Identification of a disulfide bridge essential for structure and function of the voltage-gated Ca2+ channel α2δ-1 auxiliary subunit

Abstract

Voltage-gated calcium (CaV) channels are transmembrane proteins that form Ca2+-selective pores gated by depolarization and are essential regulators of the intracellular Ca2+ concentration. By providing a pathway for rapid Ca2+ influx, CaV channels couple membrane depolarization to a wide array of cellular responses including neurotransmission, muscle contraction and gene expression. CaV channels fall into two major classes, low voltage-activated (LVA) and high voltage-activated (HVA). The ion-conducting pathway of HVA channels is the α1 subunit, which typically contains associated β and α2δ ancillary subunits that regulate the properties of the channel. Although it is widely acknowledged that α2δ-1 is post-translationally cleaved into an extracellular α2 polypeptide and a membrane-anchored δ protein that remain covalently linked by disulfide bonds, to date the contribution of different cysteine (Cys) residues to the formation of disulfide bridges between these proteins has not been investigated. In the present report, by predicting disulfide connectivity with bioinformatics, molecular modeling and protein biochemistry experiments we have identified two Cys residues involved in the formation of an intermolecular disulfide bond of critical importance for the structure and function of the α2δ-1 subunit. Site directed-mutagenesis of Cys404 (located in the von Willebrand factor-A region of α2) and Cys1047 (in the extracellular domain of δ) prevented the association of the α2 and δ peptides upon proteolysis, suggesting that the mature protein is linked by a single intermolecular disulfide bridge. Furthermore, co-expression of mutant forms of α2δ-1 Cys404Ser and Cys1047Ser with recombinant neuronal N-type (CaV2.2α1/β3) channels, showed decreased whole-cell patch-clamp currents indicating that the disulfide bond between these residues is required for α2δ-1 function.