Extended Data Figure 5: Details of the PCI ring interactions and PCI ring capping features.

a, Location of the composite β-sheet within CSN. b, c, Close-up views of the interface between the CSN1–CSN3 (b) and the CSN2–CSN4 WH subdomains (c). d, CSN8 carries a helix-turn-β2 motif in the WH subdomain that is reduced by six residues (magenta), retracting the available interaction interface for further PCI interactions (see Supplementary Data). e, Superposition of PCI subunits, highlighting the position of conserved aromatic residues (Tyr/Phe shown as sticks) interconnecting PCI subunits. CSN7 lacks an equivalent residue. Interactions between subunits within CSN: f, g, Interactions between the CSN3 and CSN8 helical repeats (f); and the CSN7 helical repeats and the proximal end of the C-terminal helical bundle (g).