Biophysical characterization of the honeybee DSC1 orthologue reveals a novel voltage-dependent Ca2+ channel subfamily: CaV4

Abstract

Bilaterian voltage-gated Na+ channels (NaV) evolved from voltage-gated Ca2+ channels (CaV). The Drosophila melanogaster Na+ channel 1 (DSC1), which features a D-E-E-A selectivity filter sequence that is intermediate between CaV and NaV channels, is evidence of this evolution. Phylogenetic analysis has classified DSC1 as a Ca2+-permeable Na+ channel belonging to the NaV2 family because of its sequence similarity with NaV channels. This is despite insect NaV2 channels (DSC1 and its orthologue in Blatella germanica, BSC1) being more permeable to Ca2+ than Na+. In this study, we report the cloning and molecular characterization of the honeybee (Apis mellifera) DSC1 orthologue. We reveal several sequence variations caused by alternative splicing, RNA editing, and genomic variations. Using the Xenopus oocyte heterologous expression system and the two-microelectrode voltage-clamp technique, we find that the channel exhibits slow activation and inactivation kinetics, insensitivity to tetrodotoxin, and block by Cd2+ and Zn2+. These characteristics are reminiscent of CaV channels. We also show a strong selectivity for Ca2+ and Ba2+ ions, marginal permeability to Li+, and impermeability to Mg2+ and Na+ ions. Based on current ion channel nomenclature, the D-E-E-A selectivity filter, and the properties we have uncovered, we propose that DSC1 homologues should be classified as CaV4 rather than NaV2. Indeed, channels that contain the D-E-E-A selectivity sequence are likely to feature the same properties as the honeybee's channel, namely slow activation and inactivation kinetics and strong selectivity for Ca2+ ions.