The M-phase regulatory phosphatase PP2A-B55δ opposes protein kinase A on Arpp19 to initiate meiotic division

Abstract

Oocytes are held in meiotic prophase for prolonged periods until hormonal signals trigger meiotic divisions. Key players of M-phase entry are the opposing Cdk1 kinase and PP2A-B55δ phosphatase. In Xenopus, the protein Arpp19, phosphorylated at serine 67 by Greatwall, plays an essential role in inhibiting PP2A-B55δ, promoting Cdk1 activation.Furthermore Arpp19 has an earlier role in maintaining the prophase arrest through a second serine (S109) phosphorylated by PKA. Prophase release, induced by progesterone, relies on Arpp19 dephosphorylation at S109, owing to an unknown phosphatase. Here we identified this phosphatase as PP2A-B55δ. In prophase, PKA and PP2A-B55δ are simultaneously active, suggesting the presence of other important targets for both enzymes. The drop in PKA activity induced by progesterone decreases S109 phosphorylation, unlocking the prophase block. Hence, PP2A-B55δ acts critically on Arpp19 on two distinct sites, opposing PKA and Greatwall to orchestrate the prophase release and M-phase entry.

Role of Protein Phosphatase-2A (PP-2A) in the Control of Mitosis and Meiosis.

“…As discussed below, the idea that Arpp19 phosphorylated on serine 109 is phosphorylated by PP-2A-B55ɗ is another fantastical proposal of the authors of Dupre et al [139]. Referring to the papers [1,2], the authors in the paper [9] stated that "PP2A associated with the B55δ subunit appears to be a major enzyme that dephosphorylates Cdk1 targets (Castilho et al, 2009;, hence controlling the downstream events of Ruling out PP-1 contradicts the seminal works of Huchon et al [149] and Foulkes and…”

“…This understanding of the regulation of PP-2A subfamily of enzymes through activation is consistent with the fact that in the cell, there are many inhibitors of PP-2A so that most of PP-2A present in the cell would be permanently inhibited and that PP-2A would be undergoing periodic moderate activation instead of periodic inhibition. Following the publication of the papers by Mochida et al [1] and Gharbi-Ayashi et al [2], several copycat papers appeared to show that PP-2A-B55ɗ was also the protein phosphatase that was responsible for the control of meiosis in Xenopus oocytes [5,9,143] These paper purported to show that the stimulation of Xenopus oocytes arrested at prophase by progesterone resulted in or was accompanied by the inhibition of PP-2A- [143] did not find α-Endosulfine in their so called acid and heat stable protein preparation from Xenopus oocytes. The presence of α-Endosulfine in Xenopus oocytes is like sore thumb.…”

“…Maller [150] which showed that a specific inhibitor of PP-1, namely protein phosphatase-1 inhibior-1 (PP-1-I1) or PP-1-I2 delays the resumption of meiosis induced by progesterone but not by MPF in Xenopus oocytes implying the importance of a form of PP-1 in the resumption of meiosis. In view of the fact that the authors in the paper/manuscript [9] did not do any work to rule out the role of any forms of PP-1 in the initiation, maintenance and ending of meiosis in Xenopus oocytes, they cannot scientifically state that no form of PP-1 was not involved. It must be noted that PP-1 exists in many forms and that the activity of PP-1 depends on targeting units [151][152][153] , other modulating proteins including 14-3-3 and whether PP-1 binds directly to its substrates or not [151][152][153][154][155][156][157][158][159][160][161].…”

“…It is first important to consider a number of conclusory statements of the authors of the paper/manuscript [9]. It is stated that: (i) Importantly, Cdk1 activation also requires the inhibition of a specific phosphatase, the PP2A-B55ɗ isoform, which counteracts Cdk1dependent phosphorylations of mitotic/meiotic substrates, including Cdc25 and Myt17.…”

“…The authors of the paper [9] proclaimed that they identified PP-2AB55ɗ as the protein phosphatase that is responsible for dephosphorylating serine 109 of ARPP19. The authors used GST-tagged Arpp19 previously in vitro phosphorylated at S109 by PKA (pS109-GST-Arpp19) as substrate to assay for protein phosphatase(s).…”