Figure 2.

Store-dependent, higher-order oligomerization of STIM1 requires the CRAC activation domain (CAD). (A) Confocal images focused at the equator of HEK 293 cells expressing GFP-labeled STIM1 mutants. Resting cells were bathed in 2 mM Ca²⁺ Ringer's solution (left), and the same cells were store-depleted with 4 μM ionomycin in Ca²⁺-free Ringer's solution for 5 min (right). Arrowheads mark locations of puncta. Bar, 5 μm. (B) FRET-E images of a HEK 293 cell expressing CFP-STIM1 and YFP-STIM1 before (left) and after (right) store depletion with 1 μM ionomycin in Ca²⁺-free Ringer's solution for 7 min. Bar, 5 μm. (C) Time course of mean FRET-E (±SEM) from cells coexpressing CFP- and YFP-labeled STIM1-WT (black, n = 8), STIM1-CAD (blue, n = 9), STIM1-CC1 (green, n = 7), or STIM1-ΔC (red, n = 14). Extracellular solution changes are indicated above the graph.