Variance–mean plot for the heterotetramers compared with the 4wt. (A) Fluctuation analysis from an ensemble of current traces (n = 300 pulses) for a 20-ms depolarization to +70 mV, followed by repolarization to −120 mV in a cell-attached patch of an oocyte expressing 4wt. The data were fitted with Eq. 2, and the single-channel current i and the number of channels N in the patches were estimated. The corresponding conductance was estimated to be of 13 and 27 pS at +70 and −120 mV, respectively. The number of channels in the patch was 213. Eq. 3 allows for the calculation of the Pomax (maximal open probability), which was calculated to be 0.73 (n = 8). (B) Fluctuation analysis from a cell-attached patch of an oocyte expressing 2wt/2mut. 450 current traces of 20-ms depolarizations to +70 mV and repolarizations to −120 mV were used for noise analysis. The number of channels in the patch was estimated at 374, the single-channel conductance to 11.7 and 25 pS at +70 and −120 mV, respectively, and the Pomax to 0.67 (n = 7). (C) Fluctuation analysis from an oocyte expressing 1wt/3mut in a cell-attached configuration. 600 current traces of a 20-ms depolarization to +70 mV and repolarization to −180 mV were averaged and analyzed. The single-channel conductance was estimated at 10.8 and 22 pS at +70 and −180 mV, respectively. The number of channels in the patch could not be estimated accurately because of the open probability of 0.47 (n = 3). The black lines are the data, and the red lines are the fitted curve. Patch clamp experiments. The external and internal solutions both contained 120 mM K+.
