Molecular chaperones of the Hsp70 family assist in the assembly of 20S proteasomes

The eukaryotic 26S proteasome is a large protease comprised of two major sub assemblies, the 20S proteasome, or core particle (CP), and the 19S regulatory particle (RP). Assembly of the CP and RP is assisted by an expanding list of dedicated assembly factors. For the CP, this includes Ump1 and the heterodimeric Pba1-Pba2 and Pba3-Pba4 proteins. It is not known how many additional proteins that assist in proteasome biogenesis remain to be discovered. Here, we demonstrate that two members of the Hsp70 family in yeast, Ssa1 and Ssa2, play a direct role in CP assembly. Ssa1 and Ssa2 interact genetically and physically with proteasomal components. Specifically, they associate tightly with known CP assembly intermediates, but not with fully assembled CP, through an extensive purification protocol. And, in yeast lacking both Ssa1 and Ssa2, specific defects in CP assembly are observed.