Calcium-triggered Membrane Fusion Proceeds Independently of Specific Presynaptic Proteins*
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Complexes of specific presynaptic proteins have been hypothesized to drive or catalyze the membrane fusion steps of exocytosis. Here we use a stage-specific preparation to test the roles of SNAREs, synaptotagmin, and SNARE-binding proteins in the mechanism of Ca2+-triggered membrane fusion. Excess exogenous proteins, sufficient to block SNARE interactions, did not inhibit either the Ca2+ sensitivity, extent, or kinetics of fusion. In contrast, despite a limited effect on SNARE and synaptotagmin densities, treatments with high doses of chymotrypsin markedly inhibited fusion. Conversely, low doses of chymotrypsin had no effect on the Ca2+ sensitivity or extent of fusion but did alter the kinetic profile, indicating a more direct involvement of other proteins in the triggered fusion pathway. SNAREs, synaptotagmin, and their immediate binding partners are critical to exocytosis at a stage other than membrane fusion, although they may still influence the triggered steps.
author list (cited authors)
Szule, J. A., Jarvis, S. E., Hibbert, J. E., Spafford, J. D., Braun, J., Zamponi, G. W., Wessel, G. M., & Coorssen, J. R.
complete list of authors
Szule, Joseph A||Jarvis, Scott E||Hibbert, Julie E||Spafford, J David||Braun, Janice EA||Zamponi, Gerald W||Wessel, Gary M||Coorssen, Jens R