Synaptotagmin Increases the Dynamic Range of Synapses by Driving Ca2+-Evoked Release and by Clamping a Near-Linear Remaining Ca2+ Sensor

Ca2+-evoked transmitter release shows a high dynamic range over spontaneous release. We investigated the role of the Ca2+ sensor protein, Synaptotagmin2 (Syt2), in both spontaneous and Ca2+-evoked release under direct control of presynaptic [Ca2+](i), using an in vivo rescue approach at the calyx of Held. Re-expression of Syt2 rescued the highly Ca2+ cooperative release and suppressed the elevated spontaneous release seen in Syt2 KO synapses. This latter release clamping function was partially mediated by the poly-lysine motif of the C2B domain. Using an aspartate mutation in the C2B domain (D364N) in which Ca2+ triggering was abolished but release clamping remained intact, we show that Syt2 strongly suppresses the action of another, near-linear Ca2+ sensor that mediates release over a wide range of [Ca2+](i). Thus, Syt2 increases the dynamic range of synapses by driving release with a high Ca2+ cooperativity, as well as by suppressing a remaining, near-linear Ca2+ sensor.