Dominant-negative calcium channel suppression by truncated constructs involves a kinase implicated in the unfolded protein response

Abstract

Expression of the calcium channel Cav2.2 is markedly suppressed by coexpression with truncated constructs of Cav2.2. Furthermore, a two-domain construct of Cav2.1 mimicking an episodic ataxia-2 mutation strongly inhibited Cav2.1 currents. We have now determined the specificity of this effect, identified a potential mechanism, and have shown that such constructs also inhibit endogenous calcium currents when transfected into neuronal cell lines. Suppression of calcium channel expression requires interaction between truncated and full-length channels, because there is inter-subfamily specificity. Although there is marked cross-suppression within the Cav2 calcium channel family, there is no cross-suppression between Cav2 and Cav3 channels. The mechanism involves activation of a component of the unfolded protein response, the endoplasmic reticulum resident RNA-dependent kinase (PERK), because it is inhibited by expression of dominant-negative constructs of this kinase. Activation of PERK has been shown previously to cause translational arrest, which has the potential to result in a generalized effect on protein synthesis. In agreement with this, coexpression of the truncated domain I of Ca v2.2, together with full-length Cav2.2, reduced the level not only of Cav2.2 protein but also the coexpressed α2δ-2. Thapsigargin, which globally activates the unfolded protein response, very markedly suppressed Cav2.2 currents and also reduced the expression level of both Cav2.2 and α2δ-2 protein. We propose that voltage-gated calcium channels represent a class of difficult-to-fold transmembrane proteins, in this case misfolding is induced by interaction with a truncated cognate Cav channel. This may represent a mechanism of pathology in episodic ataxia-2.