Modulation of voltage-gated K+ channels by the sodium channel β1 subunit

Abstract

Voltage-gated sodium (Na(V)) and potassium (K(V)) channels are critical components of neuronal action potential generation and propagation. Here, we report that Na(V)β1 encoded by SCN1b, an integral subunit of Na(V) channels, coassembles with and modulates the biophysical properties of K(V)1 and K(V)7 channels, but not K(V)3 channels, in an isoform-specific manner. Distinct domains of Na(V)β1 are involved in modulation of the different K(V) channels. Studies with channel chimeras demonstrate that Na(V)β1-mediated changes in activation kinetics and voltage dependence of activation require interaction of Na(V)β1 with the channel's voltage-sensing domain, whereas changes in inactivation and deactivation require interaction with the channel's pore domain. A molecular model based on docking studies shows Na(V)β1 lying in the crevice between the voltage-sensing and pore domains of K(V) channels, making significant contacts with the S1 and S5 segments. Cross-modulation of Na(V) and K(V) channels by Na(V)β1 may promote diversity and flexibility in the overall control of cellular excitability and signaling.

Fig. 1.

Activation kinetics at +40 mV (A, gray hatched area in pulse protocol represents first 30 ms analyzed), voltage dependence of activation (B) (mean ± SEM), and deactivation (C, gray hatched area in pulse protocol represents last 10 ms analyzed) of K_V1.2 channels expressed in Xenopus oocytes in the presence or absence of Na_Vβ1 (n ≥ 7). Only currents of comparable amplitude between channel-alone, channel plus Na_Vβ1, were selected for analysis. (D) K_V1.2 stably expressed in L929 fibroblasts exhibits use-dependent activation when repetitive depolarizing pulses at +40mV are administered at 1-s intervals. Average normalized current traces show Na_Vβ1 speeds up activation of K_V1.2 at the first pulse and to a lesser extent at the ninth pulse (K_V1.2, n = 6; K_V1.2 + Na_Vβ1, n = 9 cells). (E) Western blot of coprecipitation experiment in transfected HEK cells showing that K_V1.2 and Na_Vβ1 coassemble. (F) Mouse cerebral cortex immunostained for K_V1.2 (green; Left) and Na_Vβ1 (red; Center), showing colocalization in the axon initial segment in the merged image (Right).

Fig. 2.

Activation kinetics (A, D, G, and J), voltage dependence of activation (B, E, H, and K) (mean ± SEM), and deactivation kinetics (C, F, I, and L) of K_V1.1, K_V1.3, K_V1.6, and K_V3.1 expressed alone (black) or with Na_Vβ1 (red) in Xenopus oocytes (K_V1.1, K_V1.3, and K_V1.6) or mammalian cells (K_V3.1) (n ≥ 5). (M) Western blot of coimmunoprecipitation experiment in transfected HEK cells showing that K_V7.2 and Na_Vβ1 coprecipitate. (N) Na_Vβ1 slows activation of the K_V7.2 current in CHO cells at moderate depolarizing potentials (n = 5). (O) Relative instantaneous current of K_V7.2 shows a significant difference in the 1-s prepulse potential in the presence of Na_Vβ1 (n = 9).

Fig. 3.

Schematic representation of Na_Vβ1 (red), myelin P₀ protein (blue), and their chimeras (A). Inset, Extracellular domain of Na_Vβ1 showing the R85C and C121W epilepsy-causing mutations. (B and E) Effect on K_V1.2 and K_V1.3 activation kinetics by Na_Vβ1 versus P₀ (B and E), chimeras (C and F), and Na_Vβ1 mutants (D and G) studied in Xenopus oocytes. Current traces are averages of normalized, representative currents of comparable amplitudes and shown are the first 30-ms activating phase of the 200-ms traces.

Fig. 4.

Activation kinetics (A and B), voltage dependence of activation (D and E; mean ± SEM), cumulative inactivation (G and H), and deactivation of K_V1.1–K_V1.3 and the K_V1.3–K_V1.1 chimeras (J and K), expressed alone (black) or with Na_Vβ1 (red) in Xenopus oocytes. Schematic showing interaction between Na_Vβ1 and domains within the channel chimeras (C, F, I, and L). Current traces shown are averages of normalized currents of selective sample currents of comparable amplitudes. Side view (M) and view from the extracellular side of the membrane (N) of the model of the complex of K_V1.2 with Na_Vβ1. The channel monomers are colored in four different shades of blue, Na_Vβ1 in tan, and residues that are lipid exposed on the S1 and S5 segments and that differ in character to equivalent residues in other K_V channel families are displayed as green and orange spheres, respectively.