The non-transmembrane protein tyrosine phosphatase, PTP-S, is located predominantly in the cell nucleus in association with chromatin. Here we have analysed the expression of PTP-S upon mitogenic stimulation and during cell division cycle. During liver regeneration after partial hepatectomy, PTP-S mRNA levels increased 16-fold after 6 h (G₁ phase) and declined thereafter. Upon stimulation of serum starved cells in culture with serum, PTP-S mRNA levels increased reaching a maximum during late G₁ phase and declined thereafter. No significant change in PTP-S RNA levels was observed in growing cells during cell cycle. PTP^-S protein levels were also found to increase upon mitogenic stimulation. Upon serum starvation for 72 h, PTP-S protein disappears from the nucleus and is seen in the cytoplasm; after 96 h of serum starvation the PTP-S protein disappears from the nucleus as well as cytoplasm. Refeeding of starved cells for 6 h results in reappearance of this protein in the nucleus. Our results suggest a role of this phosphatase during cell proliferation.

PTP-S2 is a tyrosine specific protein phosphatase that binds to DNA and is localized to the nucleus in association with chromatin. It plays a role in the regulation of cell proliferation. Here we show that the subcellular distribution of this protein changes during cell division. While PTP-S2 was localized exclusively to the nucleus in interphase cells, during metaphase and anaphase it was distributed throughout the cytoplasm and excluded from condensed chromosomes. At telophase PTP-S2 began to associate with chromosomes and at cytokinesis it was associated with chromatin in the newly formed nucleus. It was hyperphosphorylated and showed retarded mobility in cells arrested in metaphase.In vitro experiments showed that it was phosphorylated by CK2 resulting in mobility shift. Using a deletion mutant we found that CK2 phosphorylated PTP-S2 in the C-terminal non-catalytic domain. A heparin sensitive kinase from mitotic cell extracts phosphorylated PTP-S2 resulting in mobility shift. These results are consistent with the suggestion that during metaphase PTP-S2 is phosphorylated (possibly by CK2 or a CK2-like enzyme), resulting in its dissociation from chromatin.