The goal of this project is to understand the mechanisms by which the synaptotagmin family functions in neuronal exocytosis, membrane trafficking and synaptic plasticity. Synaptic transmission has evolved as a highly specialized form of cellular vesicle trafficking capable of rapid calcium triggered exocytotic cycles. Modulation of neurotransmitter release in the presynaptic nerve terminal can alter the functional properties of neuronal circuits, leading to dramatic changes in behavioral outputs. Alterations in synaptic strength are also likely to underlie a variety of forms of synaptic plasticity associated with learning and memory and may have important roles in pathological states such as epilepsy, Alzheimer's disease, Parkinson's disease, psychoses and the trinucleotide repeat disorders. Synaptotagmin I is a calcium-binding synaptic vesicle protein that has been shown to play an essential, yet undefined, role in synapse function, possibly serving as the calcium sensor for synaptic exocytosis. Synaptotagmin IV is a unique member of the synaptotagmin family and an intermediate early gene that is upregulated in response to seizures. We have identified a family of two neuronal specific synaptotagmins (syt I and syt IV) and two ubiquitous synaptotagmins (syt V and syt VII) in Drosophila that form an evolutionarily conserved synaptotagmin family from invertebrates to humans. Syt IV is co-expressed on synaptic vesicles with syt I, but contains an evolutionarily conserved amino acid change in the first calcium-binding domain that abolishes its ability to bind membranes in a calcium-dependent manner. Thus, synaptic vesicles contain calcium-dependent and -independent synaptotagmin isoforms, indicating that modulation of the expression of these two isoforms or their function could provide a, unique mechanism for controlling synaptic output during episodes of synaptic plasticity or in response to seizure activity. Our analysis will combine genetic manipulations available in Drosophila with molecular, biochemical, electrophysiological and morphological approaches to investigate this possibility and define the function of the synaptotagmin family in neurotransmission. We will also perform structure-function studies using synaptotagmin I and IV mutants to determine the precise role of each isoform. The results of these studies should significantly advance our understanding of the mechanisms of synapse function and plasticity and provide novel insights into potential pathways that modulate neuronal communication.