Damage suppressor protein (Dsup) is a tardigrade-unique intrinsically disordered nuclear protein that protects chromosomal DNA from damage by reactive oxygen species (ROS) and ionizing radiation. Dsup binds preferentially to nucleosomes over free DNA, associating with the nucleosome core via a C-terminal region (aa 360-445) that shares sequence similarity with the nucleosome-binding domain of vertebrate HMGN proteins. By physically shielding chromatin, Dsup prevents hydroxyl radical-mediated DNA cleavage including single-strand breaks (SSBs) and double-strand breaks (DSBs). Expression in non-tardigrade cells (human, plant, fly) confers improved radiotolerance. Dsup is largely unstructured and forms fuzzy complexes with DNA.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005634 nucleus | IEA GO_REF:0000044 | ACCEPT | Summary: Nuclear localization of Dsup is well supported by experimental data in the discovery paper (PMID:27649274). GFP-fused Dsup co-localized with nuclear DNA in both Drosophila S2 cells and human HEK293T cells, and immunohistochemistry confirmed nuclear localization in tardigrade embryos. The IEA annotation via UniProtKB-SubCell mapping is consistent with this experimental evidence and is appropriate to retain. Reason: Although this is an IEA annotation, it is strongly supported by direct experimental evidence from PMID:27649274, where Dsup-GFP was shown to co-localize with nuclear DNA in multiple cell types, and immunohistochemistry confirmed endogenous nuclear localization in tardigrade embryos. UniProt also records this as experimentally validated subcellular location. Supporting Evidence: PMID:27649274 Only one protein, termed Damage suppressor (Dsup), co-localized with nuclear DNA (Supplementary Fig. 11) and similar co-localization was also observed in human cultured HEK 293T cells (Fig. 3a). PMID:27649274 In almost all tardigrade cells expressing Dsup, Dsup proteins co-localized with nuclear DNA |
| GO:0003677 DNA binding | EXP PMID:39358423 Structural study of the intrinsically disordered tardigrade ... | ACCEPT | Summary: DNA binding by Dsup is directly supported by PMID:39358423, which used SAXS to structurally characterize the Dsup-DNA complex, demonstrating fuzzy complex formation with free DNA. Hashimoto et al. (PMID:27649274) also showed DNA binding by gel-shift assay. While Dsup binds nucleosomes with higher affinity than free DNA (PMID:31571581), GO:0031491 (nucleosome binding) is on a separate GO branch (child of chromatin binding, not DNA binding), so both annotations are warranted. DNA binding is a genuine molecular function of Dsup. Reason: PMID:39358423 provides direct structural evidence for Dsup binding to free DNA, characterizing the Dsup-DNA complex by SAXS and showing fuzzy complex formation. Although Dsup has higher affinity for nucleosomes, nucleosome binding (GO:0031491) is not a subclass of DNA binding (GO:0003677) in GO - they are on separate branches. Both molecular functions are experimentally supported and should be annotated independently. Supporting Evidence: PMID:39358423 intrinsically disordered nature of Dsup protein with highly flexible structure was experimentally proven and characterized by the combination of small angle X-ray scattering (SAXS) technique, circular dichroism spectroscopy, and computational methods PMID:39358423 we have shown that Dsup forms fuzzy complex with DNA file:RAMVA/Dsup/Dsup-deep-research-falcon.md Dsup binds DNA non-sequence-specifically, with higher affinity for nucleosomes than free DNA |
| GO:0031491 nucleosome binding | EXP PMID:31571581 The tardigrade damage suppressor protein binds to nucleosome... | NEW | Summary: Chavez et al. (PMID:31571581) demonstrated that Dsup binds preferentially to nucleosomes over free DNA, binds primarily to the nucleosome core rather than linker DNA, and contains a conserved HMGN-like nucleosome-binding domain. The C-terminal region (aa 360-445) is required for nucleosome binding and hydroxyl radical protection. Nucleosome binding (child of chromatin binding) is a separate GO branch from DNA binding and captures a distinct molecular function of Dsup. Reason: Strong biochemical evidence from PMID:31571581 demonstrates preferential nucleosome binding via gel mobility shift assays with multiple nucleosome substrates. Mutagenesis of the HMGN-like domain confirms functional importance. This is a distinct molecular function from DNA binding and should be annotated separately. Supporting Evidence: PMID:31571581 These experiments revealed that Rv Dsup binds with a higher affinity to nucleosomes than to free DNA. PMID:31571581 It thus appears that Rv Dsup binds primarily to the nucleosome core rather than to the linker DNA. PMID:31571581 a conserved region in Dsup proteins exhibits sequence similarity to the nucleosome-binding domain of vertebrate HMGN proteins and is functionally important for nucleosome binding and hydroxyl radical protection |
