Secretory-abundant heat soluble protein 2 (SAHS2) is a tardigrade-specific secreted protein belonging to the SAHS family, which adopts a beta-barrel fold structurally similar to fatty acid-binding proteins (FABPs). SAHS proteins are abundantly expressed, heat-soluble, and are proposed to act as molecular shields that protect extracellular components during desiccation (anhydrobiosis). SAHS2 has a signal peptide (residues 1-19), three conserved SAHS domains (SAHS-c1, c2, c3), and an N-linked glycosylation site. The SAHS family is unique to tardigrades and has no homologs outside Tardigrada, except for distant structural similarity to metazoan FABPs. Crystal structures of the paralog SAHS1 (RvSAHS1) revealed two putative ligand binding sites where fatty acids can bind (PMID:28703282), establishing SAHS proteins as a new FABP-like family. SAHS2 is one of 13 SAHS paralogs encoded in the R. varieornatus genome and is constitutively and abundantly expressed (PMID:27649274).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
|
GO:0005576
extracellular region
|
IEA
GO_REF:0000044 |
ACCEPT |
Summary: SAHS2 is annotated to the extracellular region based on IEA mapping from UniProtKB subcellular location vocabulary. UniProt marks SAHS2 as "Secreted" based on similarity to SAHS1 (J7MFT5). The protein has a predicted signal peptide (residues 1-19) and was originally identified in the heat-soluble proteome of tardigrades (PMID:22937162). SAHS proteins are described as having a secretory localization distinct from the cytoplasmic CAHS family (PMID:22937162). The annotation to GO:0005576 extracellular region is well supported.
Reason: SAHS2 has a signal peptide and is described as a secretory protein. The original proteomics study identified SAHS proteins as having distinct subcellular localization from CAHS proteins, with SAHS being secretory (PMID:22937162). The name itself (Secretory Abundant Heat Soluble) reflects this localization. Extracellular region is the appropriate GO CC term for a secreted protein.
Supporting Evidence:
PMID:22937162
We named them Cytoplasmic Abundant Heat Soluble (CAHS) and Secretory Abundant Heat Soluble (SAHS) protein families, according to their localization.
PMID:28703282
Secretory abundant heat-soluble (SAHS) proteins have been reported as candidates for anhydrobiosis-related proteins in tardigrades, which seem to protect extracellular components and/or secretory organelles.
|
|
GO:0008289
lipid binding
|
ISS
PMID:28703282 Structural insights into a secretory abundant heat-soluble p... |
NEW |
Summary: SAHS2 is not currently annotated with lipid binding in GOA, but structural studies of the paralog SAHS1 (RvSAHS1) demonstrate that SAHS proteins adopt a beta-barrel fold homologous to fatty acid-binding proteins (FABPs) with two putative ligand binding sites (PMID:28703282). CDD classifies SAHS2 as containing an FABP domain (cd00742), InterPro assigns calycin fold (IPR012674), and SUPFAM classifies it in the Lipocalin superfamily (SSF50814). Crystal structure of SAHS4 showed variant binding preferences at these sites (PMID:29493034), suggesting functional diversification within the SAHS family.
Reason: Although no direct lipid binding assay has been performed on SAHS2 specifically, the structural evidence from the SAHS1 crystal structure (PMID:28703282) combined with the FABP domain classification (CDD cd00742) and calycin/lipocalin superfamily membership strongly supports lipid binding as a molecular function. SAHS1 (J7MFT5) already has an IEA lipid binding annotation via InterPro. The same structural basis applies to SAHS2, which shares the conserved SAHS domains and FABP-like fold. ISS evidence from the paralog SAHS1 crystal structure is appropriate.
Supporting Evidence:
PMID:28703282
RvSAHS1 shows a beta-barrel structure similar to fatty acid-binding proteins (FABPs), in which hydrophilic residues form peculiar hydrogen bond networks... We identified two putative ligand-binding sites: one that superimposes on those of some FABPs and the other, unique to and conserved in SAHS proteins. These results indicate that SAHS proteins constitute a new FABP family.
PMID:29493034
A previous crystallographic study revealed that a SAHS protein (RvSAHS1) from one of the toughest tardigrades, Ramazzottius varieornatus, has a beta-barrel architecture similar to fatty acid binding proteins (FABPs) and two putative ligand binding sites (LBS1 and LBS2) where fatty acids can bind.
|
|
GO:0009269
response to desiccation
|
ISS
PMID:22937162 Two novel heat-soluble protein families abundantly expressed... |
NEW |
Summary: SAHS proteins were discovered through heat-soluble proteomics specifically aimed at identifying desiccation tolerance factors in tardigrades (PMID:22937162). SAHS-family proteins change conformation from beta-structure to alpha-helical structure under water-deficient conditions, similar to LEA proteins, suggesting a protective role during desiccation. SAHS2 is constitutively and abundantly expressed, consistent with the fact that R. varieornatus tolerates rapid desiccation without significant transcriptional regulation (PMID:27649274). Functional assays on recombinant SAHS paralogs support a family-level role in extracellular protection during desiccation (PMID:38796644).
Reason: Response to desiccation (GO:0009269) is a core biological process for SAHS2, but the available support is family- and paralog-level rather than a direct SAHS2-specific assay. SAHS proteins were identified in the context of anhydrobiosis research, show water-deficiency-associated conformational change, are constitutively abundant in R. varieornatus, and recombinant SAHS paralogs protect membranous structures during drying. ISS is therefore more conservative than IDA for SAHS2.
Supporting Evidence:
PMID:22937162
proteins in both families changed their conformation to an alpha-helical structure in water-deficient conditions as LEA proteins do
PMID:38796644
Recombinantly expressed SAHS proteins prevent desiccated liposomes from fusion, and enhance desiccation tolerance of E. coli and Rhizobium tropici upon extracellular application
PMID:27649274
These abundantly expressed proteins included previously identified tardigrade-unique heat-soluble proteins, CAHS and SAHS, both of which maintain solubility even after heat treatment and are proposed to be involved in the protection of biomolecules during desiccation
file:RAMVA/SAHS2/SAHS2-deep-research-falcon.md
secreted, heat-soluble stress protein likely functioning as an extracellular protectant during desiccation/anhydrobiosis
|
Q: Does SAHS2, like other SAHS paralogs, bind fatty acids or other lipid ligands at LBS1/LBS2, and is ligand binding required for its membrane-protective activity during desiccation?
Q: Is SAHS2 primarily released into the body cavity / extracellular space from storage cells in vivo, and does its activity depend on this extracellular localization?
Q: Among the 13 R. varieornatus SAHS paralogs, what is the functional specialization of SAHS2 (e.g. in tissue distribution, ligand preference, or membrane-protection efficacy), and how redundant is its function with SAHS1, SAHS3 and other family members?
