New Determinant for the Ca sub(V) beta sub(2) Subunit Modulation of the Ca sub(V)1.2 Calcium Channel

Ca sub(v) beta subunits support voltage gating of Ca sub(v)1.2 calcium channels and play important role in excitation-contraction coupling. The common central membrane-associated guanylate kinase (MAGUK) region of Ca sub(v) beta binds to the alpha -interaction domain (AID) and the IQ motif of the pore-forming alpha sub(1C) subunit, but these two interactions do not explain why the cardiac Ca sub(v) beta sub(2) subunit splice variants differentially modulate inactivation of Ca super(2+) currents (I sub(Ca)). Previously we described beta sub(2 Delta g), a functionally active splice variant of human Ca sub(v) beta sub(2) lacking MAGUK. By deletion analysis of beta sub(2 Delta g), we have now identified a 41-amino acid C-terminal essential determinant ( beta sub(2)CED) that stimulates I sub(Ca) in the absence of Ca sub(v) beta subunits and conveys a +20-mV shift in the peak of the I sub(Ca)-voltage relationship. The beta sub(2)CED is targeted by alpha sub(1C) to the plasma membrane, forms a complex with alpha sub(1C) but does not bind to AID. Electrophysiology and binding studies point to the calmodulin-interacting LA/IQ region in the alpha sub(1C) subunit C terminus as a functionally relevant beta sub(2)CED binding site. The beta sub(2)CED interacts with LA/IQ in a Ca super(2+)- and calmodulin-independent manner and need LA, but not IQ, to activate the channel. Deletion/mutation analyses indicated that each of the three Ca sub(v) beta sub(2)/ alpha sub(1C) interactions is sufficient to support I sub(Ca). However, beta sub(2)CED does not support Ca super(2+)-dependent inactivation, suggesting that interactions of MAGUK with AID and IQ are crucial for Ca super(2+)-induced inactivation. The beta sub(2)CED is conserved only in Ca sub(v) beta sub(2) subunits. Thus, beta sub(2)CED constitutes a previously unknown integrative part of the multifactorial mechanism of Ca sub(v) beta sub(2)-subunit differential modulation of the Ca sub(v)1.2 calcium channel that in beta sub(2 Delta g) occurs without MAGUK.