Store-independent, spontaneous activity of STIM1 deletion mutants. (a and b) Ca²⁺ influx was measured in HEK cells loaded with Fura2 and alternately perfused with Ca²⁺-free and Ca²⁺-containing solutions. The traces are mean ± SEM recorded from 30–40 HEK293 cells from three experiments transfected with YFP or the indicated STIM1 deletion mutants (a) and mutants in which four lysine residues in SOAR (384–387) were mutated to alanine (b). (c) Example traces of normalized whole cell current at −100 mV measured in HEK cells expressing mCherry-Orai1 and the indicated YFP-STIM1 C-terminal deletion mutants. Pipette solution contained 10 mM BAPTA and external solution contained 10 mM Ca²⁺. Current measurement started within 10 s of break-in to evaluate the spontaneous current. The summary of multiple experiments is giver in panel d, which shows the mean ± SEM of mean spontaneous current of the indicated number of cells.