Experiment: Express and purify recombinant SAHS2 with a cleavable signal peptide, confirm secretion in a heterologous system, and test its activity in liposome (POPC) drying-rehydration DLS assays and in microbial desiccation survival assays, benchmarking against SAHS1/4/6, BSA and trehalose.
Hypothesis: Purified SAHS2 prevents desiccation-induced liposome fusion and improves survival of dried bacteria at concentrations comparable to other tested SAHS paralogs, confirming its membrane-protective function.
Type: in vitro biophysical and microbial protection assays
Experiment: Solve the crystal or cryo-EM structure of SAHS2 in apo and ligand-bound states, and test fatty acid / hydrophobic ligand binding by ITC, fluorescence and competition assays. Compare LBS1/LBS2 residue identities with SAHS1 and SAHS4 to predict and test ligand preferences.
Hypothesis: SAHS2 retains a calycin/FABP-like beta-barrel fold with two ligand binding sites whose residue substitutions (relative to SAHS1) determine a distinct hydrophobic ligand preference.
Type: structural biology / binding biochemistry
Experiment: Use TardiVec-style in vivo reporters (pRvSAHS2-mEGFP) and CRISPR knock-in of an epitope tag to determine the native tissue distribution and subcellular fate of SAHS2 in R. varieornatus (storage cells, vesicles, body cavity), with secretion confirmed by immunoblot of body cavity fluid.
Hypothesis: Endogenous SAHS2 is expressed predominantly in storage cells and is secreted into the body cavity where it accumulates and acts during anhydrobiosis.
Type: in vivo expression and secretion mapping
The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.
You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.
We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.
We are interested in where in or outside the cell the gene product carries out its function.
We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.
Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.
The target protein is SAHS2 from the tardigrade Ramazzottius varieornatus, originally defined as part of the SAHS (Secretory Abundant Heat Soluble) family discovered by heat‑soluble proteomics in R. varieornatus (Yamaguchi et al., 2012-08-22, PLoS ONE, https://doi.org/10.1371/journal.pone.0044209). In that defining work, SAHS1 and SAHS2 are explicitly classified as “secretory” proteins (i.e., the “secretory abundant heat soluble” family), and SAHS2 is reported to have a four‑exon gene structure. (yamaguchi2012twonovelheatsoluble pages 3-5, yamaguchi2012twonovelheatsoluble pages 6-7)
Critical scope note: direct experimental assays in the retrieved literature are largely performed on other R. varieornatus SAHS paralogs (e.g., SAHS1/4/6) rather than SAHS2. Therefore, SAHS2 functional annotation below distinguishes (i) SAHS2-specific evidence (gene identity and gene structure; secretory prediction) from (ii) SAHS-family evidence used to infer likely SAHS2 role. (yamaguchi2012twonovelheatsoluble pages 3-5, yamaguchi2012twonovelheatsoluble pages 6-7, lim2024tardigradesecretoryproteins pages 3-5)
SAHS proteins are a tardigrade-specific family of abundant, heat‑soluble proteins identified by extracting proteins that remain soluble after heating (92°C for 15 min) and then identifying major bands by mass spectrometry and cDNA cloning. (yamaguchi2012twonovelheatsoluble pages 2-3)
SAHS proteins were defined as “secretory” based on N‑terminal signal peptides and subcellular localization analyses, in contrast to CAHS (cytoplasmic abundant heat soluble) proteins, which were assigned to cytosol/nucleus-associated protective roles. (yamaguchi2012twonovelheatsoluble pages 3-5, yamaguchi2012twonovelheatsoluble pages 5-6)
A core concept in tardigrade stress tolerance is that some tardigrade-specific proteins (including SAHS) exhibit secondary-structure changes under water-deficient / desolvating conditions, analogous to LEA-like “molecular shield” models.
In the defining SAHS/CAHS work, SAHS proteins are described as β-structure-rich in hydrated conditions (demonstrated for SAHS1 by CD) but converting toward α-helical conformations in water-deficient conditions (family-level statement; SAHS1 requires strong desolvation mimic conditions such as >50% TFE to show α-helical conversion). (yamaguchi2012twonovelheatsoluble pages 5-6, yamaguchi2012twonovelheatsoluble pages 3-5)
Family-level functional assays (2024) strongly support that secreted SAHS proteins act as extracellular protectants of membrane-containing structures, rather than general enzyme stabilizers:
Liposome protection: In POPC liposome drying/rehydration assays, desiccation causes substantial liposome fusion/aggregation (appearance of large particles; loss of initial ~50–100 nm population). Addition of SAHS proteins (tested SAHS paralogs include R. varieornatus SAHS1/4/6 and a Hypsibius SAHS) at 0.1–10 mg/mL partially to strongly preserves small size distributions and suppresses formation of large fused particles; effects exceeded BSA controls and were comparable to trehalose in some conditions. (lim2024tardigradesecretoryproteins pages 3-5, lim2024tardigradesecretoryproteins media f52027ea)
Microbial survival: Extracellular application of SAHS proteins during drying improves survival of two bacteria (E. coli and Rhizobium tropici) after 48 h desiccation. The text reports >10-fold survival enhancement for E. coli in some conditions and summarizes performance as substantially better than BSA and (in some assays) better than trehalose. (lim2024tardigradesecretoryproteins pages 3-5, lim2024tardigradesecretoryproteins pages 6-7, lim2024tardigradesecretoryproteins media b0bc6f49)
Negative/limited enzyme protection: In the same study, SAHS proteins did not substantially outperform BSA in protecting lactate dehydrogenase activity after desiccation/rehydration, supporting specialization toward membranes/cells rather than broad enzyme chaperoning. (lim2024tardigradesecretoryproteins pages 7-8, lim2024tardigradesecretoryproteins pages 6-7)
Implication for SAHS2: Since SAHS2 is a canonical SAHS-family member in R. varieornatus and is predicted secretory like SAHS1, it is most parsimonious that SAHS2 contributes to extracellular stabilization (especially of membranes or secreted/extracellular structures) during desiccation, though direct SAHS2-specific assays were not found in the retrieved corpus. (yamaguchi2012twonovelheatsoluble pages 3-5, lim2024tardigradesecretoryproteins pages 3-5)
Lim et al. (2024-05, Communications Biology, https://doi.org/10.1038/s42003-024-06336-w) combine structural comparisons, AlphaFold-assisted analysis, and MD simulations to propose that SAHS proteins have fatty-acid-binding-protein (FABP)-like β-barrel folds with large internal cavities, and that loss of cavity solvent during desiccation destabilizes β-structure, promoting conformational rearrangements and potentially gel-/network-like assemblies that stabilize membranes and extracellular structures. (lim2024tardigradesecretoryproteins pages 1-3, lim2024tardigradesecretoryproteins pages 6-7)
This mechanistic model is consistent with the earlier observation that SAHS proteins can undergo β→α secondary-structure shifts under desolvating conditions. (yamaguchi2012twonovelheatsoluble pages 3-5, lim2024tardigradesecretoryproteins pages 6-7)
In the original SAHS-family description, SAHS1 and SAHS2 were predicted secretory (signal peptides), and SAHS1-GFP experiments supported secretion into culture media in heterologous systems, motivating the idea that SAHS proteins protect extracellular components. (yamaguchi2012twonovelheatsoluble pages 3-5, yamaguchi2012twonovelheatsoluble pages 5-6)
Tanaka et al. (2023-01, PNAS, https://doi.org/10.1073/pnas.2216739120) developed TardiVec, an in vivo expression system using R. varieornatus regulatory regions, enabling tissue-specific live imaging.
