Chaperones That SNARE Neurotransmitter Release

Science  25 Jan 2013:
Vol. 339, Issue 6118, pp. 406-407
DOI: 10.1126/science.1233801

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The cellular machinery of neurotransmitter release is well known, but the precise molecular mechanism has long been elusive (1). Its basic features are clear enough: Neuron firing causes the intracellular Ca2+ concentration to rise, triggering the fusion of synaptic vesicles carrying neurotransmitter molecules with the plasma membrane. Fusion is mediated by SNARE [soluble N-ethylmaleimide–sensitive factor (NSF) attachment protein (SNAP) receptor] proteins and a Ca2+ sensor (synaptotagmin) in a process that has been reproduced in vitro using artificial membrane vesicles (liposomes) and purified proteins. A conundrum has, however, remained: In vitro fusion does not depend critically on two additional proteins—Munc13 and Munc18—that are essential for synaptic vesicle fusion in vivo. On page 421 of this issue, Ma et al. (2) report a modified in vitro fusion assay that depends not only on SNAREs and synaptotagmin-Ca2+, but also on both Munc13 and Munc18. The results identify a mechanistic role for Munc13 and Munc18 and force reconsideration of the fundamental model for SNARE assembly.