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Protein degradation by 20S proteasomes in vivo requires ATP hydrolysis by associated hexameric AAA ATPase complexes such as PAN in archaea and the homologous ATPases in the eukaryotic 26S proteasome. This review discusses recent insights into their multistep mechanisms and the roles of ATP. We have focused on the PAN complex, which offers many advantages for mechanistic and structural studies over the more complex 26S proteasome. By single-particle EM, PAN resembles a “top-hat” capping the ends of the 20S proteasome and resembles densities in the base of the 19S regulatory complex. The binding of ATP promotes formation of the PAN–20S complex, which induces opening of a gate for substrate entry into the 20S. PAN’s C-termini, containing a conserved motif, docks into pockets in the 20S’s α ring and causes gate opening. Surprisingly, once substrates are unfolded, their translocation into …