Using a SAHS promoter (pRvSAHS1), they found SAHS-family expression is strongly enriched in “storage cells” (free-floating cells in the body cavity). RNA-seq supported that SAHS-family transcripts are ~5–20× higher in storage cells compared with whole-body measurements, indicating specialized expression of SAHS proteins in that cell type in natural settings. (tanaka2023invivoexpression pages 3-4)
They further observed SAHS1 fusion protein localizing to vesicle-like structures within storage cells, and in some cases appearing in the body cavity, leading to the interpretation that SAHS proteins are synthesized in storage cells and can be secreted/transported to protect other tissues during anhydrobiosis. (tanaka2023invivoexpression pages 6-7, tanaka2023invivoexpression pages 4-6)
Implication for SAHS2: While SAHS2 is not individually reported in these TardiVec excerpts, SAHS2 is part of the SAHS gene family; thus SAHS2 is likely expressed in or coordinated with storage-cell secretory programs and may function in the body cavity/extracellular milieu. (tanaka2023invivoexpression pages 3-4, yamaguchi2012twonovelheatsoluble pages 3-5)
The SAHS family was originally reported as conserved among tardigrades and not found in other phyla, implying lineage-specific innovation associated with anhydrobiosis. (yamaguchi2012twonovelheatsoluble pages 1-2)
A 2024 phylogenomic analysis of desiccation/temperature-related families in tardigrades reconstructed phylogenies for multiple extremotolerance-linked families, including SAHS, and inferred numerous independent gene duplications across these families, consistent with complex and repeated evolutionary adaptation to aridity in limnoterrestrial environments. (Fleming et al., 2024, Genome Biology and Evolution, https://doi.org/10.1093/gbe/evad217) (lim2024tardigradesecretoryproteins pages 1-3)
The strongest evidence for real-world application comes from direct demonstration that extracellular SAHS proteins can stabilize microbes during drying, including Rhizobium tropici, a bacterium relevant to plant-associated contexts. The authors explicitly frame this as a potential route to cell preservation and microbial stabilization (e.g., to improve survivability during desiccation-based storage/handling), and also demonstrate protection of membrane structures (liposomes), suggesting broader biostabilization uses for membrane-bound biological materials. (lim2024tardigradesecretoryproteins pages 1-3, lim2024tardigradesecretoryproteins pages 8-9, lim2024tardigradesecretoryproteins media b0bc6f49)
Recommended primary annotation (most supported):
* Molecular role: secreted, heat-soluble stress protein likely functioning as an extracellular protectant during desiccation/anhydrobiosis, with strongest evidence for membrane/membrane-structure stabilization rather than broad enzyme chaperoning. (yamaguchi2012twonovelheatsoluble pages 3-5, lim2024tardigradesecretoryproteins pages 6-7)
* Biological process: anhydrobiosis/desiccation tolerance; extracellular stabilization during dehydration and rehydration. (yamaguchi2012twonovelheatsoluble pages 1-2, lim2024tardigradesecretoryproteins pages 3-5)
* Cellular localization: secretory pathway / extracellular space (signal peptide; secretion inferred for SAHS family; in vivo expression suggests origin in storage cells and potential release into body cavity). (yamaguchi2012twonovelheatsoluble pages 3-5, tanaka2023invivoexpression pages 3-4)
* Mechanistic hypothesis: dehydration-driven destabilization of β-structure and/or cavity desolvation leads to conformational changes and higher-order assemblies that stabilize membranes and extracellular structures. (lim2024tardigradesecretoryproteins pages 6-7)
Evidence limitations (SAHS2-specific):
Direct biochemical/functional characterization in the retrieved sources is primarily for SAHS1 and other SAHS paralogs; SAHS2-specific evidence is strongest for its membership in the secretory SAHS family and gene structure (four exons) rather than direct assay results. (yamaguchi2012twonovelheatsoluble pages 6-7, lim2024tardigradesecretoryproteins pages 3-5)
| Aspect | Key findings (with numbers) | Evidence type (experiment/bioinformatics) | Primary source (authors year journal) | URL/DOI | Notes on SAHS2-specific vs family-level |
|---|---|---|---|---|---|
| Gene/protein identity | SAHS2 in Ramazzottius varieornatus is part of the SAHS = Secretory Abundant Heat Soluble family; defining study states SAHS1 and SAHS2 are secretory, and SAHS2 has 4 exons. SAHS proteins were identified among major heat-soluble proteins after heating extracts to 92°C for 15 min and analyzing heat-soluble fractions by MS/RACE (yamaguchi2012twonovelheatsoluble pages 2-3, yamaguchi2012twonovelheatsoluble pages 6-7, yamaguchi2012twonovelheatsoluble pages 3-5) | Proteomics, gene structure, localization prediction | Yamaguchi et al. 2012, PLoS ONE | https://doi.org/10.1371/journal.pone.0044209 | Directly relevant to SAHS2 identity; strongest SAHS2-specific evidence is family membership, secretory prediction, and exon structure rather than direct biochemical assay (yamaguchi2012twonovelheatsoluble pages 6-7, yamaguchi2012twonovelheatsoluble pages 3-5) |
| Localization: signal peptide and secretion | SAHS proteins contain N-terminal secretory signal peptides; TargetP/SignalP predicted SAHS1 and SAHS2 as secretory. SAHS1-GFP was detected in culture medium, supporting secretion; authors inferred SAHS proteins protect extracellular components/secretory organelles (yamaguchi2012twonovelheatsoluble pages 3-5, yamaguchi2012twonovelheatsoluble pages 5-6) | Bioinformatics plus heterologous expression/immunoblot | Yamaguchi et al. 2012, PLoS ONE | https://doi.org/10.1371/journal.pone.0044209 | SAHS2 secretion is predicted, not directly shown; direct secretion experiment was for SAHS1 (yamaguchi2012twonovelheatsoluble pages 3-5, yamaguchi2012twonovelheatsoluble pages 5-6) |
| Localization: tissue/cell specificity | In vivo TardiVec work showed SAHS-family promoter activity is highly enriched in storage cells; pRvSAHS1 drove strong expression in free-floating storage cells, and SAHS1-mEGFP localized to vesicle-like structures in storage cells and sometimes body cavity. RNA-seq showed SAHS transcripts enriched about ~5–20× in storage cells vs whole body (tanaka2023invivoexpression pages 3-4, tanaka2023invivoexpression pages 4-6, tanaka2023invivoexpression pages 2-3) | In vivo transgenesis, live imaging, RNA-seq | Tanaka et al. 2023, PNAS | https://doi.org/10.1073/pnas.2216739120 | This is SAHS1/promoter-family evidence, not SAHS2-specific; useful for likely native context of SAHS2 as a secreted storage-cell-associated protectant (tanaka2023invivoexpression pages 6-7, tanaka2023invivoexpression pages 3-4) |
