Two distinct voltage-sensing domains control voltage sensitivity and kinetics of current activation in CaV1.1 calcium channels

Abstract

CaV1.1 is a slowly activating voltage-gated Ca2+ channel that exists in two splice variants with different voltage sensitivities. By making chimeras of these variants, Tuluc et al. show that activation kinetics and voltage dependence are controlled by distinct molecular mechanisms in the voltage-sensing domains of repeats I and IV, respectively.

Role of putative voltage-sensor countercharge D4 in regulating gating properties of CaV1.2 and CaV1.3 calcium channels

“…In Ca V 1.2 and in Ca V 1.3 charge neutralization of D4 (D1327N, D1283N, respectively) caused a right-shift of V½ and a reduction of current density. Qualitatively these effects are consistent with the original observation in Ca V 1.1 [8,10] and suggest that, similarly to the situation in the skeletal muscle channel isoform, D4 in Ca V 1.2 and Ca V 1.3 might be involved in voltage sensor function and facilitate channel activation by serving as countercharge for the outer gating charges R1 and R2 in VSD IV. Previous structure modeling and mutagenesis analysis in Ca V 1.1 suggested that hydrogen bonds established between D4 and R1 during intermediate state 2 facilitate the transition into the activated state, resulting in the observed left-shifted V½.…”

“…Although in both Ca V 1.2 and Ca V 1.3 neutralization of D4 in the fourth VSD affected voltage-dependence of activation and peak current density, the magnitude of these effects was substantially smaller than that previously observed in Ca V 1.1 [8,10]. This could indicate that in Ca V 1.2 and Ca V 1.3 the putative D4-R1/R2 interaction is of minor importance for voltage-sensing, or it could indicate that the contribution of VSD IV to channel gating is smaller compared to Ca V 1.1.…”

“…Moreover, a recent study demonstrated that its voltage-dependence of activation is differentially regulated by alternative splicing of exon 32 in the IVS3-S4 linker [16]. Therefore this channel is well suited to test the hypothesis that – in analogy to the D4-R1/R2 voltage-sensing interaction recently identified in Ca V 1.1 [8,10] – D4 in the VSD IV is an important determinant of the gating properties of other L-type calcium channels as well. If in Ca V 1.3 the negatively charged D4 (D1283) also acts as countercharge for the positive gating charges R1 and R2, and if this interaction is important for determining the sensitivity of Ca V 1.3 to relatively low voltages, then charge neutralization of D4 is expected to cause a right-shift in the voltage-dependence of activation.

10.1080/19336950.2018.1482183-F0001Figure 2.Charge-neutralization of D4 in VSD IV of Ca V 1.3 affects voltage sensitivity and peak current density.

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“…When D4 or either one of the putative interaction partners (R1, R2) were mutated in Ca V 1.1e, the voltage-dependence of activation was shifted by  ̴20 mV towards more positive potentials and the current amplitude was dramatically reduced. Interestingly, this mirrored the effect of insertion of exon 29 into the extracellular loop connecting helices IVS3 and IVS4 (IVS3-S4 linker) that causes a similar 30 mV shift in voltage-dependence of activation and reduction of current density [9,10]. Therefore, this molecular interaction within VSD IV of Ca V 1.1 facilitates channel gating and provides a means for regulation of the gating properties by alternative splicing.…”

“…Our recent mutagenesis studies unambiguously identified VSD IV as the major determinant of the voltage-dependence of Ca V 1.1, whereas VSD I determines the kinetics of activation [8,10]. In contrast, recent voltage-clamp fluorometry studies demonstrated that in Ca V 1.2 voltage-dependence of activation primarily is determined by VSDs II and III, whereas VSD IV does not contribute to it at all [20].…”