Toxicity of human adenovirus E4orf4 protein in Saccharomyces cerevisiae results from interactions with the Cdc55 regulatory B subunit of PP2A

The E4orf4 protein of human adenovirus induces p53-independent apoptosis, a process that may promote cell death and viral spread. When expressed alone , E4orf4 kills transformed cells but not normal human cells. The only clear target of E4orf4 in mammalian cells is the Ba (B55) subunit of protein pho sphatase 2A (PP2A), a member of one of three classes of regulatory B subuni ts. Here we report the effects of E4orf4 in Saccharomyces cerevisiae, which encodes two PP2A regulatory B subunits, CDC55 and RTS1, that share homolog y with mammalian B and B' subunits, respectively. E4orf4 expression was fou nd to be toxic in yeast, resulting in the accumulation of cells in G2/M pha se that failed to grow upon removal of E4orf4. E4orf4-expressing yeast also displayed an elongated cell morphology similar to cdc55 deletion strains. E4orf4 required CDC55 to elicit its effect, whereas RTS1 was dispensable. T he recruitment of the PP2A holoenzyme by E4orf4 was entirely dependent on C dc55. These studies indicate that E4orf4-induced apoptosis in mammalian cel ls and cell death in yeast require functional interactions with B-type subu nits of PP2A. However, some inhibition of growth by E4orf4 was observed in the cdc55 strain and with an E4orf4 mutant that fails to interact with Cdc5 5, indicating that E4orf4 may possess a second Cdc55-independent function a ffecting cell growth.