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SNAP-25 is a SNARE protein that contributes two helices to the neuronal fusion complex, partnering with syntaxin and synaptobrevin. Lipid attachment anchors SNAP-25 to membranes despite its lack of a conventional membrane-spanning helix. Assembly helps couple recognition of a secretory carrier to membrane fusion. Its two helical contributions explain why a four-helix SNARE complex need not contain four separate protein chains. ^[ref]

A SNARE protein contributes a helical segment to a membrane-fusion assembly. Complementary SNARE proteins on opposing membranes form a SNARE complex whose assembly helps pull the membranes together. Syntaxins, SNAP-25 and synaptobrevin illustrate different components of the neuronal machinery. Purified components reconstituted into separate liposomes can promote fusion, showing that these proteins participate directly in membrane rearrangement. Cellular control additionally involves targeting and regulatory factors. ^[ref]

Synaptobrevin is a membrane-anchored SNARE protein associated with secretory vesicles, including synaptic vesicles. Its cytosolic helix joins complementary helices from syntaxin and SNAP-25 to form a SNARE complex. When the partners initially occupy opposing lipid bilayers, their assembly helps bring those lipid bilayers close enough for membrane fusion. The protein helps couple the membranes mechanically; it is not a pore through which the neurotransmitter simply leaks. ^[ref]

Reconstituting synaptobrevin and its complementary SNARE proteins into separate liposomes made fusion measurable outside a cell. The ability to accumulate docked intermediates at low temperature and trigger fusion on warming also distinguished membrane docking from the subsequent merger. Living synapses add regulation that this minimal assay does not reproduce. ^[ref]

Syntaxin is a family of membrane-anchored SNARE proteins that contributes a helix to a membrane-fusion complex. In neuronal secretion, syntaxin and SNAP-25 on the target cell membrane assemble with synaptobrevin on a synaptic vesicle. Syntaxin contributes one helix, synaptobrevin one and SNAP-25 two, producing a four-helix SNARE complex from three protein chains. Assembly across opposing lipid bilayers couples molecular binding to membrane fusion. ^[ref]

The membrane context matters. Reconstituted complementary SNARE proteins in different liposomes can promote fusion, while a complex assembled after fusion has its membrane anchors on the same lipid bilayer. Detecting a syntaxin-containing complex therefore does not by itself show that it is still bridging a vesicle to its target. ^[ref]

A t-SNARE is a SNARE protein or complementary assembly assigned to the target membrane in a particular membrane-transport event. It pairs with a v-SNARE on the incoming carrier to form a fusion-associated SNARE complex. The prefix describes the experimental transport arrangement; it does not imply that a given membrane is always a target in every trafficking pathway. The neuronal target assembly includes syntaxin and SNAP-25. ^[ref]