ER-associated retrograde SNAREs and the Dsl1 complex mediate an alternative, Sey1p-independent homotypic ER fusion pathway.
Bottom Line: However, an alternative explanation--that the observed phenotypes arose from perturbed vesicle trafficking--could not be ruled out.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.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.
Affiliation: Department of Molecular Biology, Princeton University, Princeton, NJ 08544-1014.Show MeSH
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Mentions: We next compared growth rates related only to sey1Δ, rtn1Δ, yop1Δ, and dsl1ΔE genotypic combinations (Figure 6A). As expected, all single mutants grew about as well as wild type (compare strain 1 with strains 2–5 in Figure 6A). Among double mutants (strains 6–11), sey1Δ dsl1ΔE exhibited a strong growth defect, as expected from our initial work, and yop1Δ rtn1Δ exhibited a minor growth defect, as previously reported. Surprisingly, rtn1Δ dsl1ΔE exhibited an intermediate growth defect, implying that Rtn1p and Dsl1p share functional redundancy. All triple mutants (strains 12–15) showed intermediate to severe growth defects (see Figure 6B for colony sizes after 6 d of growth). The quadruple-mutant sey1Δ yop1Δ rtn1Δ dsl1ΔE (strain 16) was almost completely inviable (four out of eight spores did not form colonies after 6 d of growth, and the remaining spores formed extremely tiny colonies; Figure 6B), suggesting that there is an unexpected partial redundancy between the function of the reticulons and the two ER–ER fusion pathways, which together are essential for cell viability.
Affiliation: Department of Molecular Biology, Princeton University, Princeton, NJ 08544-1014.