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UBA7 encodes ubiquitin-like modifier activating enzyme 7, an E1-class enzyme specialized for the ubiquitin-like protein ISG15 (www.proteinatlas.org). As the sole E1 in the ISG15 conjugation (ISGylation) pathway, UBA7 catalyzes the ATP-dependent activation of ISG15 and forms a high-energy thioester bond with the ISG15 C-terminus via its active-site cysteine (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). UBA7 then transfers ISG15 to its cognate E2 conjugating enzyme (UBE2L6, also known as UBCH8) to ultimately facilitate ISG15 attachment to target lysine residues on substrate proteins (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This mechanism is analogous to ubiquitination, involving a multi-step enzymatic cascade (E1–E2–E3), but UBA7 is highly specific for ISG15 (in contrast to the ubiquitin E1, UBA1) (pmc.ncbi.nlm.nih.gov). Structural studies confirm that while UBA7 shares a conserved domain architecture with other E1 enzymes (including an adenylation domain, a first and second catalytic cysteine half-domain, and a ubiquitin-fold domain) (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov), unique conformational arrangements underlie its strict specificity for ISG15 and its E2 partner (pmc.ncbi.nlm.nih.gov). Notably, UBA7-mediated ISG15 conjugation (termed ISGylation) can modify a broad range of interferon-stimulated proteins and other cellular proteins (e.g. p53) (www.proteinatlas.org). Through ISG15 attachment, UBA7 often alters the stability or activity of substrates – for example, ISGylation of the influenza A virus NS1 protein blocks its nuclear import, thereby inhibiting viral replication (www.proteinatlas.org). In summary, UBA7 functions as the initiating enzyme of the ISG15 system, activating ISG15 in an ATP-dependent manner and catalyzing its covalent linkage to target proteins, which is a critical post-translational modification in antiviral defense and other cellular pathways (www.proteinatlas.org) (pubmed.ncbi.nlm.nih.gov).
UBA7 protein is predominantly a cytosolic and nuclear enzyme, reflecting its role in modifying both cytoplasmic and nuclear substrates. Immunocytochemistry and proteomic analyses show that UBA7 is localized to the nucleoplasm and cytosol in human cells, with additional presence in vesicular structures (www.proteinatlas.org). Consistently, curated data list UBA7 in the nucleus (nucleoplasm) as well as the cytoplasm/cytosol compartments (www.proteinatlas.org). While sequence analysis predicted a single transmembrane region in UBA7 (suggesting a membrane association) (www.proteinatlas.org) (www.proteinatlas.org), experimental evidence does not indicate a stable membrane localization; instead UBA7 appears to be a soluble protein partitioned between cytosolic and nuclear compartments. The nuclear pool of UBA7 likely supports ISGylation of nuclear targets (such as transcription factors and PML nuclear body components), whereas cytosolic UBA7 can modify proteins in the cytoplasm or on vesicular organelles. Indeed, UBA7’s diffuse distribution aligns with the wide range of its substrates’ locations (e.g. cytosolic antiviral effectors and nuclear tumor suppressors). In summary, UBA7 is a predominantly intracellular enzyme found in the cytosol and nucleoplasm, enabling it to participate in post-translational modification processes throughout the cell (www.proteinatlas.org) (www.proteinatlas.org).
UBA7 plays a pivotal role in several biological processes, chiefly centered on protein post-translational modification and innate immune defense. Its primary role is to mediate protein ISGylation (ISG15-protein conjugation), a process that parallels ubiquitination and is part of the broader ubiquitin-like protein conjugation pathway (www.proteinatlas.org) (www.proteinatlas.org). Through ISGylation, UBA7 is intimately involved in the antiviral immune response: together with ISG15, it restricts the replication of many viruses including influenza A, rabies, Sindbis, rotavirus, and human cytomegalovirus (www.proteinatlas.org). For example, UBA7-dependent ISGylation of influenza virus NS1 protein prevents NS1 from entering the nucleus, thereby curbing viral replication (www.proteinatlas.org). Similarly, ISG15 modification of HCMV protein UL26 (catalyzed by the UBA7 pathway) destabilizes UL26 and impedes its ability to block NF-κB signaling, contributing to antiviral immunity (www.proteinatlas.org). Beyond direct antiviral effects, UBA7 orchestrates interferon-driven immune signaling; notably, ISGylation of key signaling molecules (such as STAT1/STAT2 by the UBA7–ISG15 system) amplifies chemokine production and promotes the formation of antiviral PML nuclear bodies, thereby enhancing cytotoxic T cell recruitment and anti-tumor immunity (pubmed.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov).
