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. Required for efficient chromosome segregation. Restores protein transport and ribosomal RNA stability. Binds the proteasome. Interacts with karyopherin SRP1 and proteasome subunit RPN11. Present with 2120 molecules/cell in log phase SD medium. Belongs to the cut8/STS1 family.