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![[Figure 4]](rr5143fig4mag.jpg)
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Figure 4
Cryo-EM structure of the human 20S proteasome with an inhibitor bound, showing the map and coordinates deposited in the Electron Microscopy Data Bank and Protein Data Bank with accession codes EMD-2981 and 5a0q, respectively (da Fonseca & Morris, 2015  ). (a) Overall view of the density map (mesh) and protein coordinates (cartoon). (b) The map shown in (a) cut to reveal the location in the proteasome inner cavity of the Thr1 residues (shown as spheres) at the active sites of subunits β1, β2 and β5. (c) Close-up views of the three active sites, showing the map densities (mesh representation) colour-coded according to the local resolution estimated with ResMap (Kucukelbir et al., 2014) with protein coordinates represented as a cartoon. Densities for the ligand (encircled by dashed lines) are clearly seen extending from the Thr1 residues of subunits β1, β2 and β5 (indicated by arrows). (d) Close-up view of the cryo-EM map (mesh representation) and model coordinates (shown as sticks) for the β5 subunit of the human 20S proteasome, with the L3VS moiety of the inhibitor (coloured yellow) extending from the Thr1 residue (indicated by an arrow). The ligand LLL tripeptide mimics proteasome substrate positions P1–P3, as labelled, where the side chains at positions P1 and P3 are oriented towards the ligand-binding pocket, while that at position P2 is oriented towards the inner cavity of the proteasome. (a), (b) and (c) were created using UCSF Chimera (Pettersen et al., 2004) and (d) was created using PyMOL (Schrödinger). |
![[Figure 4]](rr5143fig4.jpg)
 | STRUCTURAL BIOLOGY |
ISSN: 2059-7983
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