C-Terminal Modulatory Domain Controls Coupling of Voltage-Sensing to Pore Opening in Ca_v1.3 L-type Ca²⁺ Channels

Abstract

Activity of voltage-gated Ca_v1.3 L-type Ca²⁺ channels is required for proper hearing as well as sinoatrial node and brain function. This critically depends on their negative activation voltage range, which is further fine-tuned by alternative splicing. Shorter variants miss a C-terminal regulatory domain (CTM), which allows them to activate at even more negative potentials than C-terminally long-splice variants. It is at present unclear whether this is due to an increased voltage sensitivity of the Ca_v1.3 voltage-sensing domain, or an enhanced coupling of voltage-sensor conformational changes to the subsequent opening of the activation gate. We studied the voltage-dependence of voltage-sensor charge movement (Q_ON-V) and of current activation (I_Ca-V) of the long (Ca_v1.3_L) and a short Ca_v1.3 splice variant (Ca_v1.3_42A) expressed in tsA-201 cells using whole cell patch-clamp. Charge movement (Q_ON) of Ca_v1.3_L displayed a much steeper voltage-dependence and a more negative half-maximal activation voltage than Ca_v1.2 and Ca_v3.1. However, a significantly higher fraction of the total charge had to move for activation of Ca_v1.3 half-maximal conductance (Ca_v1.3: 68%; Ca_v1.2: 52%; Ca_v3.1: 22%). This indicated a weaker coupling of Ca_v1.3 voltage-sensor charge movement to pore opening. However, the coupling efficiency was strengthened in the absence of the CTM in Ca_v1.3_42A, thereby shifting I_Ca-V by 7.2 mV to potentials that were more negative without changing Q_ON-V. We independently show that the presence of intracellular organic cations (such as n-methyl-D-glucamine) induces a pronounced negative shift of Q_ON-V and a more negative activation of I_Ca-V of all three channels. These findings illustrate that the voltage sensors of Ca_v1.3 channels respond more sensitively to depolarization than those of Ca_v1.2 or Ca_v3.1. Weak coupling of voltage sensing to pore opening is enhanced in the absence of the CTM, allowing short Ca_v1.3_42A splice variants to activate at lower voltages without affecting Q_ON-V.