Journal: Proc Natl Acad Sci U S A / Year: 2017 Title: Near-atomic resolution cryoelectron microscopy structure of the 30-fold homooligomeric SpoIIIAG channel essential to spore formation in . Authors: Natalie Zeytuni / Chuan Hong / Kelly A Flanagan / Liam J Worrall / Kate A Theiltges / Marija Vuckovic / Rick K Huang / Shawn C Massoni / Amy H Camp / Zhiheng Yu / Natalie C Strynadka / Abstract: Bacterial sporulation allows starving cells to differentiate into metabolically dormant spores that can survive extreme conditions. Following asymmetric division, the mother cell engulfs the ...Bacterial sporulation allows starving cells to differentiate into metabolically dormant spores that can survive extreme conditions. Following asymmetric division, the mother cell engulfs the forespore, surrounding it with two bilayer membranes. During the engulfment process, an essential channel, the so-called feeding tube apparatus, is thought to cross both membranes to create a direct conduit between the mother cell and the forespore. At least nine proteins are required to create this channel, including SpoIIQ and SpoIIIAA-AH. Here, we present the near-atomic resolution structure of one of these proteins, SpoIIIAG, determined by single-particle cryo-EM. A 3D reconstruction revealed that SpoIIIAG assembles into a large and stable 30-fold symmetric complex with a unique mushroom-like architecture. The complex is collectively composed of three distinctive circular structures: a 60-stranded vertical β-barrel that forms a large inner channel encircled by two concentric rings, one β-mediated and the other formed by repeats of a ring-building motif (RBM) common to the architecture of various dual membrane secretion systems of distinct function. Our near-atomic resolution structure clearly shows that SpoIIIAG exhibits a unique and dramatic adaptation of the RBM fold with a unique β-triangle insertion that assembles into the prominent channel, the dimensions of which suggest the potential passage of large macromolecules between the mother cell and forespore during the feeding process. Indeed, mutation of residues located at key interfaces between monomers of this RBM resulted in severe defects both in vivo and in vitro, providing additional support for this unprecedented structure.
Cryogen: NITROGEN / Specimen holder model: FEI TITAN KRIOS AUTOGRID HOLDER / Temperature (max): 80 K / Temperature (min): 80 K
Image recording
Average exposure time: 0.2 sec. / Electron dose: 1.1 e/Å2 / Detector mode: SUPER-RESOLUTION / Film or detector model: GATAN K2 SUMMIT (4k x 4k) / Num. of grids imaged: 1 / Num. of real images: 2876
EM imaging optics
Energyfilter name: Gatan GIF / Energyfilter upper: 20 eV / Energyfilter lower: 0 eV
Image scans
Sampling size: 5 µm / Width: 7676 / Height: 7420 / Movie frames/image: 50 / Used frames/image: 1-50
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Processing
EM software
ID
Name
Version
Category
1
EMAN2
2
particleselection
2
RELION
1.4
particleselection
3
SerialEM
imageacquisition
5
CTFFIND4
4
CTFcorrection
11
RELION
1.4
initialEulerassignment
12
RELION
1.4
finalEulerassignment
13
RELION
1.4
classification
14
RELION
1.4
3Dreconstruction
CTF correction
Type: PHASE FLIPPING AND AMPLITUDE CORRECTION
Particle selection
Num. of particles selected: 770000
Symmetry
Point symmetry: C30 (30 fold cyclic)
3D reconstruction
Resolution: 3.5 Å / Resolution method: FSC 0.143 CUT-OFF / Num. of particles: 55000 / Algorithm: FOURIER SPACE / Symmetry type: POINT
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