Neuron
Volume 50, Issue 1, 6 April 2006, Pages 49-62
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Article
Discrete Residues in the C2B Domain of Synaptotagmin I Independently Specify Endocytic Rate and Synaptic Vesicle Size

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Summary

It has been demonstrated that synapses lacking functional synaptotagmin I (Syt I) have a decreased rate of synaptic vesicle endocytosis. Beyond this, the function of Syt I during endocytosis remains undefined. Here, we demonstrate that a decreased rate of endocytosis in sytnull mutants correlates with a stimulus-dependent perturbation of membrane internalization, assayed ultrastructurally. We then separate the mechanisms that control endocytic rate and vesicle size by mapping these processes to discrete residues in the Syt I C2B domain. Mutation of a poly-lysine motif alters vesicle size but not endocytic rate, whereas the mutation of calcium-coordinating aspartate residues (syt-D3,4N) alters endocytic rate but not vesicle size. Finally, slowed endocytic rate in the syt-D3,4N animals, but not sytnull animals, can be rescued by elevating extracellular calcium concentration, supporting the conclusion that calcium coordination within the C2B domain contributes to the control of endocytic rate.

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Present address: Laboratory of Neural Circuits and Behavior, The Rockefeller University, 1230 York Avenue, New York, New York 10021.