C-terminal splicing reveals intramolecular gating modulation in Ca_V1.3 L-type Ca²⁺ channels

Neuronal excitability and pace-making in the sinoatrial node are controlled by low-voltage activated Ca_V1.3 L-type Ca²⁺ channels. We recently found that in related Ca_V1.4 channels a highly-structured distal C-terminal motif (CTM) modulates voltage- and Ca²⁺-dependent gating (CDI). In Ca_V1.3, C-terminal splicing leads to a full-length (Ca_V1.3L) and at least 1 short (Ca_V1.3S) splice form. If a CTM would also modulate Ca_V1.3 gating it would be present in Ca_V1.3L but not Ca_V1.3S variants. We therefore compared the biophysical properties of Ca_V1.3L or Ca_V1.3S coexpressed with β3 + α2δ-1 in tsA-201 cells using the whole-cell patch-clamp technique. Ca_V1.3S channels activated at more negative potentials compared to Ca_V1.3L (~ -10 mV, p < 0.0001), inactivated faster (p < 0.01) and showed more CDI (p < 0.01). These changes resulted in a decreased window current shifted to more hyperpolarized potentials likely to cause a reduction in the channels' dynamic range. Removal of the C-terminal 158 (Ca_V1.3_Δ1158) or 76 amino acids was sufficient to induce gating properties similar to Ca_V1.3S. FRET experiments revealed interaction of the last 158 amino acids (C₁₅₈) to a proximal C-terminal domain in Ca_V1.3L. Coexpression of C₁₅₈ with Ca_V1.3_Δ1158 completely restored Ca_V1.3L gating properties confirming this protein interaction. Thus Ca_V1.3 channel gating is under control of the distal C-terminus allowing alternative splicing to fine-tune channel activity and adapt channel function to physiological needs.