Dynamic association of calcium channel subunits at the cellular membrane

Abstract

High voltage gated calcium channels (VGCCs) are composed of at least three subunits, one pore forming α1α1-subunit, an intracellular ββ-variant, and a mostly extracellular α2δα2δ-variant. Interactions between these subunits determine the kinetic properties of VGCCs. It is unclear whether these interactions are stable over time or rather transient. Here, we used single-molecule tracking to investigate the surface diffusion of α1α1- and α2δ1α2δ1-subunits at the cell surface. We found that α2δ1α2δ1-subunits show higher surface mobility than α1α1-subunits, and that they are only transiently confined together, suggesting a weak association between α1α1- and α2δ1α2δ1-subunits. Moreover, we observed that different α1α1-subunits engage in different degrees of association with the α2δ1α2δ1-subunit, revealing the tighter interaction of α2δ1α2δ1 with CaV1.2>CaV2.2>CaV2.1>CaV3.2CaV1.2>CaV2.2>CaV2.1>CaV3.2. These data indicate a distinct regulation of the α1/α2δ1α1/α2δ1 interaction in VGCC subtypes. We modeled their membrane dynamics in a Monte Carlo simulation using experimentally determined diffusion constants. Our modeling predicts that the ratio of associated α1α1- and α2δ1α2δ1-subunits mainly depends on their expression density and confinement in the membrane. Based on the different motilities of particular α1/α2δ1α1/α2δ1-subunit combinations, we propose that their dynamic assembly and disassembly represent an important mechanism to regulate the signaling properties of VGCC.