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#NSF

3 public questions tagged with this topic.

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 particle size sucrose gradient observed historically. ATP binding preorganizes NSF; cooperative hydrolysis D1 tier produces piston-like movement threading SNARE polypeptides through central tyrosine-containing pore loops unfolding superhelix releasing individual syntaxin SNAP-25 synaptobrevin. Freed syntaxin clamped by SM proteins, VAMP packaged into recycling vesicles. NSF does not function cargo receptor recognizing sorting signal, nor tether vesicles, nor phosphorylate Rab GTPases; specialized function SNARE chaperone powered by ATP. Depletion via N-ethylmaleimide or dominant negative E329Q accumulates cis complexes blocks secretion within minutes demonstrating essential housekeeping role supporting iterative fusion cycles.

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 clathrin lattice at vertices via clathrin binding motif and J domain recruiting Hsc70 ATPase 70 kDa heat shock cognate via ATPase domain. Hsc70 ATP bound initial interaction converts after auxilin-stimulated ATP hydrolysis to ADP tight binding prying apart heavy and light chain interactions dissociating triskelia into soluble pool within seconds post-fission. Sar1 initiates COPII budding not uncoating, Sec61 ribosome-associated ER translocon for protein import not vesicle coat factor, clathrin cannot auto-disassemble. Hsc70 therefore dedicated ATP-dependent uncoating enzyme recycling coats for successive rounds maintaining flux around 100 vesicles per minute per cell at steady state.

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

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Protein Sorting and Trafficking

Which factor is required for dissociation of the SNARE complex?

After fusion membrane-embedded SNARE complex ends up as cis-complex where all helices reside in same membrane representing dead-end product incapable of further fusion blocking availability. To maintain flux complex must be disassembled into monomers. Alpha-SNAP adaptor family including alpha, beta, gamma isoforms binds along four-helix bundle recognizing charge pattern; up to four molecules coat complex serving landing pad for hexameric NSF ATPase. NSF contains N-terminal substrate binding domains, D1 ATPase providing mechanical power and D2 stabilizing hexamer. ATP hydrolysis in D1 hydrolyzing up to six ATP per disassembly induces large threading movements as central pore tyrosine grips SNARE pulling through channel analogous to AAA unfoldase. Released SNAREs sort: v-SNARE synaptobrevin packaged into retrograde vesicles for return to donor, t-SNAREs syntaxin and SNAP-25 remain for next round. Dynamin mediates scission using GTP via collar, clathrin heavy chain and AP2 mu mediate sorting, but only NSF ATPase supplies disassembly energy. Alkylation by N-ethylmaleimide inactivates NSF causing rapid accumulation cis-SNARE particles and secretory block within minutes indicating essential recycling role.

Ref: Alberts et al., MBC: NSF ATPase disassembles SNARE complexes post-fusion for recycling.

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