Figure 1. Identification of STIM2 as a regulator of basal Ca²⁺ concentration.

(A) Overview of intracellular Ca²⁺ homeostasis. Basal cytosolic Ca²⁺ concentration is controlled by PM as well as ER Ca²⁺ channels and pumps.

(B) Sensitized siRNA screening assay for basal Ca²⁺ regulation. 2304 diced siRNA constructs were individually transfected into HeLa cells and cultured in 384 well plates. High and Low extracellular Ca²⁺ exposure (+10 mM and ~0.1 mM) were used for sensitization. Single cell Ca²⁺ levels were measured using automated image analysis software.

(C) Test experiments using a siRNA set targeting Ca²⁺ pumps, channels, and exchangers (performed in duplicate). Deviations from control Ca²⁺ levels are shown in units of standard deviation.

(D) Result from the sensitized siRNA screen of the human signaling proteome highlighting STIM2 and Calm1 as primary hits (performed in triplicate).

(E) Schematic representation of modular domains found in STIM2. On the luminal side: EF-hand is a Ca²⁺ binding domain and SAM is a conserved protein interaction domain. On the cytosolic side: CC and PB are a coiled-coil and a polybasic region, respectively.