| GO:0042262 DNA protection | EXP PMID:27649274 Extremotolerant tardigrade genome and improved radiotoleranc... | NEW | Summary: DNA protection is the core biological process function of Dsup. Hashimoto et al. (PMID:27649274) demonstrated that Dsup suppresses X-ray-induced DNA damage (SSBs and DSBs) and protects against ROS/hydrogen peroxide damage. Chavez et al. (PMID:31571581) showed that Dsup protects chromatin from hydroxyl radical-mediated cleavage in a purified biochemical system. This is not a DNA repair function but a direct physical shielding of chromatin. Reason: DNA protection (GO:0042262) is the central biological function of Dsup and is not currently annotated in GOA. Multiple publications provide strong experimental evidence that Dsup physically shields chromatin from damage by hydroxyl radicals, ROS, and ionizing radiation. This term is present in UniProt as a keyword-based IEA (GO:0006974, DNA damage response) but GO:0042262 is more precise for the protective (not repair) mechanism. Supporting Evidence: PMID:27649274 DNA fragmentation in Dsup-expressing cells was substantially suppressed to only 18% of total DNA in the tail (Fig. 4b), indicating that Dsup protein was able to protect DNA from ROS as well as X-rays. PMID:27649274 we concluded that the reduced number of DNA breaks in Dsup-expressing cells was due to the suppression of DNA breaks, rather than facilitation of DNA repair processes PMID:31571581 R. varieornatus Dsup is a nucleosome-binding protein that protects chromatin from hydroxyl radicals |
| GO:0003723 RNA binding | EXP PMID:37534176 The tardigrade Dsup protein enhances radioresistance in Dros... | NEW | Summary: RNA binding by Dsup is directly demonstrated in Zarubin et al. 2023 (PMID:37534176, iScience), which incubated R. varieornatus Dsup protein with total D. melanogaster RNA and showed a dose-dependent gel mobility shift of the RNA, indicating direct (sequence-non-specific) Dsup-RNA binding in vitro. This is the primary experimental source for the activity; the later structural paper PMID:39358423 (Zarubin et al. 2024, Sci Rep) only restates it as a secondary citation, so PMID:37534176 is used here as the evidence reference. RNA binding likely reflects the highly charged, intrinsically disordered nature of Dsup and may contribute to its non-specific transcriptional-repressor activity, but its precise biological significance remains uncertain. Reason: RNA binding (GO:0003723) is not currently in GOA but is supported by direct in vitro experimental evidence (gel mobility shift assay) in the primary source PMID:37534176. The original draft cited only the secondary structural paper PMID:39358423, which itself attributes the RNA-binding demonstration to its reference 17 (= PMID:37534176); the evidence reference has been corrected to the primary paper to comply with GO evidence standards. Proposed as a non-core annotation because, although experimentally observed, its biological relevance relative to Dsup's core DNA/chromatin-protection function is still unclear. Supporting Evidence: PMID:37534176 We examined in the same way whether the Dsup protein could bind to RNA. ... melanogaster RNA at various wt:wt ratios, and after gel mobility shift analysis, we observed a significantly slowed-down RNA migration depending on the Dsup dose (Figure 8), which indicated the binding of Dsup to RNA. |
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Download this section (compressed HTML)Q: What is the structural basis of Dsup's "fuzzy" nucleosome-shielding complex - which residues in the HMGN-like C-terminal region (aa 360-445) contact specific nucleosome surfaces, and can a high-resolution cryo-EM/NMR structure of the Dsup-nucleosome complex be obtained?
Q: Is the physical-shielding mechanism of Dsup sufficient to fully explain its protective phenotype in heterologous cells, or does Dsup also influence chromatin accessibility, DNA repair pathway choice, or transcription in human cells?
Q: Does the RNA-binding activity of Dsup reported in PMID:39358423 have a biological role (e.g. protecting nuclear RNA from oxidative damage, affecting chromatin-associated RNAs), or is it a non-specific consequence of its highly charged, disordered character?
Q: How conserved is the Dsup nucleosome-binding / DNA-protection function across other tardigrade species beyond R. varieornatus and H. exemplaris, and does sequence divergence correlate with differences in desiccation / radiation tolerance?
Experiment: Solve a high-resolution cryo-EM structure of the Dsup-nucleosome complex (and Dsup + nucleosome + H1) using cross-linking and tagged constructs to capture the dynamic complex, complemented by HX-MS and NMR to map the fuzzy footprint of Dsup on the nucleosome surface.
Hypothesis: Dsup binds the nucleosome via the HMGN-like RRSSR motif in the acidic patch / dyad region, partially shielding histone- and DNA-facing surfaces that are hotspots for hydroxyl-radical attack.
Type: structural biology of dynamic complexes
Experiment: Generate Dsup C-terminus (aa 360-445) deletion and RRSSR->EESSE mutant knock-ins in R. varieornatus by CRISPR, and quantify radiation tolerance, ROS-induced DNA damage (gamma-H2AX foci, COMET), and desiccation survival relative to wild-type animals.
Hypothesis: Loss or mutation of the nucleosome-binding C-terminal region abolishes Dsup-mediated DNA protection in vivo, demonstrating that nucleosome binding is required for the protective phenotype in the native organism.
Type: in vivo loss-of-function and structure-function rescue
Experiment: Profile chromatin accessibility (ATAC-seq), transcription (RNA-seq) and DNA-damage response (gamma-H2AX ChIP-seq) in human HEK293T cells expressing Dsup vs control before and after X-ray / H2O2 stress to determine whether Dsup alters genome-wide chromatin behaviour beyond direct nucleosome shielding.
Hypothesis: Dsup expression in human cells provides DNA protection primarily by direct nucleosome shielding with minimal off-target effects on chromatin accessibility or gene expression.
Type: heterologous expression + multi-omics profiling
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