Divergent actions of Ca_V1.1 voltage-sensing domain III in activating EC coupling and channel gating

The voltage-gated calcium channel Ca_V1.1 is the voltage sensor for skeletal muscle excitation-contraction (EC) coupling. Upon depolarization of the membrane, it rapidly triggers calcium release from the sarcoplasmic reticulum by conformational coupling to the type 1 ryanodine receptor. Strong depolarization further gives rise to slowly activating L-type calcium currents. Activation of these two processes at distinct voltages and with distinct kinetics is accomplished by the specific actions and properties of Ca_V1.1's four voltage-sensing domains (VSDs). Although they jointly regulate the gating of the channel pore, only a single VSD (VSD III) controls EC coupling. How Ca_V1.1 VSD III operates these two functions and to what degree it contributes to channel gating, if at all, are still incompletely known. Here, we analyze the molecular mechanism by which VSD III S3-S4 loop chimeras shift the voltage dependence of EC coupling to negative potentials without affecting the current properties. Furthermore, we report on point mutations in VSD III that shift the voltage dependence of both processes to more positive potentials and on one combined mutant construct that abolishes the currents while retaining the EC coupling function. Together, these findings demonstrate that, in addition to its primary function in EC coupling, Ca_V1.1 VSD III contributes to channel gating. Moreover, the data indicate that the mechanisms in VSD III operating the two processes can be experimentally separated from one another and probably represent two divergent state transitions initiated by voltage changes.