Alternative titles; symbols
HGNC Approved Gene Symbol: UBE2N
Cytogenetic location: 12q22 Genomic coordinates (GRCh38) : 12:93,405,684-93,441,947 (from NCBI)
UBE2N is an E2 ubiquitin-conjugating enzyme that catalyzes lys63-linked polyubiquitin chains for many important proteins. It forms an E2/E3 complex with UEV1A (UBE2V1; 602995) and TRAF6 (602355) that is involved in NF-kappa-B (see 164011) activation (summary by Chen et al., 2015).
Yamaguchi et al. (1996) cloned the cDNA encoding UBE2N, a human ubiquitin (191339)-conjugating enzyme, from an epidermoid carcinoma KB cDNA library. The UBE2N gene, which the authors referred to as UBCHBEN, encodes a protein of 152 amino acids with a calculated molecular mass of 17.1 kD. The amino acid sequence shows 80% identity with the Drosophila 'bendless' gene product (ubiquitin-conjugating enzyme E2). Northern blot analysis detected a major 1.4- and a minor 2.4-kb transcript in all tissues examined, with the highest expression in heart, skeletal muscle, and testis.
Gross (2014) mapped the UBE2N gene to chromosome 12q22 based on an alignment of the UBE2N sequence (GenBank BC000396) with the genomic sequence (GRCh38).
Yamaguchi et al. (1996) found that, when expressed in Escherichia coli, the UBE2N gene product exhibited the ability to form a thiol ester linkage with ubiquitin in a ubiquitin-activating enzyme E1 (314370)-dependent manner. They concluded that the UBE2N gene encodes a novel human E2 that may be involved in protein degradation in the muscles and testis.
In yeast, Hofmann and Pickart (1999) showed that UBE2N, which they referred to as UBC13, formed a specific heteromeric complex with MMS2, a ubiquitin-conjugating enzyme variant (UBE2V2; 603001). A ubc13 yeast strain was ultraviolet (UV) sensitive, and single, double, and triple mutants of the UBC13, MMS2, and ubiquitin genes displayed a similar phenotype. These findings supported a model in which an MMS2/UBC13 complex assembles novel polyubiquitin chains for signaling in DNA repair, and suggested that UEV proteins may act to increase diversity and selectivity in ubiquitin conjugation.
The RAD6 (179095) pathway is central to postreplicative DNA repair in eukaryotic cells. Two principal elements of this pathway are the ubiquitin-conjugating enzymes RAD6 and the MMS2-UBC13 heterodimer, which are recruited to chromatin by the RING finger proteins RAD18 (605256) and RAD5 (608048), respectively. Hoege et al. (2002) showed that UBC9 (601661), a small ubiquitin-related modifier (SUMO)-conjugating enzyme, is also affiliated with this pathway and that proliferating cell nuclear antigen (PCNA; 176740), a DNA polymerase sliding clamp involved in DNA synthesis and repair, is a substrate. PCNA is monoubiquitinated through RAD6 and RAD18, modified by lys63-linked multiubiquitination, which additionally requires MMS2, UBC13, and RAD5, and is conjugated to SUMO by UBC9. All 3 modifications affect the same lysine residue of PCNA, K164, suggesting that they label PCNA for alternative functions. Hoege et al. (2002) demonstrated that these modifications differentially affect resistance to DNA damage, and that damage-induced PCNA ubiquitination is elementary for DNA repair and occurs at the same conserved residue in yeast and humans.
Zhao et al. (2007) found that depletion of UBC13 in chicken and human cells resulted in severe growth defects due to chromosome instability, as well as hypersensitivity to UV and ionizing radiation, consistent with a conserved role for UBC13 in RAD6/RAD18-dependent postreplication repair. UBC13-deficient cells were also compromised for DNA double-strand break repair by homologous recombination. Recruitment and activation of the E3 ubiquitin ligase function of BRCA1 (113705) and subsequent formation of RAD51 (179617) nucleoprotein filaments at double-strand breaks were abolished in UBC13-deficient cells. Furthermore, in the absence of UBC13, generation of single-strand DNA/RPA (see RPA1; 179835) complexes at double-strand breaks was severely attenuated. Zhao et al. (2007) concluded that UBC13 has a role in initiation of homologous recombination during double-strand break repair.
