Ca²⁺/Calmodulin Regulates Trafficking of Ca_V1.2 Ca²⁺ Channels in Cultured Hippocampal Neurons

Abstract

As the Ca²⁺-sensor for Ca²⁺-dependent inactivation, calmodulin (CaM) has been proposed, but never definitively demonstrated, to be a constitutive Ca_V1.2 Ca²⁺ channel subunit. Here we show that CaM is associated with the Ca_V1.2 pore-forming α_1C subunit in brain in a Ca²⁺-independent manner. Within its CaM binding pocket, α_1C has been proposed to contain a membrane targeting domain. Because ion channel subunits assemble early during channel biosynthesis, we postulated that this association with CaM could afford the opportunity for Ca²⁺-dependent regulation of membrane targeting. We showed that the isolated domain functioned as a Ca²⁺/CaM regulated trafficking determinant for CD8 (a model transmembrane protein) using fluorescent-activated cell sorting analysis and, using green fluorescent protein-tagged α_1C subunits expressed in cultured hippocampal neurons, that Ca²⁺/CaM interaction with this domain accelerated trafficking of Ca_V1.2 channels to distal regions of the dendritic arbor. Furthermore, this Ca²⁺/CaM-accelerated trafficking was activity dependent. Thus, CaM imparts Ca²⁺-dependent regulation not only to mature Ca_V1.2 channels at the cell surface but also to steps during channel biosynthesis.