An adapter protein that specifically links the 26S proteasome to its substrate CDC4 which is one of the substrate recognition subunits of the SCF E3 ubiquitin ligase complex. Required for turnover of cell cycle regulatory proteins CDC4 and GRR1. Required for synthesis and nuclear export of 60S ribosomal subunits. Required for vegetative growth. Interacts with CDC4, PRE4, PRE6, RPT1 and SCL1 as part of the fully assembled 26S proteasome. Interacts with pre-ribosomal particles constituent NOP7. Present with 45900 molecules/cell in log phase SD medium.