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#membrane fusion

9 public questions tagged with this topic.

Fertilization in mammals-l

Site of sperm-egg membrane fusion called:

Sperm-egg plasma membrane fusion does not occur over entire sperm head but restricted to specialized equatorial segment posterior to acrosome where inner acrosomal membrane and plasma membrane merge post acrosome reaction. Region maintains fusogenic lipids, Izumo1 protein concentration and relative stability after acrosomal exocytosis, allowing initial attachment then merger with microvillar egg membrane containing Juno and CD9 domains. Fusion creates continuity allowing sperm nucleus entry while flagellum often left outside. Equatorial specificity limits fusion competent domain to single site

Ref: Evans, Front Cell Dev Biol 2020: Equatorial segment as specialized site of sperm-egg membrane fusion.

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Properties of Plasma Membrane

Which of the following phospholipids contributes to membrane fusion?

Stalk hypothesis posits fusion proceeds through negatively curved hourglass intermediate where proximal monolayers merge before distal creating hemifusion diaphragm. Conical lipids like phosphatidylethanolamine with small ethanolamine headgroup relatively large chain volume spontaneously adopt inverse hexagonal HII phase and stabilize stalk reducing bending energy dehydration cost from twenty to ten kBT. Sites exocytosis enrich PE via locally activated scramblases and PSD decarboxylase converting PS to PE within mitochondria and plasma membrane contact sites. Reconstituted vesicles containing

Ref: Chernomordik & Kozlov, Nature Struct Mol Biol 2008, PE and stalk fusion mechanism.

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Protein Sorting and Trafficking

What is the role of NSF in membrane fusion?

Secretory pathway maintains SNARE availability through constant recycling machinery whose energy source ATP hydrolysis by NSF. After lipid merger SNARE proteins locked in four-helix bundle cis-complex embedded single target membrane deep energy well impossible separate spontaneously. To regenerate fusion-competent monomers ATP-driven chaperone required. Hexameric N-ethylmaleimide sensitive factor NSF 76 kDa per protomer double ring structure associates via adaptor alpha-SNAP decorating outside SNARE rod tetramer coating. Each SNAP C-terminus contacts one N-domain NSF hexamer forming 20S partic

Ref: Alberts et al., MBC: NSF hydrolyzes ATP to disassemble cis-SNARE complexes.

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Protein Sorting and Trafficking

Which protein facilitates vesicle uncoating after transport?

Temporal control coat association ensures vesicle budding but permits subsequent fusion because SNAREs hidden under coat cannot engage target. After detachment coat must be shed within seconds exposing v-SNAREs for tethering. Distinct uncoating mechanisms evolved: COPII shedding triggered by Sar1 GTP hydrolysis stimulated by Sec23 GAP plus Sec13-31 destabilization; COPI shedding by ARF GAP1/2 ASAP stimulating ARF1 hydrolysis plus curvature mismatch. Clathrin uncoating for AP2 and AP1/GGA vesicles requires chaperone system unique: DnaJ co-chaperone auxilin 1/2 specifically recognizes assembled

Ref: Sousa & Lafer, Traffic 2015: Hsc70 and auxilin mediate clathrin uncoating.

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Protein Sorting and Trafficking

What is the function of α-SNAP?

Alpha-SNAP soluble attachment protein family 35 kDa containing N-terminal helical bundle and C-terminal tetratricopeptide repeats serving adaptor for NSF ATPase. After fusion four-helix cis-SNARE complex remains tightly intertwined embedded in acceptor membrane extremely stable. Alpha-SNAP tetramer binds along outer surface grooves via charged interactions each molecule providing C-terminal leucine repeat interface for one N-domain of hexameric NSF oligomer. Once six N-domains engage four SNAPs 20S supercomplex forms stabilized by ATP bound non-hydrolyzable state where SNAP orientation present

Ref: Alberts et al., MBC Chapter 13: alpha-SNAP and NSF recycle SNAREs after fusion.

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Protein Sorting and Trafficking

Which of the following regulates vesicle docking before fusion?

Before trans-SNARE zippering drives bilayer merger vesicles must be captured near correct target ensuring compartment identity maintained despite promiscuous SNAREs. Rab GTPases provide master specificity for docking stage. Active GTP-bound form on vesicle or target exposes switch regions binding effectors. Examples: Rab1-GTP recruits p115 and GM130 at ER-to-Golgi interface forming extended coiled-coil tether over 200 nm, Rab5-GTP recruits EEA1 and Rabenosyn-5 containing FYVE domains binding PI3P plus alpha helical tether, Rab6 recruits GCC185, Rab7 recruits HOPS complex VPS11-18-39-41. This p

Ref: Alberts et al., MBC: Rab GTPases regulate vesicle docking before SNARE-mediated fusion.

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Protein Sorting and Trafficking

What triggers the disassembly of SNARE complexes after membrane fusion?

Cis-SNARE complexes formed post-fusion are extremely stable four-helix bundles with melting temperature above 90 Celsius requiring energy for recycling. Disassembly machinery uses ATP. Hexameric AAA+ ATPase NSF N-ethylmaleimide sensitive factor assembles as ring with two stacked ATPase tiers D1 catalytic and D2 structural. Adaptor alpha-SNAP soluble attachment protein binds grooves along rod-like bundle via tetratricopeptide repeats and recruits NSF via N domains interaction with SNAP C-terminus. ATP hydrolysis in D1 causes power stroke threading SNARE polypeptide through central pore with tyr

Ref: Lodish et al., MCB Chapter 14: NSF and alpha-SNAP disassemble cis-SNARE complexes via ATP hydrolysis.

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Protein Sorting and Trafficking

What is the function of the SNARE complex?

Vesicle fusion fidelity beyond tethering relies on SNARE proteins providing thermodynamic driver for bilayer merger overcoming hydration repulsion. Vesicle membranes contain R-SNAREs like synaptobrevin VAMP2 with single motif and C-terminal anchor, while target membranes harbor Q-SNAREs syntaxin family with Habc regulatory domain and SNAP-25 family contributing two motifs anchored by palmitoylation. When opposed membranes brought within few nanometers by Rab tethers and multisubunit complexes, oppositely oriented SNARE motifs assemble as parallel four-helix coiled coil from N-terminus to C-ter

Ref: Jahn & Scheller, Nat Rev Mol Cell Biol 2006: SNARE four-helix bundle drives membrane fusion.

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Protein Sorting and Trafficking

What is the function of the SNARE complex in vesicular transport?

Final step of vesicular traffic requires merging of two lipid bilayers normally separated by hydration repulsion and electrostatic forces. SNARE proteins provide mechanical work through formation of extremely stable coiled-coil bundle. Vesicle-anchored R-SNAREs like VAMP2/synaptobrevin carry single SNARE motif with arginine at ionic layer, while target membrane Q-SNAREs syntaxin-1 and SNAP-25 contribute three motifs with glutamine at ionic layer; together they zipper progressively from membrane-distal N termini toward membrane-proximal C termini into parallel four-helix bundle releasing energy

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 13: SNARE Complex Mediating Membrane Fusion.

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