FIGURE 2 | Models of the complete SCF-ligase complexes with their substrate targets.

a | A model of the Skp1–Cul1–F-box-protein
(SCF)^-Trcp complex bound to the phosphorylated destruction-box peptide of -catenin, which is shown in atom-coloured, space-filling depiction (carbon, grey; nitrogen, blue; oxygen, red). Each protein subunit is labelled by name in the same colour as its stucture. The location of the 59-Å space between the predicted ubiquitin site and the F-box substrate-binding interface (the 'hot zone') is indicated. Adapted from Ref. 66. b | Equivalent view of SCF^Skp2 next to a model of the Cks1–Cdk2–p27 complex (where Cks1 stands for Cdc28-protein-kinase regulatory subunit-1 and Cdk2 stands for cyclin-dependent kinase-2), which was produced by superimposition of the CDK domains from the Cks1–Cdk2 structure and the p27–Cdk2–cyclin-A structure. In the underlying crystallographic structure, p27 is a fragment of the protein amino-terminal to residue Thr187. A dashed red line indicates how the backbone up to residue Thr187 might extend in the complex to the Cks1–Skp2 bimolecular interface, possibly forming a circular complex. Adapted from Ref. 67. Cul1, cullin-1; Rbx1, RING-box protein-1; Skp, S-phase-kinase-associated protein; Ub, ubiquitin; UBC, ubiquitin-conjugating enzyme (or E2 ubiquitin ligase).

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