Mechanism of Auxiliary Subunit Modulation of Neuronal α_1E Calcium Channels

Voltage-gated calcium channels are composed of a main pore-forming α₁ moiety, and one or more auxiliary subunits (β, α₂δ) that modulate channel properties. Because modulatory properties may vary greatly with different channels, expression systems, and protocols, it is advantageous to study subunit regulation with a uniform experimental strategy. Here, in HEK 293 cells, we examine the expression and activation gating of α_1E calcium channels in combination with a β (β₁–β₄) and/or the α₂δ subunit, exploiting both ionic- and gating-current measurements. Furthermore, to explore whether more than one auxiliary subunit can concomitantly specify gating properties, we investigate the effects of cotransfecting α₂δ with β subunits, of transfecting two different β subunits simultaneously, and of COOH-terminal truncation of α_1E to remove a second β binding site. The main results are as follows. (a) The α₂δ and β subunits modulate α_1E in fundamentally different ways. The sole effect of α₂δ is to increase current density by elevating channel density. By contrast, though β subunits also increase functional channel number, they also enhance maximum open probability (G_max/Q_max) and hyperpolarize the voltage dependence of ionic-current activation and gating-charge movement, all without discernible effect on activation kinetics. Different β isoforms produce nearly indistinguishable effects on activation. However, β subunits produced clear, isoform-specific effects on inactivation properties. (b) All the β subunit effects can be explained by a gating model in which subunits act only on weakly voltage-dependent steps near the open state. (c) We find no clear evidence for simultaneous modulation by two different β subunits. (d) The modulatory features found here for α_1E do not generalize uniformly to other α₁ channel types, as α_1C activation gating shows marked β isoform dependence that is absent for α_1E. Together, these results help to establish a more comprehensive picture of auxiliary-subunit regulation of α_1E calcium channels.