Frat Is a Phosphatidylinositol 3-Kinase/Akt-Regulated Determinant of Glycogen Synthase Kinase 3 Subcellular Localization in

Suppressing the activity of Gsk3 is critical for maintenance of murine pluripotent stem cells. In murine embryonic stem cells (mESCs), Gsk3 is inhibited by multiple mechanisms, including its inhibitory phosphorylation on serine 9 by protein kinase B (Akt), a major effector of the canonical phosphatidylinositol 3-kinase (PI3K) pathway. A second PI3K/Akt-regulated mechanism promotes the nuclear export of Gsk3, thereby restricting its access to nuclear substrates such as c-myc and -catenin. Although Gsk3 shuttles between the nucleus and cytoplasm under self-renewing conditions, its localization is primarily cytoplasmic be-cause its rate of nuclear export exceeds its rate of nuclear import. In this report, we show that Gsk3 is exported from the nu-cleus in a complex with Frat. Loss of PI3K/Akt activity results in dissociation of this complex and retention of Gsk3 in the nu-cleus. Frat continues to shuttle between the nucleus and cytoplasm under these conditions and remains predominantly in the cytoplasm. These results indicate that Frat carries Gsk3 out of the nucleus under self-renewing conditions and that PI3K regu-lates this by promoting its association with Frat. These findings provide new links between PI3K/Akt signaling and regulation of Gsk3 activity by Frat, an oncogene previously shown to cooperate withMyc in tumorigenesis. Suppression of Gsk3 activity in murine embryonic stem cells(mESCs) is required for stabilization of the pluripotent state and for maintenance of self-renewing capacity (5, 13, 14). This is achieved by the action of canonical phosphatidylinositol 3-kinase (PI3K) signaling through its key effector, protein kinase B (Akt)