| Influx of calcium into the neuron triggers fusion of synaptic vesicles with the nerve terminal membrane and release of neurotransmitter into the synaptic cleft. A calcium receptor on the synaptic vesicle that mediates this fusion event is synaptotagmin. Interactions between calcium and synaptotagmin promote key events that stimulate neurotransmitter release, but the molecular details that describe this process are not well understood. Our efforts have focused on understanding the mechanism(s) of calcium activation of synaptotagmin proteins, and how lead affects the activation process. Towards this end, biophysical and biochemical experiments were conducted on two isoforms of synaptotagmin, namely synaptotagmin I and synaptotagmin II. Here, we present detailed studies on syt that show that synaptotagmin I and synaptotagmin II are activated by calcium via distinct mechanisms. Calcium binding to synaptotagmin I triggers a global change in the structure of the protein that triggers rearrangement of its C2 domains. By contrast, calcium binding results in a concomitant self-association and conformational change of synaptotagmin II. Whereas lead inhibits the ability of calcium to trigger a conformational change in synaptotagmin I, lead strongly potentiates oligomerization of synaptotagmin II. Taken together, these studies provide detailed molecular insights into how calcium triggers neurotransmission and how lead interferes with this process. The different behaviors of synaptotagmin I and synaptotagmin II suggest that differential regulation of exocytosis in different cell types could be controlled by varying the expression levels of different synaptotagmin isoforms. |
