*A–D*, whole-cell K^{+} currents from tsA-201 cells transfected with Kv4.2 (*A*), Kv4.2 and DPPX-S (*B*), Kv4.2 and KChIP1 (*C*) or Kv4.2 + KChIP1 and DPPX-S (*D*), evoked by step depolarizations from −109 to +61 mV in 10 mV increments from a holding potential of −149 mV. Note that DPPX accelerates and KChIP1 slows down inactivation kinetics. *E*, voltage dependence of the rate of inactivation of Kv4.2 channels. Plots of the half-inactivation time (*t*_{1/2}) against membrane potential from the currents recorded in tsA-201 cells expressing Kv4.2 alone (•), Kv4.2 and DPPX-S (○), Kv4.2 and KChIP1 (▵) and Kv4.2 plus KChIP1 and DPPX-S (grey triangles). Shown are means s.e.m. (*n* = 4, 4, 10 and 10, respectively). Note that, for currents recorded in the presence of KChIP1 and KChIP1 plus DPPX-S, the rate of inactivation increases with strong depolarizations. However, DPPX accelerates the rate of inactivation between −60 and −10 mV, increasing the voltage range over which the *t*_{1/2} increases with progressive depolarization. *F*, recoveries from inactivation of Kv4.2 alone (•), Kv4.2 and DPPX-S (○), Kv4.2 and KChIP1 (▵) and Kv4.2 plus KChIP1 and DPPX-S (grey triangles); the recovery voltages were −115, −128, −140 and −140 mV, respectively. These voltages were adjusted to account for shifts in the voltage dependence of steady-state inactivation (not shown), and therefore are 40 mV more negative than the midpoint voltage of the corresponding steady-state inactivation curve. The curves shown are means s.e.m. (*n* = 3, 3, 8 and 9, respectively) and the lines are best-fit single exponentials with the following time constants: 165, 40, 96 and 45 ms, for Kv4.2, Kv4.2:DPPX-S, Kv4.2:KChIP-1 and Kv4.2:KChIP-1:DPPX-S, respectively.

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