Voltage-dependent G-protein regulation of CaV2.2 (N-type) channels

How G proteins inhibit N-type, voltage-gated, calcium-selective channels (Ca(V)2.2) during presynaptic inhibition is a decades-old question. G proteins G beta gamma bind to intracellular Ca(V)2.2 regions, but the inhibition is voltage dependent. Using the hybrid electrophysiological and optical approach voltage-clamp fluorometry, we show that G beta gamma acts by selectively inhibiting a subset of the four different Ca(V)2.2 voltage-sensor domains (VSDs I to IV). During regular "willing" gating, VSD-I and -IV activations resemble pore opening, VSD III activation is hyperpolarized, and VSD II appears unresponsive to depolarization. In the presence of G beta gamma, Ca(V)2.2 gating is "reluctant": pore opening and VSD I activation are strongly and proportionally inhibited, VSD IV is modestly inhibited, while VSD III is not. We propose that G beta gamma inhibition of VSDs I and IV underlies reluctant Ca(V)2.2 gating and subsequent presynaptic inhibition.