Effective gating charges per channel in voltage-dependent K+ and Ca2+ channels

In voltage-dependent ion channels, the gating of the channels is determined by the movement of the voltage sensor. This movement reflects the rearrangement of the protein in response to a voltage stimulus, and it can be thought of as a net displacement of elementary charges (e(0)) through the membrane (z: effective number of elementary charges). In this paper, we measured z in Shaker IR (inactivation removed) K+ channels, neuronal alpha(1E) and alpha(1A), and cardiac alpha(1C) Ca2+ channels using two methods: (a) limiting slope analysis of the conductance-voltage relationship and (b) variance analysis, to evaluate the number of active channels in a patch, combined with the measurement of charge movement in the same patch. We found that in Shaker IR K+ channels the two methods agreed with a z similar or equal to 13. This suggests that all the channels that gate can open and that all the measured charge is coupled to pore opening in a strictly sequential kinetic model. For all Ca2+ channels the limiting slope method gave consistent results regardless of the presence or type of beta subunit tested (z = 8.6). However, as seen with alpha(1E), the variance analysis gave different results depending on the beta subunit used. alpha(1E) and alpha(1E)beta(1a) gave higher z values (z = 14.77 and z = 15.13, respectively) than alpha(lE)beta(2a) (z = 9.50, which is similar to the limiting slope results). Both the beta(1a) and beta(2a) subunits, coexpressed with alpha(1E) Ca2+ channels facilitated channel opening by shifting the activation curve to more negative potentials, but only the beta(2a) subunit increased the maximum open probability. The higher z using variance analysis in alpha(1E) and alpha(1E)beta(1a) can be explained by a set of charges not coupled to pore opening. This set of charges moves in transitions leading to nulls thus not contributing to the ionic current fluctuations but eliciting gating currents. Coexpression of the beta(2a) subunit would minimize the fraction of nulls leading to the correct estimation of the number of channels and z.