Tardigrade Stress Response Protein Curation Project

MATURE BIOLOGY_DOMAINFLAGSHIP

Species: RAMVA

Genes: Dsup CAHS1 CAHS2 CAHS3 SAHS1 SAHS2 MAHS RvLEAM RvY_00650 RvY_00651 RvY_01767 RvY_03754 RvY_03757 RvY_09480 RvY_10893 RvY_13070 RvY_15948 RvY_17310

Warnings (1)

Tardigrade Stress Response Protein Curation Project

Bottom line: Ramazzottius varieornatus survives near-total desiccation,
and with it radiation, vacuum and extreme temperature, using a set of
intrinsically disordered proteins that each protect a different cellular
compartment, plus a much-expanded superoxide dismutase family. We reviewed all
18 of its curated proteins: the chromatin shield Dsup, the cytosolic CAHS,
secretory SAHS and mitochondrial MAHS families, the LEA protein RvLEAM, the Mn-SOD
RvY_01767, and all 9 Cu/Zn-SOD paralogs. Across 83 annotation rows the actions
were 38 ACCEPT, 17 NEW, 11 KEEP_AS_NON_CORE, 10 MARK_AS_OVER_ANNOTATED, 3 MODIFY,
3 UNDECIDED and 1 REMOVE. Two results stand out: GO:0009269 response to
desiccation was missing from every disordered protectant and was proposed as
NEW on seven genes, and a sequence plus PROSITE analysis of the SOD family
(genes/RAMVA/RvY_13070/RvY_13070-bioinformatics/) found one pseudoenzyme
(RvSOD15, whose Val87 in place of a copper-ligand histidine is confirmed by
crystal structure, PMID:37358501; the cached record, which holds only the
abstract and introduction, reports no activity measurement), two probably
impaired paralogs, one unresolved paralog, one copper chaperone and four
likely functional enzymes.

We did this because the organism is both biologically unusual and almost
entirely electronically annotated, which makes it a good place to see what
family-based propagation gets wrong. The SOD result is the sharp case: about
half of an expanded antioxidant family may not be catalytic, so "more gene
copies means more antioxidant capacity" does not hold as stated.

Project Start Date: 2026-04-09
Organism: Ramazzottius varieornatus (RAMVA)
Focus: Anhydrobiosis and extremotolerance - tardigrade-unique stress response proteins

Project Overview

This project curates stress-response proteins from the extremotolerant
tardigrade Ramazzottius varieornatus: the reviewed Swiss-Prot TDP/RvSOD15
set plus unreviewed SOD-family candidates needed to interpret the expanded
antioxidant repertoire. The protectants represent a remarkable evolutionary
innovation: a suite of tardigrade-unique intrinsically disordered proteins
(TDPs) that protect different cellular compartments during anhydrobiosis
(near-complete desiccation), plus conventional stress-response enzymes. The
same mechanisms that enable desiccation tolerance also confer extraordinary
resistance to ionizing radiation, extreme temperatures, vacuum, and high
pressure.

The TDP families partition by cellular compartment:

Gene List and Curation Status

Nuclear Protectant

Gene Symbol UniProt Status Deep Research Review Notes
Dsup P0DOW4 COMPLETE falcon COMPLETE Nucleosome-binding chromatin shield; 2 GOA annots, 3 NEW proposed

Cytosolic Protectants (CAHS Family)

Gene Symbol UniProt Status Deep Research Review Notes
CAHS1 J7M799 REVIEWED falcon REVIEWED 2 GOA ACCEPT, 1 NEW (response to desiccation)
CAHS2 J7MDG6 REVIEWED falcon REVIEWED 1 GOA ACCEPT, 2 NEW (response to desiccation, acquisition of desiccation tolerance)
CAHS3 J7M3T1 REVIEWED falcon REVIEWED 1 GOA ACCEPT, 2 NEW (response to desiccation, protein stabilization)

Secretory Protectants (SAHS Family)

