title: How and why intralumenal membrane fragments form during vacuolar lysosome fusion.
abstract: Lysosomal membrane fusion mediates the last step of the autophagy and endocytosis pathways and supports organelle remodeling and biogenesis. Because fusogenic proteins and lipids concentrate in a ring at the vertex between apposing organelle membranes, the encircled area of membrane can be severed and internalized within the lumen as a fragment upon lipid bilayer fusion. How or why this intralumenal fragment forms during fusion, however, is not entirely clear. To better understand this process, we studied fragment formation during homotypic vacuolar lysosome membrane fusion in Saccharomyces cerevisiae Using cell-free fusion assays and light microscopy, we find that GTPase activation and trans-SNARE complex zippering have opposing effects on fragment formation and verify that this affects the morphology of the fusion product and regulates transporter protein degradation. We show that fragment formwation is limited by stalk expansion, a key intermediate of the lipid bilayer fusion reaction. Using electron microscopy, we present images of hemifusion diaphragms that form as stalks expand and propose a model describing how the fusion machinery regulates fragment formation during lysosome fusion to control morphology and protein lifetimes.
authors.affiliation: Department of Biology, Concordia University, Montréal, QC H4B 1R6, Canada.
authors.forename: Sevan
authors.initials: S
authors.lastname: Mattie
authors.affiliation: Department of Biology, Concordia University, Montréal, QC H4B 1R6, Canada.
authors.forename: Erin K
authors.initials: EK
authors.lastname: McNally
authors.affiliation: Department of Biology, Concordia University, Montréal, QC H4B 1R6, Canada.
authors.forename: Mahmoud A
authors.initials: MA
authors.lastname: Karim
authors.affiliation: Department of Anatomy and Cell Biology, McGill University, Montréal, QC H3A 0C7, Canada.
authors.forename: Hojatollah
authors.initials: H
authors.lastname: Vali
authors.affiliation: Department of Biology, Concordia University, Montréal, QC H4B 1R6, Canada christopher.brett@concordia.ca.|Department of Anatomy and Cell Biology, McGill University, Montréal, QC H3A 0C7, Canada.
authors.forename: Christopher L
authors.initials: CL
authors.lastname: Brett
mesh_terms: D002352:Carrier Proteins / Q000378:metabolism; D020549:GTP Phosphohydrolase Activators / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism; D008051:Lipid Bilayers / Q000378:metabolism; D051907:Lysosomal Membrane Proteins / Q000378:metabolism; D008247:Lysosomes / Q000378:metabolism; D008561:Membrane Fusion / Q000502:physiology*; D008566:Membranes / Q000378:metabolism; D050600:SNARE Proteins / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism; D014617:Vacuoles / Q000378:metabolism
chemical_list: D002352:Carrier Proteins; D020549:GTP Phosphohydrolase Activators; D008051:Lipid Bilayers; D051907:Lysosomal Membrane Proteins; D050600:SNARE Proteins; D029701:Saccharomyces cerevisiae Proteins