Position-dependent attenuation by Kv1.6 of N-type inactivation of Kv1.4-containing channels

Assembly of distinct α subunits of Kv1 (voltage-gated K+ channels) into tetramers underlies the diversity of their outward currents in neurons. Kv1.4-containing channels normally exhibit N-type rapid inactivation, mediated through an NIB (N-terminal inactivation ball); this can be over-ridden if associated with a Kv1.6 α subunit, via its NIP (N-type inactivation prevention) domain. Herein, NIP function was shown to require positioning of Kv1.6 adjacent to the Kv1.4 subunit. Using a recently devised gene concatenation, heterotetrameric Kv1 channels were expressed as single-chain proteins on the plasmalemma of HEK (human embryonic kidney)-293 cells, so their constituents could be arranged in different positions. Placing the Kv1.4 and 1.6 genes together, followed by two copies of Kv1.2, yielded a K+ current devoid of fast inactivation. Mutation of critical glutamates within the NIP endowed rapid inactivation. Moreover, separating Kv1.4 and 1.6 with a copy of Kv1.2 gave a fast-inactivating K+ current with steady-state inactivation shifted to more negative potentials and exhibiting slower recovery, correlating with similar inactivation kinetics seen for Kv1.4-(1.2)3. Alternatively, separating Kv1.4 and 1.6 with two copies of Kv1.2 yielded slow-inactivating currents, because in this concatamer Kv1.4 and 1.6 should be together. These findings also confirm that the gene concatenation can generate K+ channels with α subunits in pre-determined positions.