ER-associated retrograde SNAREs and the Dsl1 complex mediate an alternative, Sey1p-independent homotypic ER fusion pathway.

Jason V Rogers, Conor McMahon, Anastasia Baryshnikova, Frederick M Hughson, Mark D Rose

Molecular biology of the cell 2014 Nov 01

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The peripheral endoplasmic reticulum (ER) network is dynamically maintained by homotypic (ER-ER) fusion. In Saccharomyces cerevisiae, the dynamin-like GTPase Sey1p can mediate ER-ER fusion, but sey1Δ cells have no growth defect and only slightly perturbed ER structure. Recent work suggested that ER-localized soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) mediate a Sey1p-independent ER-ER fusion pathway. However, an alternative explanation--that the observed phenotypes arose from perturbed vesicle trafficking--could not be ruled out. In this study, we used candidate and synthetic genetic array (SGA) approaches to more fully characterize SNARE-mediated ER-ER fusion. We found that Dsl1 complex mutations in sey1Δ cells cause strong synthetic growth and ER structure defects and delayed ER-ER fusion in vivo, additionally implicating the Dsl1 complex in SNARE-mediated ER-ER fusion. In contrast, cytosolic coat protein I (COPI) vesicle coat mutations in sey1Δ cells caused no synthetic defects, excluding perturbed retrograde trafficking as a cause for the previously observed synthetic defects. Finally, deleting the reticulons that help maintain ER architecture in cells disrupted for both ER-ER fusion pathways caused almost complete inviability. We conclude that the ER SNAREs and the Dsl1 complex directly mediate Sey1p-independent ER-ER fusion and that, in the absence of both pathways, cell viability depends upon membrane curvature-promoting reticulons. © 2014 Rogers, McMahon, et al. This article is distributed by The American Society for Cell Biology under license from the author(s). Two months after publication it is available to the public under an Attribution–Noncommercial–Share Alike 3.0 Unported Creative Commons License (http://creativecommons.org/licenses/by-nc-sa/3.0).

Citation

Jason V Rogers, Conor McMahon, Anastasia Baryshnikova, Frederick M Hughson, Mark D Rose. ER-associated retrograde SNAREs and the Dsl1 complex mediate an alternative, Sey1p-independent homotypic ER fusion pathway. Molecular biology of the cell. 2014 Nov 01;25(21):3401-12