Fig. 1: Stepwise assembly of the 20S proteasome is mediated by multiple chaperone interactions.

a Cryo-EM structures of human 20S intermediate complexes depicting the stepwise assembly and maturation of the proteasome. Assembly begins with formation of the α-ring, aided by PAC1-4. The β-ring is formed on the α-ring, aided by POMP. Dimerization of α-ring/β-ring half-proteasomes triggers cleavage of the β pro-peptides (pro-β) and dissociation of POMP and PAC1/PAC2 to yield the mature 20S proteasome. Scale bar: 20 Å. b Interaction map between proteasome chaperones and 20S subunits. Each chaperone makes extensive contacts with multiple 20S subunits, with PAC1-4 interacting primarily with the α4-7 subunits while POMP interactions are spread out across many α and β subunits. c Root mean square deviation (r.m.s.d.) values of the α subunit atomic positions across the PAC1-4/α-ring (I), pre-13S (II), mixed-13S (III), pre-20S (IV), and mature 20S (V, PDB 7NAN) structures. d R.m.s.d. values of proteasome chaperone atomic positions across the PAC1-4/α-ring (I), pre-13S (II), mixed-13S (III), and pre-20S (IV) structures.