Module 3: Figure capacitative Ca2+ entry
Capacitative Ca2+ entry activated by depletion of Ca2+ in localized regions of the endoplasmic reticulum (ER) close to the plasma membrane.
The hypothesis outlined in this figure suggests that capacitative Ca2+ entry occurs at localized regions where there is close apposition of the endoplasmic reticulum (ER) and plasma membrane. The ER shown in red is a network of tubules connected to the nuclear membrane. Short arms of this ER network come into contact with the plasma membrane to form subsurface cisternae (SSC). The narrow gap between the SSC and the plasma membrane is the site where the signal from the empty ER is transferred to the store-operated channels (SOCs). Agonists acting on cell-surface receptors (green) in the plasma membrane stimulate phospholipase C (PLC) to produce a microdomain of inositol 1,4,5-trisphosphate (InsP3) (mauve), which functions to deplete Ca2+ in localized SSC. The latter then send a message to the SOCs in the plasma membrane to promote entry. This signal might occur through a protein–protein interaction, as outlined in the conformational coupling hypothesis. The box outlines the region where this mechanism might be located, as illustrated in Module 3: Figure conformational coupling hypothesis.