More Gating Charges are Needed to Open a Shaker K+ Channel than are Needed to Open an rBIIA Na+ Channel

This study presents what is, to our knowledge, a novel technique by means of which the ratio of the single gating charges of voltage-gated rat brain IIA (rBIIA) sodium and Shaker potassium ion channels was estimated. In the experiment, multiple tandems of enhanced green fluorescent protein were constructed and inserted into the C-terminals of Na+ and K+ ion channels. cRNA of Na+ and K+ ion channels was injected and expressed in Xenopus laevis oocytes. The two electrode voltage-clamp technique allowed us to determine the total gating charge of sodium and potassium ion channels, while a relative measure of the amount of expressed channels could be established on the basis of the quantification of the fluorescence intensity of membrane-bound channels marked by enhanced green fluorescent proteins. As a result, gating charge and fluorescence intensity were found to be positively correlated. A relative comparison of the single gating charges of voltage-gated sodium and potassium ion channels could thus be established: the ratio of the single gating charges of the Shaker potassium channel and the rBIIA sodium channel was found to be 2.5 ± 0.4. Assuming the single channel gating charge of the Shaker K+ channel to be ∼13 elementary charges (well supported by other studies), this leads to approximately six elementary charges for the rBIIA sodium channel, which includes a fraction of gating charge that is missed during inactivation.