Function of concatenated homotetrameric channels. (A) Steady-state activation relationships in the absence and presence of 50 µM cAMP for w4 channels. The data points were fitted with Eq. 1, yielding for the parameters Vh and zδ −116.5 ± 1.8 mV (n = 10) and 6.57 ± 0.40 (n = 10); −94.8 ± 1.5 mV (n = 9) and 6.42 ± 0.48 (n = 9), respectively. (B) Same as A for wwww channels. The respective parameters for Vh and zδ are −123.9 ± 2.3 mV (n = 17) and 6.41 ± 0.26 (n = 17); −103.9 ± 3.0 mV (n = 12) and 6.43 ± 0.26 (n = 12). (C) Comparison of the Vh values for the three wild-type and three mutated channels; 0 cAMP (control), black squares, 50 µM cAMP, green squares. (D) Comparison of the cAMP-induced voltage shift of Vh, ΔVh. (E) Comparison of zδ. Symbols correspond to C. (F) Comparison of cAMP-evoked current increase, ΔI−150mV. 4–17 recordings per data point were included for analyses in C to F. In the cartoons here and below, green circles represent functional binding sites, whereas empty crossed circles represent mutated binding sites. Error bars indicate SEM.
