Two distinct effects on neurotransmission in a temperature‐sensitive SNAP‐25 mutant

Vesicle fusion in eukaryotic cells is mediated by SNAREs (soluble N ‐ethylmaleimide‐sensitive factor attachment protein receptors). In neurons, the t‐SNARE SNAP‐25 is essential for synaptic vesicle fusion but its exact role in this process is unknown. We have isolated a SNAP‐25 temperature‐sensitive paralytic mutant in Drosophila , SNAP‐25 ts . The mutation causes a Gly50 to Glu change in SNAP‐25's first amphipathic helix. A similar mutation in the yeast homologue SEC9 also results in temperature sensitivity, implying a conserved role for this domain in secretion. In vitro ‐generated 70 kDa SNARE complexes containing SNAP‐25 ts are thermally stable but the mutant SNARE multimers (of ∼120 kDa) rapidly dissociate at 37°C. The SNAP‐25 ts mutant has two effects on neurotransmitter release depending upon temperature. At 22°C, evoked release of neurotransmitter in SNAP‐25 ts larvae is greatly increased, and at 37°C, the release of neurotransmitter is reduced as compared with controls. Our data suggest that at 22°C the mutation causes the SNARE complex to be more fusion competent but, at 37°C the same mutation leads to SNARE multimer instability and fusion incompetence.