FIGURE 16-28 Generation of phosphatidylinositol 3-phosphates. The enzyme phosphatidylinositol-3 kinase (PI-3 kinase) is recruited to the membrane by many activated receptor tyrosine kinases (RTKs) and cytokine receptors. The 3-phosphate added by this enzyme, to yield PI(3,4)P₂ or PI(3,4,5)P₃, is a binding site for various signal-transducing proteins, such as the PH domain of protein kinase B. Phosphatidylinositol 4,5-bisphosphate is also the substrate of phospholipase C (see Figure 15-33). See L. Rameh and L. C. Cantley, 1999, J. Biol. Chem. 274:8347.