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#vesicle trafficking

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

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 presents SNAREs to central pore. Upon ATP binding then hydrolysis in D1 ring pore loops containing conserved aromatic residues exert pulling force threading SNARE polypeptide through central channel unfolding coiled coil into monomers recycling syntaxin retained and VAMP for retrograde trafficking. Alpha-SNAP couples NSF ATPase to SNARE disassembly. Mitochondrial import uses separate TIM/TOM PAM motors, Rab docking uses effector tethers EEA1, degradation not normal fate. In vitro purified alpha-SNAP plus NSF plus ATP sufficient to disassemble neuronal SNAREs demonstrating dedicated role as SNARE recycling chaperone after membrane fusion events.

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

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Protein Sorting and Trafficking

What is the main function of ARF-GTP in vesicle formation?

Budding of AP1, AP3 and GGA clathrin-coated vesicles at trans-Golgi network and endosomes requires activated ARF1 small GTPase cycling between GDP and GTP states. ARF1-GDP cytosolic bound to GDI-like factor; activation by large ARF GEFs BIG1-BIG2 at TGN and GBF1 at Golgi exchange GDP for GTP upon membrane recruitment mediated by HDS domains sensing curvature and lipid composition. ARF-GTP undergoes structural rearrangement exposing N-terminal myristoylated amphipathic helix inserting into outer leaflet and effector binding interface. Effectors include heterotetrameric AP1 via gamma subunit trunk, AP3 via delta subunit, AP4, and monomeric GGAs via GAT domain, plus activation of PI4KIIIbeta generating PI4P enhancing adaptor affinity and phospholipase D producing phosphatidic acid promoting negative curvature. Coordinated recruitment concentrates lysosomal cargo such as mannose-6-phosphate receptors bearing acidic cluster dileucine motifs. ARF does not hydrolyze ATP; ATP independent. Scission involves dynamin-family or Arf GAP-induced curvature together with BAR proteins, not SNARE scaffolding. Hydrolysis by ARF GAP1 later triggers partial uncoating for fusion competence and recycling.

Ref: Lodish et al., MCB Chapter 14: ARF1-GTP recruits AP1/GGA to initiate clathrin coat at TGN.

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