STIM1_R304W enhances CRAC channel activity in transfected cells. (A) Time course of ICRAC in HEK 293 cells transiently cotransfected with ORAI1 plus STIM1_WT (OS_WT; black, n = 5), ORAI1 plus STIM1_R304W (OS_R304W; red, n = 7), ORAI1 plus STIM1_K(384-6)Q [OS_K(384-6)Q; dark yellow, n = 5], ORAI1 plus STIM1_R304W_K(384-6)Q [OS_R304W_K(384-6)Q; blue, n = 6], ORAI1 plus STIM1_D76A (OS_D76A; green, n = 7), or ORAI1 plus a 1:1 mix of STIM1_WT and STIM1_R304W (OS_WT/R304W; burgundy, n = 8) induced by 10 mM EGTA and suppressed by 20 µM La3+. (B and C) Current–voltage curves taken at 50 (B) or 200 s (C) after break-in in cells transfected with plasmid combinations shown in A. (D) Representative step currents generated from hyperpolarizing pulses at the indicated test potentials at 200 s following break-in in cells transfected with OS_WT, OS_R304W, OS_D76A, or OS_WT/R304W in 10 mM extracellular Ca2+. Duration of the pulse was 200 ms. (E) Quantification of inactivation as determined by the ratio (R195ms) of the peak current at the beginning of the pulse (I0) to tail current at the end of the pulse (I195) in cells transfected with OS_WT (n = 6), OS_R304W (n = 7), D76A (n = 8), or OS_WT/R304W (n = 8).