Intracellular calcium (Ca2+) inhibits the opening of L-type (alpha 1C) Ca2+ channels, providing physiological control of Ca2+ entry into a wide variety of cells. A structural determinant of this Ca(2+)-sensitive inactivation was revealed by chimeric Ca2+ channels derived from parental alpha 1C and alpha 1E channels, the latter of which is a neuronal channel lacking Ca2+ inactivation. A consensus Ca(2+)-binding motif (an EF hand), located on the alpha 1C subunit, was required for Ca2+ inactivation. Donation of the alpha 1C EF-hand region to the alpha 1E channel conferred the Ca(2+)-inactivating phenotype. These results strongly suggest that Ca2+ binding to the alpha 1C subunit initiates Ca2+ inactivation.

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 α1E 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 α1E to remove a second β binding site. The main results are as follows. (a) The α2δ and β subunits modulate α1E 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 (Gmax/Qmax) 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 α1E do not generalize uniformly to other α1 channel types, as α1C activation gating shows marked β isoform dependence that is absent for α1E. Together, these results help to establish a more comprehensive picture of auxiliary-subunit regulation of α1E calcium channels.

The physiological and pharmacological properties of the ␣ 1E calcium (Ca) channel subtype do not exactly match any of the established categories described for native neuronal Ca currents. Many of the key diagnostic features used to assign cloned Ca channels to their native counterparts, however, are dependent on a number of factors, including cellular environment, ␤ subunit coexpression, and modulation by second messengers and G-proteins. Here, by examining the intrinsic pore characteristics of a family of transiently expressed neuronal Ca channels, we demonstrate that the permeation properties of ␣ 1E closely resemble those described for a subset of lowthreshold Ca channels. The ␣ 1A (P-/Q-type), ␣ 1B (N-type), and ␣ 1C (L-type) high-threshold Ca channels all exhibit larger whole-cell currents with barium (Ba) as the charge carrier as compared with Ca or strontium (Sr). In contrast, macroscopic ␣ 1E currents are largest in Sr, followed by Ca and then Ba. The unique permeation properties of ␣ 1E are maintained at the single-channel level, are independent of the nature of the expression system, and are not affected by coexpression of ␣ 2 and ␤ subunits. Overall, the permeation characteristics of ␣ 1E are distinct from those described for R-type currents and share some similarities with native low-threshold Ca channels.