Toward an atomic model of the 26S proteasome

Abstract

Since the discovery of the 26S proteasome, much progress has been made in determining the structure of this large dynamic protein complex. Until now, a vast amount of structural information of the proteasome has been obtained from all kinds of structure determination techniques, and the function of the protease core is well understood at atomic detail. Yet our understanding of the entire 26S proteasome structure, particularly its 19S regulatory complex, is still limited at a low-resolution blob-ology level. In this review, we highlight the recent progress made in understanding the mechanism of 20S gate opening by the proteasomal activators. We also emphasized the recent methodological advances, particularly in achieving the near atomic resolution by single particle electron cryomicroscopy, and the possible approaches that will enable more detailed structural analysis of the entire 26S proteasome.

Figure 1

Architecture of the 20S CP. (a): A ribbon diagram of the yeast 20S proteasome (pdb accession code: 1RYP) [–6]. Two inner β-rings and two outer α-rings are marked. (b) Volume of the yeast 20S CP, calculated from atomic coordinates and filtered to 20 Å. The volume is cut in half, showing three chambers within the 20S CP. (c) The top surface of the α-ring. The N-termini of the α-subunits form a closed gate block the entrance to the inner chamber of the 20S CP.

Figure 2

Gate opening of the 20S CP by proteasomal activators. (a) Ribbon diagram of 20S-PA26 complex (PDB accession code: 1YA7) [17]. (b) Top view of the 20S CP’s α-ring, showing both a closed gate (light gray, PDB accession code: 3C92) and an open gate (yellow) in the presence of the PA26 (blue). The locations of the PA26 C-terminus and activation loop are marked. Notice that the 20S reverse turn loop (pointed in (c)) is moved by the PA26 activation loop. (c) CryoEM structure of 20S-PAN peptides, with an open-gate conformation (light blue, PDB accession code: 3C91). The structure of a closed gate is shown in light gray (PDB access code: 3C92) [33]. The densities correspond to the PAN C-terminal peptide, indicates the binding site of PAN’s C-termini in the 20S CP.

Figure 3

Structure of the 26S proteasome. (a) and (b) 3D reconstructions of the 26S proteasome, by single particle EM (reprinted with permission from [7] and [9]). (c) Comparison of the 20S-PAN complex with the 26S proteasome shows the similarity between the PAN ATPase complex (colored in gold) and the 19S base subunit (colored in gold) (reprinted with permission from [28]). The 20S-PAN complex may be used as a simpler model system to dissect the mechanism of substrates unfolding and translocation by the eukaryotic proteasomal ATPases.