Figure 5.

Immobile IP₃ receptor clusters initiate Ca²⁺signals. (A) HeLa cells with endogenous IP₃R1 tagged with EGFP showing IP₃R puncta (green) within endoplasmic reticulum (ER) membranes (red). (B) Diffraction-limited image of a punctum recorded using total internal reflection fluorescence (TIRF) microscopy, and the superimposed super-resolution (STORM) image showing IP₃Rs (red) within the punctum. The magenta square indicates the approximate size of a single tetrameric IP₃R. (C) IP₃Rs appear to be diffusively distributed within a punctum, suggesting the need for a scaffold to corral IP₃Rs. (D) TIRF images of a HeLa cell in which the ER has been depleted of Ca²⁺ by treatment with thapsigargin, showing the distribution of STIM1 (red) and IP₃R (green). The enlarged image shows that stromal interaction molecule (STIM) puncta form alongside the ER where the immobile IP₃Rs that are licensed to respond are parked. (Data results for A–D are from Thillaiappan et al. 2017.) (E) Licensed IP₃Rs parked alongside the ER–PM junctions where store-operated Ca²⁺ entry (SOCE) occurs may allow substantial loss of Ca²⁺ from that part of the ER without causing appreciable Ca²⁺ loss from the remaining ER. The ER–PM junction with its associated licensed IP₃Rs may constitute the basic functional unit for SOCE.