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.