Nonconserved Ca²⁺/Calmodulin Binding Sites in Munc13s Differentially Control Synaptic Short-Term Plasticity

Abstract

Munc13s are presynaptic proteins that mediate synaptic vesicle priming and thereby control the size of the readily releasable pool of vesicles. During high synaptic activity, Munc13-1 and its closely related homolog, ubMunc13-2, bind Ca²⁺/calmodulin, resulting in enhanced priming activity and in changes of short-term synaptic plasticity characteristics. Here, we studied whether bMunc13-2 and Munc13-3, two remote isoforms of Munc13-1 with a neuronal subtype-specific expression pattern, mediate synaptic vesicle priming and regulate short-term synaptic plasticity in a Ca²⁺/calmodulin-dependent manner. We identified a single functional Ca²⁺/calmodulin binding site in these isoforms and provide structural evidence that all Munc13s employ a common mode of interaction with calmodulin despite the lack of sequence homology between their Ca²⁺/calmodulin binding sites. Electrophysiological analysis showed that, during high-frequency activity, Ca²⁺/calmodulin binding positively regulates the priming activity of bMunc13-2 and Munc13-3, resulting in an increase in the size of the readily releasable pool of vesicles and subsequently in strong short-term synaptic enhancement of neurotransmission. We conclude that Ca²⁺/calmodulin-dependent regulation of priming activity is structurally and functionally conserved in all Munc13 proteins, and that the composition of Munc13 isoforms in a neuron differentially controls its short-term synaptic plasticity characteristics.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://dx.doi.org/10.1128/MCB.00933-12.

ACKNOWLEDGMENTS

N.L. was a recipient of a Feodor Lynen Fellowship of the Minerva Foundation.

We thank H. Kawabe for helpful discussion and A. Poulopoulos for critically reading the manuscript. We are grateful to F. Benseler, I. Thanhäuser, D. Schwerdtfeger, A. Galinski, L. van Werven, M. Uecker, and T. Liepold for excellent technical support and to the staff of the MPIEM animal facility for the management of mouse colonies.

We declare no conflicts of interest.