Contrasting the roles of the I-II loop gating brake in CaV3.1 and CaV3.3 calcium channels

Abstract

Low-voltage-activated CaV3 channels are distinguished among other voltage-activated calcium channels by the most negative voltage activation threshold. The voltage dependence of current activation is virtually identical in all three CaV3 channels while the current kinetics of the CaV3.3 current is one order slower than that of the CaV3.1 and CaV3.2 channels. We have analyzed the voltage dependence and kinetics of charge (Q) movement in human recombinant CaV3.3 and CaV3.1 channels. The voltage dependence of voltage sensor activation (Qon-V) of the CaV3.3 channel was significantly shifted with respect to that of the CaV3.1 channel by +18.6 mV and the kinetic of Qon activation in the CaV3.3 channel was significantly slower than that of the CaV3.1 channel. Removal of the gating brake in the intracellular loop connecting repeats I and II in the CaV3.3 channel in the ID12 mutant channel shifted the Qon-V relation to a value even more negative than that for the CaV3.1 channel. The kinetic of Qon activation was not significantly different between ID12 and CaV3.1 channels. Deletion of the gating brake in the CaV3.1 channel resulted in a GD12 channel with the voltage dependence of the gating current activation significantly shifted toward more negative potentials. The Qon kinetic was not significantly altered. ID12 and GD12 mutants did not differ significantly in voltage dependence nor in the kinetic of voltage sensor activation. In conclusion, the putative gating brake in the intracellular loop connecting repeats I and II controls the gating current of the CaV3 channels. We suggest that activation of the voltage sensor in domain I is limiting both the voltage dependence and the kinetics of CaV3 channel activation.

Role of individual S4 segments in gating of Cav3.1 T-type calcium channel by voltage

“…The importance of voltage sensors in domains I and II was suggested in several studies showing that removal of the so-called gating brake in the intracellular loop connecting domains I and II increases opening probability of both Ca V 3.1 and Ca V 3.3 channels [13,16,17] and shifts voltage dependencies of gating currents towards more negative potentials. Hence, S4 segments in proximity of the gating brake play an important role in channel activation, and molecular modeling suggests the voltage sensor in the domain I as a possible candidate [14,23].…”

“…The lack of an effect of charge removal in the IVS4 segment of the Ca V 3.1 channel on voltage dependence of gating current was demonstrated [15]. Removal of the gating brake in the I-II loop of Ca V 3.1 and Ca V 3.3 channels shifted voltage dependences of both gating and ion currents towards even more negative voltages suggesting a prominent role of domain I in Ca V 3 channel activation [16,17]. …”

T-type channels: release a brake, engage a gear

“…1). 4 In addition, the kinetics of the ON-charge movement were found significantly accelerated upon deletion of the gating brake in the Ca V 3.3 channel, suggesting an increased mobility of the voltage-sensor.…”

“…2 Deletion of this molecular determinant gives rise to channels that activate at even more hyperpolarized potentials and present faster activation and inactivation kinetics. 3 In our recent study, 4 we have extended the functional characterization of the gating brake by contrasting its importance within the Ca V 3 family members. Activation of voltage-gated calcium channels (VGCC) proceeds in 2 steps (Fig.…”

“…5 In addition, mobilization of the voltagesensor occurs at significantly more positive voltages for Ca V 3.3 channel compared to Ca V 3.1 channel, as evidenced by a depolarized shift the voltage-dependence of the ON-charge (Q ON -V). 4 From a thermodynamic point of view, this indicates that more energy is needed to mobilize the voltage-sensor of Ca V 3.3 channels than of Ca V 3.1 channels. On the other hand, because the voltage-dependences of pore opening are nearly identical, it implies that less charge transfer is needed to open the pore of Ca V 3.3 channels than of Ca V 3.1 channels suggesting a more efficient coupling or "gear" between the activation of the voltage-sensor and the pore opening.…”