The Voltage-dependent Calcium Channel β Subunit Contains Two Stable Interacting Domains

Voltage-dependent calcium channels selectively enable Ca²⁺ ion movement through cellular membranes. These multiprotein complexes are involved in a wide spectrum of biological processes such as signal transduction and cellular homeostasis. α₁ is the membrane pore-forming subunit, whereas β is an intracellular subunit that binds to α₁, facilitating and modulating channel function. We have expressed, purified, and characterized recombinant β₃ and β_2a using both biochemical and biophysical methods, including electrophysiology, to better understand the β family's protein structural and functional correlates. Our results indicate that the β protein is composed of two distinct domains that associate with one another in a stable manner. The data also suggest that the polypeptide regions outside these domains are not structured when β is not in complex with the channel. In addition, the β structural core, comprised of just these two domains without other sequences, binds tightly to the α interaction domain (AID) motif, a sequence derived from the α₁ subunit and the principal anchor site of β. Domain II is responsible for this binding, but domain I enhances it.