FIGURE 15-33 Synthesis of second messengers DAG and IP₃ from phosphatidylinositol (PI). Each membrane-bound PI kinase places a phosphate (yellow circles) on a specific hydroxyl group on the inositol ring, producing the phosphorylated derivatives PI(4)P and PI(4,5)P₂. Cleavage of PI(4,5)P₂ by phospholipase C yields the two important second messengers DAG and IP₃. Signaling is terminated when a phosphatase removes the 5-phosphate from IP₃; a second phosphatase removes the 1-phosphate, and the inositol 4-phosphate is reused to synthesize PI 4-phosphate. See A. Toker and L. C. Cantley, 1997, Nature 387:673, and C. L. Carpenter and L. C. Cantley, 1996, Curr. Opin. Cell Biol. 8:153.