Figure 1.

Extracellular TEA accelerates slow inactivation in T449F Shaker channels. (A) KcsA crystal structure with Y82 shown in space filling representation; PDB, 1K4C. (B) Outward potassium currents in response to a 50-s depolarizing pulse to +40 mV from a holding potential of −100 mV for control and in the presence of the indicated TEA concentration. Traces are normalized to their peak current value to facilitate comparison of the inactivation kinetics. As opposed to the traditional slowing behavior of TEA on inactivation, here the accelerating effect motivates the terminology “spring-in-the-door.” (C) The rate of inactivation is the inverse of the time constant for a single-exponential fit to the relaxation and is well fit by a three-state model (inset) in which inactivation is more rapid from the blocked state. Let k_OI be the inactivation rate constant from the open state (O) to the inactivated state (I), and k_BI the inactivation rate constant from the blocked state (B). The equilibrium probability of fast block in the absence of inactivation is P_block =(1 + K_i/[TEA])⁻¹. The observed inactivation rate is then defined as ρ = k_BIP_block + k_OI(1 − P_block). Values of k_OI and P_block were estimated separately, leaving only k_BI to be estimated from the data in C. We propose that electrostatic attraction contributes to TEA collapse in the single mutant due to the en face orientation of the aromatic rings.