Calmodulin-dependent gating of Cav1.2 calcium channels in the absence of Cavβ subunits

It is generally accepted that to generate calcium currents in response to depolarization, Cav1.2 calcium channels require association of the pore-forming α₁C subunit with accessory Cavβ and α₂δ subunits. A single calmodulin (CaM) molecule is tethered to the C-terminal α₁C-LA/IQ region and mediates Ca²⁺-dependent inactivation of the channel. Cavβ 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/α₂δ in COS1 cells in the absence of Cavβ 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 Cavβ subunits nor coimmunoprecipitation analysis with exogenous α₁C revealed an induction of endogenous Cavβ subunits that could be linked to the effect of CaMex. Coexpression of a calcium-insensitive CaM mutant CaM₁₂₃₄ also facilitated gating of Cavβ-free Cav1.2 channels but did not support Ca²⁺-dependent inactivation. Our results show there is a functional matchup between CaMex and Cavβ subunits that, in the absence of Cavβ, 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 Cavβ. We suggest that CaMex affects specific Cavβ-free conformations of the channel that are not available to endogenous CaM.