FIGURE 5.

Putative kinetic schemes of Kv4:KChIP-1 channel gating: 6T models. These kinetic schemes include six transitions in the activation pathway and are based on the models proposed by Ayer and Sigworth, Beck and Covarrubias, Bähring et al., Klemic et al., and Kurata et al. (31,43,46,50,51). Sequential transitions between C₀ and C₄ correspond to activation steps governed by the strongly voltage-dependent rate constants α and β. Opening occurs in two cooperative and weakly voltage-dependent steps as proposed by Schoppa and Sigworth (49). This is the difference between Schemes II (Fig. 4) and IV. Scheme V assumes an additional closing transition outside the main activation pathway as explained in Fig. 4 legend; and Scheme VI further assumes that this transition may occur equally well from all states in the main activation pathway. The forward rate constant (k_xp) of the transitions between the C_i pathway and the P_i pathway decreases as external [K⁺] increases. Thus, the latter predicts virtual elimination of P₆ at saturating concentrations of external K⁺; and therefore, Schemes V and VI reduce to Scheme IV.