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A Novel Assay for Protein Phosphatase 2A (PP2A) Complexes In Vivo Reveals Differential Effects of Covalent Modifications on Different Saccharomyces cerevisiae PP2A Heterotrimers

Matthew S. Gentry,^1, Yikun Li,² Huijun Wei,^2, Farhana F. Syed,¹ Sameer H. Patel,² Richard L. Hallberg,¹ and David C. Pallas^2*

Department of Biology, Syracuse University, Syracuse, New York 13244,¹ Department of Biochemistry and Winship Cancer Institute, Emory University School of Medicine, Atlanta, Georgia 30322²

Received 30 March 2005/ Accepted 14 April 2005

Protein phosphatase 2A (PP2A) catalytic subunit can be covalently modified at its carboxy terminus by phosphorylation or carboxymethylation. Determining the effects of these covalent modifications on the relative amounts and functions of different PP2A heterotrimers is essential to understanding how these modifications regulate PP2A-controlled cellular processes. In this study we have validated and used a novel in vivo assay for assessing PP2A heterotrimer formation in Saccharomyces cerevisiae: the measurement of heterotrimer-dependent localization of green fluorescent protein-PP2A subunits. This assay relies on the fact that the correct cellular localization of PP2A requires that it be fully assembled. Thus, reduced localization would occur as the result of the inability to assemble a stable heterotrimer. Using this assay, we determined the effects of PP2A C-subunit phosphorylation mimic mutations and reduction or loss of PP2A methylation on the formation and localization of PP2A_B/Cdc55p and PP2A_B'/Rts1p heterotrimers. Collectively, our findings demonstrate that phosphorylation and methylation of the PP2A catalytic subunit can influence its function both by regulating the total amount of specific PP2A heterotrimers within a cell and by altering the relative proportions of PP2A_B/Cdc55p and PP2A_B'/Rts1p heterotrimers up to 10-fold. Thus, these posttranslational modifications allow flexible, yet highly coordinated, regulation of PP2A-dependent signaling pathways that in turn modulate cell growth and function.

Present address: Department of Pharmacology, University of California, San Diego, San Diego, CA 92093.

Present address: Department of Medicine, Duke University Medical Center, Durham, NC 27710.

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