Involved in ubiquitin-mediated protein degradation. Regulatory factor in the ubiquitin/proteasome pathway that controls the turnover of proteasome substrates. Targets proteasomes to the nucleus and facilitates the degradation of nuclear proteins like mitotic cyclin and cut2. Required for normal progression of anaphase. {ECO:0000269|PubMed:11084332; ECO:0000269|PubMed:16096059; ECO:0000269|PubMed:8065367}.