Ho endonuclease initiates a mating type switch by making a double-strand break at the mating type locus, MAT. Ho is marked by phosphorylation for rapid destruction by functions of the DNA damage response, MEC1, RAD9, and CHK1. Phosphorylated Ho is recruited for ubiquitylation via the SCF ubiquitin ligase complex by the F-box protein, Ufo1. Here we identify a further DNA damage-inducible protein, the UbL-UbA protein Ddi1, specifically required for Ho degradation. Ho interacts only with Ddi1; it does not interact with the other UbL-UbA proteins, Rad23 or Dsk2. Ho must be ubiquitylated to interact with Ddi1, and there is no interaction when Ho is produced in mec1 or ⌬ufo1 mutants that do not support its degradation. Ddi1 binds the proteasome via its N-terminal ubiquitinlike domain (UbL) and interacts with ubiquitylated Ho via its ubiquitin-associated domain (UbA); both domains of Ddi1 are required for association of ubiquitylated Ho with the proteasome. Despite being a nuclear protein, Ho is exported to the cytoplasm for degradation. In the absence of Ddi1, ubiquitylated Ho is stabilized and accumulates in the cytoplasm. These results establish a role for Ddi1 in the degradation of a natural ubiquitylated substrate. The specific interaction between Ho and Ddi1 identifies an additional function associated with DNA damage involved in its degradation.

SCF complexes are E3 ubiquitin-protein ligases that mediate degradation of regulatory and signaling proteins and control G 1 /S cell cycle progression by degradation of G 1 cyclins and the cyclin-dependent kinase inhibitor, Sic1. Interchangeable F-box proteins bind the core SCF components; each recruits a specific subset of substrates for ubiquitylation. The F-box proteins themselves are rapidly turned over by autoubiquitylation, allowing rapid recycling of SCF complexes. Here we report a role for the UbL-UbA protein Ddi1 in the turnover of the F-box protein, Ufo1. Ufo1 is unique among F-box proteins in having a domain comprising multiple ubiquitin-interacting motifs (UIMs) that mediate its turnover. Deleting the UIMs leads to stabilization of Ufo1 and to cell cycle arrest at G 1 /S of cells with long buds resembling skp1 mutants. Cells accumulate substrates of other F-box proteins, indicating that the SCF pathway of substrate ubiquitylation is inhibited. Ufo1 interacts with Ddi1 via its UIMs, and ⌬ddi1 cells arrest when full-length UFO1 is overexpressed. These results imply a role for the UIMs in turnover of SCF Ufo1 complexes that is dependent on Ddi1, a novel activity for an UbL-UbA protein.

Ho endonuclease of Saccharomyces cerevisiae is a homing endonuclease that makes 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-26S proteasome system through two ubiquitin-conjugating enzymes UBC2 Rad6 and UBC3 Cdc34 . UBC2 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 degradation pathway involving UBC3 Cdc34 goes through the Skp1͞Cdc53͞F-box (SCF) ubiquitin ligase complex and identify a F-box protein, Yml088w, that is required for Ho degradation. Components of a defined pathway of the DNA damage response, MEC1, RAD9, and CHK1, are also necessary for Ho degradation, whereas functions of the RAD24 epistasis group and the downstream effector RAD53 have no role in degradation of Ho. Our results indicate a link between the endonuclease function of Ho and its destruction.

Yeast mating switch Ho endonuclease is rapidly degraded by the ubiquitin system and this depends on the DNA damage response functions, MEC1, RAD9, and CHK1. A PEST sequence marks Ho for degradation. Here we show that the novel F-box receptor, Ufo1, recruits phosphorylated Ho for degradation. Mutation of PEST residue threonine 225 stabilizes Ho, yet HoT225A still binds Ufo1 in vitro. Stable HoT225A accumulates within the nucleus, whereas HoT225E is degraded. Deletion of the nuclear exportin Msn5 traps native Ho in the nucleus and extends its half-life. These experiments suggest that Ho is degraded in the cytoplasm. In mec1 mutants stable Ho accumulates within the nucleus; Ho produced in mec1 cells does not bind Ufo1. Thus the MEC1 pathway has functions both in phosphorylation of Thr-225 for nuclear export and in additional phosphorylations for binding Ufo1. Cells with HO under its genomic promoter, but stabilized by deletion of the Msn5 exportin, proliferate, but are multibudded. These experiments elucidate some of the links between the DNA damage response and degradation of Ho by the ubiquitin system.