It is one of the G protein-coupled receptor signaling pathways. In the phosphatidylinositol signaling pathway, the extracellular signaling molecule binds to the G protein-coupled receptor on the cell surface, activates phospholipase C (PLC-β) on the plasma membrane, and hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) on the plasma membrane into 1,4,5-trisphosphate inositol (IP3) and diacylglycerol (DG). Two second messengers, extracellular signals are converted to intracellular signals (Figure 8-21), this signaling system is also known as "double messenger system" (double messenger system).

IP3 binds to IP3 ligand-gated calcium channels in the endoplasmic reticulum and opens the calcium channels, increasing the intracellular Ca2+ concentration. Activates various calcium ion-dependent proteins. Treatment of cells with the Ca2+ carrier ionomycin produces similar results (Fig. 8-22).

DG binds to the plasma membrane and activates plasma membrane-bound Protein Kinase C (PKC).PKC is distributed in the cytosol in an inactive form, and when the cell receives a stimulus that produces IP3, which increases the Ca2+ concentration, PKC translocates to the inner surface of the plasma membrane, where it is activated by DG (Fig. 8-22), and PKC can cause protein Phosphorylation of serine/threonine residues is different cells produce different responses, such as cell secretion, muscle contraction, cell proliferation and differentiation, etc. The role of DG can be simulated by phorbol ester.

Ca2+ activates a variety of Ca2+-binding proteins to elicit cellular responses. calmodulin (CaM) consists of a single peptide chain with four calcium ion binding sites. Binding calcium ions undergoes a conformational change that activates calmodulin-dependent kinase (CaM-Kinase). The cellular response to Ca2+ depends on the type of intracellular calcium-binding protein and calmodulin-dependent kinase. For example, calmodulin-dependent kinase II is abundant at the synapses of mammalian brain neurons and is associated with memory formation. Mice with point mutations in this protein show marked memory incompetence.

IP3 signaling is terminated by dephosphorylation to form IP2, or by phosphorylation to form IP4. Ca2+ is pumped out of the cell by Ca2+ pumps on the plasma membrane and Na+-Ca2+ exchangers, or into the endoplasmic reticulum by calcium pumps on the endoplasmic reticulum membranes

DG terminates its messenger action by two pathways: either it is phosphorylated to phosphatidic acid by DG-kinase, and enters the phospholipid phosphatidylinositol cycle; and second, hydrolyzed by DG esterase to monoesterylglycerol. Due to the short cycle of DG metabolism, it is impossible to maintain PKC activity for a long period of time, while changes in cell proliferation or differentiation behavior require the effects produced by long-term PKC activity. Another pathway of DG production, that is, DG produced by phospholipase-catalyzed rupture of phosphatidylglycerol at the plasma membrane, has now been identified to maintain the long-term effects of PKC.