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

Low-voltage-activated Ca(V)3 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 Ca(V)3 channels while the current kinetics of the Ca(V)3.3 current is one order slower than that of the Ca(V)3.1 and Ca(V)3.2 channels. We have analyzed the voltage dependence and kinetics of charge (Q) movement in human recombinant Ca(V)3.3 and Ca(V)3.1 channels. The voltage dependence of voltage sensor activation (Q(on)-V) of the Ca(V)3.3 channel was significantly shifted with respect to that of the Ca(V)3.1 channel by +18.6 mV and the kinetic of Q(on) activation in the Ca(V)3.3 channel was significantly slower than that of the Ca(V)3.1 channel. Removal of the gating brake in the intracellular loop connecting repeats I and II in the Ca(V)3.3 channel in the ID12 mutant channel shifted the Q(on)-V relation to a value even more negative than that for the Ca(V)3.1 channel. The kinetic of Q(on) activation was not significantly different between ID12 and Ca(V)3.1 channels. Deletion of the gating brake in the Ca(V)3.1 channel resulted in a GD12 channel with the voltage dependence of the gating current activation significantly shifted toward more negative potentials. The Q(on) 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 Ca(V)3 channels. We suggest that activation of the voltage sensor in domain I is limiting both the voltage dependence and the kinetics of Ca(V)3 channel activation.