Characterization of the Munc13-calmodulin interaction by photoaffinity labeling.

Abstract Sensing of and response to transient increases in the residual presynaptic Ca 2+ levels are important adaptive mechanisms that define the short-term plasticity characteristics of neurons. Due to their essential function in synaptic vesicle priming and in the modulation of synaptic strength, Munc13 proteins have emerged as key regulators of these adaptive mechanisms. Indeed, Munc13-1 and ubMunc13-2 contain a conserved calmodulin (CaM) binding site and the Ca 2+ -dependent interaction of these Munc13 isoforms with CaM constitutes a molecular mechanism that transduces residual Ca 2+ signaling to the synaptic exocytotic machinery. Here, we used Munc13-derived model peptides in photoaffinity labeling (PAL) experiments to demonstrate the stoichiometric and Ca 2+ -dependent CaM binding of the other members of the Munc13 family, bMunc13-2 and Munc13-3, via structurally distinct non-conserved binding sites. A PAL-based Ca 2+ titration assay revealed that all Munc13 isoforms can form a complex with CaM already at low Ca 2+ concentrations just above resting levels, underscoring the Ca 2+ sensor/effector function of this interaction in short-term synaptic plasticity phenomena.