New Determinant for the CaVβ2 Subunit Modulation of the CaV1.2 Calcium Channel

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