Snapin Specifically Up-Regulates Ca v 1.3 Ca 2+ Channel Variant with a Long Carboxyl Terminus.

Ca ²⁺ entry through Ca _v 1.3 Ca ²⁺ channels plays essential roles in diverse physiological events. We employed yeast-two-hybrid (Y2H) assays to mine novel proteins interacting with Ca _v 1.3 and found Snapin2, a synaptic protein, as a partner interacting with the long carboxyl terminus (CT _L ) of rat Ca _v 1.3 _L variant. Co-expression of Snapin with Ca _v 1.3 _L /Ca _v β ₃ /α ₂ δ ₂ subunits increased the peak current density or amplitude by about 2-fold in HEK-293 cells and Xenopus oocytes, without affecting voltage-dependent gating properties and calcium-dependent inactivation. However, the Snapin up-regulation effect was not found for rat Ca _v 1.3 _S containing a short CT (CT _S ) in which a Snapin interaction site in the CT _L was deficient. Luminometry and electrophysiology studies uncovered that Snapin co-expression did not alter the membrane expression of HA tagged Ca _v 1.3 _L but increased the slope of tail current amplitudes plotted against ON-gating currents, indicating that Snapin increases the opening probability of Ca _v 1.3 _L . Taken together, our results strongly suggest that Snapin directly interacts with the CT _L of Ca _v 1.3 _L , leading to up-regulation of Ca _v 1.3 _L channel activity via facilitating channel opening probability.