Three-dimensional Structure of Ca(V)3.1: COMPARISON WITH THE CARDIAC L-TYPE VOLTAGE-GATED CALCIUM CHANNEL MONOMER ARCHITECTURE

Calcium entry through voltage-gated calcium channels has widespread cellular effects upon a host of physiological processes including neuronal excitability, muscle excitation-contraction coupling, and secretion. Using single particle analysis methods, we have determined the first three-dimensional structure, at 23 Å resolution, for a member of the low voltage-activated voltage-gated calcium channel family, Ca(V)3.1, a T-type channel. Ca(V)3.1 has dimensions of ∼115 × 85 × 95 Å, composed of two distinct segments. The cytoplasmic densities form a vestibule below the transmembrane domain with the C terminus, unambiguously identified by the presence of a His tag being ∼65 Å long and curling around the base of the structure. The cytoplasmic assembly has a large exposed surface area that may serve as a signaling hub with the C terminus acting as a “fishing rod” to bind regulatory proteins. We have also determined a three-dimensional structure, at a resolution of 25 Å, for the monomeric form of the cardiac L-type voltage-gated calcium (high voltage-activated) channel with accessory proteins β and α(2)δ bound to the ion channel polypeptide Ca(V)1.2. Comparison with the skeletal muscle isoform finds a good match particularly with respect to the conformation, size, and shape of the domain identified as that formed by α(2). Furthermore, modeling of the Ca(V)3.1 structure (analogous to Ca(V)1.2 at these resolutions) into the heteromeric L-type voltage-gated calcium channel complex volume reveals multiple interaction sites for β-Ca(V)1.2 binding and for the first time identifies the size and organization of the α(2)δ polypeptides.