| Structural/domain info | Early work found SAHS proteins are beta-structure-rich in hydrated conditions and shift toward alpha-helix under water-deficient/desolvating conditions; SAHS1 had CD minimum at 215 nm and required >50% TFE for alpha-helical conversion. Later work describes SAHS proteins as FABP-like/calycin-like beta-barrels with large internal cavities and structures solved for RvSAHS1/4; 2024 analyses propose dehydration-induced cavity solvent loss destabilizes the beta-barrel and promotes alternative protective conformations/networks (yamaguchi2012twonovelheatsoluble pages 5-6, lim2024tardigradesecretoryproteins pages 7-8, lim2024tardigradesecretoryproteins pages 6-7, roseteenriquez2025survivingdesiccationkey pages 13-15, yamaguchi2012twonovelheatsoluble pages 3-5) | CD spectroscopy, structural biology, MD simulation, comparative modeling | Yamaguchi et al. 2012, PLoS ONE; Lim et al. 2024, Communications Biology | https://doi.org/10.1371/journal.pone.0044209; https://doi.org/10.1038/s42003-024-06336-w | No direct SAHS2 structure in cited evidence; inference is family-level. UniProt/J7MAN2 annotation of calycin is consistent with later family-level FABP-like beta-barrel evidence (lim2024tardigradesecretoryproteins pages 7-8, lim2024tardigradesecretoryproteins pages 6-7) |
| Functional assay: membrane/liposome protection | Drying/rehydration of POPC liposomes caused major fusion/aggregation: initial diameters ~50–100 nm, but after drying <10% remained in that range and large particles around ~360 and 4000 nm formed. With SAHS proteins at 10, 1, or 0.1 mg/mL, major peaks stayed near ~60–100 nm and no particles >200 nm were observed; performance exceeded BSA and was comparable to trehalose at 10 mg/mL (lim2024tardigradesecretoryproteins pages 1-3, lim2024tardigradesecretoryproteins pages 3-5, lim2024tardigradesecretoryproteins pages 6-7, lim2024tardigradesecretoryproteins media f52027ea) | In vitro DLS liposome assay | Lim et al. 2024, Communications Biology | https://doi.org/10.1038/s42003-024-06336-w | Assayed proteins included RvSAHS1/4/6 and HeSAHS4, not SAHS2 specifically; supports extracellular membrane-protection role for SAHS family that may extend to SAHS2 (lim2024tardigradesecretoryproteins pages 3-5, lim2024tardigradesecretoryproteins media f52027ea) |
| Functional assay: microbial desiccation protection | Extracellular addition of SAHS proteins improved microbial survival after 48 h drying. For E. coli, 0.5 mg/mL RvSAHS1 gave >10-fold survival enhancement; across assays SAHS proteins gave roughly ~50-fold improvement overall, at least ~10-fold better than BSA and ~3-fold better than trehalose. A single SAHS protein also stabilized Rhizobium tropici (lim2024tardigradesecretoryproteins pages 3-5, lim2024tardigradesecretoryproteins pages 8-9, lim2024tardigradesecretoryproteins pages 6-7, lim2024tardigradesecretoryproteins media b0bc6f49) | Desiccation survival/CFU assays | Lim et al. 2024, Communications Biology | https://doi.org/10.1038/s42003-024-06336-w | Family-level evidence only; SAHS2 not individually tested in the cited excerpts (lim2024tardigradesecretoryproteins pages 8-9, lim2024tardigradesecretoryproteins media b0bc6f49) |
| Functional assay: enzyme protection | SAHS proteins did not significantly outperform BSA in protecting lactate dehydrogenase (LDH) after desiccation/rehydration; only slight protection was observed, indicating SAHS may preferentially protect membranes/cells rather than soluble enzymes (lim2024tardigradesecretoryproteins pages 8-9, lim2024tardigradesecretoryproteins pages 7-8, lim2024tardigradesecretoryproteins pages 6-7) | Enzyme activity assay | Lim et al. 2024, Communications Biology | https://doi.org/10.1038/s42003-024-06336-w | Negative/limited family-level result; no SAHS2-specific enzyme assay in cited evidence |
| Evolutionary context | SAHS proteins are tardigrade-specific and not found outside the phylum in the original study; homologs were found across tardigrades, with abundant expression in Hypsibius dujardini ESTs (100 ESTs for SAHS vs 15 for actin and 64 for EF1a). A 2024 phylogenomic study found complex histories with multiple independent duplications of SAHS family genes in tardigrades; R. varieornatus and related taxa show SAHS diversification linked to extremotolerance evolution (yamaguchi2012twonovelheatsoluble pages 1-2, yamaguchi2012twonovelheatsoluble pages 3-5) | Comparative genomics, transcriptomics, phylogenomics | Yamaguchi et al. 2012, PLoS ONE; Fleming et al. 2024, Genome Biology and Evolution | https://doi.org/10.1371/journal.pone.0044209; https://doi.org/10.1093/gbe/evad217 | SAHS2 is one member of a broader duplicated family; evolutionary conclusions are family-level rather than gene-specific |
| Comparative genomics in R. varieornatus | Comparative genome analysis reported that no SAHS2 ortholog was found in Hypsibius dujardini, emphasizing lineage-specific expansion/retention in R. varieornatus and related anhydrobiotic lineages (from retrieved snippet) | Comparative genomics | Yoshida et al. 2017, PLOS Biology | https://doi.org/10.1371/journal.pbio.2002266 | Supports distinct SAHS2 evolutionary trajectory; evidence is orthology-level rather than direct function |
| Key quantitative data points for annotation | 92°C for 15 min heat-solubility screen; SAHS2 gene has 4 exons; SAHS-family transcripts enriched ~5–20× in storage cells; hydrated SAHS1 CD minimum at 215 nm and alpha-helical conversion above >50% TFE; liposomes initially ~50–100 nm, dried samples produced ~360 and 4000 nm aggregates, but SAHS-treated samples retained ~60–100 nm peaks with no particles >200 nm; E. coli survival improved >10-fold with 0.5 mg/mL RvSAHS1, with some assays showing ~50-fold gains overall after 48 h drying (yamaguchi2012twonovelheatsoluble pages 2-3, yamaguchi2012twonovelheatsoluble pages 6-7, lim2024tardigradesecretoryproteins pages 3-5, lim2024tardigradesecretoryproteins pages 6-7, lim2024tardigradesecretoryproteins media f52027ea, tanaka2023invivoexpression pages 3-4) | Mixed: proteomics, gene structure, imaging, CD, DLS, CFU assays | Yamaguchi et al. 2012, Tanaka et al. 2023, Lim et al. 2024 | https://doi.org/10.1371/journal.pone.0044209; https://doi.org/10.1073/pnas.2216739120; https://doi.org/10.1038/s42003-024-06336-w | Most numeric values are family-level or SAHS1-centered; direct SAHS2-specific numbers remain sparse, so functional annotation should distinguish direct evidence from family inference |
Table: This table compiles the strongest available evidence for functional annotation of SAHS2 (UniProt J7MAN2) in Ramazzottius varieornatus, separating SAHS2-specific findings from broader SAHS family-level inference. It is useful for assigning likely localization, structural class, and protective role while being explicit about evidentiary limits.
