Functions of the DNA damage response pathway target Ho endonuclease of yeast for degradation via the ubiquitin-26S proteasome system

Ho endonuclease of Saccharomyces cerevisiae is a homing endonuclease that m akes a site-specific double-strand break in the MAT gene in late G(1), Here we show that Ho is rapidly degraded via the ubiquitin-265 proteasome syste m through two ubiquitin-conjugating enzymes UBC2(Rad6) and UBC3(Cdc34). UBC Z(Rad6) is complexed with the ring finger DNA-binding protein Rad18, and we find that Ho is stabilized in rad18 mutants. We show that the Ho degradati on pathway involving UBC3Cdc34 goes through the Skp1/Cdc53/F-box (SCF) ubiq uitin ligase complex and identify a F-box protein, Ym1088w, that is require d for Ho degradation, Components of a defined pathway of the DNA damage res ponse, MEC1, RAD9, and CHK1, are also necessary for Ho degradation, whereas functions of the RAD24 epistasis group and the downstream effector RAD53 h ave no role in degradation of Ho, Our results indicate a link between the e ndonuclease function of Ho and its destruction.