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. Author manuscript; available in PMC: 2010 Nov 1.
Published in final edited form as: Trends Biochem Sci. 2009 Oct 7;34(11):562–570. doi: 10.1016/j.tibs.2009.07.002

Figure 1. Assembly of Cullin–RING ubiquitin ligases (CRLs).

Figure 1

The cullin-ROC family of E3 ligases regulates the ubiquitylation of many substrates by assembling into multiple distinct E3 ligases. Each cullin uses a modular assembly to recruit different substrates to a common catalytic core by varying its substrate receptor. Cullin family members (green) from different organisms share similar mechanisms for assembling a multi-subunit complex to ubiquitylate specific substrate protein (light blue). An N-terminal domain interacts either directly with a protein motif (orange) present in substrate receptors (black) or via a linker (blue). Separately, a C-terminal domain binds with a small RING protein (ROC1 or ROC2, yellow) which recruits and allosterically activates an E2 enzyme (red) that transfers ubiquitin (Ub; color) to the substrate. Cullins are activated by the covalent conjugation with a ubiquitin-like modifier, NEDD8 (Nd8; purple). Human cells express an estimated 78 F-box proteins, 205 BTB proteins, about 90 DWD proteins and an estimated 50 BC-box proteins.