Abstract
Voltage-dependent calcium (Ca(V)) 1.3 channels are involved in the control of cellular excitability and pacemaking in neuronal, cardiac, and sensory cells. Various proteins interact with the alternatively spliced channel C-terminus regulating gating of Ca(V)1.3 channels. Binding of a regulatory calcium-binding protein calmodulin (CaM) to the proximal C-terminus leads to the boosting of channel activity and promotes calcium-dependent inactivation (CDI). The C-terminal modulator domain (CTM) of Ca(V)1.3 channels can interfere with the CaM binding, thereby inhibiting channel activity and CDI. Here, we compared single-channel gating behavior of two natural Ca(V)1.3 splice isoforms: the long Ca(V)1.3(42) with the full-length CTM and the short Ca(V)1.3(42A) with the C-terminus truncated before the CTM. We found that CaM regulation of Ca(V)1.3 channels is dynamic on a minute timescale. We observed that at equilibrium, single Ca(V)1.3(42) channels occasionally switched from low to high open probability, which perhaps reflects occasional binding of CaM despite the presence of CTM. Similarly, when the amount of the available CaM in the cell was reduced, the short Ca(V)1.3(42A) isoform showed patterns of the low channel activity. CDI also underwent periodic changes with corresponding kinetics in both isoforms. Our results suggest that the competition between CTM and CaM is influenced by calcium, allowing further fine-tuning of Ca(V)1.3 channel activity for particular cellular needs.