Once PLC-γ has been recruited to the inner face of the plasma membrane and has been activated, it can catalyze the breakdown of the membrane lipid PIP2 (see Section 7-4 and Fig. 7.5) to generate two products, the membrane lipid diacylglycerol (DAG) and the diffusible second messenger inositol 1,4,5-trisphosphate (IP3) (not to be confused with the membrane lipid PIP3) (Fig. 7.17). DAG is confined to the membrane, but diffuses in the plane of the membrane and serves as a molecular target that recruits other signaling molecules to the membrane. IP3 diffuses into the cytosol and binds to IP3 receptors on the endoplasmic reticulum (ER) membrane. These receptors are Ca2+ channels, which open and release calcium stored in the ER into the cytosol. The consequent low levels of calcium in the ER then cause a conformational change in the transmembrane protein STIM1, promoting its clustering within the ER membrane. The STIM1 oligomers bind to the plasma membrane, where they interact directly with ORAI1, the plasma membrane calcium channel (also known as the CRAC channel: calcium release-activated calcium channel). Binding of STIM1 to ORAI1 triggers calcium channel opening, allowing extracellular calcium to flow into the cell to activate further signaling pathways and to replenish ER calcium stores.
The activation of PLC-γ marks an important step in T-cell activation, because after this point the PLC-γ signaling module splits into three distinct branches— the stimulation of Ca2+ entry, the activation of Ras, and the activation of protein kinase C-θ (PKC-θ)—each of which ends in the activation of a different transcription factor. These signaling pathways are utilized by many cell types, in addition to lymphocytes. Their importance in T-cell activation is shown by the observation that treatment of T cells with phorbol myristate acetate (an analog of DAG) and ionomycin (a pore-forming drug that allows extracellular calcium to flow into the cell) can reconstitute many of the effects of T-cell receptor stimulation. Additionally, deficiencies in several of these signaling components, including Lck, Zap-70, Itk, CD45, Carma1, and ORAI1, have been found to be mutated in cases of severe combined immunodeficiency (SCID).
7-14 Ca2+ entry activates the transcription factor NFAT.
One of the three signaling pathways leading from PLC-γ results in an influx of calcium ions into the cytosol. An important outcome of the increased cytosolic Ca2+ resulting from T-cell receptor signaling via PLC-γ is the activation of a family of transcription factors called NFAT (nuclear factor of activated
T cells). NFAT is something of a misnomer, because the five members of this family are expressed in many different tissues. In the absence of activating