Cdc55p-mediated E4orf4 growth inhibition in Saccharomyces cerevisiae is mediated only in part via the catalytic subunit of protein phosphatase 2A.
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.
Li
Yikun
Y
Department of Biochemistry, Emory University School of Medicine, 1510 Clifton Road, Atlanta, GA 30322, USA.
Wei
Huijun
H
Hsieh
Tung-Chin
TC
Pallas
David C
DC
D020134:Catalytic Domain / Q000502:physiology*; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D016297:Mutagenesis, Site-Directed; D017354:Point Mutation; D011496:Protein Methyltransferases / Q000235:genetics* / Q000378:metabolism; D054648:Protein Phosphatase 2 / Q000235:genetics / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000201:enzymology / Q000235:genetics / Q000254:growth & development*; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics* / Q000378:metabolism*; D017384:Sequence Deletion; D014764:Viral Proteins / Q000235:genetics / Q000378:metabolism*
C071246:CDC55 protein, S cerevisiae; D018797:Cell Cycle Proteins; C121517:E4orf4 protein, adenovirus; D029701:Saccharomyces cerevisiae Proteins; D014764:Viral Proteins; C442645:Ppm1 protein, S cerevisiae; D011496:Protein Methyltransferases; D054648:Protein Phosphatase 2; C104343:SIT4 protein, S cerevisiae