Evidence that a phosphatidylinositol 3,4,5-trisphosphate-binding protein can function in nucleus

PIP3BP is a phosphatidylinositol 3,4,5-trisphosphate-binding protein (PIP3BP) abundant in brain, containing a zinc finger motif and two pleckstrin homology (PH) domains. Staining of rat brain cells with anti-PIP3BP antibody and determination of localization of PIP3BP fused to the green fluorescent protein (GFP-PIP3BP) revealed that PIP3BP was targeted to the nucleus. Targeting was dependent on a putative nuclear localization signal in PIP3BP. Generation of PIP3 in the nucleus was detected in H2O2-treated 293T cells, nerve growth factor (NGF)-treated PC12 cells, and platelet-derived growth factor (PDGF)-treated NIH 3T3 cells. Translocation of phosphatidylinositol 3-kinase (PI 3-kinase) to the nucleus and enhanced activity of PI 3-kinase in the nucleus fraction were observed after H2O2 treatment of 293T cells, suggesting that PI 3-kinase can be activated in the nucleus as well as in the membrane after appropriate stimulation of the cells. Co-expression of the constitutively active PI 3-kinase with PIP3BP resulted in exportation of the protein from the nucleus to the cytoplasm, suggesting that PIP3BP can function as a PIP3-binding protein in the intact cells. These results imply that there may be an unknown function of PI 3-kinase in the nucleus.