14-3-3ζ regulates nuclear trafficking of protein phosphatase 1α (PP1α) in HEK-293 cells

Protein phosphatase 1 (PP1) is one of the major Ser/Thr phosphatases in mammalian cells. There are four isoforms of PP1 namely, PP1α, PP1β/δ, PP1γ1 and PP1γ2. PP1γ and PP1β translocate to the nucleus by binding to a co-transporter that contains a nuclear localization signal. The mechanism by which PP1α shuttles between the nucleus and the cytosol is not known. In this study, we found that PP1α co-immunoprecipitates with 14-3-3ζ from HEK-293 cell lysates. By co-immunoprecipitation and GST pull-down assay, we determined that 14-3-3ζ binds to both PP1α (WT) and PP1α (T320A), and that phosphorylation of PP1α is not required for binding. Using PP1α deletion mutants, we located the 14-3-3ζ binding region within PP1α residues 159-279. An in vitro assay showed that 14-3-3ζ does not affect PP1α activity. When HEK-293 cells expressing PP1α and 14-3-3ζ were subjected to subcellular fractionation, the ratio of cytosolic vs. nuclear PP1α was significantly higher in cells expressing PP1α and 14-3-3ζ than those expressing PP1α alone. In cells expressing a dominant negative 14-3-3ζ (K49E), PP1α accumulated in the nucleus. Our results show that 14-3-3ζ binds to PP1α and causes its retention in the cytosol which suggests that 14-3-3ζ regulates nuclear trafficking of PP1α in mammalian cells.