Structural determinants of L-type channel activation in segment IIS6 revealed by a retinal disorder

The mechanism of channel opening for voltage-gated calcium channels is poorly understood. The importance of a conserved isoleucine residue in the pore-lining segment IIS6 has recently been highlighted by functional analyses of a mutation (I745T) in the Ca(V)1.4 channel causing severe visual impairment (Hemara-Wahanui, A., Berjukow, S., Hope, C. I., Dearden, P. K., Wu, S. B., Wilson-Wheeler, J., Sharp, D. M., Lundon-Treweek, P., Clover, G. M., Hoda, J. C., Striessnig, J., Marksteiner, R., Hering, S., and Maw, M. A. ( 2005) Proc. Natl. Acad. Sci. U. S. A. 102, 7553 - 7558). In the present study we analyzed the influence of amino acids in segment IIS6 on gating of the Ca(V)1.2 channel. Substitution of Ile-781, the Ca(V)1.2 residue corresponding to Ile-745 in Ca(V)1.4, by residues of different hydrophobicity, size and polarity shifted channel activation in the hyperpolarizing direction (I781P > I781T > I781N > I781A> I781L). As I781P caused the most dramatic shift ( - 37 mV), substitution with this amino acid was used to probe the role of other residues in IIS6 in the process of channel activation. Mutations revealed a high correlation between the midpoint voltages of activation and inactivation. A unique kinetic phenotype was observed for residues 779 - 782 (LAIA) located in the lower third of segment IIS6; a shift in the voltage dependence of activation was accompanied by a deceleration of activation at hyperpolarized potentials, a deceleration of deactivation at all potentials ( I781P and I781T), and decreased inactivation. These findings indicate that Ile-781 substitutions both destabilize the closed conformation and stabilize the open conformation of Ca(V)1.2. Moreover there may be a flexible center of helix bending at positions 779 - 782 of Ca(V)1.2. These four residues are completely conserved in high voltage-activated calcium channels suggesting that these channels may share a common mechanism of gating.