Fig. 7. The non-canonical cluster. (A) Stereoview showing overlap of a canonical (yellow) and the α1/α2 non-canonical (cyan) clusters. Overlap was performed according to the 7-fold symmetry of the PA26 heptamer. All six canonical clusters overlap closely with the α6/α7 cluster shown here. View direction is from the position of the 7-fold axis, slightly above the top of the proteasome α-subunits. Hydrogen bonds of canonical clusters are shown as dotted lines. Potential steric clashes with α-2 Phe10 that exclude aspartate from position 9 of this subunit and displace Tyr8 of subunit α-1 are indicated with black lines. (B) Deviations from perfect conservation of cluster residues. All cluster residues of all subunits are invariant unless indicated here. Hs, Homo sapiens; Mm, Mus musculus; Rn, Rattus norvegicus; Dm, Drosophila melanogaster; Ce, Caenorhabditis elegans; At, Arabidopsis thaliana; Sp, Schizosaccharomyces pombe; Sc, Saccharomyces cerevisiae. Residue identities corresponding to the canonical sequences are shown on a yellow background.