Fig. 4.

A schematic model of mammalian 20S proteasome assembly. The PAC1–PAC2 and PAC3–PAC4 heterodimeric complexes are involved in the formation of the α-ring. Then, sequential incorporation of the β subunits begins with the binding of β2 and hUmp1 on the α-ring. hUmp1 is required for the association of β2 in early assembly intermediates. PAC3–PAC4, which is released at the time of β3 association, maintains the structural integrity of the intermediates until β3 is incorporated on the α-ring. The subsequent ordered incorporation of other β subunits is assisted by intramolecular chaperones, such as the propeptides of β2 and β5 and the C-terminal tail of β2. Dimerization of half-mers (i.e., half-proteasomes lacking β7) is assisted by the C-terminal tail of β7. This is followed by removal of the β subunit propeptides (β1, β2, β5, β6 and β7) as well as hUmp1 and PAC1–PAC2 degradation. Note that the role of Ump1 for dimerization of half-proteasomes or checkpoint of half-mers is emphasized in yeast, but its exact role is somewhat difference in mammals (for details, see text).