Inversion of both gating polarity and CO2 sensitivity of voltage gating with D3N mutation of Cx50.

The effect of CO(2)-induced acidification on transjunctional voltage (V(j)) gating was studied by dual voltage-clamp in oocytes expressing mouse connexin 50 (Cx50) or a Cx50 mutant (Cx50-D3N), in which the third residue, aspartate (D), was mutated to asparagine (N). This mutation inverted the gating polarity of Cx50 from positive to negative. CO(2) application greatly decreased the V(j) sensitivity of Cx50 channels, and increased that of Cx50-D3N channels. CO(2) also affected the kinetics of V(j) dependent inactivation of junctional current (I(j)), decreasing the gating speed of Cx50 channels and increasing that of Cx50-D3N channels. In addition, the D3N mutation increased the CO(2) sensitivity of chemical gating such that even CO(2) concentrations as low as 2.5% significantly lowered junctional conductance (G(j)). With Cx50 channels G(j) dropped by 78% with a drop in intracellular pH (pH(i)) to 6.83, whereas with Cx50-D3N channels G(j) dropped by 95% with a drop in pH(i) to just 7.19. We have previously hypothesized that the way in which V(j) gating reacts to CO(2) might be related to connexin's gating polarity. This hypothesis is confirmed here by evidence that the D3N mutation inverts the gating polarity as well as the effect of CO(2) on V(j) gating sensitivity and speed.