Munc13-1 acts as a priming factor for large dense-core vesicles in bovine chromaffin cells.
In chromaffin cells the number of large dense-core vesicles (LDCVs) which can be released by brief, intense stimuli represents only a small fraction of the 'morphologically docked' vesicles at the plasma membrane. Recently, it was shown that Munc13-1 is essential for a post-docking step of synaptic vesicle fusion. To investigate the role of Munc13-1 in LDCV exocytosis, we overexpressed Munc13-1 in chromaffin cells and stimulated secretion by flash photolysis of caged calcium. Both components of the exocytotic burst, which represent the fusion of release-competent vesicles, were increased by a factor of three. The sustained component, which represents vesicle maturation and subsequent fusion, was increased by the same factor. The response to a second flash, however, was greatly reduced, indicating a depletion of release-competent vesicles. Since there was no apparent change in the number of docked vesicles, we conclude that Munc13-1 acts as a priming factor by accelerating the rate constant of vesicle transfer from a pool of docked, but unprimed vesicles to a pool of release-competent, primed vesicles.
Ashery
U
U
Max-Planck-Institute for Biophysical Chemistry, Department of Membrane Biophysics, Am Fassberg 11, 37077 Göttingen, Germany.
Varoqueaux
F
F
Voets
T
T
Betz
A
A
Thakur
P
P
Koch
H
H
Neher
E
E
Brose
N
N
Rettig
J
J
D000818:Animals; D001921:Brain; D002118:Calcium / Q000378:metabolism; D002395:Catecholamines / Q000378:metabolism; D002417:Cattle; D002478:Cells, Cultured; D019439:Chromaffin Cells / Q000166:cytology* / Q000378:metabolism; D003198:Computer Simulation; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000378:metabolism; D004553:Electric Conductivity; D005089:Exocytosis; D007700:Kinetics; D008561:Membrane Fusion; D008854:Microscopy, Electron; D008954:Models, Biological; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D010782:Photolysis; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D051381:Rats; D013572:Synaptic Vesicles / Q000378:metabolism; D013574:Synaptosomes / Q000378:metabolism; D014617:Vacuoles / Q000378:metabolism*
D002395:Catecholamines; D009419:Nerve Tissue Proteins; D020033:Protein Isoforms; C103912:Unc13a protein, rat; C085112:2-nitrophenyl-EGTA; D004533:Egtazic Acid; D002118:Calcium