Regulatory mechanisms of HECT E3 enzymes.
(A) Structure of the crystallographic trimer formed by a truncated HECT domain construct of E6AP (viewed along the 3-fold axis). For clarity, the N-lobes of the three subunits are shown as surfaces, the C-lobes as ribbons. The insert provides details of the N-lobe-N-lobe interaction between two subunits, focusing on Phe 727 (the subject of mutational analyses) and its inter-subunit contacts within a distance of 5 Å. (B) The ubiquitin-binding ‘exosite’ of NEDD4 subfamily enzymes, illustrated by a structure of the non-covalent complex of the NEDD4-1 HECT domain and ubiquitin (PDB ID: 2XBB; Maspero et al., 2011). (C) Auto-inhibitory interaction of the 2,3-linker with the HECT domain of WWP2, as seen in a crystal structure of a WW-2-(2,3-linker)-HECT fusion construct (PDB ID: 5TJ7; Chen et al., 2017). The HECT domain is shown in the same orientation as in (B) to illustrate that the position of the 2,3-linker occludes the exosite. (D) Crystal structure of the asymmetric dimer formed by an extended HECT domain construct of HUWE1 (PDB ID: 5LP8; Sander et al., 2017). Dimerization is mediated by an α-helical region that flanks the HECT domain N-terminally. The green subunit, in which the C-lobe is locked conformationally at the dimer interface, adopts an auto-inhibited state.