Abstract

Genome wide experiments indicate both proteasome- and vacuole-mediated protein degradation modulate sensitivity to classical DNA-damaging agents. Here, we show that global protein degradation is significantly increased upon methyl methanesulfonate (MMS) exposure. In addition, global protein degradation is similarly increased upon exposure to 4-nitroquinoline-N-oxide (4NQO) and UV and, to a lesser extent, tert-butyl hydroperoxide. The proteasomal inhibitor MG132 decreases both MMS-induced and 4NQO-induced protein degradation, while addition of the vacuolar inhibitor phenylmethanesulfonyl fluoride does not. The addition of both inhibitors grossly inhibits cell growth upon MMS exposure over and above the growth inhibition induced by MMS alone. The MMS-induced protein degradation response remains unchanged in several ubiquitin-proteasome and vacuolar mutants, presumably because these mutants are not totally deficient in either essential pathway. Furthermore, MMS-induced protein degradation is independent of Mec1, Mag1, Rad23, and Rad6, suggesting that the protein degradation response is not transduced through the classical Mec1 DNA damage response pathway or through repair intermediates generated by the base excision, nucleotide excision, or postreplication-DNA repair pathways. These results identify the regulation of protein degradation as an important factor in the recovery of cells from toxicity induced by classical DNA-damaging agents.