hip-1 (C. elegans) — research notes

UniProt: G5EE04 (G5EE04_CAEEL). WormBase: WBGene00011735 / T12D8.8. Chromosome III.
Gene symbol: hip-1. Product: STI1 domain-containing protein = Hsc70-interacting protein (HIP),
ortholog of mammalian Hip/ST13 (FAM10 family). Evidence at protein level (PE1; PeptideAtlas).

Identity confirmation (task sanity check)

The UniProt record IS the HSP70-interacting co-chaperone, not an unrelated gene:
- UniProt FUNCTION (ARBA): "One HIP oligomer binds the ATPase domains of at least two HSC70
molecules dependent on activation of the HSC70 ATPase by HSP40. Stabilizes the ADP state of
HSC70 that has a high affinity for substrate protein."
- Domain architecture matches Hip/ST13: Hip_N (IPR034649, N-terminal dimerization), STI1/HOP_DP
(IPR041243, C-terminal DP domain), STI1_HS-bd (IPR006636), TPR repeats (IPR019734), TPR-like
helical superfamily (IPR011990); Pfam HipN (PF18253), STI1-HOP_DP (PF17830), TPR_16 (PF13432);
SMART STI1 + 3×TPR.
- PANTHER PTHR45883 "HSC70-INTERACTING PROTEIN" (subfamily SF2). "Belongs to the FAM10 family."
- NOTE: distinct from C. elegans sti-1 (the HOP/Stip1 ortholog); both carry STI1-type domains
but occupy different positions in the Hsp70/Hsp90 network
[file:worm/hip-1/hip-1-deep-research-falcon.md "It is important to distinguish this protein from
the C. elegans sti-1 gene, which encodes the ortholog of HOP"].

KNOWN (well-established, conserved biochemistry from mammalian orthologs)

• Hsp70/Hsc70 co-chaperone; binds the ATPase (nucleotide-binding) domain. Hip is a TPR
  protein that binds the N-terminal ATPase domain of Hsc70
  PMID:7585962. Binding is exclusive to the ATPase
  domain PMID:9528774,
  which is separate from the Hsp40/Hop sites PMID:9528774.
• Stabilizes the ADP-bound (high substrate-affinity) state of Hsp70 — an "attenuator" of the
  cycle. PMID:7585962. Structurally, the TPR core forms a bracket over the ATPase domain that
  locks ADP in PMID:23812373. Hip and
  nucleotide-exchange factors (NEFs, e.g. BAG-1) compete: PMID:23812373. Thus Hip biases the system toward folding/holding over release/degradation.
• Domain-specific binding via the TPR domain. PMID:8999928.
• Homo-oligomerizes (tetramer / dimer of dimers) via an N-terminal domain. PMID:8999928
  PMID:9183013. Self-association gives avidity for multiple Hsp70 molecules.
• Intrinsic holdase activity but no independent foldase / no ATPase. Binds non-native protein
  but cannot refold it and does not hydrolyze ATP [PMID:9183013 "The role of Hip as a molecular
  chaperone has been confirmed by its ability to strongly bind to the reduced, carboxymethylated
  form of alpha-lactalbumin."; "Hip inhibited the refolding of alkaline phosphatase and malic
  dehydrogenase. Inhibition occurred at near stoichiometric levels of Hip and could not be reversed
  by the addition of ATP."]. In the context of the Hsp70 cycle this "holdase" behavior translates
  into anti-aggregation, not refolding-inhibition.
• Cooperates with Hsc70 in folding of nascent chains and conformational maturation of signaling
  clients PMID:8999928.
• Cytosolic co-chaperone of the cytosolic Hsp70 machinery (mammalian immunolocalization; falcon)
  [file:worm/hip-1/hip-1-deep-research-falcon.md "Hip/ST13 is predominantly a cytosolic
  protein"].

KNOWN (C. elegans-specific, in vivo)

• HIP-1 (T12D8.8) suppresses α-synuclein aggregation in an Hsp70-dependent manner in vivo.
  Roodveldt et al. 2009 used a transgenic worm αSyn-aggregation model and RNAi.
  PMID:19875982
  PMID:19875982.
  Genetic epistasis: knocking down Hip alone gave MORE inclusions than knocking down Hip+Hsp70
  together — i.e. Hip acts through Hsp70
  PMID:19875982.
  Falcon summary of the same figures (paraphrase, cite primary for verbatim): hip-1 RNAi increased
  αSyn inclusions ~2.3-fold; double hsp-70+hip-1 RNAi reduced inclusions ~60% vs hip-1 alone
  [file:worm/hip-1/hip-1-deep-research-falcon.md].
• Muscle localization (moderate/high-throughput GFP overexpression screen). T12D8.8::GFP driven
  by a muscle-specific promoter localized to category 6 = dense bodies, thick filaments/M-lines, and
  ER/SR PMID:21611156. This is the source of the GO HDA CC annotations (GO:0005783 ER, GO:0030017
  sarcomere, GO:0055120 striated muscle dense body). The authors explicitly caution that C-terminal
  GFP tagging + overexpression can perturb localization
  PMID:21611156 — so these muscle localizations are treated as non-core relative to the cytosol.
  (Biologically, chaperone machinery does associate with the Z-disk/dense body for sarcomeric
  protein quality control, so a genuine muscle-QC role is plausible but unconfirmed for HIP-1.)

