EFFECTIVE GATING CHARGES PER CHANNEL IN VOLTAGE-DEPENDENT K+ AND CA2+CHANNELS

In voltage-dependent ion channels, the gating of the channels is deter mined by the movement of the voltage sensor. This movement reflects th e 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+ chan nels, neuronal alpha(1E) and alpha(1A), and cardiac alpha(1C) Ca2+ cha nnels using two methods: (a) limiting slope analysis of the conductanc e-voltage relationship and (b) variance analysis, to evaluate the numb er of active channels in a patch, combined with the measurement of cha rge 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 ch arge is coupled to pore opening in a strictly sequential kinetic model . For all Ca2+ channels the limiting slope method gave consistent resu lts regardless of the presence or type of beta subunit tested (z = 8.6 ). However, as seen with alpha(1E), the variance analysis gave differe nt 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 slo pe results). Both the beta(1a) and beta(2a) subunits, coexpressed with alpha(1E) Ca2+ channels facilitated channel opening by shifting the a ctivation curve to more negative potentials, but only the beta(2a) sub unit increased the maximum open probability. The higher z using varian ce analysis in alpha(1E) and alpha(1E)beta(1a) can be explained by a s et of charges not coupled to pore opening. This set of charges moves i n transitions leading to nulls thus not contributing to the ionic curr ent fluctuations but eliciting gating currents. Coexpression of the be ta(2a) subunit would minimize the fraction of nulls leading to the cor rect estimation of the number of channels and z.