In addition to immunity, UBA7 is involved in protein homeostasis and degradation pathways. By tagging proteins with ISG15 or ubiquitin-like modifiers, UBA7 can target aberrant or regulatory proteins for turnover (a modification-dependent protein catabolic process) (www.proteinatlas.org). For instance, in the context of retinoic acid signaling in leukemia, UBA7 (as UBE1L) conjugates ISG15 to the PML-RARα oncoprotein, marking it for proteasomal degradation (pmc.ncbi.nlm.nih.gov). This promotes apoptosis of acute promyelocytic leukemia cells and is a crucial mechanism by which all-trans retinoic acid therapy reverses the oncogenic block in differentiation (www.pnas.org). Likewise, UBA7-mediated ISGylation of the cell-cycle regulator cyclin D1 leads to cyclin D1’s accelerated degradation, resulting in G1 arrest and growth suppression in lung epithelial cells (pubmed.ncbi.nlm.nih.gov). Through such actions, UBA7 influences cell cycle and apoptotic processes, especially in stressed or cancerous cells. There is also evidence linking UBA7 to the DNA damage response: ISG15 modification of p53 (a DNA damage-responsive tumor suppressor) suggests that UBA7 might modulate p53 stability or activity under genotoxic stress (www.proteinatlas.org). Overall, UBA7 is involved in innate immune processes (antiviral defense and interferon signaling), protein modification and catabolism, and regulation of cell proliferation and survival. These contributions are reflected in its Gene Ontology annotations, which include ISG15 conjugation (GO:0032020), protein ubiquitination (GO:0016567), protein modification by small protein transfer and related processes (www.proteinatlas.org).
Given its role in protein regulation and immunity, UBA7 has been implicated as a tumor suppressor and is associated with several diseases. In cancer biology, loss or reduced expression of UBA7 is frequently observed. The UBA7 gene resides on chromosome 3p21.3, a region that undergoes loss of heterozygosity (LOH) in many lung cancers: about 70–80% of non-small cell lung cancers and up to ~90–100% of small cell lung cancers show deletion of 3p21.3 (pmc.ncbi.nlm.nih.gov). Correspondingly, UBA7 expression is notably downregulated in lung cancer cell lines (pmc.ncbi.nlm.nih.gov). Functional studies indicate UBA7 exerts tumor-suppressive effects in lung epithelial cells by promoting ISGylation and degradation of oncogenic proteins – for example, UBA7-driven ISG15 conjugation of cyclin D1 leads to cyclin D1 repression, and ectopic UBA7 can also reduce EGFR levels in bronchial cells (pmc.ncbi.nlm.nih.gov). Consistent with a tumor suppressor role, re-introduction of UBA7 (UBE1L) in lung cancer models inhibits cell growth, and retinoid treatments (e.g. bexarotene) that upregulate UBA7 result in lowered cyclin D1 and proliferation indices in tumors (pubmed.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov).
