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 |
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Download this section (compressed HTML)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
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