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STK15 is an Aurora/Ipl-1 related serine/threonine kinase that is associated with centrosomes and induces aneuploidy when overexpressed in mammalian cells. It is well known that phosphorylation and dephosphorylation of kinases are important for regulation of their activity. But mechanisms by which STK15 activity is regulated have not been elucidated. We report that STK15 contains two functional binding sites for protein phosphatase type 1 (PP1), and the binding of these proteins is cell cycle-regulated peaking at mitosis. Activated STK15 at mitosis phosphorylates PP1 and inhibits PP1 activity in vitro. In vivo, PP1 activity co-immunoprecipitated with STK15 is also reduced. These data indicate that STK15 inhibits PP1 activity during mitosis. Also, PP1 is shown to dephosphorylate active STK15 and abolish its activity in vitro. Furthermore, we show that non-binding mutants of STK15 for PP1 are superphosphorylated, but their kinase activities are markedly reduced. Cells transfected with these non-binding mutants manifest aberrant chromosome alignment during mitosis. Our results suggest that a feedback regulation through phosphorylation/dephosphorylation events between STK15 kinase and PP1 phosphatase operates through the cell cycle. Deregulation of this balance may contribute to anomalous segregation of chromosomes during mitotic progression of cancer cells.
Katayama et al. (Sat,) studied this question.