UBA7’s tumor-suppressive impact extends to other cancers as well. In breast cancer, UBA7 is significantly under-expressed in tumor tissues compared to normal, and low UBA7 levels correlate with more aggressive disease features and worse patient outcomes (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Patients whose tumors have low UBA7 expression have poorer prognosis (shorter overall and relapse-free survival), whereas higher UBA7 levels are associated with improved survival (pmc.ncbi.nlm.nih.gov). A recent study identified UBA7 as an interferon-stimulated gene that acts as a tumor suppressor in breast cancer, facilitating an immune-friendly tumor microenvironment: UBA7-dependent ISGylation of STAT1/2 in tumor cells boosts chemokine secretion and T-cell infiltration, restraining tumor growth and metastasis (a finding mirrored by better survival in breast cancer patients with robust UBA7/ISG15 pathway activity) (pubmed.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov). UBA7 is thus being investigated as a prognostic biomarker and potential therapeutic target in breast carcinoma (pmc.ncbi.nlm.nih.gov).
In hematological malignancy, acute promyelocytic leukemia (APL) is a key example where UBA7 is involved in disease response. UBA7 (historically called UBE1L) is strongly induced by all-trans retinoic acid in APL cells and mediates ISG15 conjugation to the PML-RARα fusion protein (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This leads to degradation of PML-RARα and contributes to the differentiation and apoptosis of APL cells in therapy (www.pnas.org). Experimental overexpression of UBA7 in APL cell lines causes selective elimination of PML-RARα and cell death, confirming that UBA7 can phenocopy retinoid treatment by targeting the oncogene for destruction (www.pnas.org). These findings underscore UBA7’s role in the pathogenesis and treatment response of APL.
Beyond cancer, recent research suggests UBA7 may be relevant in certain myelodysplastic syndromes. In MDS patients with SF3B1 splicing-factor mutations, aberrant alternative splicing of UBA7 has been observed, leading to reduced UBA7 expression (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This spliceosome-mediated downregulation of UBA7 is associated with poorer prognosis in MDS and chronic lymphocytic leukemia, highlighting UBA7 as a potential disease modifier in these contexts (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Additionally, inherited or de novo loss-of-function mutations in UBA7 are rare (no Mendelian disease is currently attributed to UBA7), but functional knockout of Uba7 in mice produces notable phenotypes: Uba7-null mice fail to ISGylate proteins and show increased susceptibility to viral infections (pmc.ncbi.nlm.nih.gov). For example, Ube1L^(-/-) mice are significantly more vulnerable to influenza virus infection, underscoring the enzyme’s critical role in antiviral defense in vivo (pmc.ncbi.nlm.nih.gov). Overall, UBA7’s dysregulation is linked to oncogenesis (lung, breast cancers, leukemia), and its activity is vital for controlling viral disease, aligning with its function in immune surveillance and protein quality control.
UBA7 is a 1012–amino acid protein (≈111 kDa) comprising several conserved domains characteristic of E1 enzymes (www.proteinatlas.org). The N-terminal portion contains a bipartite adenylation domain that binds ATP and the C-terminal glycine of ISG15, catalyzing the formation of ISG15–AMP (adenylate) (pmc.ncbi.nlm.nih.gov). This adenylation domain consists of an active adenylation subdomain (AAD) and an inactive adenylation subdomain (IAD), which together cradle ISG15 and ATP during the activation step (pmc.ncbi.nlm.nih.gov). UBA7 also harbors two distinct catalytic cysteine domains: the FCCH (First Catalytic Cysteine Half-domain) and the SCCH (Second Catalytic Cysteine Half-domain) (pmc.ncbi.nlm.nih.gov). The FCCH helps recognize and position the ISG15 adenylate, while the SCCH contains the critical active-site cysteine residue (Cys^m) that forms a thioester bond with the ISG15 C-terminus (pmc.ncbi.nlm.nih.gov). These domains undergo dramatic conformational changes during UBA7’s catalytic cycle; for instance, the SCCH domain rotates from an “open” to a “closed” conformation to bring the catalytic cysteine into alignment for thioester formation (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Following ISG15 activation, UBA7’s C-terminal ubiquitin fold domain (UFD) engages the E2 enzyme (UBE2L6), facilitating E2 docking and thioester transfer of ISG15 from UBA7 to the E2’s active cysteine (pmc.ncbi.nlm.nih.gov). This modular domain organization – adenylation domain (for ATP binding and ISG15 adenylation), catalytic cysteine domains (for thioester generation), and UFD (for E2 recruitment) – is similar to that of the ubiquitin E1 (UBA1), reflecting their common evolutionary origin (pmc.ncbi.nlm.nih.gov).
