Figure 5. The β2N terminus (β2N) causes inactivation of BK currents from α-only channels expressed in heterologous cells.

(a) Representative macroscopic current traces from HEK293 cells expressing BK_SRKR channels, a daytime BK variant previously cloned from SCN (ref. ⁶⁰). β2N (0–100 μM), corresponding to the first 45 amino acids of the N terminus, was dissolved in recording solution and applied to the intracellular side via the patch pipette. (b) Representative currents in the presence of 50 μM β2N^ΔFIW, a peptide mutating the three residues required for inactivation⁴². There was no macroscopic current decay with β2N^ΔFIW. (c) Representative traces from the +90 mV voltage step at each concentration of β2N. The current peaks were scaled to illustrate the dose-dependent speeding of inactivation. (d) τ_inact values were calculated from the current elicited at +90 mV and plotted as a function of β2N concentration. τ_inact was plotted for BK_i and β2 KO SCN BK currents for cross-comparison. (e) τ_inact versus voltage for BK channels co-expressed the β2 subunit, or with 50 μM β2N. There is no difference in the voltage dependence of activation using the isolated β2N compared with the intact β2 subunit. All values are mean±s.e.m. For currents from HEK293 patches, n=4 at each concentration of β2N or β2N^ΔFIW peptide. For currents from daytime SCN neurons, BK_i (n=18) and β2 KO+β2N (n=10).