| Enzymatic Property | Molecular Details | Citations |
|---|---|---|
| Enzyme classification | UBE2L6 (also called UbcH8) is an E2 ubiquitin/ubiquitin-like conjugating enzyme of the ubiquitin-conjugating enzyme family. In the ISG15 pathway it functions as the cognate E2 that receives activated ISG15 from the E1 enzyme UBA7/UBE1L; it can also participate in ubiquitination reactions. | (pqac-00000001, pqac-00000007, pqac-00000014) |
| Core catalytic role | UBE2L6 acts in the middle of the E1-E2-E3 cascade: activated ISG15 is first adenylated and linked to UBA7, then transferred to the active-site cysteine of UBE2L6, and finally passed to an E3 ligase or directly positioned for ligation to substrate lysines. This is the canonical conjugating-enzyme step of ISGylation. | (pqac-00000001, pqac-00000006, pqac-00000008) |
| Catalytic mechanism | The mechanism is thiol-based chemistry typical of E2 enzymes: UBA7 forms a thioester with ISG15, UBE2L6 accepts ISG15 by transthiolation/transesterification onto its catalytic cysteine, and downstream transfer yields an isopeptide bond between the C-terminus of ISG15 and a substrate lysine. | (pqac-00000006, pqac-00000008, pqac-00000001) |
| E1 enzyme specificity | Biochemical and structural work shows that UBE2L6 has much higher affinity for the ISG15 E1 UBA7/UBE1L than for the canonical ubiquitin E1 UBA1, supporting functional specificity for ISGylation in cells even though the enzyme can catalyze ubiquitin transfer in vitro. | (pqac-00000007, pqac-00000001, pqac-00000000) |
| UBA7-UBE2L6 structural interface | Cryo-EM and biochemical studies show that UBA7/UBE1L recruits UBE2L6 through its ubiquitin-fold domain (UFD), while additional contacts involve the SCCH domain and crossover loop. These contacts position the active sites for transfer of activated ISG15 and help explain pathway fidelity. | (pqac-00000002, pqac-00000003, pqac-00000000) |
| Structural basis of specificity | Recent structural studies identified determinants that enforce UBA7-UBE2L6 selectivity, including strong UFD-mediated affinity, features of the catalytic cysteine capping loop, and favorable active-site cysteine chemistry. These determinants distinguish UBE2L6 from the closely related UBE2L3. | (pqac-00000000, pqac-00000003) |
| Active-site chemistry | UBE2L6 contains the catalytic cysteine required for thioester formation with ISG15 (and ubiquitin in ubiquitination settings). Experimental structural trapping used a C86-only UBE2L6 construct, underscoring that this catalytic cysteine is essential for E1-to-E2 transfer. | (pqac-00000002, pqac-00000000) |
| E1-E2 reaction intermediate | Structural capture of a chemically trapped UBE1L-UBE2L6 complex bound to activated ISG15 revealed how the first transfer steps occur and provided evidence for the transient E1-E2 intermediate that precedes substrate modification. | (pqac-00000003, pqac-00000002) |
| Disulfide/thioester-related chemistry | In addition to the normal thioester transfer step, UBA7 and UBE2L6 can form a disulfide-linked complex under biochemical conditions; recent work suggests this reflects strong and specific E1-E2 recognition and redox-sensitive control of the ISG15 transfer cascade. | (pqac-00000000) |
| E3 ligase partners | The main E3 ligases functioning with UBE2L6 in ISGylation are HERC5, TRIM25/EFP, and HHARI/ARIH1. HERC5 is broadly active and relatively promiscuous, whereas TRIM25 and HHARI show more substrate selectivity. | (pqac-00000001, pqac-00000007, pqac-00000008) |
| HERC5-associated mechanism | In the HERC5 pathway, the UBE2L6~ISG15 thioester transfers ISG15 onward for covalent attachment to host or viral substrates. HERC5 can act co-translationally at polysomes, making UBE2L6 part of a ribosome-associated antiviral modification system. | (pqac-00000004, pqac-00000008, pqac-00000006) |
| Substrate specificity: ISG15 versus ubiquitin | UBE2L6 is best described as bifunctional: it is the principal E2 for ISG15 conjugation but is also capable of ubiquitin conjugation. In cells, its stronger biochemical preference for UBA7 and the interferon-induced ISGylation machinery make ISG15 transfer its dominant defined role. | (pqac-00000007, pqac-00000010, pqac-00000014) |
| Substrate attachment chemistry | The final product of UBE2L6-driven ISGylation is typically mono-ISGylation at substrate lysines via an isopeptide bond between the ISG15 C-terminal glycine motif and the ε-amino group of lysine on the target protein. | (pqac-00000006, pqac-00000008) |
| Ubiquitination capacity | Beyond ISGylation, UBE2L6 can function as a ubiquitin-conjugating enzyme and has been implicated in ubiquitin-proteasome-mediated degradation of signaling proteins, especially in infection contexts where it promotes K48-linked ubiquitination of innate immune sensors. | (pqac-00000010, pqac-00000013) |
| Ubiquitin chain linkage types | Experimental infection studies indicate that UBE2L6 can promote K48-linked ubiquitination of RIG-I and MDA5, leading to proteasome-dependent degradation. Other literature also supports broader involvement of UBE2L6 in ubiquitin and ISG15 pathway cross-talk, though linkage specificity is best established for K48 in this context. | (pqac-00000010, pqac-00000013) |
| Domain features important for function | UBE2L6 is built around the conserved E2/UBC catalytic core. Its function depends on the canonical E2 fold, catalytic cysteine, and precise E1-recognition surfaces that allow discrimination between ISG15 and ubiquitin pathways. | (pqac-00000007, pqac-00000015, pqac-00000000) |
| Pathway fidelity | Structural and biochemical analyses show that fidelity in ISG15 signaling depends on selective UBA7-UBE2L6 pairing and recognition of the ISG15 C-terminal ubiquitin-like domain, preventing indiscriminate crossover with the ubiquitin system. | (pqac-00000003, pqac-00000000, pqac-00000001) |


*Table: This table summarizes the enzymatic properties and molecular mechanism of UBE2L6, emphasizing its role as a dual ubiquitin/ISG15 E2 enzyme and highlighting structural evidence from recent cryo-EM and biochemical studies.*