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A trans-SNARE complex links SNARE proteins anchored in different lipid bilayers. Its formation can bring membranes together before complete membrane fusion.

A vesicle is a small compartment enclosed by a lipid bilayer. Its vesicle lumen can carry soluble cargo, while its lipid bilayer carries lipids and membrane proteins. In a transport pathway, budding separates a carrier from one compartment and membrane fusion joins it to another. Orientation matters: fusion of a secretory vesicle with the cell membrane connects its vesicle lumen to the extracellular space while incorporating its membrane proteins into the surface. ^[ref]

A vesicle is therefore more than a freely diffusing package. Recognition, movement and membrane fusion require distinct interactions. In a reconstitution experiment, complementary SNARE proteins placed in separate liposomes brought opposing lipid bilayers together and promoted fusion; the experiment isolated one part of transport from the rest of a living cell. ^[ref]

Vesicle docking brings a transport vesicle into close, stable association with its target membrane before membrane fusion. Recognition factors and SNARE proteins help organize the contact. Docking and fusion are experimentally distinct: a carrier can remain attached without exchanging its internal contents with the target compartment. Reconstituted liposomes bearing appropriate SNARE proteins allowed such docked intermediates to be distinguished from subsequent fusion. ^[ref]

Vesicle trafficking moves membrane and cargo between cellular compartments through transport carriers. Cargo selection and budding are followed by targeting, vesicle docking and membrane fusion. A carrier from the endoplasmic reticulum can deliver newly synthesized proteins to the Golgi apparatus, while other carriers retrieve resident components. Directional transport maintains distinct compartments despite continual membrane exchange; it depends on selective recognition rather than random mixing. ^[ref]

A v-SNARE is a SNARE protein assigned to a transport vesicle in a specified fusion event. Complementary interaction with a t-SNARE on the target produces a membrane-bridging SNARE complex. Synaptobrevin is a familiar example in neuronal secretion. The label describes which membrane initially carries the component; after membrane fusion, both sets of components occupy the same membrane and must be reorganized for reuse. ^[ref]