The following extracted figure crops contain the quantitative DLS liposome distributions and microbial survival plots referenced above. (lim2024tardigradesecretoryproteins media f52027ea, lim2024tardigradesecretoryproteins media b0bc6f49)
References
(yamaguchi2012twonovelheatsoluble pages 3-5): Ayami Yamaguchi, Sae Tanaka, Shiho Yamaguchi, Hirokazu Kuwahara, Chizuko Takamura, Shinobu Imajoh-Ohmi, Daiki D. Horikawa, Atsushi Toyoda, Toshiaki Katayama, Kazuharu Arakawa, Asao Fujiyama, Takeo Kubo, and Takekazu Kunieda. Two novel heat-soluble protein families abundantly expressed in an anhydrobiotic tardigrade. PLoS ONE, 7:e44209, Aug 2012. URL: https://doi.org/10.1371/journal.pone.0044209, doi:10.1371/journal.pone.0044209. This article has 195 citations and is from a peer-reviewed journal.
(yamaguchi2012twonovelheatsoluble pages 6-7): Ayami Yamaguchi, Sae Tanaka, Shiho Yamaguchi, Hirokazu Kuwahara, Chizuko Takamura, Shinobu Imajoh-Ohmi, Daiki D. Horikawa, Atsushi Toyoda, Toshiaki Katayama, Kazuharu Arakawa, Asao Fujiyama, Takeo Kubo, and Takekazu Kunieda. Two novel heat-soluble protein families abundantly expressed in an anhydrobiotic tardigrade. PLoS ONE, 7:e44209, Aug 2012. URL: https://doi.org/10.1371/journal.pone.0044209, doi:10.1371/journal.pone.0044209. This article has 195 citations and is from a peer-reviewed journal.
(lim2024tardigradesecretoryproteins pages 3-5): Samuel Lim, Charles B. Reilly, Zeina Barghouti, Benedetto Marelli, Jeffrey C. Way, and Pamela A. Silver. Tardigrade secretory proteins protect biological structures from desiccation. Communications Biology, May 2024. URL: https://doi.org/10.1038/s42003-024-06336-w, doi:10.1038/s42003-024-06336-w. This article has 17 citations and is from a peer-reviewed journal.
(yamaguchi2012twonovelheatsoluble pages 2-3): Ayami Yamaguchi, Sae Tanaka, Shiho Yamaguchi, Hirokazu Kuwahara, Chizuko Takamura, Shinobu Imajoh-Ohmi, Daiki D. Horikawa, Atsushi Toyoda, Toshiaki Katayama, Kazuharu Arakawa, Asao Fujiyama, Takeo Kubo, and Takekazu Kunieda. Two novel heat-soluble protein families abundantly expressed in an anhydrobiotic tardigrade. PLoS ONE, 7:e44209, Aug 2012. URL: https://doi.org/10.1371/journal.pone.0044209, doi:10.1371/journal.pone.0044209. This article has 195 citations and is from a peer-reviewed journal.
(yamaguchi2012twonovelheatsoluble pages 5-6): Ayami Yamaguchi, Sae Tanaka, Shiho Yamaguchi, Hirokazu Kuwahara, Chizuko Takamura, Shinobu Imajoh-Ohmi, Daiki D. Horikawa, Atsushi Toyoda, Toshiaki Katayama, Kazuharu Arakawa, Asao Fujiyama, Takeo Kubo, and Takekazu Kunieda. Two novel heat-soluble protein families abundantly expressed in an anhydrobiotic tardigrade. PLoS ONE, 7:e44209, Aug 2012. URL: https://doi.org/10.1371/journal.pone.0044209, doi:10.1371/journal.pone.0044209. This article has 195 citations and is from a peer-reviewed journal.
(lim2024tardigradesecretoryproteins media f52027ea): Samuel Lim, Charles B. Reilly, Zeina Barghouti, Benedetto Marelli, Jeffrey C. Way, and Pamela A. Silver. Tardigrade secretory proteins protect biological structures from desiccation. Communications Biology, May 2024. URL: https://doi.org/10.1038/s42003-024-06336-w, doi:10.1038/s42003-024-06336-w. This article has 17 citations and is from a peer-reviewed journal.
(lim2024tardigradesecretoryproteins pages 6-7): Samuel Lim, Charles B. Reilly, Zeina Barghouti, Benedetto Marelli, Jeffrey C. Way, and Pamela A. Silver. Tardigrade secretory proteins protect biological structures from desiccation. Communications Biology, May 2024. URL: https://doi.org/10.1038/s42003-024-06336-w, doi:10.1038/s42003-024-06336-w. This article has 17 citations and is from a peer-reviewed journal.
(lim2024tardigradesecretoryproteins media b0bc6f49): Samuel Lim, Charles B. Reilly, Zeina Barghouti, Benedetto Marelli, Jeffrey C. Way, and Pamela A. Silver. Tardigrade secretory proteins protect biological structures from desiccation. Communications Biology, May 2024. URL: https://doi.org/10.1038/s42003-024-06336-w, doi:10.1038/s42003-024-06336-w. This article has 17 citations and is from a peer-reviewed journal.
(lim2024tardigradesecretoryproteins pages 7-8): Samuel Lim, Charles B. Reilly, Zeina Barghouti, Benedetto Marelli, Jeffrey C. Way, and Pamela A. Silver. Tardigrade secretory proteins protect biological structures from desiccation. Communications Biology, May 2024. URL: https://doi.org/10.1038/s42003-024-06336-w, doi:10.1038/s42003-024-06336-w. This article has 17 citations and is from a peer-reviewed journal.
