Level of expression controls modes of gating of a K⁺ channel

Several distinct subfamilies of K⁺ channel genes have been discovered by molecular cloning, however, in some cases the structural differences among them do not account for the diversity of K⁺ current types, ranging from transient A-type to slowly inactivating delayed rectifier-type, as members within each subfamily have been shown to code for K⁺ channels of different inactivation kinetics and pharmacological properties. We show that a single K⁺ channel cDNA of the Shaker subfamily (ShH4) can express in Xenopus oocytes not only a transient A-type K⁺ current but also, upon increased level of expression, slowly inactivating K⁺ currents with markedly reduced sensitivity to tetraethylammonium. In correlation with the macroscopic currents there are single-channel gating modes ranging from the fast-inactivation mode which underlies the transient A-type current, to slow-inactivation modes characterized by bursts of longer openings, and corresponding to the slowly inactivating macroscopic currents.