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 (By similarity). Binds the proteasome. Interacts with karyopherin SRP1 and Proteasome subunit RPN11. Belongs to the cut8/STS1 family.