(lim2024tardigradesecretoryproteins pages 1-3): Samuel Lim, Charles B. Reilly, Zeina Barghouti, Benedetto Marelli, Jeffrey C. Way, and Pamela A. Silver. Tardigrade secretory proteins protect biological structures from desiccation. Communications Biology, May 2024. URL: https://doi.org/10.1038/s42003-024-06336-w, doi:10.1038/s42003-024-06336-w. This article has 17 citations and is from a peer-reviewed journal.
(tanaka2023invivoexpression pages 3-4): Sae Tanaka, Kazuhiro Aoki, and Kazuharu Arakawa. In vivo expression vector derived from anhydrobiotic tardigrade genome enables live imaging in eutardigrada. Proceedings of the National Academy of Sciences, Jan 2023. URL: https://doi.org/10.1073/pnas.2216739120, doi:10.1073/pnas.2216739120. This article has 27 citations and is from a highest quality peer-reviewed journal.
(tanaka2023invivoexpression pages 6-7): Sae Tanaka, Kazuhiro Aoki, and Kazuharu Arakawa. In vivo expression vector derived from anhydrobiotic tardigrade genome enables live imaging in eutardigrada. Proceedings of the National Academy of Sciences, Jan 2023. URL: https://doi.org/10.1073/pnas.2216739120, doi:10.1073/pnas.2216739120. This article has 27 citations and is from a highest quality peer-reviewed journal.
(tanaka2023invivoexpression pages 4-6): Sae Tanaka, Kazuhiro Aoki, and Kazuharu Arakawa. In vivo expression vector derived from anhydrobiotic tardigrade genome enables live imaging in eutardigrada. Proceedings of the National Academy of Sciences, Jan 2023. URL: https://doi.org/10.1073/pnas.2216739120, doi:10.1073/pnas.2216739120. This article has 27 citations and is from a highest quality peer-reviewed journal.
(yamaguchi2012twonovelheatsoluble pages 1-2): Ayami Yamaguchi, Sae Tanaka, Shiho Yamaguchi, Hirokazu Kuwahara, Chizuko Takamura, Shinobu Imajoh-Ohmi, Daiki D. Horikawa, Atsushi Toyoda, Toshiaki Katayama, Kazuharu Arakawa, Asao Fujiyama, Takeo Kubo, and Takekazu Kunieda. Two novel heat-soluble protein families abundantly expressed in an anhydrobiotic tardigrade. PLoS ONE, 7:e44209, Aug 2012. URL: https://doi.org/10.1371/journal.pone.0044209, doi:10.1371/journal.pone.0044209. This article has 195 citations and is from a peer-reviewed journal.
(lim2024tardigradesecretoryproteins pages 8-9): Samuel Lim, Charles B. Reilly, Zeina Barghouti, Benedetto Marelli, Jeffrey C. Way, and Pamela A. Silver. Tardigrade secretory proteins protect biological structures from desiccation. Communications Biology, May 2024. URL: https://doi.org/10.1038/s42003-024-06336-w, doi:10.1038/s42003-024-06336-w. This article has 17 citations and is from a peer-reviewed journal.
(tanaka2023invivoexpression pages 2-3): Sae Tanaka, Kazuhiro Aoki, and Kazuharu Arakawa. In vivo expression vector derived from anhydrobiotic tardigrade genome enables live imaging in eutardigrada. Proceedings of the National Academy of Sciences, Jan 2023. URL: https://doi.org/10.1073/pnas.2216739120, doi:10.1073/pnas.2216739120. This article has 27 citations and is from a highest quality peer-reviewed journal.
(roseteenriquez2025survivingdesiccationkey pages 13-15): María Rosete-Enríquez, Victor Rivelino Juárez-González, Esmeralda Escobar-Muciño, Jesús Muñoz-Rojas, and Verónica Quintero-Hernández. Surviving desiccation: key factors underlying tolerance in prokaryotes and eukaryotes. Protoplasma, Nov 2025. URL: https://doi.org/10.1007/s00709-025-02134-1, doi:10.1007/s00709-025-02134-1. This article has 2 citations and is from a peer-reviewed journal.
id: J7MAN2
gene_symbol: SAHS2
product_type: PROTEIN
status: IN_PROGRESS
taxon:
id: NCBITaxon:947166
label: Ramazzottius varieornatus
description: >-
Secretory-abundant heat soluble protein 2 (SAHS2) is a tardigrade-specific secreted protein
belonging to the SAHS family, which adopts a beta-barrel fold structurally similar to fatty
acid-binding proteins (FABPs). SAHS proteins are abundantly expressed, heat-soluble, and are
proposed to act as molecular shields that protect extracellular components during desiccation
(anhydrobiosis). SAHS2 has a signal peptide (residues 1-19), three conserved SAHS domains
(SAHS-c1, c2, c3), and an N-linked glycosylation site. The SAHS family is unique to
tardigrades and has no homologs outside Tardigrada, except for distant structural similarity
to metazoan FABPs. Crystal structures of the paralog SAHS1 (RvSAHS1) revealed two putative
ligand binding sites where fatty acids can bind (PMID:28703282), establishing SAHS proteins
as a new FABP-like family. SAHS2 is one of 13 SAHS paralogs encoded in the R. varieornatus
genome and is constitutively and abundantly expressed (PMID:27649274).
existing_annotations:
- term:
id: GO:0005576
label: extracellular region
evidence_type: IEA
original_reference_id: GO_REF:0000044
review:
summary: >-
SAHS2 is annotated to the extracellular region based on IEA mapping from UniProtKB
subcellular location vocabulary. UniProt marks SAHS2 as "Secreted" based on similarity
to SAHS1 (J7MFT5). The protein has a predicted signal peptide (residues 1-19) and was
originally identified in the heat-soluble proteome of tardigrades (PMID:22937162).
SAHS proteins are described as having a secretory localization distinct from the
cytoplasmic CAHS family (PMID:22937162). The annotation to GO:0005576 extracellular
region is well supported.
action: ACCEPT
reason: >-
SAHS2 has a signal peptide and is described as a secretory protein. The original
proteomics study identified SAHS proteins as having distinct subcellular localization
from CAHS proteins, with SAHS being secretory (PMID:22937162). The name itself
(Secretory Abundant Heat Soluble) reflects this localization. Extracellular region is
the appropriate GO CC term for a secreted protein.
supported_by:
- reference_id: PMID:22937162
supporting_text: >-
We named them Cytoplasmic Abundant Heat Soluble (CAHS) and Secretory Abundant
Heat Soluble (SAHS) protein families, according to their localization.
- reference_id: PMID:28703282
supporting_text: >-
Secretory abundant heat-soluble (SAHS) proteins have been reported as candidates
for anhydrobiosis-related proteins in tardigrades, which seem to protect
extracellular components and/or secretory organelles.
