Biophysical Society Thematic Meeting| Padova 2019

Quantitative Aspects of Membrane Fusion and Fission

Poster Abstracts

22-POS Board 22 THE MOLECULAR MACHINERY OF MEMBRANE FUSION: SNARES VERSUS SYNAPTOTAGMIN Cameron B Gundersen 1 ; 1 David Geffen UCLA School of Medicine, Molecular & Medical Pharmacology, Los Angeles, California, USA Fast, synchronous exocytosis at mammalian nerve terminals takes place in <0.1 msec (Sabatini & Regehr, 1996; Nature 384:170-172). While it is clear that synaptotagmin-1 (syt-1) is the Ca 2+ sensor for this process (Sudhof, 2014; Angew. Chem. Int. 53:12696-717.), the mechanism by which syts engage other proteins, including the SNAREs, munc-13, munc-18 and complexin, to catalyze membrane fusion remains unclear. This presentation will compare and contrast two models for how syts (without SNAREs) or the SNARE proteins, synaptobrevin-2 (syb-2) and syntaxin 1A/B (syx), might drive the membrane fusion step of fast, synchronous exocytosis. A key feature of these models is that specific domains of syt-1 or syb-2 and syx need to adopt a b- strand conformation. We proposed (Gundersen & Umbach, 2013; J. Theor. Biol. 332:149-60) that the presence of b-strand in the 12 residues (which include the fatty acylated cysteine residues of syt-1) following the membrane-affiliated a-helix would enable a quartet of syts to be positioned at the region of contact between a synaptic vesicle and the plasma membrane. By executing a sequence of steps (see: Gundersen & Umbach, 2013), these syts could serve as templates for Ca 2+ -triggered membrane fusion. Although this syt-based proposal remains hypothetical, recent experiments using a syt-1 peptide support the idea that the fatty acylated domain of syt-1 adopts b-structure in lipid bilayers. Nevertheless, further testing of this model is necessary. At the same time, it also became evident that intra-membrane b-structure could be exploited to turn syb-2 and syx into “membrane-fusion” machines (Gundersen, 2017; Int. J. Mol. Sci. 18: E1582). This model demands that the domains of syb-2 and syx currently represented as membrane-spanning a-helices temporarily adopt intra-membrane b-structure. If these b-strands are suitably arrayed at the vesicle-plasma membrane interface, a b-to-a transition (triggered by SNARE zippering) can initiate membrane fusion (see: Gundersen, 2017).

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