DESCRIPTION: The majority of intracellular phosphatase activity is attributed to the serine/threonine protein phosphatases types 1 (PP1) and 2A (PP2A). PP2A is involved in the transcriptional regulation of various growth regulatory genes as well as post-translational control of cell cycle regulatory kinases. The overall objectives of the proposed research are to characterize the mechanisms that control PP2A expression and to determine the role of PP2A function in cellular growth and transformation. Specific Aim 1 is to characterize regulation of PP2A gene expression at the transcription and post-transcriptional levels and to determine whether autoregulation of PP2A expression occurs. Cloned promoters for both the alpha and beta isoforms will be studied in standard transfection and deletional analyses. The stability of the mRNAs for the alpha and beta isoforms will be examined, and the contributions of the respective 3' untranslated regions (3'-UTR) in controlling mRNA stability will be assessed. The effects of PP2A overexpression and underexpression on promoter activities and mRNA stabilities will be determined to assess autoregulation. Specific Aim 2 is to evaluate the effects of overexpression and underexpression on cellular phenotype. Strong viral promoters will be utilized to drive overexpression of the two PP2A isoforms, both of which will be tagged at their amino-termini to allow discrimination from the corresponding endogenous proteins. Underexpression of the two PP2A isoforms will be accomplished through use of PP2A-specific ribozymes expressed from an inducible promoter. The effects of overexpression and underexpression of the PP2A isoforms on cell proliferation, the function of cell cycle regulatory kinases and gene expression will be examined. Specific Aim 3 is to evaluate the role of PP2A in cellular transformation. The relationship between the level of PP2A expression and the ability of okadaic acid, an inhibitor of PP2A, and various oncogenes to transform cells will be evaluated. Transformation will be assessed by examining growth in soft agar.