Role for protein phosphatases in the cell-cycle-regulated phosphorylation of stathmin

Stathmin is a major cytosolic phosphoprotein that regulates microtubule dynamics during the assembly of the mitotic spindle. The activity of stathmin itself is regulated by changes in its state of phosphorylation during the transition from interphase to metaphase. For a better understanding of the regulation of stathmin activity during the cell cycle, we explored the mechanism(s) responsible for the decrease in the level of phosphorylation of stathmin as cells complete mitosis and enter a new G₁ phase. We show that stathmin mRNA and protein are expressed constitutively throughout the different phases of the cell cycle. This suggests that the non-phosphorylated stathmin that predominates during G₁ is not generated by degradation of phosphorylated stathmin in mitosis and synthesis of new unphosphorylated stathmin as cells enter a new G₁ phase. This suggested that protein phosphatases might be responsible for dephosphorylating stathmin as cells enter a new cell cycle. Okadaic acid-mediated inhibition of protein phosphatases in vivo showed a major increase in the level of phosphorylation of stathmin. Dephosphorylation studies in vitro showed differential patterns of site-specific dephosphorylaton of stathmin to protein phosphatase type 1, protein phosphatase type 2A and protein phosphatase type 2B. Thus stathmin might be a target for okadaic acid-sensitive protein phosphatase(s), and its activity in eukaryotic cells might be modulated by the sequential activity of specific protein kinases and phosphatases.