Drosophila mutants in the 55 kDa regulatory subunit of protein phosphatase 2A show strongly reduced ability to dephosphorylate substrates of p34cdc2

Abstract

The 55 kDa regulatory subunit of Drosophila protein phosphatase 2A is located in the cytoplasm at all cell cycle stages, by the criterion of immunofluorescence. We are unable to detect significant change in protein phosphatase activity during the nuclear division cycle of syncytial embryos. However, cell cycle function of the enzyme is suggested by the mitotic defects exhibited by two Drosophila mutants, aar1 and twinsP, defective in the gene encoding the 55 kDa subunit. The reduced levels of the 55 kDa subunit correlate with the loss of protein phosphatase 2A-like, okadaic acid-sensitive phosphatase activity of brain extracts against caldesmon and histone H1 phosphorylated by p34cdc2/cyclin B kinase, but not against phosphorylase a. Thus the mitotic defects of aar1 and twinsP are likely to result from the lack of dephosphorylation of specific substrates by protein phosphatase 2A.

Greatwall-phosphorylated Endosulfine is both an inhibitor and a substrate of PP2A-B55 heterotrimers

“…To distinguish which form of PP2A is anti-Endos, we took advantage of the fact that the Drosophila genome has a near-minimal set of genes encoding PP2A regulatory subunits (for example, one B55-type subunit as opposed to four in mammals; and two B56class (B') subunits versus five in mammals). Null or strongly hypomorphic mutations are available for almost all of these fly genes (11,27,52,83,84); animals homozygous for these mutations usually survive until third instar larval or pupal stages because maternal contributions are sufficient for earlier stages of development (21).…”

“…These M phase-specific phosphorylations must eventually be removed so that mitotic/meiotic cells can reset to interphase. PP2A-B55 (heterotrimeric protein phosphatase 2A composed of a catalytic C subunit, a structural A subunit, and a B55-type regulatory subunit) is the phosphatase responsible for removing many key CDK-generated phosphorylations at the conclusion of M phase (9,12,52,55,56,72). The circuitry's basic logic suggests that PP2A-B55 activity must be downregulated when cells go into M phase; if not, the phosphorylations catalyzed by MPF would be prematurely removed, preventing M phase entry (12,47).…”

Dephosphorylation in nuclear reassembly after mitosis

“…To execute their functions in mitotic exit, PP1 and PP2A associate with specific regulatory subunits. PP2A in complex with its B55 regulatory subunits (PP2A-B55) efficiently targets sites phosphorylated by CDKs ( Agostinis et al, 1992 ; Ferrigno et al, 1993 ; Mayer-Jaekel et al, 1994 ; Castilho et al, 2009 ; Mochida et al, 2009 ; Amin et al, 2021 ). PP2A-B55 enzymes also dephosphorylate sites phosphorylated by other mitotic kinases ( Amin et al, 2021 ).…”

Structural, enzymatic and spatiotemporal regulation of PP2A-B55 phosphatase in the control of mitosis

“…However, data from the last years have established the prominent role of the modulation of these enzymes in the control of protein function and cell signaling. Phosphatases such as PP1, PP2A, PP4 or PP6 have been shown to play a primordial role in the control of mitotic progression ( Mayer-Jaekel et al, 1994 ; Chen et al, 2007 ; Schmitz et al, 2010 ; Wurzenberger et al, 2012 ).…”

“…So far, two different kinases, Cyclin B/Cdk1 in MEFs ( Alvarez-Fernandez et al, 2013 ) and Plk1 in the Drosophila model ( Wang et al, 2013 ) have been proposed as responsible of this phosphorylation. Unlike Gwl, B55 is localized in the cytoplasm throughout the cell cycle ( Mayer-Jaekel et al, 1994 ; Santos et al, 2012 ; Larouche et al, 2021 ). Finally, different localizations have been reported for ENSA (Endos in Drosophila ), being cytoplasmic for the Drosophila Endos ( Larouche et al, 2021 ) and nuclear/cytoplasmic in human cells ( Charrasse et al, 2017 ).…”