Gene Symbol UniProt Status Deep Research Review Notes
SAHS1 J7MFT5 REVIEWED falcon REVIEWED 2 GOA ACCEPT, 1 NEW (response to desiccation); FABP-like beta-barrel
SAHS2 J7MAN2 REVIEWED falcon REVIEWED 1 GOA ACCEPT, 2 NEW (lipid binding, response to desiccation)

Mitochondrial Protectants

Gene Symbol UniProt Status Deep Research Review Notes
MAHS A0A1D1V3Z0 REVIEWED falcon REVIEWED 1 GOA ACCEPT, 3 NEW (desiccation, osmotic stress, protein stabilization)
RvLEAM A0A0E4AVP3 REVIEWED falcon REVIEWED 1 GOA MODIFY (mitochondrial matrix), 2 NEW (desiccation, osmotic stress)

ROS Scavenging - Cu/Zn Superoxide Dismutase Paralog Family

R. varieornatus has an expanded family of 9 Cu/Zn SOD paralogs in UniProt
(plus 1 Mn/Fe-SOD, RvY_01767). Sim & Inoue (2023) identified RvSOD15 as a
likely pseudoenzyme and noted that "some other RvSODs" may also have lost
function. We applied bioinformatic analysis (sequence conservation + PROSITE
motif matching + Pfam membership) to all 9 Cu/Zn paralogs to assess
catalytic capability. The first-pass sequence analysis is in
genes/RAMVA/RvY_13070/RvY_13070-bioinformatics/RESULTS.md; the current
reviews supersede two calls there by reclassifying RvY_01767 as a Mn/Fe-SOD
instead of a degraded Cu/Zn-SOD and moving RvY_17310 to UNDECIDED after an
OpenScientist active-site-geometry run conflicted with the PROSITE-failure
verdict.

Gene Symbol UniProt UniProt curation Verdict Notes
RvY_13070 (RvSOD15) A0A1D1VU85 Reviewed PSEUDOENZYME Val87 replaces catalytic His48; confirmed structurally (PMID:37358501); 4 SOD activity annots OVER-ANNOTATED
RvY_00650 A0A1D1UDY8 Unreviewed IMPAIRED All Cu His present but PROSITE PS00087 fails (loop context divergent); 292 aa with N-terminal extension
RvY_03757 A0A1D1UP59 Unreviewed IMPAIRED Same: residues OK, PROSITE PS00087 fails
RvY_17310 A0A1D1W3Y1 Unreviewed UNRESOLVED 475 aa (3x normal); PROSITE PS00087 fails, but active-site residues and AlphaFold geometry are intact; SOD-derived rows UNDECIDED
RvY_15948 A0A1D1VWP9 Unreviewed CCS chaperone NOT a SOD; copper chaperone homolog; H46→A, H48→C; correctly lacks SOD activity in GOA
RvY_00651 A0A1D1UKR0 Unreviewed Likely functional 66% identity to human SOD1 (highest); all sequence + PROSITE checks pass
RvY_03754 A0A1D1UP68 Unreviewed Likely functional 63% identity; all checks pass
RvY_09480 A0A1D1VEY6 Unreviewed Likely functional All checks pass
RvY_10893 A0A1D1VE88 Unreviewed Likely functional All checks pass

Mn/Fe Superoxide Dismutase

Gene Symbol UniProt UniProt curation Notes
RvY_01767 A0A1D1USM4 Unreviewed Mitochondrial Mn-SOD (different family from Cu/Zn-SODs); 5 ACCEPT, 1 KEEP_AS_NON_CORE, 1 REMOVE (incorrect respiratory chain complex annotation from ARBA rule)

Scientific Background

Anhydrobiosis as the Central Selective Pressure

Tardigrades can survive near-complete desiccation by entering a dormant tun state. During desiccation, massive ROS accumulation causes oxidative damage across all cellular compartments. The TDP protein families represent a coordinated evolutionary response: each family protects a different compartment (nucleus, cytosol, mitochondria, extracellular space) during the desiccation-rehydration cycle.

