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

A specific form of Protein Phosphatase-2A (PP-2A), namely PP2A-B55δ was proposed to occupy a central role in the control of mitosis entry and exit, and meiosis in Xenopus oocytes [1,3]. It was held that PP2A-B55δ is responsible for dephosphorylating substrates of cdc2/Cdk1 and that inhibition of PP2A-B55δ by Arpp-19 phosphorylated at serine 67 by Greatwall kinase triggers entry of both mitosis and meiosis in Xenopus oocytes. It was further declared that the phosphorylation of Arpp19 at serine 109 by PKA underlies the blockade of meiotic division and that dephosphorylation of serine 109 of Arpp19 triggers resumption of meiotic division in Xenopus oocytes [4]. Recently two groups have stated that PP2A-B55δ is the protein phosphatase that is responsible for dephosphorylating both serine 67 and serine 109 of Arpp19 [4,5] However, unfortunately for the authors concerned [1-5], no verifiable scientific evidence exists that shows that Arpp19 is a specific inhibitor of PP-2AB55ɗ when Arpp19 is phosphorylated at serine 67 by Greatwall kinase and that Arpp-19 phosphorylated at serine 67 and Arpp19 phosphorylated at 109 are both specifically dephosphorylated by PP-2AB55ɗ Arpp19. The idea that Arpp-19 phosphorylated at serine 67 is both an inhibitor and a substrate of PP-2AB55ɗ has more to do with science fiction than science. The role of other Protein Phosphatases, including, PP-2A-B'56ɗ and Protein Phosphatase-1 I (PP-1 I) cannot be ignored. Evidenceis presented and discussed here...