title: Lipid-anchored Synaptobrevin Provides Little or No Support for Exocytosis or Liposome Fusion.
abstract: SNARE proteins catalyze many forms of biological membrane fusion, including Ca(2+)-triggered exocytosis. Although fusion mediated by SNAREs generally involves proteins anchored to each fusing membrane by a transmembrane domain (TMD), the role of TMDs remains unclear, and previous studies diverge on whether SNAREs can drive fusion without a TMD. This issue is important because it relates to the question of the structure and composition of the initial fusion pore, as well as the question of whether SNAREs mediate fusion solely by creating close proximity between two membranes versus a more active role in transmitting force to the membrane to deform and reorganize lipid bilayer structure. To test the role of membrane attachment, we generated four variants of the synaptic v-SNARE synaptobrevin-2 (syb2) anchored to the membrane by lipid instead of protein. These constructs were tested for functional efficacy in three different systems as follows: Ca(2+)-triggered dense core vesicle exocytosis, spontaneous synaptic vesicle exocytosis, and Ca(2+)-synaptotagmin-enhanced SNARE-mediated liposome fusion. Lipid-anchoring motifs harboring one or two lipid acylation sites completely failed to support fusion in any of these assays. Only the lipid-anchoring motif from cysteine string protein-α, which harbors many lipid acylation sites, provided support for fusion but at levels well below that achieved with wild type syb2. Thus, lipid-anchored syb2 provides little or no support for exocytosis, and anchoring syb2 to a membrane by a TMD greatly improves its function. The low activity seen with syb2-cysteine string protein-α may reflect a slower alternative mode of SNARE-mediated membrane fusion.
authors.affiliation: From the Physiology Ph.D. Graduate Training Program.
authors.forename: Che-Wei
authors.initials: CW
authors.lastname: Chang
authors.affiliation: Biophysics Ph.D. Program.
authors.forename: Chung-Wei
authors.initials: CW
authors.lastname: Chiang
authors.affiliation: Howard Hughes Medical Institute, and.
authors.forename: Jon D
authors.initials: JD
authors.lastname: Gaffaney
authors.affiliation: Biophysics Ph.D. Program, Howard Hughes Medical Institute, and Department of Neuroscience, University of Wisconsin, Madison, Wisconsin 53705.
authors.forename: Edwin R
authors.initials: ER
authors.lastname: Chapman
authors.affiliation: From the Physiology Ph.D. Graduate Training Program, Biophysics Ph.D. Program, Department of Neuroscience, University of Wisconsin, Madison, Wisconsin 53705 mbjackso@wisc.edu.
authors.forename: Meyer B
authors.initials: MB
authors.lastname: Jackson
mesh_terms: D020816:Amino Acid Motifs; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002462:Cell Membrane / Q000235:genetics / Q000378:metabolism*; D005089:Exocytosis / Q000502:physiology*; D058635:Lipid-Linked Proteins / Q000235:genetics / Q000378:metabolism*; D008081:Liposomes; D008561:Membrane Fusion / Q000502:physiology*; D051379:Mice; D018345:Mice, Knockout; D050988:Vesicle-Associated Membrane Protein 2 / Q000235:genetics / Q000378:metabolism*
chemical_list: D058635:Lipid-Linked Proteins; D008081:Liposomes; D050988:Vesicle-Associated Membrane Protein 2; C494808:vesicle-associated membrane protein 2, mouse; D002118:Calcium
keywords: exocytosis; lipid vesicle; membrane fusion; membrane trafficking; synapse