Drosophila mutants in the 55 kDa regulatory subunit of protein phosphatase 2A show strongly reduced ability to dephosphorylate substrates of p34^(cdc2)
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 twins^P, 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 p34^(cdc2)/cyclin B kinase, but not against phosphorylase a. Thus the mitotic defects of aar¹ and twins^P are likely to result from the lack of dephosphorylation of specific substrates by protein phosphatase 2A.
© 1994 by Company of Biologists. (Received 14 March 1994 - Accepted 12 May 1994) We thank Bob MacKintosh and Patric Turowski for providing [³²P]phosphorylase a, Greg Moorhead for providing the p34^(cdc2)/cyclin B, and Stanislaw Zolnierowicz, Patric Turowski and Carl Smythe for commenting on the manuscript. R.M.-J. was supported for part of these studies by a short-term EMBO fellowship. H.O. and D.M.G. are grateful to the Cancer Research Campaign and the HSFP for support. P.F. was supported by grants from the MRC and HSFP (awarded to P. Cohen). K.S. was supported by a Fellowship of the Japan Society for the Promotion of Science for Junior Scientists; and T.U. is grateful to research grants from the Ministry of Education, Science and Culture of Japan.
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