Enhanced expression of a chromatin associated protein tyrosine phosphatase during G0 to S transition

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.