Nuclear Localization of PTEN Is Regulated by Ca2+ through a Tyrosil Phosphorylation–Independent Conformational Modification in Major Vault Protein

Abstract

We have recently shown in MCF-7 cells that nuclear phosphatase and tensin homologue deleted on chromosome 10 (PTEN) down-regulates phosphorylation of p44/42 and cyclin D1 and induces G(1) cell cycle arrest, whereas cytoplasmic PTEN down-regulates phosphorylation of Akt, up-regulates p27, and induces apoptosis. In this manner, nucleocytoplasmic partitioning of PTEN seems to differentially regulate the cell cycle and apoptosis. We have also reported that PTEN has nuclear localization signal-like sequences required for major vault protein (MVP)-mediated nuclear translocation. To date, several other proteins are reported to interact with MVP, including extracellular signal-regulated kinases and steroid receptors, suggesting that MVP is likely to be involved in signal transduction through nucleocytoplasmic transport. However, the exact mechanism of MVP-mediated nucleocytoplasmic shuttling remains elusive. PTEN reportedly interacts in vitro with the EF hand-like motif of MVP in a Ca(2+)-dependent manner. The current study shows that small interfering RNA-mediated MVP silencing decreases the nuclear localization of PTEN and increases phosphorylation of nuclear p44/42. We show in situ that PTEN-MVP interaction is Ca(2+) dependent and is abolished by Mg(2+). Nuclear localization of PTEN is decreased by increasing Ca(2+) levels in culture medium in a dose-dependent manner. Ca(2+) ionophore A23187 increases nuclear localization of PTEN and decreases phosphorylation of nuclear p44/42. Finally, we show that Ca(2+)-dependent PTEN-MVP interaction is not related to MVP's tyrosil phosphorylation but rather due to its conformational modification. Our observations suggest that Ca(2+) regulates PTEN's nuclear entry through a tyrosil phosphorylation-independent conformational change in MVP. Collectively, our data present evidence of a novel crosstalk between the Ca(2+) signaling-mediated regulation of the cell cycle and MVP-mediated nuclear PTEN localization and function.