Direct observation of molecular arrays in the organized smooth endoplasmic reticulum.
BACKGROUND
Tubules and sheets of endoplasmic reticulum perform different functions and undergo inter-conversion during different stages of the cell cycle. Tubules are stabilized by curvature inducing resident proteins, but little is known about the mechanisms of endoplasmic reticulum sheet stabilization. Tethering of endoplasmic reticulum membranes to the cytoskeleton or to each other has been proposed as a plausible way of sheet stabilization.


RESULTS
Here, using fluorescence microscopy we show that the previously proposed mechanisms, such as membrane tethering via GFP-dimerization or coiled coil protein aggregation do not explain the formation of the calnexin-induced organized smooth endoplasmic reticulum membrane stacks. We also show that the LINC complex proteins known to serve a tethering function in the nuclear envelope are excluded from endoplasmic reticulum stacks. Finally, using cryo-electron microscopy of vitreous sections methodology that preserves cellular architecture in a hydrated, native-like state, we show that the sheet stacks are highly regular and may contain ordered arrays of macromolecular complexes. Some of these complexes decorate the cytosolic surface of the membranes, whereas others appear to span the width of the cytosolic or luminal space between the stacked sheets.


CONCLUSIONS
Our results provide evidence in favour of the hypothesis of endoplasmic reticulum sheet stabilization by intermembrane tethering.
Korkhov
Vladimir M
VM
MRC Laboratory of Molecular Biology, Cambridge, UK. vkorkhov@mol.biol.ethz.ch
Zuber
Benoît
B
D002460:Cell Line; D020285:Cryoelectron Microscopy; D018871:Endoplasmic Reticulum, Smooth / Q000378:metabolism / Q000648:ultrastructure*; D017930:Genes, Reporter; D006801:Humans; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D055503:Protein Multimerization
C081876:CKAP4 protein, human; D008565:Membrane Proteins
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