Practice question

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-terminus zippering releasing substantial free energy. Formation pulls membranes into point contact inducing hemifusion stalk then fusion pore opening delivering content efficiently. Specificity arises from compatible QabcR combinations respecting ionic central layer; neuronal exocytosis uses syntaxin1-SNAP25-VAMP2, ER-Golgi uses Sed5-membrin-Bet1-Bos1-Sec22, endosome uses syntaxin7-8-Vti1b-VAMP8. They do not mediate budding or ATP hydrolysis or clathrin recruitment; those involve coats and GTPases. After fusion cis complex awaits disassembly by NSF ATPase AAA family for reuse throughout secretory pathway.