New Determinant for the CaVbeta2 subunit modulation of the CaV1.2 calcium channel

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