Oxidative Stress-Induced Destruction of the Yeast C-Type Cyclin Ume3p Requires Phosphatidylinositol-Specific Phospholipase C and the 26S Proteasome

Abstract

The yeast UME3 (SRB11/SSN3) gene encodes a C-type cyclin that represses the transcription of the HSP70family member SSA1. To relieve this repression, Ume3p is rapidly destroyed in cells exposed to elevated temperatures. This report demonstrates that Ume3p levels are also reduced in cultures subjected to ethanol shock, oxidative stress, or carbon starvation or during growth on nonfermentable carbons. Of the three elements (RXXL, PEST, and cyclin box) previously shown to be required for heat-induced Ume3p destruction, only the cyclin box regulates Ume3p degradation in response to these stressors. The one exception observed was growth on nonfermentable carbons, which requires the PEST region. These findings indicate that yeast cells contain multiple, independent pathways that mediate stress-induced Ume3p degradation. Ume3p destruction in response to oxidative stress, but not to ethanol treatment, requires DOA4 and UMP1, two factors required for 26S proteasome activity. This result for the first time implicates ubiquitin-mediated proteolysis in C-type cyclin regulation. Similarly, the presence of a membrane stabilizer (sorbitol) or the loss of phosphatidylinositol-specific phospholipase C (PLC1) protects Ume3p from oxidative-stress-induced degradation. Finally, a ume3 null allele suppresses the growth defect of plc1 mutants in response to either elevated temperature or the presence of hydrogen peroxide. These results indicate that the growth defects observed in plc1mutants are due to the failure to downregulate Ume3p. Taken together, these findings support a model in which Plc1p mediates an oxidative-stress signal from the plasma membrane that triggers Ume3p destruction through a Doa4p-dependent mechanism.

ACKNOWLEDGMENTS

We thank M. Gustin and J. Flick for helpful discussions and J. Thorner for bringing the plc1 mutant phenotypes to our attention. We thank J. Broach and J. Cannon for activated RAS alleles, J. Thorner for plc1 mutant strains, F. Cross for the triple cln deletion strain, R. J. Dohmen for the ump1 mutant strain, and V. Guacci for tubulin antibodies. We also thank E. Golemis and J. Chernoff for critical reading of the manuscript.

K.F.C. was supported by NIH grant CA-09035-23. This work was supported by NSF grant MCB-9513479 to R.S. and an appropriation from the Commonwealth of Pennsylvania.