Cytokine signaling is thought to require assembly of multicomponent signaling complexes at cytoplasmic segments of membrane-embedded receptors, in which receptor-proximal protein kinases are activated. Matsuzawa et al. (2008) reported that, upon ligation, CD40 (109535) formed a complex containing adaptor molecules TRAF2 (601895) and TRAF3 (601896), ubiquitin-conjugating enzyme UBC13, cellular inhibitor of apoptosis protein-1 (CIAP1, or BIRC2; 601712) and -2 (CIAP2, or BIRC3; 601721), IKK-gamma (IKBKG; 300248), and MEKK1 (MAP3K1; 600982). TRAF2, UBC13, and IKK-gamma were required for complex assembly and activation of MEKK1 and MAP kinase cascades. However, the kinases were not activated unless the complex was translocated from the membrane to the cytosol upon CIAP1/CIAP2-induced degradation of TRAF3. Matsuzawa et al. (2008) proposed that this 2-stage signaling mechanism may apply to other innate immune receptors and may account for spatial and temporal separation of MAPK and IKK signaling.
Shembade et al. (2010) showed that A20 (191163) inhibits the E3 ligase activities of TRAF6, TRAF2, and cIAP1 by antagonizing interactions with E2 ubiquitin-conjugating enzymes UBC13 and UBCH5C (602963). A20, together with the regulatory molecule TAX1BP1 (605326), interacted with UBC13 and UBCH5C and triggered their ubiquitination and proteasome-dependent degradation. These findings suggested a mechanism of A20 action in the inhibition of inflammatory signaling pathways.
Nakada et al. (2010) reported that OTUB1 (608337), a deubiquitinating enzyme, is an inhibitor of double-stranded break-induced chromatin ubiquitination. Surprisingly, they found that OTUB1 suppresses RNF168 (612688)-dependent polyubiquitination independently of its catalytic activity. OTUB1 does so by binding to and inhibiting UBC13, the cognate E2 enzyme for RNF168. Nakada et al. (2010) suggested that this unusual mode of regulation is unlikely to be limited to UBC13 because analysis of OTUB1-associated proteins revealed that OTUB1 binds to E2s of the UBE2D and UBE2E subfamilies. Finally, OTUB1 depletion mitigates the double-stranded break repair defect associated with defective ATM (607585) signaling, indicating that pharmacologic targeting of the OTUB1-UBC13 interaction might enhance the DNA damage response.
Pertel et al. (2011) demonstrated that TRIM5 (608487) promotes innate immune signaling and that this activity is amplified by retroviral infection and interaction with the capsid lattice. Acting with the heterodimeric ubiquitin-conjugating enzyme UBC13-UEV1A, TRIM5 catalyzes the synthesis of unattached K63-linked ubiquitin chains that activate the TAK1 (602614) kinase complex and stimulate AP1 (see 165160) and NF-kappa-B signaling. Interaction with the HIV-1 capsid lattice greatly enhanced the UBC13-UEV1A-dependent E3 activity of TRIM5, and challenge with retroviruses induced the transcription of AP1- and NF-kappa-B-dependent factors with a magnitude that tracked with TRIM5 avidity for the invading capsid. Finally, TAK1 and UBC13-UEV1A contribute to capsid-specific restriction by TRIM5. Pertel et al. (2011) concluded that the retroviral restriction factor TRIM5 has 2 additional activities that are linked to restriction: it constitutively promotes innate immune signaling, and it acts as a pattern recognition receptor specific for the retrovirus capsid lattice.
By infecting a human epithelial cell line with wildtype Shigella flexneri or an S. flexneri mutant lacking the glutamine deamidase OspI, Sanada et al. (2012) showed that OspI selectively deamidated gln100 of UBC13 to glu. Consequently, the E2 ubiquitin-conjugating activity required for TRAF6 activation was inhibited, allowing OspI to modulate the diacylglycerol-NFKB signaling pathway, which is mediated through the CARD11 (607210)-BCL10 (603517)-MALT1 (604860) (CBM) complex and TRAF6. Sanada et al. (2012) proposed that S. flexneri inhibits acute inflammatory responses by targeting the UBC13-TRAF6 complex.
