Does the Effect of Mutation of the MIDAS on Ca2+-Channel Currents Involve Binding of Extracellular Divalent Cations?
Because MIDAS motifs bind divalent cations (Fig. 1a), we wished to examine whether the effect of mutation of the MIDAS affected channel permeation. We therefore removed all divalent cations from the external medium and measured Na+ flux through CaV2.2 channels in tsA-201 cells. This protocol produced the same result: the amplitude of the CaV2.2/β1b Na+ currents was enhanced >4-fold by α2δ-2 but not by α2δ-2 μMIDAS, whose currents were slightly smaller than those in the absence of any α2δ-2 (Fig. 2 d–f). Therefore, if Mg2+ (or Ca2+) binding to the MIDAS in α2δ-2 is involved in its function in relation to enhancing currents through CaVα1 channels, it must be during the process of channel assembly or trafficking rather than during the permeation process, once the channels have reached the cell surface.
