C-terminal Fragments of the α_1C (Ca_V1.2) Subunit Associate with and Regulate L-type Calcium Channels Containing C-terminal-truncated α_1C Subunits

L-type Ca²⁺ channels in native tissues have been found to contain a pore-forming α₁, subunit that is often truncated at the C terminus. However, the C terminus contains many important domains that regulate channel function. To test the hypothesis that C-terminal fragments may associate with and regulate C-terminal-truncated α_1C (Ca _V1.2) subunits, we performed electrophysiological and biochemical experiments. In tsA201 cells expressing either wild type or C-terminal-truncated α_1C subunits in combination with β_2a subunit, truncation of the α_1C subunit by as little as 147 amino acids led to a 10-15-fold increase in currents compared with those obtained from control, full-length α_1C subunits. Dialysis of cells expressing the truncated α_1C subunits with C-terminal fragments applied through the patch pipette reconstituted the inhibition of the channels seen with full-length α_1C subunits. In addition, C-terminal deletion mutants containing a tethered C terminus also exhibited the C-terminal-induced inhibition. Immunoprecipitation assays demonstrated the association of the C-terminal fragments with truncated α_1C subunits. In addition, glutathione S-transferase pull-down assays demonstrated that the C-terminal inhibitory fragment could associate with at least two domains within the C terminus. The results support the hypothesis the C-terminal fragments of the α_1C subunit can associate with C-terminal-truncated α_1C subunits and inhibit the currents through L-type Ca²⁺ channels.