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.

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.

SAHS proteins are secretory, rich in beta-structure in hydrated conditions, and shift to alpha-helical conformation under water-deficient conditions.

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.

R. varieornatus genome encodes 13 SAHS genes; SAHS family members are constitutively and abundantly expressed.

SAHS proteins maintain solubility after heat treatment and are proposed to protect biomolecules during desiccation.

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.

Hydrophilic residues form peculiar hydrogen bond networks in the SAHS1 structure, which may provide better tolerance against dehydration.

SAHS4 has different amino acid residues at ligand binding sites compared to SAHS1, preferring uncharged molecules, suggesting functional diversification within the SAHS family.

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.

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.

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.

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.

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.

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.