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A) Structural model of HECT-domain E3 complex. The mechanism for the conjugation of Ub to a substrate by a HECT-domain E3 is shown. The E2 binds the N-terminal lobe of the HECT-domain E3 (top) and transfers Ub to the C-terminal lobe via a thiolester linkage (middle). The C-terminal lobe swivels on a hinge-loop and catalyzes the transfer of Ub to the substrate protein (bottom). B) Model of U-box or monomeric RING finger E3s. The U-box or RING finger domains of the E3 are directly involved in binding the E2. C) Model of SCF complexes. The N-terminus of CUL-1 binds the adaptor Skp1 (SKR proteins in C. elegans), while the C-terminus binds the RING finger protein Rbx1, which binds the E2. The substrate recognition subunit (SRS) binds to Skp1 through an F-box motif. D) Model of CUL-2-based E3 complexes. The N-terminus of CUL-2 binds to the adaptor elongin C (ELC-1), which is in complex with elongin B (ELB-1). The SRS binds to elongin C through a BC-box motif. E) Model of CUL-3-based E3 complexes. The N-terminus of CUL-3 binds directly to the SRS, which utilizes a BTB/POZ domain to bind to CUL-3.