Secretory abundant heat-soluble protein 1 (SAHS1) is a tardigrade-specific secreted protein that functions as a molecular shield under water-deficient conditions, contributing to desiccation tolerance (anhydrobiosis). SAHS1 adopts a beta-barrel fold structurally similar to fatty acid-binding proteins (FABPs), but with unique hydrophilic hydrogen bond networks that provide enhanced tolerance against dehydration. It is one of the most abundantly expressed proteins in Ramazzottius varieornatus and undergoes conformational change from beta-structure to alpha-helical structure upon desiccation. The protein is secreted and proposed to protect extracellular components during desiccation.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005576 extracellular region | IEA GO_REF:0000044 | ACCEPT | Summary: SAHS1 is a secretory protein with a signal peptide (residues 1-19) that is cleaved to produce the mature protein. Yamaguchi et al. (2012) demonstrated experimentally that SAHS1-GFP fusion protein was detected in the culture medium rather than cell bodies by immunoblot analysis (PMID:22937162), confirming secretion. UniProt annotates the subcellular location as "Secreted" with experimental evidence (ECO:0000269|PubMed:22937162). The IEA annotation via UniProtKB-SubCell mapping to GO:0005576 (extracellular region) is well-supported. Reason: The extracellular region annotation is strongly supported by experimental evidence. SAHS1 contains a signal peptide and was shown to be secreted into culture medium when expressed as a GFP fusion in mammalian cells (PMID:22937162). This is also consistent with the protein name (Secretory Abundant Heat Soluble). The IEA mapping from UniProtKB-SubCell is accurate. Supporting Evidence: PMID:22937162 We named them Cytoplasmic Abundant Heat Soluble (CAHS) and Secretory Abundant Heat Soluble (SAHS) protein families, according to their localization. |
| GO:0008289 lipid binding | IEA GO_REF:0000002 | ACCEPT | Summary: The lipid binding annotation is inferred electronically from the InterPro FABP domain (IPR000463). Fukuda et al. (2017) solved the crystal structure of SAHS1 (PDB: 5XN9, 5XNA) and confirmed that it adopts a beta-barrel fold similar to FABPs, with two putative ligand-binding sites (PMID:28703282). However, the authors noted that SAHS1 has unique hydrophilic hydrogen bond networks within the barrel that are distinct from canonical FABPs, adapted for desiccation tolerance rather than lipid transport. No experimental evidence of lipid binding has been demonstrated for SAHS1. The structural similarity to FABPs makes the annotation plausible but unconfirmed. Reason: While no direct lipid binding has been experimentally demonstrated for SAHS1, the crystal structure confirms a genuine FABP-like beta-barrel fold with two putative ligand-binding sites that superimpose on those of canonical FABPs (PMID:28703282). The InterPro-based IEA annotation is a reasonable inference from the structural similarity to the FABP family. It is possible that lipid binding is an ancestral function retained or repurposed in SAHS proteins, even though the primary biological role appears to be desiccation protection. The IEA annotation accurately reflects the computational inference and should be retained pending experimental testing. 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, which would provide RvSAHS1 with better tolerance against dehydration. PMID:28703282 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:22937162 We named them Cytoplasmic Abundant Heat Soluble (CAHS) and Secretory Abundant Heat Soluble (SAHS) protein families, according to their localization. |
| GO:0009269 response to desiccation | IDA PMID:22937162 Two novel heat-soluble protein families abundantly expressed... | NEW | Summary: SAHS1 is a major heat-soluble protein identified by proteomics in the anhydrobiotic tardigrade R. varieornatus. It is constitutively and abundantly expressed and undergoes conformational change from beta-structure to alpha-helical structure under water-deficient conditions (PMID:22937162). UniProt describes its function as a "molecular shield in water-deficient condition" (ECO:0000269|PubMed:22937162). The protein is proposed to protect extracellular components during desiccation, acting analogously to LEA proteins in other anhydrobiotic organisms. This biological process annotation is critical for capturing the core function of SAHS1. Reason: This is the core biological process for SAHS1. The protein was identified specifically through heat-soluble proteomics aimed at understanding anhydrobiosis (PMID:22937162). It undergoes desiccation-induced conformational changes and is proposed to act as a molecular shield protecting extracellular components during desiccation (PMID:22937162, PMID:27649274). The GO term GO:0009269 (response to desiccation) accurately captures this biological role. This annotation is conspicuously absent from the current GOA set and should be added. Supporting Evidence: PMID:22937162 Both protein families were conserved among tardigrades, but not found in other phyla. Although CAHS protein was intrinsically unstructured and SAHS protein was rich in beta-structure in the hydrated condition, proteins in both families changed their conformation to an alpha-helical structure in water-deficient conditions as LEA proteins do. PMID:22937162 proteins in both families changed their conformation to an alpha-helical structure in water-deficient conditions as LEA proteins do 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 |
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