Fig. 1. Steps in the delivery of vesicles to the correct organelle. (1) An intracellular transport vesicle approaches its destination organelle either by diffusion or motor-mediated directed transport. (2) The vesicle is then proposed to be tethered to the organelle by protein complexes and long coiled-coil proteins. (3) A v-SNARE protein on the vesicle then engages a t-SNARE on the target, forming a four-helical bundle whose assembly drives the two bilayers into close proximity, (4) thereby causing membrane fusion. Both vesicle tethering and SNARE assembly have been referred to by others as `docking', so to avoid confusion we use only the former terms here.