Crystallographic and cryo-EM analyses have revealed that UBA7’s overall tertiary structure is highly ordered, with the ISG15 and E2 binding sites spatially coordinated. Notably, recent cryo-EM structures of the human UBA7–UBE2L6–ISG15 complex captured UBA7 in action, bound to ISG15 (in adenylate and thioester states) and the E2 enzyme (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). These structures highlight subtle but important differences from the canonical ubiquitin E1: for example, in UBA7 the ISG15 molecule is oriented with a ~17° rotation relative to how ubiquitin binds UBA1, and UBA7’s FCCH domain is rotated ~25° compared to UBA1 (pmc.ncbi.nlm.nih.gov). Such differences explain UBA7’s “exquisite specificity” for ISG15 and UBE2L6 (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). The active-site region of UBA7 accommodates ISG15’s two-domain (tandem ubiquitin-like) structure, and specific residues in UBA7 contact ISG15’s C-terminal LRLRGG motif (shared with ubiquitin) to ensure correct positioning (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). UBA7 also contains sequence motifs for ATP binding (e.g. a conserved Adenosine-binding loop) and for magnesium coordination, typical of adenylate-forming enzymes. There is no signal peptide in UBA7 and it is not a secreted protein (www.proteinatlas.org). Some algorithms predict a single hydrophobic segment in UBA7 (hence “Predicted location: membrane”) (www.proteinatlas.org) (www.proteinatlas.org), but this likely represents an internal helix rather than a true transmembrane anchor, as experimental data show UBA7 is cytosolic/nuclear. In summary, UBA7’s structure comprises the necessary domains for a two-step enzyme: an adenylation domain (for ISG15 activation), a catalytic cysteine domain (for thioester formation), and a UFD domain (for E2 interaction), all arranged to carry out ISG15 transfer with high specificity (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).
UBA7 is expressed ubiquitously in human tissues with generally low tissue specificity, although its expression levels can vary under certain physiological conditions. Baseline RNA and protein profiles show that UBA7 is present in most tissue types, with a slight enrichment in immune-related organs. For example, UBA7’s tissue expression cluster is associated with the spleen and immune response pathways (www.proteinatlas.org). It is also expressed in various cell types; single-cell RNA data indicate relatively higher UBA7 expression in granulocytes, intestinal enterocytes, and gastric mucus-secreting cells, among others (www.proteinatlas.org). In the blood, UBA7 is not detected as a secreted factor (consistent with it being an intracellular enzyme) (www.proteinatlas.org). The gene shows low regional specificity in the brain and is broadly expressed in both neural and non-neural cells (www.proteinatlas.org). At the protein level, immunohistochemistry confirms moderate to high nuclear staining in most tissues (www.proteinatlas.org), aligning with UBA7’s role as a ubiquitin-like modifier enzyme active in many cell types.
Regulation of UBA7 expression is tightly linked to immune signaling and differentiation cues. Type I interferons (IFN-α/β) robustly induce UBA7 transcription, as UBA7 is itself an interferon-stimulated gene (ISG) (pmc.ncbi.nlm.nih.gov). Upon IFN treatment or viral infection, UBA7 levels rise alongside ISG15 and other components of the ISGylation system, ensuring the cell is equipped to initiate ISG15 conjugation (pubmed.ncbi.nlm.nih.gov). This coordinated induction is part of the innate immune program; indeed, one study identified UBA7 as part of a network of ISGs whose upregulation leads to enhanced T cell–mediated tumor killing (pubmed.ncbi.nlm.nih.gov). UBA7 is also known to be a retinoic acid-inducible gene: exposure to all-trans retinoic acid or certain synthetic retinoids (e.g. bexarotene) can significantly increase UBA7 mRNA and protein expression in cells (www.pnas.org) (pubmed.ncbi.nlm.nih.gov). In APL cells, retinoid signaling through RAR/RXR receptors relieves transcriptional repression on the UBA7 gene (UBE1L), causing a rapid rise in UBA7 transcripts within hours of treatment (www.pnas.org). This induction is critical for the therapeutic degradation of PML-RARα in APL, as discussed above. Additionally, UBA7 expression can be modulated by certain chemicals; for example, polyphenols like curcumin were reported to upregulate UBA7 in bronchial epithelial cells, leading to downstream effects on EGFR levels (pmc.ncbi.nlm.nih.gov).