- term:
id: GO:0008289
label: lipid binding
evidence_type: ISS
original_reference_id: PMID:28703282
review:
summary: >-
SAHS2 is not currently annotated with lipid binding in GOA, but structural studies of
the paralog SAHS1 (RvSAHS1) demonstrate that SAHS proteins adopt a beta-barrel fold
homologous to fatty acid-binding proteins (FABPs) with two putative ligand binding sites
(PMID:28703282). CDD classifies SAHS2 as containing an FABP domain (cd00742), InterPro
assigns calycin fold (IPR012674), and SUPFAM classifies it in the Lipocalin superfamily
(SSF50814). Crystal structure of SAHS4 showed variant binding preferences at these sites
(PMID:29493034), suggesting functional diversification within the SAHS family.
action: NEW
reason: >-
Although no direct lipid binding assay has been performed on SAHS2 specifically, the
structural evidence from the SAHS1 crystal structure (PMID:28703282) combined with the
FABP domain classification (CDD cd00742) and calycin/lipocalin superfamily membership
strongly supports lipid binding as a molecular function. SAHS1 (J7MFT5) already has an
IEA lipid binding annotation via InterPro. The same structural basis applies to SAHS2,
which shares the conserved SAHS domains and FABP-like fold. ISS evidence from the
paralog SAHS1 crystal structure is appropriate.
additional_reference_ids:
- PMID:29493034
supported_by:
- reference_id: PMID:28703282
supporting_text: >-
RvSAHS1 shows a beta-barrel structure similar to fatty acid-binding proteins
(FABPs), in which hydrophilic residues form peculiar hydrogen bond networks...
We identified two putative ligand-binding sites: one that superimposes on those
of some FABPs and the other, unique to and conserved in SAHS proteins. These
results indicate that SAHS proteins constitute a new FABP family.
- reference_id: PMID:29493034
supporting_text: >-
A previous crystallographic study revealed that a SAHS protein (RvSAHS1) from one
of the toughest tardigrades, Ramazzottius varieornatus, has a beta-barrel
architecture similar to fatty acid binding proteins (FABPs) and two putative
ligand binding sites (LBS1 and LBS2) where fatty acids can bind.
- term:
id: GO:0009269
label: response to desiccation
evidence_type: ISS
original_reference_id: PMID:22937162
review:
summary: >-
SAHS proteins were discovered through heat-soluble proteomics specifically aimed at
identifying desiccation tolerance factors in tardigrades (PMID:22937162). SAHS-family
proteins change conformation from beta-structure to alpha-helical structure under
water-deficient conditions, similar to LEA proteins, suggesting a protective role during
desiccation. SAHS2 is constitutively and abundantly expressed, consistent with the fact
that R. varieornatus tolerates rapid desiccation without significant transcriptional
regulation (PMID:27649274). Functional assays on recombinant SAHS paralogs support a
family-level role in extracellular protection during desiccation (PMID:38796644).
action: NEW
reason: >-
Response to desiccation (GO:0009269) is a core biological process for SAHS2, but the
available support is family- and paralog-level rather than a direct SAHS2-specific
assay. SAHS proteins were identified in the context of anhydrobiosis research, show
water-deficiency-associated conformational change, are constitutively abundant in
R. varieornatus, and recombinant SAHS paralogs protect membranous structures during
drying. ISS is therefore more conservative than IDA for SAHS2.
additional_reference_ids:
- PMID:27649274
- PMID:38796644
supported_by:
- reference_id: PMID:22937162
supporting_text: >-
proteins in both families changed their conformation to an alpha-helical structure
in water-deficient conditions as LEA proteins do
- reference_id: PMID:38796644
supporting_text: >-
Recombinantly expressed SAHS proteins prevent desiccated liposomes from fusion,
and enhance desiccation tolerance of E. coli and Rhizobium tropici upon
extracellular application
- reference_id: PMID:27649274
supporting_text: >-
These abundantly expressed proteins included previously identified
tardigrade-unique heat-soluble proteins, CAHS and SAHS, both of which maintain
solubility even after heat treatment and are proposed to be involved in the
protection of biomolecules during desiccation
- reference_id: file:RAMVA/SAHS2/SAHS2-deep-research-falcon.md
supporting_text: "secreted, heat-soluble stress protein likely functioning as an extracellular protectant during desiccation/anhydrobiosis"
references:
- id: GO_REF:0000044
title: >-
Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location
vocabulary mapping, accompanied by conservative changes to GO terms applied by
UniProt
findings:
- statement: >-
UniProt SUBCELLULAR LOCATION for SAHS2 (J7MAN2) records the protein as
"Secreted" by similarity to SAHS1 (J7MFT5); SAHS2 carries a predicted
signal peptide (residues 1-19). This mapping reference converts the
controlled-vocabulary "Secreted" location into the GO:0005576
(extracellular region) annotation with IEA evidence.
- id: PMID:22937162
title: >-
Two novel heat-soluble protein families abundantly expressed in an anhydrobiotic
tardigrade
findings:
- statement: >-
SAHS proteins were identified via heat-soluble proteomics of R. varieornatus
as one of two novel protein families (CAHS and SAHS) abundantly expressed in
anhydrobiotic tardigrades.
- statement: >-
SAHS proteins are secretory, rich in beta-structure in hydrated conditions,
and shift to alpha-helical conformation under water-deficient conditions.
- statement: >-
The "molecular shield" role (via amphiphilic alpha-helical 19-mer motifs) is
proposed in this paper specifically for CAHS proteins, not SAHS. SAHS proteins
are shown to share the beta-to-alpha conformational change under water-deficient
conditions as LEA proteins do, but the molecular-shield mechanism is not
attributed to SAHS in this study.
- id: PMID:27649274
title: >-
Extremotolerant tardigrade genome and improved radiotolerance of human cultured
cells by tardigrade-unique protein
findings:
- statement: >-
R. varieornatus genome encodes 13 SAHS genes; SAHS family members are
constitutively and abundantly expressed.
- statement: >-
SAHS proteins maintain solubility after heat treatment and are proposed to
protect biomolecules during desiccation.
- id: PMID:28703282
title: >-
Structural insights into a secretory abundant heat-soluble protein from an
anhydrobiotic tardigrade, Ramazzottius varieornatus
findings:
- statement: >-
Crystal structure of RvSAHS1 reveals a beta-barrel fold similar to FABPs with
two putative ligand binding sites, establishing SAHS as a new FABP family.
- statement: >-
Hydrophilic residues form peculiar hydrogen bond networks in the SAHS1 structure,
which may provide better tolerance against dehydration.
- id: PMID:29493034
title: >-
Crystal structure of secretory abundant heat soluble protein 4 from one of the
toughest water bears micro-animals Ramazzottius Varieornatus
findings:
- statement: >-
SAHS4 has different amino acid residues at ligand binding sites compared to SAHS1,
preferring uncharged molecules, suggesting functional diversification within the
SAHS family.
