A, mean (±SEM) inward ARC channel currents measured at −80 mV in cells expressing the Lck-STIM1-Δ448 construct, or the same construct bearing the T389A mutation. Values are mean ± SEM with values of n (number of cells) shown in parentheses. **P = 0.0005 as indicated by Student's unpaired two-tailed t test. B, representative current–voltage relationships for the corresponding ARC channel currents in cells expressing the Lck-STIM1-Δ448 construct (left), or the same construct bearing the T389A mutation (right). C, representative image of cells confirming the plasma membrane expression of a Cerulean-tagged Lck-STIM1-Δ448 construct bearing the T389A mutation. Scale bar = 10 μm. D, representative images of the cellular FRET signal obtained on co-expression of an N-terminal Venus-tagged Orai3 construct with either a C-terminal Cerulean-tagged Lck-STIM1-Δ448(T389A) construct (top), or the wild-type Lck-STIM1-Δ448 construct (bottom). Scale bar = 5 μm. Also shown are the mean (±SEM) of the corresponding FRET values.