C-Terminal Modulatory Domain Controls Coupling of Voltage-Sensing to Pore Opening in Cav1.3 L-type Ca2+ Channels

Activity of voltage-gated Ca(v)1.3 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(v)1.3 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(v)1.3(L)) and a short Ca(v)1.3 splice variant (Ca(v)1.3(42A)) expressed in tsA-201 cells using whole cell patch-clamp. Charge movement (Q(ON)) of Ca(v)1.3(L) displayed a much steeper voltage-dependence and a more negative half-maximal activation voltage than Ca(v)1.2 and Ca(v)3.1. However, a significantly higher fraction of the total charge had to move for activation of Ca(v)1.3 half-maximal conductance (Ca(v)1.3: 68%; Ca(v)1.2: 52%; Ca(v)3.1: 22%). This indicated a weaker coupling of Ca(v)1.3 voltage-sensor charge movement to pore opening. However, the coupling efficiency was strengthened in the absence of the CTM in Ca(v)1.3(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(v)1.3 channels respond more sensitively to depolarization than those of Ca(v)1.2 or Ca(v)3.1. Weak coupling of voltage sensing to pore opening is enhanced in the absence of the CTM, allowing short Ca(v)1.3(42A) splice variants to activate at lower voltages without affecting Q(ON)-V.