Allosteric modulation of the presynaptic Ca2+ sensor for vesicle fusion

Neurotransmitter release is triggered by an increase in the cytosolic Ca 2+ concentration ([Ca 2+ ] i ), but it is unknown whether the Ca 2+ -sensitivity of vesicle fusion is modulated during synaptic plasticity. We investigated whether the potentiation of neurotransmitter release by phorbol esters 1 , 2 , 3 , which target presynaptic protein kinase C (PKC)/munc-13 signalling cascades 4 , 5 , 6 , exerts a direct effect on the Ca 2+ -sensitivity of vesicle fusion. Using direct presynaptic Ca 2+ -manipulation and Ca 2+ uncaging at a giant presynaptic terminal, the calyx of Held, we show that phorbol esters potentiate transmitter release by increasing the apparent Ca 2+ -sensitivity of vesicle fusion. Phorbol esters potentiate Ca 2+ -evoked release as well as the spontaneous release rate. We explain both effects by an increased fusion ‘willingness’ in a new allosteric model of Ca 2+ -activation of vesicle fusion. In agreement with an allosteric mechanism, we observe that the classically high Ca 2+ cooperativity in triggering vesicle fusion (∼ 4) is gradually reduced below 3 µM [Ca 2+ ] i , reaching a value of