Synaptotagmin interaction with SNAP-25 governs vesicle docking, priming, and fusion triggering

J Neurosci. 2013 Sep 4;33(36):14417-30. doi: 10.1523/JNEUROSCI.1236-13.2013.

Abstract

SNARE complex assembly constitutes a key step in exocytosis that is rendered Ca(2+)-dependent by interactions with synaptotagmin-1. Two putative sites for synaptotagmin binding have recently been identified in SNAP-25 using biochemical methods: one located around the center and another at the C-terminal end of the SNARE bundle. However, it is still unclear whether and how synaptotagmin-1 × SNARE interactions at these sites are involved in regulating fast neurotransmitter release. Here, we have used electrophysiological techniques with high time-resolution to directly investigate the mechanistic ramifications of proposed SNAP-25 × synaptotagmin-1 interaction in mouse chromaffin cells. We demonstrate that the postulated central binding domain surrounding layer zero covers both SNARE motifs of SNAP-25 and is essential for vesicle docking, priming, and fast fusion-triggering. Mutation of this site caused no further functional alterations in synaptotagmin-1-deficient cells, indicating that the central acidic patch indeed constitutes a mechanistically relevant synaptotagmin-1 interaction site. Moreover, our data show that the C-terminal binding interface only plays a subsidiary role in triggering but is required for the full size of the readily releasable pool. Intriguingly, we also found that mutation of synaptotagmin-1 interaction sites led to more pronounced phenotypes in the context of the adult neuronal isoform SNAP-25B than in the embryonic isoform SNAP-25A. Further experiments demonstrated that stronger synaptotagmin-1 × SNAP-25B interactions allow for the larger primed vesicle pool supported by SNAP-25 isoform B. Thus, synaptotagmin-1 × SNARE interactions are not only required for multiple mechanistic steps en route to fusion but also underlie the developmental control of the releasable vesicle pool.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Motifs
  • Amino Acid Sequence
  • Animals
  • Cells, Cultured
  • Chromaffin Cells / metabolism
  • Mice
  • Molecular Sequence Data
  • Mutation
  • Protein Binding
  • Protein Interaction Domains and Motifs
  • Protein Isoforms
  • Protein Transport*
  • Synaptosomal-Associated Protein 25 / chemistry
  • Synaptosomal-Associated Protein 25 / genetics
  • Synaptosomal-Associated Protein 25 / metabolism*
  • Synaptotagmin I / chemistry
  • Synaptotagmin I / genetics
  • Synaptotagmin I / metabolism*
  • Transport Vesicles / metabolism*

Substances

  • Protein Isoforms
  • Snap25 protein, mouse
  • Synaptosomal-Associated Protein 25
  • Synaptotagmin I
  • Syt1 protein, mouse