The extraordinary radiation tolerance of tardigrades is likely a byproduct of desiccation tolerance, since both stresses generate hydroxyl radicals that damage biomolecules. This is supported by the observation that Dsup protects against both X-ray and H2O2-induced damage through the same nucleosome-shielding mechanism.

Intrinsic Disorder as a Common Theme

Most TDPs are intrinsically disordered proteins (IDPs). This is functionally important: disorder allows flexible, multivalent interactions with target substrates (chromatin for Dsup, cytoskeletal elements for CAHS) and enables reversible phase transitions (hydrogel formation for CAHS) that may physically stabilize cellular structures during water loss.

Evolutionary Uniqueness

The TDP families (CAHS, SAHS, MAHS, Dsup) are tardigrade-specific with no homologs outside the phylum. RvLEAM is the exception - LEA proteins are found in plants, nematodes, and other anhydrobiotic organisms, representing convergent evolution of desiccation tolerance. RvSOD15 is a conventional Cu-Zn superoxide dismutase-family protein that has lost canonical SOD activity.

Key Findings from Curation

Annotation gaps across the desiccation protectants

The most consistent gap is the absence of GO:0009269 (response to desiccation) from the CAHS, SAHS, MAHS and LEA annotations. This is the core biological process for those protectants and was proposed as NEW for seven genes. Similarly, response to osmotic stress was proposed for several genes where experimental gain-of-function data exists (MAHS, RvLEAM in human cells).

RvSOD15 is a confirmed pseudoenzyme - and it's not alone

The crystal structure (PMID:37358501) revealed that Val87 replaces a critical
histidine copper ligand in RvSOD15. We extended this analysis to all 9
Cu/Zn-SOD paralogs in R. varieornatus using sequence + PROSITE motif analysis
(see genes/RAMVA/RvY_13070/RvY_13070-bioinformatics/RESULTS.md):

This validates Sim & Inoue's claim that "some other RvSODs are also unusual
SODs" and provides a more precise count: three of nine Cu/Zn-SOD-fold proteins
are currently pseudoenzymatic or probably impaired, one is unresolved, one is
CCS-like, and four pass the sequence/PROSITE checks. The picture of "gene
duplication = more antioxidant capacity" is only partially correct - roughly
half the expanded SOD repertoire may be non-catalytic.

The bioinformatic analysis pipeline (sequence conservation + PROSITE PS00087/
PS00332 + Pfam membership) is reusable: analyze_sods.py and check_prosite.py
in the bioinformatics folder.

Annotation propagation errors are systematic

Standard automated pipelines (InterPro2GO, EC2GO, UniRule, ARBA) assigned
GO:0004784 (SOD activity) to eight Cu/Zn-SOD enzyme-family proteins based on
Pfam membership, including the RvSOD15 pseudoenzyme and two probably impaired
paralogs, while correctly avoiding SOD activity on the CCS-like RvY_15948.
RvY_17310 remains unresolved pending direct assay. RvY_01767 (Mn-SOD) also
received an incorrect "respiratory chain complex" annotation from an ARBA rule,
since mitochondrial Mn-SOD is a soluble matrix protein, not a respiratory
chain component.

Dsup DNA binding vs nucleosome binding

The original GO:0003677 (DNA binding) annotation was initially proposed for MODIFY to nucleosome binding, but was corrected to ACCEPT after recognizing that GO:0031491 (nucleosome binding) is NOT a child of DNA binding in GO (it's under chromatin binding). Both are valid independent MF annotations for Dsup.

SAHS proteins have FABP-like folds

SAHS1 and SAHS2 adopt beta-barrel folds homologous to fatty acid-binding proteins, with crystal structures available. Lipid binding was annotated for SAHS1 but not SAHS2 - proposed to add for SAHS2.

Key References