Abstract
The mitotic kinase Aurora B regulates the condensation of chromatin into chromosomes by phosphorylating chromatin proteins during early mitosis, whereas the phosphatase PP1γ performs the opposite function. The roles of Aurora B and PP1γ must be tightly coordinated to maintain chromosomes at a high phosphorylation state, but the precise mechanisms regulating their function remain largely unclear. Here, mainly through immunofluorescence microscopy and co-immunoprecipitation assays, we find that dissociation of PP1γ from chromosomes is essential for maintaining chromosome phosphorylation. We uncover that PP1γ is recruited to mitotic chromosomes by its regulatory subunit Repo-Man in the absence of Aurora B activity and that Aurora B regulates dissociation of PP1γ by phosphorylating and disrupting PP1γ-Repo-Man interactions on chromatin. Overexpression of Repo-Man mutants that cannot be phosphorylated or inhibition of Aurora B kinase activity resulted in the retention of PP1γ on chromatin and prolonged the chromatin condensation process; a similar outcome was caused by the ectopic targeting of PP1γ to chromatin. Together, our findings reveal a novel regulation mechanism of chromatin condensation in which Aurora B counteracts PP1γ activity by releasing PP1γ from Repo-Man and may have important implications for understanding the regulations of dynamic structural changes of the chromosomes in mitosis.
Highlights
The mitotic kinase Aurora B regulates the condensation of chromatin into chromosomes by phosphorylating chromatin proteins during early mitosis, whereas the phosphatase PP1g performs the opposite function
To investigate how protein kinases and phosphatases cooperate to maintain chromosome phosphorylation in early mitosis, we first tested the localization of Aurora B and PP1g, as it was reported that their localization is dynamic [29, 30]
By expressing GFP-fused PP1g and immunostaining for Aurora B, we found that, when the prophase cells went into metaphase, Aurora B accumulated on the chromosomes, whereas PP1g dissociated from the chromatin/chromosomes and relocated to the cytoplasm, mitotic spindle, and kinetochores (Fig. 1A)
Summary
The mitotic kinase Aurora B regulates the condensation of chromatin into chromosomes by phosphorylating chromatin proteins during early mitosis, whereas the phosphatase PP1g performs the opposite function. We uncover that PP1g is recruited to mitotic chromosomes by its regulatory subunit Repo-Man in the absence of Aurora B activity and that Aurora B regulates dissociation of PP1g by phosphorylating and disrupting PP1g–RepoMan interactions on chromatin.
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