oligomeric interface disrupted ring formation in vitro and impaired forespore gene expression and efficient spore formation in vivo. Taken together, our data provide strong support for the model in which the A-Q transenvelope complex contains a conduit that connects the mother cell and forespore. We propose that a set of stacked rings spans the intermembrane space, as has been found for type III secretion systems.

KEYWORDS:

EscJ/PrgK/FliF; SigG; SpoIIIAG; sporulation; type III secretion system

PMID:
27681621
PMCID:
PMC5068255
DOI:
10.1073/pnas.1609604113
[Indexed for MEDLINE]
Free PMC Article

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