Fig. 4. Modular composition and CAND1-mediated regulation of cullin-based E3 ligase activity. Cullin 1 (Cul1) uses F-box proteins to degrade the substrates as adaptors that recognize a variety of substrates through Skp1 linker protein. Representative substrates of Cul1-dependent E3 ligase include p27, IkB and Cyclins. Likewise, Cul2/5 recruits SOCS proteins as adaptor to recognize a variety of substrates, including HIF1α using Elongin B/C as linker proteins. Without a linker protein, Cul3 utilize BTB proteins (for example, Keap1) as an adaptor and recruits many protein substrates, including Nrf2. Cul4 recruits DDB1 as a linker and proteins with WD-40 motif as an adaptor protein to recognize and degrade the substrates, including Cdt1 and Histone 3/4 (H3/4) (A). CAND1 protein inhibits the activity of Cullins by enabling linker or adaptor proteins inaccessible to Cullins at the N-terminus and Roc1 and E2 inaccessible to Cullins at the C-terminus. When a substrate is signaled for degradation, CAND1 is removed from Cullins and Nedd8 is conjugated to Cullins (Neddylation) by ubiquitin-conjugating enzymes (Ubcs), which helps to recruit the E1 and E2 ligase components to E3 enzymes and subject substrates to poly-ubiquitin (Ub) chain formation and proteolysis (B).