Fig. 3. Phe³⁹¹ is a critical residue for the Orai1 activation function of STIM1. (A), Representative FLIPR traces of SOCE in HEKO1 STIM1^-/- cells transiently transfected with mCherry (Control, red), mCherry-STIM1-∆K (STIM1-∆K, black), mCherry-STIM1-∆K F391W (STIM1-∆K F391W, blue), mCherry-STIM1-∆K F391L (STIM1-∆K F391L, green), mCherry-STIM1-∆K F391V (STIM1-∆K F391V, purple) or mCherry-STIM1-∆K F391A (STIM1- ∆K F391A, aqua). 1 µM thapsigargin (Tg) and 2 mM CaCl₂ (Ca²⁺) administrations are indicated. (B), Quantified area under the curve (AUC) and (C), slope of the SOCE traces after extracellular Ca²⁺ application in HEKO1 STIM1^-/- cells transiently transfected with control (red), STIM1-∆K (black), STIM1-∆K F391W (blue), STIM1-∆K F391L (green), STIM1-∆K F391V (purple) or STIM1-∆K F391A (aqua). The data was normalized by setting the AUC and slope of the Ca²⁺ entry traces to 100 (n = 18; mean ± standard deviation). (D), average current density (CD) of the I_CRAC recordings (STIM1-∆K, n = 14; STIM1-∆K F391W, n = 8; STIM1-∆K F391L, n = 5; STIM1-∆K F391V, n = 6; STIM1-∆K F391A, n = 4; mean - SEM) from HEKO1 STIM1^-/- cells. (E), average current-voltage (I-V) curves of the I_CRAC recordings at t = 2 min from the traces shown in D. (F), quantified average CD values at t = 2 min (STIM1-∆K, n = 14; STIM1-∆K F391W, n = 8; STIM1-∆K F391L, n = 5; STIM1-∆K F391V, n = 6; STIM1-∆K F391A, n = 4; mean ± SEM) from HEKO1 STIM1^-/- cells transiently overexpressing STIM1-∆K (black), STIM1-∆K F391W (blue), STIM1-∆K F391L (green), STIM1-∆K F391V (purple) or STIM1-∆K F391A (aqua). p value (p) of each of the STIM1-∆K Phe³⁹¹ mutant compared to the STIM1-∆K group is indicated above the respective bar as non-significant (n.s.) for p > 0.05, "*" for p ≤ 0.05, "**" for p ≤ 0.01 or "***" for p ≤ 0.001.