title: ER-associated retrograde SNAREs and the Dsl1 complex mediate an alternative, Sey1p-independent homotypic ER fusion pathway.
abstract: 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.
authors.affiliation: Department of Molecular Biology, Princeton University, Princeton, NJ 08544-1014.
authors.forename: Jason V
authors.initials: JV
authors.lastname: Rogers
authors.affiliation: Department of Molecular Biology, Princeton University, Princeton, NJ 08544-1014.
authors.forename: Conor
authors.initials: C
authors.lastname: McMahon
authors.affiliation: Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, NJ 08544-1014.
authors.forename: Anastasia
authors.initials: A
authors.lastname: Baryshnikova
authors.affiliation: Department of Molecular Biology, Princeton University, Princeton, NJ 08544-1014.
authors.forename: Frederick M
authors.initials: FM
authors.lastname: Hughson
authors.affiliation: Department of Molecular Biology, Princeton University, Princeton, NJ 08544-1014 mdrose@princeton.edu.
authors.forename: Mark D
authors.initials: MD
authors.lastname: Rose
mesh_terms: D020755:Coat Protein Complex I / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism* / Q000648:ultrastructure; D046912:Multiprotein Complexes; D009154:Mutation; D050600:SNARE Proteins / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism*; D033921:Vesicular Transport Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: D020755:Coat Protein Complex I; C428365:DSL1 protein, S cerevisiae; D046912:Multiprotein Complexes; C472231:SEY1 protein, S cerevisiae; D050600:SNARE Proteins; D029701:Saccharomyces cerevisiae Proteins; D033921:Vesicular Transport Proteins