1. Center for Basic Neuroscience, Department of Molecular Genetics, and Howard Hughes Medical Institute, The University of Texas Southwestern Medical Center, Dallas, Texas 75390-9111, USA
2. Max-Planck-Institut für experimentelle Medizin, and
3. Max-Planck-Institut für biophysikalische Chemie, 37070 Göttingen, Germany
4. Departments of Biochemistry and Pharmacology, The University of Texas Southwestern Medical Center, Dallas, Texas 75390, USA
5. The Salk Institute, and Howard Hughes Medical Institute, La Jolla, California 92037, USA
6. These authors contributed equally to this work

Correspondence to: Thomas C. Südhof¹ Correspondence and requests for materials should be addressed to T.C.S. (e-mail: Email: Thomas.Sudhof@UTSouthwestern.edu).

Abstract

In all synapses, Ca²⁺ triggers neurotransmitter release to initiate signal transmission. Ca²⁺ presumably acts by activating synaptic Ca²⁺ sensors, but the nature of these sensors—which are the gatekeepers to neurotransmission—remains unclear. One of the candidate Ca²⁺ sensors in release is the synaptic Ca²⁺-binding protein synaptotagmin I. Here we have studied a point mutation in synaptotagmin I that causes a twofold decrease in overall Ca²⁺ affinity without inducing structural or conformational changes. When introduced by homologous recombination into the endogenous synaptotagmin I gene in mice, this point mutation decreases the Ca²⁺ sensitivity of neurotransmitter release twofold, but does not alter spontaneous release or the size of the readily releasable pool of neurotransmitters. Therefore, Ca²⁺ binding to synaptotagmin I participates in triggering neurotransmitter release at the synapse.