Molecular characterization of a two-domain form of the neuronal voltage-gated P/Q-type calcium channel α₁2.1 subunit

We characterized the neuronal two-domain (95kD-α₁2.1) form of the α₁2.1 subunit of the voltage-gated calcium channels using genetic and molecular analysis. The 95kD-α₁2.1 is absent in neuronal preparations from CACNA1A null mouse demonstrating that α₁2.1 and 95kD-α₁2.1 arise from the same gene. A recombinant two-domain form (α_1AI-II) of α₁2.1 associates with the β subunit and is trafficked to the plasma membrane. Translocation of the α_1AI-II to the plasma membrane requires association with the β subunit, since a mutation in the α_1AI-II that inhibits β subunit association reduces membrane trafficking. Though the α_1AI-II protein does not conduct any voltage-gated currents, we have previously shown that it generates a high density of non-linear charge movements [Ahern et al., Proc. Natl. Acad. Sci. USA 98 (2001) 6935-6940]. In this study, we demonstrate that co-expression of the α_1AI-II significantly reduces the current amplitude of α₁2.1/β_1a/α₂δ channels, via competition for the β subunit. Taken together, our results demonstrate a dual functional role for the α_1AI-II protein, both as a voltage sensor and modulator of P/Q-type currents in recombinant systems. These studies suggest an in vivo role for the 95kD-α₁2.1 in altering synaptic activity via protein-protein interactions and/or regulation of P/Q-type currents.