Introduction into Cav2.1 of the homologous mutation of Cav1.2 causing the Timothy syndrome questions the role of V421 in the phenotypic definition of P-type Ca2+ channel

The Timothy syndrome is a multisystem disorder associated with the mutation of a Gly residue (G402 or G406) in the Cav1.2 Ca2+ channel. G406 is localized at the end of the IS6 segment and just before the intracellular I–II loop, which is important for the regulation of channel inactivation and the binding of the Cavβ subunit. This Gly residue is conserved in all Cav1 and Cav2 channels, and the G to R exchange produces a strong decrease of inactivation not only in Cav1.2 but also in Cav2.3. Here, we show that the mutation into Arg or Glu of the homologous Gly residue in Cav2.1 (G363) produces also a slowing of inactivation. However, the G-to-A exchange that decreases the inactivation rate in Cav1.2 and Cav2.3 increases inactivation in Cav2.1. Each mutation affects specifically the gating properties of Cav2.1 that remain nevertheless modulated by the co-expressed β subunit as with wild-type channel. The strong decrease of inactivation produced by the G363R or G363E mutations was reminiscent to that previously described for a specific splice variant of Cav2.1 that contains a single Val residue inserted in the I–II loop (V421). We unexpectedly found that the V421 insertion does not affect the inactivation rate of Cav2.1 and that the effects previously attributed to this insertion, including those on G-protein regulation, can be reproduced by the G363E mutation. Altogether, our results highlight the role of G363 in gating properties, inactivation kinetics, and G-protein regulation of Cav2.1 and the lack of effect of V421 insertion on inactivation.