Voltage-dependent G-protein regulation of Ca2.2 (N-type) channels.

How G proteins inhibit N-type, voltage-gated, calcium-selective channels (Ca2.2) during presynaptic inhibition is a decades-old question. G proteins Gβγ bind to intracellular Ca2.2 regions, but the inhibition is voltage dependent. Using the hybrid electrophysiological and optical approach voltage-clamp fluorometry, we show that Gβγ acts by selectively inhibiting a subset of the four different Ca2.2 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 Gβγ, Ca2.2 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 Gβγ inhibition of VSDs I and IV underlies reluctant Ca2.2 gating and subsequent presynaptic inhibition.