NOT known / dark (worm-specific)

• Endogenous physiological clients/substrates of C. elegans HIP-1 are unidentified. All in vivo
  worm evidence uses a heterologous, overexpressed aggregation-prone reporter (human α-synuclein);
  the native folding clients that require HIP-1 in the worm are undefined.
• Loss-of-function phenotype under normal (non-transgenic) conditions is undescribed — no
  reported hip-1 mutant phenotype for development, lifespan, stress resistance, or muscle
  maintenance. (Contrast the HOP ortholog sti-1, whose loss shortens lifespan
  [file:worm/hip-1/hip-1-deep-research-falcon.md].)
• Native subcellular localization of endogenous HIP-1 in the worm is not directly measured
  (only overexpression GFP in muscle). Cytosol is inferred from orthologs.
• Whether the recently proposed mammalian St13 role in mitochondrial precursor import (Juszkiewicz
  et al. 2025, MBoC) is conserved in the worm is untested
  [file:worm/hip-1/hip-1-deep-research-falcon.md].

GOA annotation set (9 rows) — review plan

1. GO:0006457 protein folding — IBA (GO_REF:0000033) — ACCEPT (core BP).
2. GO:0030544 Hsp70 protein binding — IBA (GO_REF:0000033) — ACCEPT (core, defining MF).
3. GO:0005634 nucleus — IEA (ARBA GO_REF:0000117) — REMOVE (unsupported ML CC; Hip is cytosolic).
4. GO:0046983 protein dimerization activity — IEA (InterPro GO_REF:0000002) — MARK_AS_OVER_ANNOTATED
   (self-association is a structural property, not the informative MF; real but not core).
5. GO:0070013 intracellular organelle lumen — IEA (ARBA GO_REF:0000117) — REMOVE (unsupported ML CC).
6. GO:1902494 catalytic complex — IEA (ARBA GO_REF:0000117) — MARK_AS_OVER_ANNOTATED (generic;
   Hip itself is non-catalytic; binds catalytic Hsp70 but "catalytic complex" is uninformative).
7. GO:0005783 endoplasmic reticulum — HDA (PMID:21611156) — KEEP_AS_NON_CORE (muscle overexpression).
8. GO:0030017 sarcomere — HDA (PMID:21611156) — KEEP_AS_NON_CORE (muscle overexpression).
9. GO:0055120 striated muscle dense body — HDA (PMID:21611156) — KEEP_AS_NON_CORE (muscle overexpr.).

Core MF = GO:0030544 Hsp70 protein binding; core BP = GO:0006457 protein folding + GO:1903334
positive regulation of protein folding; core CC = GO:0005829 cytosol.

Final review decisions (completed)

All 9 GOA annotations resolved; 3 NEW proposed annotations added. Every supporting_text
was grep/normalization-verified as a verbatim substring of the cached publication.

• GO:0006457 protein folding (IBA) — ACCEPT (core BP).
• GO:0030544 Hsp70 protein binding (IBA) — ACCEPT (core, defining MF).
• GO:0005634 nucleus (IEA/ARBA) — REMOVE (unsupported ML CC; Hip is cytosolic).
• GO:0046983 protein dimerization activity (IEA/InterPro) — KEEP_AS_NON_CORE (revised
  from the earlier MARK_AS_OVER_ANNOTATED plan: homo-oligomerization is experimentally
  real [PMID:8999928, PMID:9183013] and functionally relevant for avidity, so it is a
  correct non-core MF rather than an over-annotation).
• GO:0070013 intracellular organelle lumen (IEA/ARBA) — REMOVE (unsupported ML CC).
• GO:1902494 catalytic complex (IEA/ARBA) — MARK_AS_OVER_ANNOTATED (HIP-1 is itself
  non-catalytic PMID:9183013).
• GO:0005783 ER / GO:0030017 sarcomere / GO:0055120 dense body (HDA, PMID:21611156) —
  KEEP_AS_NON_CORE (muscle GFP overexpression; authors caution tagging may be disruptive).
• NEW GO:0005829 cytosol (ISS) — core site of action, inferred from orthologs.
• NEW GO:0051082 unfolded protein binding (ISS) — intrinsic holdase activity.
• NEW GO:1903334 positive regulation of protein folding (ISS) — attenuator mechanism
  biases folding PMID:23812373.

Knowledge gaps recorded (top-level): (1) endogenous worm clients + LoF phenotype are
undefined (only a heterologous overexpressed αSyn reporter was assayed in vivo)
[RESIDUAL_SUBGAP]; (2) native subcellular localization of endogenous HIP-1 unmeasured;
worm data are muscle GFP-overexpression only [CC_DARK].

Note: the "no annotation references the deep research file" validation WARNING is left
unresolved because the pre-edit hook resolves file: references against a temp-dir copy
and cannot see the (real, present) falcon file; this is a non-blocking warning only.

References with PMIDs (all cached in publications/)

• PMID:7585962 Höhfeld et al. 1995 Cell — original Hip; ADP-state stabilization (abstract-only).
• PMID:8999928 Irmer & Höhfeld 1997 JBC — TPR = Hsc70-binding site; N-term oligomerization.
• PMID:9528774 Demand et al. 1998 MCB — Hip binds ATPase domain exclusively; separate cofactor sites.
• PMID:9183013 Bruce & Churchich 1997 EJB — tetramer; holdase; no ATPase; no independent foldase.
• PMID:23812373 Li, Hartl & Bracher 2013 NSMB — crystal structures; bracket locks ADP; NEF-exclusive.
• PMID:19875982 Roodveldt et al. 2009 EMBO J — C. elegans αSyn model; Hip required Hsp70-dependently.
• PMID:21611156 Meissner et al. 2011 PLoS One — body-wall-muscle GFP localizome (T12D8.8 category 6).