Fig. 4. STIM1 oligomerization and STIM1 clustering upon strong ER Ca²⁺ depletion, impeded ER Ca²⁺ replenishment at high and low [Ca²⁺]_cyto levels. (A-C) Representative tracings (left graphs) and statistical summary (right graphs) of [Ca²⁺]_cyto (A, n=12), [Ca²⁺]_ER (B, n=9) and STIM1 oligomerization (C, n=28) in response to stimulation with 100 µM histamine and the SERCA inhibitor BHQ (15 µM). (D) Subcellular distribution of STIM1 at a TIRF-like plane (left images) and at a middle z-plane (right images) under resting conditions (`prior stimulation', upper panel), under cell stimulation with 100 µM histamine and 15 µM BHQ in 2 mM Ca<sup>2+</sup> (`His/BHQ in Ca²⁺', upper middle panel) and cell stimulation in EGTA (`His/BHQ in EGTA', lower middle panel). Along the lines indicated, fluorescence intensity of YFP-STIM1 is presented at the corresponding z-plane and activation stages in the right graphs. The lower images and graphs show overlays of the upper illustrations. (E) Average data and respective statistical evaluation of the experiments displayed in D (n=11 for each condition). (F) The time course of STIM1 clustering (n=3 for each condition). ^*P<0.05 versus the respective compared data set.