UBE2L6 (UbcH8) is a 153-residue E2 conjugating enzyme made of a single UBC fold. It carries an active-site cysteine (Cys86) that accepts a ubiquitin-like modifier from an E1 enzyme as a thioester and hands it to an E3 ligase for attachment to substrate lysines. In mammals it is the main E2 of the interferon-induced ISG15 conjugation (ISGylation) pathway. It is charged with ISG15 by the ISG15-specific E1 UBA7 (UBE1L) and works with E3 ligases such as HERC5 to ISGylate host and viral proteins during the type I interferon response. It also works as a ubiquitin E2: it is charged by the ubiquitin E1 and supports polyubiquitination by HECT and RBR (RING-between-RING) E3 ligases. The rabbit protein is a full-length ortholog of human UBE2L6 and keeps the catalytic cysteine. No experiments have been done on the rabbit protein, so its function is inferred from the human and mouse orthologs.
Summary: UniRule-derived ATP binding (UR000141827), which matches the ATP-binding and Nucleotide-binding keywords on the record. E2 enzymes have no nucleotide-binding site. ATP is consumed only by the E1 enzyme, which adenylates the modifier before passing it on to the E2 cysteine. The reaction UniProt gives for this protein is an E1-to-E2 transthioesterification that uses no ATP.
Reason: This electronic mapping is demonstrably wrong for E2 enzymes. ATP is used by the E1 (UBA7 for ISG15, UBA1 for ubiquitin) and not by UBE2L6. In the human UbcH8 assays, ATP is supplied to drive the E1 step and the E2 is charged only when the E1 is present.
Summary: UniRule-derived ligase activity (UR000414843). GO places E2 activity under transferase activity (GO:0061631 and GO:0042296 are both is_a GO:0019787 ubiquitin-like protein transferase activity, under GO:0016740), and the EC number for this activity, 2.3.2.23, is a transferase number. The historic "E2 ubiquitin ligase" usage is not a ligase (EC 6) activity.
Reason: Wrong enzyme class. UBE2L6 performs transthioesterification and modifier transfer, which GO classifies as transferase activity. It does not join two molecules coupled to nucleoside triphosphate hydrolysis.
Summary: EC-mapped ubiquitin E2 activity. Human UbcH8 was first characterised as a ubiquitin E2, and the paper that identified it as the ISG15 E2 also showed that it supports efficient Rsp5-catalysed polyubiquitination of WBP2. It binds RBR E3s (HHARI, H7-AP1). The broad E2 PAINT node PTN002514495, which includes the UBE2L3 paralog among its seeds, gives this activity by IBA to human UBE2L6. G1TUN6 is a full-length ortholog with the catalytic Cys86 conserved (bioinformatics RESULTS.md).
Reason: This is a correct, precise molecular function. UBE2L6 works with two modifiers, ubiquitin and ISG15, and the ubiquitin E2 activity is a demonstrated activity of the orthologous enzyme, not a stray family transfer. The earlier review kept it as non-core. Here it is accepted as the second of two core catalytic activities, while ISG15 transfer remains the dominant in vivo role.
The active-site cysteine (UniProt ACT_SITE 86, glycyl
file:RABIT/G1TUN6/G1TUN6-uniprot.txt
Belongs to the ubiquitin-conjugating enzyme family
GO:0019787 ubiquitin-like protein transferase activity
IEA GO_REF:0000120
MODIFY
Summary: Correct but general. GO:0019787 is the common parent of GO:0042296 ISG15 transferase activity and GO:0061631 ubiquitin conjugating enzyme activity (both verified as is_a descendants in OLS). The gene already carries both descendants, and the experimental evidence on the ortholog supports both.
Reason: The term is true but less specific than the evidence supports. Both supported descendants, which this gene already carries, are given as replacements.
Summary: Ensembl Compara transfer from human O14933. Human UbcH8 was purified as the E2 charged by E1-ISG15 in extracts of interferon-treated cells, and siRNA knockdown blocked interferon-induced ISGylation in vivo. PAINT places the same process (with GO:0042296) at the Mammalia node PTN000631065, seeded by human UBE2L6 and mouse Ube2l6, so the function is inherited across mammalian UBE2L6 orthologs rather than specific to humans. UBE2L6 does part of the work of this process: it carries the ISG15 thioester and transfers it.
Reason: This is the core process. Orthology is established by Ensembl Compara and CTD 9246, the protein is full length, and the catalytic cysteine is conserved. The PAINT ancestral node covers mammalian UBE2L6.
Summary: This is the core molecular function. Human UbcH8 accepts ISG15 from E1-ISG15 as a DTT-sensitive thioester, and a UbcH8-competent E3 transfers ISG15 onward to a substrate. Related E2s cannot substitute: UbcH5b forms no complex, and UbcH7, the closest paralog, forms only about 5% as much. This is why the activity can be transferred only to UBE2L6 orthologs and not to the UBE2L3 paralog. PAINT node PTN000631065 (Mammalia) carries this term.
Reason: This is the most specific supported activity. G1TUN6 is the rabbit UBE2L6 ortholog, about 80% identical to human UBE2L6 against about 52% for UBE2L3, and keeps Cys86.
Summary: Ensembl Compara transfer of a human UniProt IDA from PMID:28724761. That paper studies ISG15 in hepatitis E virus replication. Its only experimental statement about UBE2L6 is negative: loss of UBE2L6 (or UBE1L) did not change the virus's sensitivity to type I interferon. When PAINT curators placed the UBE2L6-specific functions at node PTN000631065, they propagated ISG15 transferase activity and ISG15-protein conjugation but not innate immune response. We read that omission as deliberate. ISGylation is an interferon-induced antiviral modification, so UBE2L6 contributes to interferon-driven defense through GO:0032020. A separate innate immune process term adds no participation evidence beyond that.
Reason: The donor evidence does not show that UBE2L6 takes part in an innate immune response, and the cited paper reports that UBE2L6 loss had no effect in its assay. The immunological relevance is captured more accurately by ISG15-protein conjugation. This is not REMOVE, because ISGylation is part of the interferon antiviral program and other studies may support a role.
PAINT did not propagate innate immune response at PTN000631065 or at any other
Core Functions
ISG15-specific E2 activity. UBE2L6 is charged with ISG15 by the E1 UBA7 on its active-site Cys86 and transfers ISG15 to substrate proteins with E3 ligases such as HERC5. This makes it the principal E2 of the interferon-induced ISGylation pathway.
PTN000631065 (taxon Mammalia, 40674): GO:0042296 ISG15 transferase activity and
Ubiquitin E2 activity. UBE2L6 is charged with ubiquitin by the ubiquitin E1 and supports substrate polyubiquitination by HECT and RBR E3 ligases. This activity is shared with the broader UBC family.
Q: The human O14933 IDA for innate immune response (UniProt, PMID:28724761) rests on a paper whose UBE2L6 result is negative. Should the IDA be re-examined or replaced with a study that directly shows UBE2L6-dependent antiviral defense?
Q: Does rabbit UBE2L6 show the same strong preference for UBA7 over UBA1 as the human enzyme? Which residues determine this E1 selectivity, and are they conserved in lagomorphs?
Suggested Experiments
Experiment: Run in vitro E1-to-E2 thioester transfer assays with recombinant rabbit UBE2L6 (wild type and C86A) and rabbit or human UBA7 and UBA1, comparing charging with ISG15 and with ubiquitin.
Type: biochemical assay
External Prediction Reviews
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"