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A more recent version of this article appeared on April 1, 2004
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Submitted on September 21, 2003
Revised on January 7, 2004
Accepted on January 8, 2004
1 Department of Anatomy and Cell Biology, Columbia University, College of Physicians and Surgeons, New York, N. Y. 10032
2 Department of Physiology, University of Wisconsin, Madison, WI 53706, USA.
* Corresponding author. E-mail address: gb74{at}columbia.edu.
Ca2+-triggered exocytosis of secretory granules mediates the release of hormones from endocrine cells and neurons. The plasma membrane protein SNAP-25 is thought to be a key component of the membrane fusion apparatus that mediates exocytosis in neurons. Recently, homologues of SNAP-25 have been identified, including SNAP-23, which is expressed in many tissues, albeit at different levels. At present, little is known concerning functional differences among members of this family of proteins. Using an in vitro assay, we show here that SNAP-25 and SNAP-23 mediate the docking of secretory granules with the plasma membrane at high (1 µM) and low (100 nM) Ca2+ levels, respectively, by interacting with different members of the synaptotagmin family. In intact endocrine cells, expression of exogenous SNAP-23 leads to high levels of hormone secretion under basal conditions. Thus, the relative expression levels of SNAP-25 and SNAP-23 might control the mode (regulated vs. basal) of granule release by forming docking complexes at different Ca2+ thresholds.
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