20S proteasome assembly is orchestrated by two distinct pairs of chaperones in yeast and in mammals.

The 20S proteasome is the catalytic core of the 26S proteasome, a central enzyme in the ubiquitin-proteasome system. Its assembly proceeds in a multistep and orderly fashion. Ump1 is the only well-described chaperone dedicated to the assembly of the 20S proteasome in yeast. Here, we report a phenotype related to the DNA damage response that allowed us to isolate four other chaperones of yeast 20S proteasomes, which we named Poc1-Poc4. Poc1/2 and Poc3/4 form two pairs working at different stages in early 20S proteasome assembly. We identify PAC1, PAC2, the recently described PAC3, and an uncharacterized protein that we named PAC4 as functional mammalian homologs of yeast Poc factors. Hence, in yeast as in mammals, proteasome assembly is orchestrated by two pairs of chaperones acting upstream of the half-proteasome maturase Ump1. Our findings provide evidence for a remarkable conservation of a pairwise chaperone-assisted proteasome assembly throughout evolution.