Cdc55p-Mediated E4orf4 Growth Inhibition in Saccharomyces cerevisiae Is Mediated Only in Part via the Catalytic Subunit of Protein Phosphatase 2A

• DOI: 10.1128/jvi.02435-07
• Publication Date: 2008-01-24T01:34:06Z
• Authors: Yikun Li; Huijun Wei; Tung-Chin Hsieh; David C. Pallas
• Keywords: 0301 basic medicine; Saccharomyces cerevisiae Proteins; Cell Cycle Proteins; Saccharomyces cerevisiae; 3. Good health; Viral Proteins; 03 medical and health sciences; Catalytic Domain; Mutagenesis, Site-Directed; Humans; Point Mutation; Protein Methyltransferases; Protein Phosphatase 2; Sequence Deletion

ABSTRACT

The adenovirus early region 4 open reading frame 4 (E4orf4) protein specifically induces p53-independent cell death of transformed but not normal human cells, suggesting that elucidation of its mechanism may provide important new avenues for cancer therapy. Wild-type E4orf4 and mutants that retain cancer cell toxicity also induce growth inhibition in Saccharomyces cerevisiae , which provides a genetically tractable system for studying E4orf4 function. Interaction with the protein phosphatase 2A (PP2A) B regulatory subunit is required for E4orf4's effects, suggesting that E4orf4 may function by regulating B subunit-containing heterotrimeric PP2A holoenzymes (PP2A BAC ), which consist of a B subunit complexed with the PP2A structural (A) and catalytic (C) subunits. However, it is not known whether E4orf4-induced growth inhibition requires interaction with the PP2A C subunit or whether E4orf4 might have PP2A B subunit-dependent effects that are independent of PP2A BAC holoenzyme formation. To test these possibilities in S. cerevisiae , we disrupted the stable formation of PP2A BAC heterotrimers and thus E4orf4/C subunit association by PP2A C subunit point mutations or by deletion of the gene for the PP2A methyltransferase, Ppm1p, and assayed for effects on E4orf4-induced growth inhibition. Our results support a model in which E4orf4 mediates growth inhibition and cell killing both through PP2A BAC heterotrimers and through a B regulatory subunit-dependent pathway(s) that is independent of stable complex formation with the PP2A C subunit. They also indicate that Ppm1p has a function other than regulating the assembly of PP2A heterotrimers and suggest that selective PP2A trimer inhibitors and PP6 inhibitors may be useful as adjuvant anticancer therapies.