Figure 1. Proteasomes from eukaryotes and archea. Magenta, regulatory complex; blue, core complex; green, slice surface; red dots, active sites; cyan dots, N-termini. In Panels 1c and 2f, cyan indicates the residues visualized that are closest to the N-termini (Thr13 and Ser11, respectively). (a) Electron micrograph of proteasome holoenzyme from a representative eukaryote (Xenopus laevis); (b) medial cut-away view of the T. acidophilum proteasome core. The lumen is divided into three chambers, and the central chamber contains the peptidase active sites (red); (c) ribbon diagram of two T. acidophilum subunits, showing the structure of the pore; (d) Cut-away view of the S. cerevisiae proteasome core; (e) ribbon diagram of two S. cerevisiae subunits (left: Pre10/Prs1; right: Pre9/Y13). The N-termini of these subunits are shown to occlude the channel. Adapted from Larsen and Finley (1997). © 1997, with permission from Elsevier.

Figure 2. The core particle gate. (a) Top view of the inactive proteasome core particle. The subunit N-termini seal the gate into the proteolytic chamber; (b) top view of activated proteasome core particle after binding of PA26. By docking of PA26 on to the apical surface of the proteasome core particle the subunit N-termini are restructured to open the gate. Reprinted from Forster and Hill (2003). © 2003, with permission from Elsevier.

Figure 3. HslVU and ClpAP proteases. (a) Electron micrograph of HslVU. The identity of the uppermost, asymmetrically distributed mass is unknown. Blue, HslV core particle; magenta, HslU regulatory particle; (b) cut-away view of the crystal structure of HslV. Adapted with permission from Larsen and Finley (1997); (c) ribbon diagram of two HslV subunits. The C-terminus has been truncated to reduce the image size. Adapted with permission from Larsen and Finley (1997); (d) electron micrograph of ClpAP. Each cap particle has a distal and proximal ring of mass density. These may correspond to the two ATPase domains of ClpA. Blue, ClpP core particle; magenta, ClpA regulatory particle; (e) cut-away view of ClpP; (f) ribbon diagram of two ClpP subunits.

Figure 4. Crystal structure of the HslVU holocomplex. The C-termini of HslU dock into surface clefts of the apical surface of HslV. Adapted from Groll et al. (2005). Reproduced with permission from Wiley-VCH.

Figure 5. Cut-away views of compartmentalized ATP-independent proteases. (a) Crystal structure of the tricorn protease with one subunit superimposed. Adapted from Groll et al. (2005). Reproduced with permission from Wiley-VCH. Note that in Figure 3a and b the 2-fold symmetry of the image results from the placement of the section plane, which is not a symmetry plane. The red dots indicate the active site positions; (b) S. cerevisiae Gal6; (c) Bovine leucine aminopeptidase.