UBE2L6 (also known as UBCH8) is an E2 ubiquitin-conjugating enzyme belonging to the UBE2L family. It functions as the principal E2 enzyme in the ISG15 conjugation (ISGylation) pathway, an interferon-stimulated ubiquitin-like modification system that is part of the innate immune defense against viral infection. UBE2L6 accepts activated ISG15 from the E1 enzyme UBA7 and transfers it to target proteins in conjunction with E3 ligases such as HERC5. The protein contains a single UBC catalytic domain with a conserved active-site cysteine that forms a thioester intermediate with ubiquitin or ISG15. While UBE2L6 can also function as an E2 for ubiquitin conjugation in vitro, its primary physiological role is in ISGylation during the type I interferon response.
Summary: ATP binding is not a direct molecular function of E2 ubiquitin-conjugating enzymes. E2 enzymes accept activated ubiquitin (or ubiquitin-like proteins) from E1 activating enzymes via a transthioesterification reaction; it is the E1 enzyme that requires ATP to adenylate ubiquitin. This annotation likely arose from an automated rule transfer that conflates the ATP requirement of the E1 step with the E2 enzyme itself.
Reason: E2 ubiquitin-conjugating enzymes do not bind or hydrolyze ATP. The ATP dependence belongs to the E1 activating enzyme (UBA7/UBE1L for ISG15, or UBA1 for ubiquitin). This is an erroneous IEA annotation.
Summary: The term 'ligase activity' (GO:0016874) is too broad and arguably misleading for an E2 enzyme. E2 enzymes are transferases (EC 2.3.2.23) that transfer ubiquitin or ubiquitin-like modifiers from an E1 thioester to a substrate. True ligase activity in the ubiquitin system is associated with E3 ubiquitin ligases. The EC number 2.3.2.23 assigned to this protein is in the transferase class, not the ligase class.
Reason: E2 enzymes are classified as transferases (EC 2.3.2.23), not ligases. Calling an E2 a ligase is misleading; the ligase function belongs to E3 enzymes that catalyze the final transfer to substrate lysine residues.
Summary: UBE2L6 is an E2 ubiquitin-conjugating enzyme with a conserved UBC domain and active-site cysteine. While its primary physiological role is in ISGylation rather than ubiquitin conjugation, UBE2L6 retains E2 enzymatic activity for ubiquitin conjugation and is classified under EC 2.3.2.23. This annotation correctly captures the general enzymatic activity of the protein.
Reason: The ubiquitin conjugating enzyme activity is a valid biochemical activity for this E2 enzyme, but the primary physiological role of UBE2L6 is as an ISG15-specific E2, making this a secondary rather than core function.
Supporting Evidence:
file:RABIT/G1TUN6/G1TUN6-uniprot.txt
Belongs to the ubiquitin-conjugating enzyme family
file:RABIT/G1TUN6/G1TUN6-uniprot.txt
EC=2.3.2.23
GO:0019787 ubiquitin-like protein transferase activity
IEA GO_REF:0000120
ACCEPT
Summary: UBE2L6 is indeed a ubiquitin-like protein transferase, as it transfers ISG15 (a ubiquitin-like protein) to substrates. This annotation is correct and captures the broader enzymatic category that encompasses both the ubiquitin and ISG15 transferase activities of this E2 enzyme. It is a parent term of the more specific ISG15 transferase activity.
Reason: This correctly captures UBE2L6 as a transferase for ubiquitin-like proteins. Since the more specific ISG15 transferase activity is also annotated, this parent term is redundant but not incorrect. Keeping it as it accurately represents the general enzymatic class.
Summary: ISG15-protein conjugation (ISGylation) is the core biological process that UBE2L6 participates in. This annotation was transferred from the well-characterized human ortholog (O14933/UBE2L6), which has been experimentally demonstrated to be the dedicated E2 enzyme for ISGylation. The rabbit ortholog, identified by Ensembl Compara, would be expected to retain this conserved function.
Reason: ISGylation is the primary biological process for UBE2L6, supported by ortholog transfer from the experimentally characterized human protein.
Summary: ISG15 transferase activity is the core molecular function of UBE2L6. The human ortholog (O14933) has been experimentally shown to be the dedicated E2 enzyme that transfers ISG15 to substrate proteins in conjunction with E3 ligases. This annotation, transferred via Ensembl Compara and combined automated methods, is well-supported by the conserved UBC domain and orthologous relationship.
Reason: This is the most specific and informative molecular function annotation for UBE2L6, representing its primary enzymatic activity as the ISG15- specific E2 conjugating enzyme.
Summary: UBE2L6/UBCH8 participates in the innate immune response through its role in ISGylation. ISG15 conjugation is an interferon-stimulated defense mechanism that modifies viral and host proteins as part of the antiviral innate immune response. This annotation, transferred from the human ortholog via Ensembl Compara, correctly places UBE2L6 in the broader biological context of innate immunity.
Reason: While UBE2L6 does participate in the innate immune response through ISGylation, this is a high-level biological process term. The more specific ISG15-protein conjugation term (GO:0032020) more precisely captures the direct biological role. This annotation provides useful biological context but is not the most informative process-level annotation.
Supporting Evidence:
file:RABIT/G1TUN6/G1TUN6-goa.tsv
UniProtKB:O14933|ensembl:ENSP00000287156
Core Functions
UBE2L6 functions as the dedicated E2 conjugating enzyme for ISG15, an interferon-stimulated ubiquitin-like modifier. It accepts activated ISG15 from the E1 enzyme UBA7 via a thioester bond at its active-site cysteine and transfers it to substrate proteins in cooperation with E3 ligases.
These computational predictions are reviewed separately from the GOA annotation set used for this review. The assessments below are from this project and do not constitute official GO annotations or endorsement by GO/UniProt. They are not included in the existing annotation review above.
LSP — Less precise than existing annotation Review score: 2/2
Prediction method: ProtNLM2 · Version: UniProt 2024_06 pilot
Review rationale: The target has a UBC conjugating-enzyme domain and the active-site cysteine annotation, consistent with its UBE2L6-family assignment. Experiments on UbcH8/UBE2L6 establish E2-mediated ubiquitin and ISG15 conjugation (PMID:15131269), supporting transfer of the broad catalytic class. Cached target annotations already include ISG15 transferase and ubiquitin-like protein transferase activities. Generic transferase activity is supported but less precise than those functions.
Supporting Evidence:
file:RABIT/G1TUN6/G1TUN6-uniprot.txt: "ID G1TUN6_RABIT Unreviewed; 153 AA. ... DR InterPro; IPR000608; UBC. ... DR InterPro; IPR023313; UBQ-conjugating_AS. ... FT DOMAIN 2..149 ... FT /note="UBC core" ... FT ACT_SITE 86"
PMID:15131269: "UbcH8 is a major E2 enzyme for ISG15 conjugation"