Cdc48 chaperone and adaptor Ubx4 distribute the proteasome in the nucleus for anaphase proteolysis

The cell cycle transition is driven by abrupt degradation of key regulators. While ubiquitylation of these proteins has been extensively studied, the requirement for the proteolytic step is less understood. By analyzing the cell cycle function of Cdc48 in the budding yeast Saccharomyces cerevisiae, we found that double mutations in Cdc48 and its adaptor Ubx4 cause mitotic arrest with sustained Clb2 and Cdc20 proteins that are normally degraded in anaphase. The phenotype is neither caused by spindle checkpoint activation nor a defect in the assembly or the activity of the ubiquitylation machinery and the proteasome. Interestingly, the 26S proteasome is mislocalized into foci, which are colocalized with nuclear envelope anchor Sts1 in cdc48-3 ubx4 cells. Moreover, genetic analysis reveals that ubx4 deletion mutant dies in the absence of Rpn4, a transcriptional activator for proteasome subunits, and the proteasome chaperone Ump1, indicating that an optimal level of the proteasome is required for survival. Overexpression of Rpn4 indeed can rescue cell growth and anaphase proteolysis in cdc48-3 ubx4 cells. Biochemical analysis further shows that Ubx4 interacts with the proteasome. Our data propose that Cdc48-Ubx4 acts on the proteasome and uses the chaperone activity to promote its nuclear distribution, thereby optimizing the proteasome level for efficient degradation of mitotic regulators.