Notice the difference between this use of phosphoinositides and their use described earlier, in which PI(4,5)P2 is cleaved by PLCβ (in the case of GPCRs) or PLCγ(in the case of RTKs) to generate soluble IP3 and membrane-bound diacylglycerol (see Figures 15–28 and 15–29). By contrast, PI(3,4,5)P3 is not cleaved by either PLC. It is made from PI(4,5)P2 and then remains in the plasma membrane until specific phosphoinositide phosphatases dephosphorylate it. Prominent among these is the PTEN phosphatase, which dephosphorylates the 3 position of the inositol ring. Mutations in PTEN are found in many cancers: by prolonging signaling by PI 3-kinase, they promote uncontrolled cell growth.
There are various types of PI 3-kinases. Those activated by RTKs and GPCRs belong to class I. These are heterodimers composed of a common catalytic subunit and different regulatory subunits. RTKs activate class Ia PI 3-kinases, in which the regulatory subunit is an adaptor protein that binds to two phosphotyrosines on activated RTKs through its two SH2 domains (see Figure 15–46A). GPCRs activate class Ib PI 3-kinases, which have a regulatory subunit that binds to the βγ complex of an activated trimeric G protein (Gq) when GPCRs are activated by their extracellular ligand. The direct binding of activated Ras can also activate the common class I catalytic subunit.
6 1 2 3 4 5 diacylglycerol IP3 PLC CATALYZED BY PI 3-KINASE phosphatidylinositol (PI) PI 4-phosphate [PI(4)P] PI 3,4-bisphosphate [PI(3,4)P2] PI 4,5-bisphosphate [PI(4,5)P2] P P P P PP P P P P P P P P P
Figure 15–52 The generation of phosphoinositide docking sites by PI 3-kinase. PI 3-kinase phosphorylates the inositol ring on carbon atom 3 to generate the phosphoinositides shown at the bottom of the figure (diverting them away from the pathway leading to IP3 and diacylglycerol; see Figure 15–28). The most important phosphorylation (indicated in red) is of PI(4,5)P2 to PI(3,4,5)P3, which can serve as a docking site for signaling proteins with PI(3,4,5)P3-binding PH domains. Other inositol phospholipid kinases (not shown) catalyze the phosphorylations indicated by the green arrows.
PI 3-phosphate PI 3,4,5-trisphosphate [PI(3)P] [PI(3,4,5)P3]
Intracellular signaling proteins bind to PI(3,4,5)P3 produced by activated PI 3-kinase via a specific interaction domain, such as a pleckstrin homology (PH) domain, first identified in the platelet protein pleckstrin. PH domains function mainly as protein–protein interaction domains, and it is only a small subset of them that bind to PI(3,4,5)P3; at least some of these also recognize a specific membrane-bound protein as well as the PI(3,4,5)P3, which greatly increases the specificity of the binding and helps to explain why the signaling proteins with PI(3,4,5) P3-binding PH domains do not all dock at all PI(3,4,5)P3 sites. PH domains occur in about 200 human proteins, including the Ras-GEF Sos discussed earlier (see Figure 15–11).