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Protein phosphatase 2A (PP2A) plays a major role in dephosphorylating the targets of the major mitotic kinase Cdk1 at mitotic exit, yet how it is regulated in mitotic progression is poorly understood. Here we show that mutations in either the catalytic or regulatory twins/B55 subunit of PP2A act as enhancers of  gwl Scant  , a gain-of-function allele of the Greatwall kinase gene that leads to embryonic lethality in  Drosophila  when the maternal dosage of the mitotic kinase Polo is reduced. We also show that heterozygous mutant  endos  alleles suppress heterozygous  gwl Scant  ; many more embryos survive. Furthermore, heterozygous  PP2A  mutations make females heterozygous for the strong mutation  polo 11   partially sterile, even in the absence of  gwl Scant  . Heterozygosity for an  endos  mutation suppresses this  PP2A / polo 11   sterility. Homozygous mutation or knockdown of  endos  leads to phenotypes suggestive of defects in maintaining the mitotic state. In accord with the genetic interactions shown by the  gwl Scant   dominant mutant, the mitotic defects of Endos knockdown in cultured cells can be suppressed by knockdown of either the catalytic or the Twins/B55 regulatory subunits of PP2A but not by the other three regulatory B subunits of  Drosophila  PP2A. Greatwall phosphorylates Endos at a single site, Ser68, and this is essential for Endos function. Together these interactions suggest that Greatwall and Endos act to promote the inactivation of PP2A-Twins/B55 in  Drosophila . We discuss the involvement of Polo kinase in such a regulatory loop.

Suppression of Scant Identifies Endos as a Substrate of Greatwall Kinase and a Negative Regulator of Protein Phosphatase 2A in Mitosis