Protein Structure from Scientific Research

Crystal structures of Bpa (PDB 5LFJ) and the 20S CP (PDB 5LZP) were rigid-body fitted to the cryo-EM map (left). The association between the Bpa dodecamer (pink) and the 20S CP α-subunit heptamer (yellow) involves seven neighboring Bpa protomers, leaving five protomers unbound. This binding mode allows for considerable tilting of the Bpa dodecamer relative to the 20S CP α-subunit heptamer and contributes to particle heterogeneity. The cryo-EM map was modified using OccuPy 65 to amplify the density of Bpa. In addition, the same map was low-pass-filtered to attenuate high-frequency signal and was colored according to the local scale estimated by OccuPy (right). Two views of the same map show Bpa (orange density) tethered asymmetrically to the α-ring of 20S CP (green and blue density) with all 20S CP binding sites occupied by Bpa.

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