Editing G protein-coupled receptor (section)

===Phosphatidylinositol signal pathway===

{{Main|IP3/DAG pathway}}
In the [[phosphatidylinositol]] signal pathway, the extracellular signal molecule binds with the G-protein receptor (G<sub>q</sub>) on the cell surface and activates [[phospholipase C]], which is located on the [[cell membrane|plasma membrane]]. The [[lipase]] hydrolyzes [[phosphatidylinositol 4,5-bisphosphate]] (PIP2) into two second messengers: [[inositol trisphosphate|inositol 1,4,5-trisphosphate (IP3)]] and [[diacylglycerol]] (DAG). IP3 binds with the [[IP3 receptor]] in the membrane of the smooth endoplasmic reticulum and mitochondria to open Ca<sup>2+</sup> channels. DAG helps activate [[protein kinase C]] (PKC), which phosphorylates many other proteins, changing their catalytic activities, leading to cellular responses.

The effects of Ca<sup>2+</sup> are also remarkable: it cooperates with DAG in activating PKC and can activate the [[Ca2+/calmodulin-dependent protein kinase|CaM kinase]] pathway, in which calcium-modulated protein [[calmodulin]] (CaM) binds Ca<sup>2+</sup>, undergoes a change in conformation, and activates CaM kinase II, which has unique ability to increase its binding affinity to CaM by autophosphorylation, making CaM unavailable for the activation of other enzymes. The kinase then phosphorylates target enzymes, regulating their activities. The two signal pathways are connected together by Ca<sup>2+</sup>-CaM, which is also a regulatory subunit of adenylyl cyclase and phosphodiesterase in the cAMP signal pathway.