One of this year's Nobel Laureates in physiology or medicine, Thomas C. Südhof of Stanford University, has proposed a new mechanism to explain neurotransmitter release in the journal Neuron. Researchers knew that vesicles containing neurotransmitters formed at the end of the axon, and that the neurotransmitters had to travel from the vesicle, through the membrane and into the synapse. Südhof found that the previous understanding of how this happened (SNARE proteins on the outside of the vesicle and on the axon would cause the vesicle and cell membrane to form a pore through which the neurotransmitters were released) was incorrect. Instead, he characterized a new model, in which the SNARE proteins cause the vesicle (pictured: blue sphere) and cell membrane into very close contact, causing them to fuse and spill the neurotransmitters into the synapse. In this model, the role of the SNARE proteins isn't fusion, it's to force the vesicle and cell membrane into close contact, from which fusion follows. Years of research indicating SNARE proteins functioned by forming pores make these new results controversial, but also show just how much scientists don't know about neurotransmitter release.
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Journal article: Lipid-Anchored SNAREs Lacking Transmembrane Regions Fully Support Membrane Fusion during Neurotransmitter Release. Neuron, 2013.