Figure 8.

(A) Gβ₁γ₅ binding to the K_ACh channel. The probability of Gβ₁γ₅ binding, P, is plotted against Gβ₁γ₅ concentration. The points and error bars represent the mean ± SEM of three to five separate experiments. The solid line through the data represents the least-squares fit with a hyperbolic equation: P = P _max[Gβγ]/ ([Gβγ]+ K _d), with P _max = 0.63 and K _d = 1.29 nM. (B) Gβ₁γ₅-concentration dependence of the K_ACh channel activation. The steady state K_ACh channel open probability, P _o, is normalized to the open probability of mode 4, P _o4Gβγ, determined in each experiment, and the P _o/P _o4Gβγ ratio is plotted against Gβ₁γ₅ concentration. Symbols and bars are mean ± SEM of three to five separate experiments. The continuous line represents the least-squares fit with the Hill equation (Eq. 3) and yields a Hill coefficient of 1.73 and an apparent k _d of 1.09 nM. The predicted P _o/P _o4Gβγ ratio for a K_ACh channel with four gating modes arising from the binding of a different number of Gβγ subunits to four binding sites in the channel structure (Eqs. 1 and 2) is plotted for comparison as a dotted line.