In cancer, downregulation of UBA7 is commonly observed, as noted. Large-scale analyses (e.g. TCGA data) show that tumors such as breast, lung, liver, and others often have lower UBA7 expression compared to normal tissue, and this low expression frequently correlates with worse clinical outcomes (pmc.ncbi.nlm.nih.gov). This suggests that in many cancers the UBA7 gene is suppressed (via chromosomal loss, epigenetic silencing, or splicing defects), potentially to evade its growth-suppressive ISGylation effects. On the other hand, leukemia cell lines (especially APL or others under inflammatory stimulation) can exhibit elevated UBA7 levels, fitting with the observation that UBA7 is cancer-enhanced in certain leukemia contexts (www.proteinatlas.org) where differentiation therapy or interferon pathways are active. Overall, UBA7 is broadly expressed and is dynamically regulated by immune signals (IFNs) and differentiation signals (retinoids). Its expression pattern mirrors its functional role – high during immune activation and in certain normal immune cells, but often lost or reduced in malignancies, underlining its importance in normal cellular defense and tumor suppression (pmc.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov).
UBA7 and its substrate ISG15 represent a relatively recent innovation in vertebrate evolution. Both genes are found exclusively in vertebrates and are absent in invertebrate genomes (pubmed.ncbi.nlm.nih.gov). Phylogenetic analysis suggests that UBA7 arose by duplication of the ancestral ubiquitin-activating enzyme gene (UBA1) early in vertebrate evolution (pubmed.ncbi.nlm.nih.gov). Likewise, ISG15 appears to have originated from a duplication of a ubiquitin gene (UBB/UBC) around the same time (pubmed.ncbi.nlm.nih.gov). This paired emergence allowed the co-evolution of a dedicated ISG15 conjugation system. In fish (teleosts), Uba7 retains some of the ancestral promiscuity of UBA1 – notably, zebrafish Uba7 can activate ubiquitin as well as ISG15, and can function in the ubiquitin conjugation cascade in vitro and in vivo (pubmed.ncbi.nlm.nih.gov). This indicates that in early vertebrates, UBA7 was a less specialized E1 enzyme that could service multiple ubiquitin-like modifiers. However, during the evolution of tetrapods (amphibians, reptiles, birds, mammals), UBA7 became highly specialized for ISG15, losing significant ability to activate ubiquitin (pubmed.ncbi.nlm.nih.gov). This increased specificity likely provided a selective advantage by refining the interferon-induced ISG15 pathway independent of the ubiquitin system. The “ubiquitin-like” nature of ISG15 is reflected in its two-domain structure (tandem ubiquitin folds) and a C-terminal LRLRGG motif identical to ubiquitin’s; UBA7 evolved to recognize these features of ISG15 with high fidelity (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Across mammalian species, UBA7 is well conserved: human UBA7 shares high sequence identity with Uba7 in other mammals (e.g. rodents, ~80–90% identity at the amino acid level), and the key catalytic residues and domains are invariant, underscoring their essential function. Even in more distant vertebrates (birds, fish), UBA7 orthologs are recognizable and contain the same domain architecture and active-site motifs (pmc.ncbi.nlm.nih.gov). This conservation pattern – presence in all jawed vertebrates but absence from insects, worms, etc. – suggests that the ISG15/UBA7 system likely evolved alongside the interferon-based immune system unique to vertebrates. It serves as an example of how gene duplication followed by functional divergence created a new pathway: an ancestral ubiquitin E1 (UBA1) gave rise to UBA7 which diverged to specifically support a novel antiviral modification (ISGylation) (pubmed.ncbi.nlm.nih.gov). The evolutionary trajectory of UBA7 exemplifies how an “old” enzyme gained a “new” specific function through natural selection, with UBA7 becoming a dedicated ISG15 activator that augments vertebrate immune responses (pubmed.ncbi.nlm.nih.gov).
