Figure 6.

Ensemble variance analysis of hexameric Orai1 concatemers. (A) A thapsigargin-pretreated cell coexpressing 6xOrai1 and mCh-STIM1 was transiently exposed to a series of DVF solutions containing various amounts of free Ca²⁺ to partially block monovalent current. Current at a holding potential of −100 mV is displayed, with the gray line indicating 2 mM Ca²⁺_o and colored bars indicating DVF with added Ca²⁺. (B) 200-ms current sweeps collected at the times indicated by the colored dots in (A). Current variance increases and then declines in response to increasing block by Ca²⁺. (C) Ca²⁺-dependent block of Na⁺ current in two cells expressing 1xOrai1 (black) or 6xOrai1 (red). Curves represent best fits of the Hill equation, $block=1/[1+{(K_{1/2}/\left[\text{Ca}\right])}^{n_H}]$, with K_1/2 = 17.6 μM and n_H = 1.3 (1xOrai1), or K_1/2 = 16.2 μM and n_H = 1.1 (6xOrai1). (D) Plots of baseline-corrected current variance versus amplitude in two cells expressing 1xOrai1 (black) or 6xOrai1 (red) and exposed to DVF with partially blocking Ca²⁺ concentrations. Parabolic fits indicate the unitary current amplitude (–87 fA for 6xOrai1, –91 fA for 1xOrai1) and channel open probability in the absence of Ca²⁺ (P_o = 0.82 for 6xOrai1, 0.74 for 1xOrai1) (see Materials and Methods). To see this figure in color, go online.