Munc18: a presynaptic transmitter release site N type (CaV2.2) calcium channel interacting protein.

Munc18 is a presynaptic protein that is essential for transmitter release. Recent studies have indicated that this protein is involved in secretory vesicle docking but its binding partners in this role remain a mystery. We demonstrate using the isolated calyx-type presynaptic terminal of the chick ciliary ganglion that staining for Munc18 colocalizes and covaries with that for transmitter release site N type calcium channels (CaV2.2), consistent with elements of a common release site complex. Biochemical analysis demonstrated that the protein coprecipitates with CaV2.2 from lysates of rat or chick brain, including its synaptic, long-splice variant; presynaptic terminal surface membrane proteins, and a cell line coexpressing Munc18 and CaV2.2. Munc18 bound with high affinity to the CaV2.2 II-III intracellular loop, low affinity to the I-II loop but not to other channel intracellular regions. Over-expression of Munc18 in dorsal root ganglion neurons did not affect CaV2.2 current amplitude or fast kinetics but siRNA-knockdown resulted in a negative shift in the steady state inactivation curve, an effect attributed to an indirect action via syntaxin 1. Recombinant Munc18 also coprecipitated strongly with the v-SNARE synaptotagmin, but only weakly with other SNAREs. Thus, the calcium channel may serve as a surface membrane platform anchoring a Munc18-containing bridge to synaptotagmin and the synaptic vesicle.