Calmodulin-dependent gating of Ca v 1.2 calcium channels in the absence of Ca v β subunits

It is generally accepted that to generate calcium currents in response to depolarization, Cav 1.2 calcium channels require association of the pore-forming α1C subunit with accessory Cav β and α2 δ subunits. A single calmodulin (CaM) molecule is tethered to the C-terminal α1C -LA/IQ region and mediates Ca2+ -dependent inactivation of the channel. Cav β subunits are stably associated with the α1C -interaction domain site of the cytoplasmic linker between internal repeats I and II and also interact dynamically, in a Ca2+ -dependent manner, with the α1C -IQ region. Here, we describe a surprising discovery that coexpression of exogenous CaM (CaMex ) with α1C /α2 δ in COS1 cells in the absence of Cav β subunits stimulates the plasma membrane targeting of α1C , facilitates calcium channel gating, and supports Ca2+ -dependent inactivation. Neither real-time PCR with primers complementary to monkey Cav β subunits nor coimmunoprecipitation analysis with exogenous α1C revealed an induction of endogenous Cav β subunits that could be linked to the effect of CaMex . Coexpression of a calcium-insensitive CaM mutant CaM1234 also facilitated gating of Cav β-free Cav 1.2 channels but did not support Ca2+ -dependent inactivation. Our results show there is a functional matchup between CaMex and Cav β subunits that, in the absence of Cav β, renders Ca2+ channel gating facilitated by CaM molecules other than the one tethered to LA/IQ to support Ca2+ -dependent inactivation. Thus, coexpression of CaMex creates conditions when the channel gating, voltage- and Ca2+ -dependent inactivation, and plasma-membrane targeting occur in the absence of Cav β. We suggest that CaMex affects specific Cav β-free conformations of the channel that are not available to endogenous CaM.