PAC4 or proteasome assembly chaperone 4 protein promotes assembly of the 20S proteasome. It interacts with PSMG3. It associates with alpha subunits of the 20S proteasome [1]. At the very C-terminal is a crucial HbYX or hydrophobic-tyrosine-X sequence motif that, in proteasome activators, opens the 20S proteasome entry pore [2].

1: 20S proteasome assembly is orchestrated by two distinct pairs of chaperones in yeast and in mammals. Le Tallec B, Barrault MB, Courbeyrette R, Guerois R, Marsolier-Kergoat MC, Peyroche A; Mol Cell. 2007;27:660-674. PMID:17707236

2: A conserved 20S proteasome assembly factor requires a C-terminal HbYX motif for proteasomal precursor binding. Kusmierczyk AR, Kunjappu MJ, Kim RY, Hochstrasser M; Nat Struct Mol Biol. 2011;18:622-629. PMID:21499243