Differential regulation of transmitter release by alternatively spliced forms of synaptotagmin I.

We discovered a novel alternatively spliced form of synaptotagmin I (Syt I). This splicing event is conserved over evolution and, in Aplysia, results in a two amino acid insert in the juxtamembrane domain of Syt I (Syt IVQ). Both Syt I and Syt IVQ are localized to synaptic vesicles; however, we also observed punctae that contained one or the other spliced products. Both Syt I and Syt IVQ are phosphorylated at the adjacent PKC site. Overexpression of Syt IVQ, but not of Syt I, in Aplysia neurons blocked the ability of serotonin to reverse synaptic depression. This effect is upstream of PKC activation, because neither Syt IVQ nor Syt I blocked the effects of phorbol esters on reversing synaptic depression or the effects of serotonin on facilitating nondepressed synapses. Our results demonstrate a physiological role for splicing in the juxtamembrane domain of Syt I.