Disruption of PKB signaling restores polarity to cells lacking tumor suppressor PTEN

By limiting phosphotidylinositol 3,4,5-triphosphate (PIP₃) levels, tumor suppressor PTEN not only controls cell growth but also maintains cell polarity required for cytokinesis and chemotaxis. To identify the critical targets of PIP₃ that link it to the cytoskeleton, we deleted secondary genes to reverse the deficiencies of pten^- cells in Dictyostelium. The polarity defects in pten^- cells correlate with elevated phosphorylations of PKB substrates. Deletion of AKT orthologue, PkbA, or a subunit of its activator TORC2, reduced the phosphorylations and suppressed the cytokinesis and chemotaxis defects in pten^- cells. In these double mutants, the excessive PIP₃ levels and, presumably, activation of other PIP₃-binding proteins had little or no effect on the cytoskeleton. In bands with increased phosphorylation in pten- cells, we found PKB substrates, PI5K, GefS, GacG, and PakA. Disruption of PakA in pten- cells restored a large fraction of the cells to normal behavior. Consistently, expression of phosphomimetic PakA in pten^- cells exacerbated the defects but nonphosphorylatable PakA had no effect. Thus, among many putative PTEN- and PIP₃-dependent events, phosphorylation of PKB substrates is the key downstream regulator of cell polarity.