The SNARE protein SNAP-25 is linked to fast calcium triggering of exocytosis.
Synchronous neurotransmission depends on the tight coupling between Ca(2+) influx and fusion of neurotransmitter-filled vesicles with the plasma membrane. The vesicular soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein synaptobrevin 2 and the plasma membrane SNAREs syntaxin 1 and synaptosomal protein of 25 kDa (SNAP-25) are essential for calcium-triggered exocytosis. However, the link between calcium triggering and SNARE function remains elusive. Here we describe mutations in two sites on the surface of the SNARE complex formed by acidic and hydrophilic residues of SNAP-25 and synaptobrevin 2, which were found to coordinate divalent cations in the neuronal SNARE complex crystal structure. By reducing the net charge of the site in SNAP-25 we identify a mutation that interferes with calcium triggering of exocytosis when overexpressed in chromaffin cells. Exocytosis was elicited by photorelease of calcium from a calcium cage and evaluated by using patch-clamp capacitance measurements at millisecond time resolution. We present a method for monitoring the dependence of exocytotic rate upon calcium concentration at the release site and demonstrate that the mutation decreased the steepness of this relationship, indicating that the number of sequential calcium-binding steps preceding exocytosis is reduced by one. We conclude that the SNARE complex is linked directly to calcium triggering of exocytosis, most likely in a complex with auxiliary proteins.
Sørensen
Jakob B
JB
Max-Planck-Institut für Biophysikalische Chemie, Am Fassberg 11, 37077 Göttingen, Germany. jsoeren@gwdg.de
Matti
Ulf
U
Wei
Shun-Hui
SH
Nehring
Ralf B
RB
Voets
Thomas
T
Ashery
Uri
U
Binz
Thomas
T
Neher
Erwin
E
Rettig
Jens
J
D019943:Amino Acid Substitution; D000818:Animals; D002118:Calcium / Q000502:physiology*; D020013:Calcium Signaling / Q000502:physiology*; D005089:Exocytosis / Q000502:physiology*; D049452:Green Fluorescent Proteins; D007700:Kinetics; D008164:Luminescent Proteins / Q000235:genetics; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D009419:Nerve Tissue Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D010957:Plasmids; D011487:Protein Conformation; D050683:R-SNARE Proteins; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D050825:Synaptosomal-Associated Protein 25
D008164:Luminescent Proteins; D008565:Membrane Proteins; D009419:Nerve Tissue Proteins; D050683:R-SNARE Proteins; D011993:Recombinant Fusion Proteins; C494210:Snap25 protein, rat; D050825:Synaptosomal-Associated Protein 25; D049452:Green Fluorescent Proteins; D002118:Calcium