Affinage

PIH1D2

PIH1 domain-containing protein 2 · UniProt Q8WWB5

Length
315 aa
Mass
36.0 kDa
Annotated
2026-06-10
7 papers in source corpus 4 papers cited in narrative 4 extracted findings
Cross-family judge faithfulness: 4/4 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

PIH1D2 is an adaptor subunit of the R2SP chaperone complex, a testis-enriched R2TP-like machine that promotes the quaternary folding and assembly of specific protein clients (PMID:29844425). Within R2SP, PIH1D2 partners with SPAG1 (an RPAP3-like protein) and the AAA+ ATPases RUVBL1 and RUVBL2, with SPAG1 binding both PIH1D2 and RUVBL1/2 and recruiting HSP70/HSP90 chaperones through its TPR domains (PMID:29844425, PMID:31118266). SPAG1 itself regulates the nucleotide hydrolysis activity of its HSP and RUVBL1/2 partners rather than acting as an autonomous GTPase (PMID:31118266). The complex is required for the expression and assembly of clients such as liprin-α2 (PMID:29844425), and in vivo loss of PIH1D2 abolishes the assembly of a defined subset of axonemal dynein arms in sperm flagella and Kupffer's vesicle cilia, producing abnormal motility (PMID:29741156). This links PIH1D2 to the building of motile-ciliary and flagellar dynein machinery through chaperone-assisted protein assembly.

Mechanistic history

Synthesis pass · year-by-year structured walk · 4 steps
  1. 2018 High

    Established that PIH1D2 is not a free-standing protein but the adaptor subunit of an R2TP-like assembly chaperone (R2SP), defining its molecular context and a first client.

    Evidence Systematic co-IP/pulldown interaction analyses, RPAP3 C-terminal domain structure, mass-spec client identification, and depletion with liprin-α2 assembly readout

    PMID:29844425

    Open questions at the time
    • Full client repertoire beyond liprin-α2 not defined
    • Mechanism by which PIH1D2 selects clients not resolved
  2. 2018 High

    Demonstrated an in vivo function for PIH1D2 in assembling specific axonemal dynein subtypes, connecting the chaperone adaptor to motile cilia/flagellar biology.

    Evidence Zebrafish loss-of-function mutants with cryo-electron tomography of sperm axonemal dynein and motility assays

    PMID:29741156

    Open questions at the time
    • Which dynein subunits are direct R2SP clients not established
    • Relationship to R2SP-mediated folding in this context not directly shown
  3. 2019 Medium

    Clarified how chaperone activity is recruited and regulated within the PIH1D2 complex, showing SPAG1 brings in HSP70/HSP90 and tunes hydrolysis of partner ATPases.

    Evidence ITC, NMR, molecular dynamics, and in vitro GTP hydrolysis assays on SPAG1 within the R2SP complex

    PMID:31118266

    Open questions at the time
    • Single-lab biochemical characterization
    • PIH1D2's own contribution to ATPase regulation not isolated
  4. 2025 Medium

    Provided a structural model of the human R2SP complex, showing how PIH1D2 and SPAG1 dock onto the RUVBL1/2 core and how its ATPase regulation differs from canonical R2TP.

    Evidence Cryo-EM, NMR, structural mass spectrometry, and biochemical ATPase assays of human R2SP (preprint)

    PMID:bio_10.1101_2025.01.27.635100

    Open questions at the time
    • Preprint, not peer-reviewed
    • Client-bound structure not determined
    • Functional consequence of distinct ATPase regulation untested

Open questions

Synthesis pass · forward-looking unresolved questions
  • How PIH1D2 recognizes and selects its diverse clients (liprin-α2 versus axonemal dyneins) and how client specificity is encoded in the complex remains unresolved.
  • No client-recognition determinant mapped on PIH1D2
  • Tissue-specific regulation of R2SP activity uncharacterized

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0044183 protein folding chaperone 2 GO:0060090 molecular adaptor activity 2
Pathway
R-HSA-392499 Metabolism of proteins 1
Partners
RUVBL1RUVBL2SPAG1
Complex memberships
R2SP

Evidence

Reading pass · 4 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2018 PIH1D2 forms an R2TP-like complex termed R2SP together with SPAG1 (an RPAP3-like protein), RUVBL1, and RUVBL2. Systematic interaction analyses showed that SPAG1 binds PIH1D2 and RUVBL1/2. R2SP is enriched in testis, functions in quaternary protein folding of specific clients (e.g., liprin-α2), and is required for liprin-α2 expression and assembly of liprin-α2 complexes. Systematic interaction analyses (co-immunoprecipitation/pulldown), structure determination of RPAP3-C-terminal domain, client identification by mass spectrometry, functional validation by R2SP depletion with client expression and complex assembly readout Nature Communications High 29844425
2018 PIH1D2 (zebrafish pih1d2) is specifically required for the assembly of a defined subset of axonemal dynein subtypes in sperm flagella; loss of pih1d2 results in loss of specific dynein arms as visualized by cryo-electron tomography, leading to abnormal sperm motility and defects in Kupffer's vesicle cilia motility. Zebrafish mutant generation and cryo-electron tomography of axonemal dynein structure in mutant spermatozoa, motility assays eLife High 29741156
2019 Within the R2SP complex, PIH1D2 partners with SPAG1, RUVBL1, and RUVBL2. SPAG1's TPR domains recruit HSP70 and HSP90 chaperones to the complex, and SPAG1 regulates nucleotide hydrolysis activity of HSP and RUVBL1/2 partners rather than possessing autonomous GTPase activity. Biochemical assays, isothermal titration calorimetry (ITC), NMR spectroscopy, molecular dynamics simulations, in vitro GTP hydrolysis assay The Biochemical Journal Medium 31118266
2025 Cryo-EM, NMR, and structural mass spectrometry of the human R2SP complex (RUVBL1, RUVBL2, SPAG1, PIH1D2) reveal a three-dimensional organization similar to the canonical R2TP complex but with distinct differences in the ATPase activity of the RUVBL1/2 core and in the mode of binding of adaptors SPAG1 and PIH1D2. Cryo-EM, NMR spectroscopy, structural mass spectrometry, biochemical ATPase assays bioRxivpreprint Medium bio_10.1101_2025.01.27.635100

Source papers

Stage 0 corpus · 7 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2018 The RPAP3-Cterminal domain identifies R2TP-like quaternary chaperones. Nature communications 63 29844425
2018 Systematic studies of all PIH proteins in zebrafish reveal their distinct roles in axonemal dynein assembly. eLife 54 29741156
2022 Omics and Male Infertility: Highlighting the Application of Transcriptomic Data. Life (Basel, Switzerland) 30 35207567
2020 Transcriptomic analysis of female and male gonads in juvenile snakeskin gourami (Trichopodus pectoralis). Scientific reports 14 32251302
2019 Binding properties of the quaternary assembly protein SPAG1. The Biochemical journal 13 31118266
2010 Novel germline SDHD deletion associated with an unusual sympathetic head and neck paraganglioma. Head & neck 4 20310044
2025 Genetic Nurture Effects on Type 2 Diabetes Among Chinese Han Adults: A Family-Based Design. Biomedicines 0 39857704

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