The C terminus of the Ca channel α_1B subunit mediates selective inhibition by G-protein-coupled receptors

Inhibition of calcium channels by G-protein-coupled receptors depends on the nature of the Gα subunit, although the Gβγ complex is thought to be responsible for channel inhibition. Ca currents in hypothalamic neurons and N-type calcium channels expressed in HEK-293 cells showed robust inhibition by G_i/G_o-coupled galanin receptors (GalR1), but not by Gq-coupled galanin receptors (GalR2). However, deletions in the C terminus of α_1B-1 produced Ca channels that were inhibited after activation of both GalR1 and GalR2. Inhibition of protein kinase C (PKC) also revealed Ca current modulation by GalR2. Imaging studies using green fluorescent protein fusions of the C terminus of α_1B demonstrated that activation of the GalR2 receptor caused translocation of the C terminus of α_1B-1 to the membrane and co-localization with Gαq and PKC. Similar translocation was not seen with a C-terminal truncated splice variant, α_1B-2. Immunoprecipitation experiments demonstrated that Gαq interacts directly with the C terminus of the α_1B subunit. These results are consistent with a model in which local activation of PKC by channel-associated Gαq blocks modulation of the channel by Gβγ released by Gq-coupled receptors.