FIGURE 16-29 Recruitment and activation of protein kinase B (PKB) in PI-3 kinase pathways. In unstimulated cells (step 1), PKB is in the cytosol with its PH domain bound to its catalytic kinase domain, inhibiting its activity. Hormone stimulation leads to activation of PI-3 kinase and subsequent formation of PI 3-phosphates (see Figure 16-28). The 3-phosphate group serves as a docking site on the plasma membrane for the PH domain of PKB (step 2) and another kinase, PDK1. Full activation of PKB requires phosphorylation both in the activation loop by PDK1 and at the C-terminus by a second kinase, PDK2 (step 3). See A. Toker and A. Newton, 2000, Cell 103:185, and S. Sarbassov et al., 2005, Curr. Opin. Cell Biol. 17:596.