- id: PMID:38796644
title: >-
Tardigrade secretory proteins protect biological structures from desiccation.
findings:
- statement: >-
Recombinantly expressed SAHS proteins prevent desiccated liposomes from
fusion and enhance desiccation tolerance of E. coli and Rhizobium tropici
upon extracellular application. They also preserve LDH activity during
desiccation, but this soluble-enzyme protection is not preferential relative
to BSA, contrasting with the SAHS-specific effect on membranous structures.
- statement: >-
Molecular dynamics and comparative structural analysis suggest SAHS
proteins undergo a structural transition upon desiccation in which
removal of water and solutes from the large internal cavity destabilizes
the beta-sheet structure.
- id: PMID:36693101
title: >-
In vivo expression vector derived from anhydrobiotic tardigrade genome
enables live imaging in Eutardigrada.
findings:
- statement: >-
Using the TardiVec in vivo expression system, SAHS proteins were shown
to be expressed exclusively in storage cells - tardigrade-specific
free-floating cells in the body cavity - whereas CAHS genes are mainly
expressed in epidermal cells with cytosolic localization, refuting the
prior single-cell hypothesis and supporting an extracellular,
storage-cell origin for SAHS proteins.
- id: file:RAMVA/SAHS2/SAHS2-deep-research-falcon.md
title: Deep research synthesis on SAHS2 (J7MAN2) from R. varieornatus
findings:
- statement: >-
SAHS proteins prevent desiccation-induced membrane / liposome fusion
in vitro (POPC liposomes retain ~60-100 nm size after drying with SAHS,
whereas drying without SAHS gives ~360 and ~4000 nm aggregates) and
provide >10-fold survival improvement to dried E. coli, supporting an
extracellular membrane-stabilization role for the SAHS family that
likely extends to SAHS2; SAHS proteins also preserve LDH activity during
desiccation, but not preferentially compared with BSA controls.
- statement: >-
SAHS-family promoter activity (pRvSAHS1) is highly enriched in tardigrade
storage cells, with SAHS transcripts ~5-20x enriched in storage cells
vs whole body; SAHS1-mEGFP localizes to vesicle-like structures and
can be detected in the body cavity, suggesting SAHS proteins originate
in storage cells and are secreted into the extracellular space.
- statement: >-
SAHS proteins are beta-structure-rich in hydrated conditions and shift
toward alpha-helix under water-deficient/desolvating conditions (CD
minimum at 215 nm; alpha-helical conversion above >50% TFE), consistent
with a stress-induced conformational change that may underlie
protective activity.
core_functions:
- description: >-
SAHS2 is a tardigrade-specific, secreted heat-soluble protein whose family
adopts an FABP-like beta-barrel (calycin / lipocalin) fold with two
putative ligand-binding sites and is proposed to function as an
extracellular molecular shield protecting membranes and extracellular
components during anhydrobiosis. In hydrated conditions SAHS proteins are
beta-rich, and they shift to alpha-helical conformation under water-
deficient conditions, analogous to LEA proteins. Family-level functional
assays show SAHS proteins stabilize liposomes against drying-induced
fusion and dramatically improve microbial survival after desiccation. The
same assays show LDH protection during desiccation, but without a
SAHS-specific advantage over BSA, implicating membrane and bilayer-structure
protection as the distinctive SAHS-family role rather than broad soluble-enzyme
chaperoning.
directly_involved_in:
- id: GO:0009269
label: response to desiccation
locations:
- id: GO:0005576
label: extracellular region
supported_by:
- reference_id: PMID:22937162
supporting_text: "proteins in both families changed their conformation to an alpha-helical structure in water-deficient conditions as LEA proteins do"
- reference_id: PMID:28703282
supporting_text: "Secretory abundant heat-soluble (SAHS) proteins have been reported as candidates for anhydrobiosis-related proteins in tardigrades, which seem to protect extracellular components and/or secretory organelles."
- reference_id: PMID:38796644
supporting_text: "Recombinantly expressed SAHS proteins prevent desiccated liposomes from fusion, and enhance desiccation tolerance of E. coli and Rhizobium tropici upon extracellular application"
- reference_id: PMID:36693101
supporting_text: "SAHS proteins are expressed exclusively in storage cells, which are tardigrade-specific free-floating cells in the body cavity"
- reference_id: file:RAMVA/SAHS2/SAHS2-deep-research-falcon.md
supporting_text: "secreted, heat-soluble stress protein likely functioning as an extracellular protectant during desiccation/anhydrobiosis"
suggested_questions:
- question: >-
Does SAHS2, like other SAHS paralogs, bind fatty acids or other lipid
ligands at LBS1/LBS2, and is ligand binding required for its
membrane-protective activity during desiccation?
- question: >-
Is SAHS2 primarily released into the body cavity / extracellular space
from storage cells in vivo, and does its activity depend on this
extracellular localization?
- question: >-
Among the 13 R. varieornatus SAHS paralogs, what is the functional
specialization of SAHS2 (e.g. in tissue distribution, ligand preference,
or membrane-protection efficacy), and how redundant is its function with
SAHS1, SAHS3 and other family members?
suggested_experiments:
- description: >-
Express and purify recombinant SAHS2 with a cleavable signal peptide,
confirm secretion in a heterologous system, and test its activity in
liposome (POPC) drying-rehydration DLS assays and in microbial desiccation
survival assays, benchmarking against SAHS1/4/6, BSA and trehalose.
hypothesis: >-
Purified SAHS2 prevents desiccation-induced liposome fusion and improves
survival of dried bacteria at concentrations comparable to other tested
SAHS paralogs, confirming its membrane-protective function.
experiment_type: in vitro biophysical and microbial protection assays
- description: >-
Solve the crystal or cryo-EM structure of SAHS2 in apo and ligand-bound
states, and test fatty acid / hydrophobic ligand binding by ITC,
fluorescence and competition assays. Compare LBS1/LBS2 residue identities
with SAHS1 and SAHS4 to predict and test ligand preferences.
hypothesis: >-
SAHS2 retains a calycin/FABP-like beta-barrel fold with two ligand binding
sites whose residue substitutions (relative to SAHS1) determine a
distinct hydrophobic ligand preference.
experiment_type: structural biology / binding biochemistry
- description: >-
Use TardiVec-style in vivo reporters (pRvSAHS2-mEGFP) and CRISPR knock-in
of an epitope tag to determine the native tissue distribution and
subcellular fate of SAHS2 in R. varieornatus (storage cells, vesicles,
body cavity), with secretion confirmed by immunoblot of body cavity
fluid.
hypothesis: >-
Endogenous SAHS2 is expressed predominantly in storage cells and is
secreted into the body cavity where it accumulates and acts during
anhydrobiosis.
experiment_type: in vivo expression and secretion mapping