page 45). This provides the first link between the long-range process of tethering and the short-range process of docking.
Vesicle transfer consists of four successive reactions: vesicle formation, tethering, docking, and fusion. Tethering of COP1 vesicles to the Golgi requires the coiled-coil protein p115, which links the Golgins GM130 (on the Golgi) and Giantin (on the vesicle). Subsequent docking of vesicles is a SNARE-dependent event, requiring the correct assembly of cognate SNAREs comprising one vesicle SNARE (v-SNARE) and three target SNARES (t-SNAREs). The resultant SNAREpin appears to force fusion of the two membranes.
Now, it seems that, in addition to tethering the Golgins, p115 also facilitates vesicle docking. Shorter et al. showed that p115, via one of its coiled-coil domains, binds to specific Golgi SNARE proteins that form SNAREpins containing the t-SNARE syntaxin-5. The binding is reflected in a functional assay with either isolated Golgi membranes or Golgi detergent extracts, where p115 stimulates the assembly of cognate SNARE complexes.Once established, maintenance of the SNAREpin structure does not require p115, and the protein is not consumed during the process, suggesting that p115 is a catalyst of SNAREpin formation. Thus, p115 is able to couple tethering and docking physically by first linking donor and acceptor membranes through associations with Golgins and then catalyzing SNAREpin assembly through direct interaction with the appropriate SNARE proteins. ▪