Auxiliary subunits operate as a molecular switch in determining gating behaviour of the unitary N‐type Ca2+ channel current in Xenopus oocytes

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Abstract
1 We systematically examined the biophysical properties of ω-conotoxin GVIA-sensitive neuronal N-type channels composed of various combinations of the α1B, α2/δ and β1b subunits in Xenopus oocytes. 2 Whole-cell recordings demonstrated that coexpression of the β1b subunit decelerated inactivation, whereas the α2/δ accelerated both activation and inactivation, and cancelled the kinetic effects of the β1b. The α2/δ and the β1b controlled voltage dependence of activation differently: the β1b significantly shifted the current-voltage relationship towards the hyperpolarizing direction; however, the α2/δ shifted the relationship only slightly in the depolarizing direction. The extent of voltage-dependent inactivation was modified solely by the β1b. 3 Unitary currents measured using a cell-attached patch showed stable patterns of opening that were markedly different among subunit combinations in their kinetic parameters. The α2/δ and the β1b subunits also acted antagonistically in regulating gating patterns of unitary N-type channels. Open time was shortened by the α2/δ, while the fraction of long opening was enhanced by the β1b. The α2/δ decreased opening probability (Po), while the β1b increased Po. α1Bα2/δβ1b produced unitary activity with an open time distribution value in between those of α1Bα2/δ and α1Bβ1b. However, both the α2/δ and the β1b subunits reduced the number of null traces. 4 These results suggest that the auxiliary subunits alone and in combination contribute differently in forming gating apparatuses in the N-type channel, raising the possibility that subunit interaction contributes to the generation of functional diversity of N-type channels in native neuronal preparations also.