Synaptotagmin-1 Docks Secretory Vesicles to Syntaxin-1/SNAP-25 Acceptor Complexes

Abstract

Docking, the initial association of secretory vesicles with the plasma membrane, precedes formation of the SNARE complex, which drives membrane fusion. For many years, the molecular identity of the docked state, and especially the vesicular docking protein, has been unknown, as has the link to SNARE complex assembly. Here, using adrenal chromaffin cells, we identify the vesicular docking partner as synaptotagmin-1, the calcium sensor for exocytosis, and SNAP-25 as an essential plasma membrane docking factor, which, together with the previously known docking factors Munc18-1 and syntaxin, form the minimal docking machinery. Moreover, we show that the requirement for Munc18-1 in docking, but not fusion, can be overcome by stabilizing syntaxin/SNAP-25 acceptor complexes. These findings, together with cross-rescue, double-knockout, and electrophysiological data, lead us to propose that vesicles dock when synaptotagmin-1 binds to syntaxin/SNAP-25 acceptor complexes, whereas Munc18-1 is required for the downstream association of synaptobrevin to form fusogenic SNARE complexes.

Large α-synuclein oligomers inhibit neuronal SNARE-mediated vesicle docking

“…S6), thereby limiting the number of v-vesicles available for docking. Recently, it was suggested that synaptobrevin-2 is not required for the initial docking of synaptic vesicles to the plasma membrane in vivo (46,47). Our results show that large α-Syn oligomers specifically inhibit SNARE complex formation between synaptobrevin-2 and t-SNAREs after initial docking by other synaptic factors, such as synaptotagmin-1, in the neuron.…”

The Diversity of Calcium Sensor Proteins in the Regulation of Neuronal Function

“…Synaptotagmin can bind to both syntaxin and SNAP-25, and fast neurotransmitter release requires synaptotagmin (Geppert et al 1994) probably prebound to assembled or partially assembled SNARE complexes (Schiavo et al 1997;Rickman et al 2006) so that Ca 2þ -induced interaction with phospholipids can occur rapidly (Xue et al 2008). It is still under debate how important synaptotagmin is in vesicle docking (de Wit et al 2009) and how it acts at the plasma membrane in fusion itself (Tang et al 2006;Hui et al 2009). Synaptotagmin could act as a brake on fusion that is relieved on Ca 2þ binding or have a positive role in membrane fusion (Chicka et al 2008).…”

Organelle docking: R-SNAREs are late

“…Using a proteoliposome clustering assay, they conclude that the R-SNARE Nyv1p is dispensable for vacuole docking, whereas the three Q-SNAREs are required. A very similar conclusion was recently reached for secretory vesicle docking in intact mammalian cells (12). This is an exciting convergence of new findings, which together suggest that establishing/stabilizing a cis3Q-SNARE complex may be a central step for docking to occur (see Fig.…”

“…Evidence for a tethering step upstream of secretory vesicle fusion is sketchy and is therefore dimmed in the diagram. ment of SM-proteins for docking in the vacuole system as shown for Munc18-1 requirement in secretory vesicle docking (12). Although dispensable for the initial docking step, R-SNAREs remain essential for the downstream fusion reaction in both yeast vacuole and other systems.…”

“…Moreover, other factors than the R-SNARE may perform the initial docking reaction and bind to the cis3Q-SNARE complex. In mammalian secretory vesicles this is the vesicular protein synaptotagmin-1 (12). It would be very interesting to see whether the vacuole SM-protein Vps33p promotes the existence/stability of cis3Q-SNARE complexes in a similar manner as the mammalian SM-protein Munc18-1 does for the mammalian cis3Q-SNARE complex (12) and whether artificial stabilization of the 3Q SNARE complex using a short C-terminal R-SNARE peptide (see ref.…”