Molecular Function:
- ISG15 activating enzyme activity (GO:0019782): Catalysis of the activation of ISG15 (covalently linking ISG15 to E1 via thioester bond) (www.proteinatlas.org).
- Ubiquitin-like modifier activating enzyme activity (GO:0008641): General E1 activity for ubiquitin-like proteins, as performed by UBA7 in ISGylation (www.proteinatlas.org).
- ATP binding (GO:0005524): Ability to bind ATP, required for UBA7’s adenylation of ISG15 (www.proteinatlas.org).
- Protein binding (GO:0005515): Interacts with proteins such as E2 conjugating enzymes and substrates during the ISGylation process (www.proteinatlas.org).
(Additional relevant MF terms include “ligase activity” (GO:0016874) and “ubiquitin-protein transferase activity” (GO:0004842) indicating UBA7’s role in transferring ubiquitin-like modifiers (www.proteinatlas.org).)
Biological Process:
- ISG15-protein conjugation (GO:0032020): The process of covalently attaching ISG15 to target proteins (protein ISGylation), which UBA7 initiates (www.proteinatlas.org).
- Protein ubiquitination (GO:0016567): Involvement in ubiquitin-dependent protein modification; UBA7 is analogous to ubiquitin E1 and functionally links to protein degradation pathways (www.proteinatlas.org).
- Protein modification by small protein conjugation (GO:0032446): A broad category encompassing attachment of small proteins like ubiquitin/ISG15 to targets (www.proteinatlas.org).
- Modification-dependent protein catabolic process (GO:0019941): Protein degradation mediated by prior modification (such as ISG15 tagging of PML-RARα or cyclin D1 leading to proteasomal degradation) (www.proteinatlas.org).
- Cellular response to DNA damage stimulus (GO:0006974): Part of the DNA damage response network (likely via ISG15 modification of p53 and other factors) (www.proteinatlas.org).
(UBA7 also contributes to “defense response to virus” and interferon-mediated signaling, which are reflected in its role in antiviral ISGylation, although these may be annotated under related GO terms or UniProt keywords (www.proteinatlas.org) (pubmed.ncbi.nlm.nih.gov).)
Cellular Component:
- Nucleus (GO:0005634) & Nucleoplasm (GO:0005654): UBA7 is localized in the nucleus, particularly the nucleoplasm, where it can modify nuclear proteins (www.proteinatlas.org) (www.proteinatlas.org).
- Cytoplasm (GO:0005737) & Cytosol (GO:0005829): UBA7 is also found in the cytosolic compartment, modifying cytoplasmic proteins and participating in cytosolic antiviral complexes (www.proteinatlas.org) (www.proteinatlas.org).
- Vesicles (e.g. GO:0031982): Detected in association with vesicular structures inside cells (as per Human Protein Atlas), though not a membrane protein per se (www.proteinatlas.org).
(These localizations support UBA7’s function in both major intracellular compartments, consistent with its broad range of protein targets.)
Each of the above GO terms is supported by experimental evidence and curation: UBA7’s enzymatic activity, cellular distribution, and biological roles have been documented in the literature and captured in Gene Ontology annotations (www.proteinatlas.org) (www.proteinatlas.org), aiding functional annotation for this gene in the context of genome databases and the Gene Ontology project.