Regulation of Voltage Sensing Structures of CaV1.2 Calcium Channels by the Auxiliary β-Subunit (β3)

Abstract

High Voltage-Activated channels (HVA) are composed of four non-identical repeats with distinct voltage-sensing domain (VSD). The cardiac HVA CaV1.2 is modulated by at least two auxiliary subunits, α2δ and β. The β-subunit is known to facilitate channel opening without altering gating currents or single channel conductance. By tracking the movement of individual voltage sensors in human CaV1.2 channels using voltage clamp fluorometry we learned that individual voltage-sensors contribute to channel opening and gating currents to a different degree (Pantazis et al. 2014, PNAS). Taking advantage that rabbit CaV1.2 can be expressed without auxiliary subunits, we explored the impact that β subunits exert on voltage-sensing or coupling and found that β3 regulates Voltage-sensor I (VSD I) activity. Voltage-sensors of all members of the voltage-gated ion channel family have three main conformations: resting, active and relaxed. VSD transition from the resting to the active conformation is thought to be coupled to channel opening, while the relaxed configuration is populated during prolonged depolarizations. Once in the relaxed state, strong hyperpolarizations are necessary to return to the resting state. When rabbit CaV1.2 channels are co-expressed without β subunit the voltage-dependency of VSD I indicates that near 50% of VSD I are in the relaxed state even at −90mV holding potential. In contrast, with β3 present, VSD I dwells between resting and active state. The augmented preference of VSD I for the relaxed state in the absence of β3 may explain why CaV1.2 channels lacking the β-subunit are less willing to open and together with our results with human CaV2.1 suggest that VSD I participates in inactivation rather than activation. We hypothesize that when in the relaxed state, VSD I keeps channels from opening or in an “unwilling” state.