Using overexpression and silencing experiments in human NKL cells, Chen et al. (2015) showed that ERADP (616869) promoted IFN-gamma (IFNG; 147570) production via NF-kappa-B pathway activation. Yeast 2-hybrid analysis of a human spleen cDNA library and coimmunoprecipitation experiments in 293T and NKL cells showed that ERADP interacted with UBC13, and NK-cell activation increased interaction strength. Immunoprecipitation assays in 293T cells revealed that ERADP strengthened the association of UBC13 with its E2/E3 ubiquitin complex components UEV1A and TRAF6. ERADP overexpression in 293T cells increased TRAF6- and UBC13-mediated lys63-linked ubiquitination of NEMO (IKBKG), resulting in increased NF-kappa-B activity. In an in vitro assay, ERADP increased the ubiquitin charging activity of UBC13 in a dose-dependent manner. Using UBC13 knockdown experiments, Chen et al. (2015) showed that UBC13 was required for ERADP-mediated IFN-gamma production in NKL cells.
Thorslund et al. (2015) elucidated how RNF8 (611685) and UBC13 promote recruitment of RNF168 (612688) and downstream factors to double-strand break sites in human cells. Thorslund et al. (2015) established that UBC13-dependent K63-linked ubiquitylation at double-strand break sites is predominantly mediated by RNF8 but not RNF168, and that H1-type linker histones, but not core histones, represent major chromatin-associated targets of this modification. The RNF168 module (UDM1) recognizing RNF8-generated ubiquitylations is a high-affinity reader of K63-ubiquitylated H1, mechanistically explaining the essential roles of RNF8 and UBC13 in recruiting RNF168 to double-strand breaks. Consistently, reduced expression or chromatin association of linker histones impair accumulation of K63-linked ubiquitin conjugates and repair factors at double-strand break-flanking chromatin. These results identified histone H1 as a key target of RNF8-UBC13 in double-strand break signaling and expanded the concept of the histone code by showing that posttranslational modifications of linker histones can serve as important marks for recognition by factors involved in genome stability maintenance.
Crystal Structure
Wiener et al. (2012) described structural and biochemical studies elucidating how OTUB1 inhibits UBC13 and other E2 enzymes. They unexpectedly found that OTUB1 binding to UBC13-Ub (the ubiquitin thiolester) is allosterically regulated by free ubiquitin, which binds to a second site in OTUB1 and increases its affinity for UBC13-Ub, while at the same time disrupting interactions with UEV1A in a manner that depends on the OTUB1 N terminus. Crystal structures of an OTUB1-UBC13 complex and of OTUB1 bound to ubiquitin aldehyde and a chemical UBC13-Ub conjugate showed that binding of free ubiquitin to OTUB1 triggers conformational changes in the OTU domain and formation of a ubiquitin-binding helix in the N terminus, thus promoting binding of the conjugated donor ubiquitin in UBC13-Ub to OTUB1. The donor ubiquitin thus cannot interact with the E2 enzyme, which has been shown to be important for ubiquitin transfer. The N-terminal helix of OTUB1 is positioned to interfere with UEV1A binding to UBC13, as well as with attack on the thiolester by an acceptor ubiquitin, thereby inhibiting K63Ub synthesis. OTUB1 binding also occludes the RING E3 binding site on UBC13, thus providing a further component of inhibition. Wiener et al. (2012) concluded that the general features of the inhibition mechanism explained how OTUB1 inhibits other E2 enzymes in a noncatalytic manner.
Yamamoto et al. (2006) generated mice with B cell-specific deletion of Ubc13. These mice were viable and had no obvious abnormalities, whereas Ubc13-deficient mice died early in utero. The absence of Ubc13 in B cells resulted in defective development of marginal zone B cells and B1 cells, as well as impaired humoral immunity. Nfkb activation and Tak1 (NR2C2; 601426) phosphorylation were essentially normal in Ubc13-deficient cells, but MAP kinase activation was substantially impaired in response to all stimuli tested except Tnf (191160). Ubc13-induced MAP kinase activation was mediated partially through ubiquitination of Ikkg, which was abolished in Ubc13-deficient cells. Yamamoto et al. (2006) concluded that UBC13 is important in the induction of immune responses.
Fukushima et al. (2007) found that homozygous Ubc13 deletion in mice caused early embryonic lethality, whereas Ubc13 +/- heterozygotes appeared normal with no alterations in immune cell populations. Ubc13 +/- mice resisted lipopolysaccharide-induced lethality, showed reduced ubiquitination of Traf6, produced lower levels of Tnf, Il6 (147620), and Ifng, and exhibited selectively impaired activation of signal transduction pathways initiated by Tnfr and Tlr family members. Fukushima et al. (2007) proposed that reducing UBC13 activity may have therapeutic uses in controlling inflammatory responses.
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