Fig. 1

A Classification of E3 ubiquitin ligases by domain architecture. The substrate protein (green) and E2 conjugating enzyme (red) may bind different regions of the enzyme. In HECT and RBR E3 ligases, ubiquitin is transferred to the E3 enzyme, while RING and U-Box ligases directly transfer ubiquitin to the substrate. B The Cbl family of RING E3 ubiquitin ligases. The N-terminal tyrosine kinase binding (TKB) domain contains four-helix bundles (4H), an EF hand (EF), and a Src homology (SH2) domain. This is followed by the RING domain, responsible for E3 activity, and the proline-rich domain for substrate recognition. The C-terminal ubiquitin-associated domain (UAB) facilitates ubiquitin binding and dimerization. Cbl-c lacks both the proline-rich and UAB domains, whereas c-Cbl and Cbl-b contain all domains. Hakai has an inverted domain order, and its RING domain is near the N-terminus, followed by the PTB domain and lacking the UAB domain. ZNF645 shares high homology and similar domain architecture with Hakai but is shorter. Both form an atypical pocket called the Hakai phospho-tyrosine binding (HYB) domain upon homodimerization, involving the RING and PTB domains. The HYB domain in Hakai dictates substrate specificity and is a potential drug target. C Comparison of Hakai HYB domain dimer PDBID 3VK6 [13, 111] and c-Cbl PDBID 2Y1M [112, 113] structures. The Hakai HYB domain dimer structure, with zinc coordination from both monomers (green and pink), is unique to Hakai and absent in other RING E3 ligases. D Structure alignment of HYB domain (orange) and c-Cbl (blue). The overlapping region corresponds to the RING domains (left). The structures were retrieved from the protein databank [114], (http://www.rcsb.org/) and were created with BioRender.com