hairy (h / hry, CG6494, UniProt P14003, HAIR_DROME) — curation notes

Reviewing hairy = P14003, the ~337 aa pair-rule bHLH-Orange (HES-family) transcriptional
repressor of Drosophila melanogaster. This is a DISTINCT gene from Hairless (H, Q02308);
no Hairless data is used here. Directory aliased hairy to avoid the h/H collision.

1. Identity, domains, and architecture

Hairy is the founding member of the Drosophila Hairy/Enhancer-of-split (HES) subfamily of
basic helix-loop-helix–Orange (bHLH-O) transcription factors. The UniProt record
(hairy-uniprot.txt) gives the domain layout: a bHLH domain at residues 31–88, an
Orange domain at 107–136, disordered/low-complexity C-terminal regions, and a terminal
WRPW motif at residues 334–337. The bHLH is of the "repressor-specific" type: UniProt notes
"Has a particular type of basic domain (presence of a helix-interrupting proline) that binds to
the N-box (CACNAG), rather than the canonical E-box (CANNTG)." Family assignment is corroborated
by InterPro (IPR050370 HES_HEY; IPR003650 Orange), Pfam PF07527 (Hairy_orange) + PF00010 (HLH),
and PANTHER PTHR10985 (HES-related bHLH), plus CDD cd18913 "bHLH-O_hairy_like."

UniProt FUNCTION: "Pair-rule protein that regulates embryonic segmentation and adult bristle
patterning. Transcriptional repressor of genes that require a bHLH protein for their transcription
(e.g. ftz)." SUBCELLULAR LOCATION: Nucleus.

2. Molecular function: sequence-specific DNA-binding transcriptional repressor

Hairy is a sequence-specific DNA-binding transcriptional repressor, established directly in
vivo/in vitro: [PMID:7958929 "h, a basic-helix-loop-helix (bHLH) protein, is a sequence-specific"
DNA-binding protein and transcriptional repressor]. The same study identified achaete as a
direct target: PMID:7958929, and mutation of a single conserved high-affinity Hairy site
upstream of ac produced ectopic sensory organs, showing [PMID:7958929 "direct repression of ac
by h plays an essential role in" pattern formation in the PNS].

Hairy-related proteins are defined by "both a repressor-specific bHLH DNA binding domain and a
carboxyl-terminal WRPW (Trp-Arg-Pro-Trp) motif" PMID:8649374. Binding-site
preference: although a canonical E-box binding IDA exists (FlyBase, from PMID:7958929),
UniProt emphasises Hairy's proline-interrupted basic region binds the N-box (CACNAG)
preferentially; the achaete high-affinity site is a Hairy "class C" site. Both the E-box IDA
and the N-box preference reflect genuine sequence-specific DNA binding — they are not
contradictory, and the more informative MF here is "DNA-binding transcription repressor activity,
RNA polymerase II-specific" (GO:0001227).

Dimerization: bHLH proteins generally dimerize (InterPro IEA GO:0046983). Note, however, that in a
comprehensive yeast two-hybrid survey PMID:9371806 — Hairy did not heterodimerize with proneural/E(spl)/Da bHLH
proteins; its key partner is the non-bHLH corepressor Groucho. So the generic "protein
dimerization activity" IEA is defensible at the family level (bHLH-O homodimerization) but Hairy
does not promiscuously heterodimerize with class I/II bHLH factors.

3. Repression mechanism: Groucho recruitment via WRPW (and CtBP)

The core mechanistic function is recruitment of the Groucho (Gro/TLE) corepressor through the
C-terminal WRPW tetrapeptide: PMID:8649374. In transgenic embryos,
PMID:9892668 and
Hairy is one of "the two long-range repressors, Hairy and Dorsal, [that] recruit a different
corepressor protein, Groucho" PMID:9892668. A direct Hairy–Groucho physical interaction was also shown in a
neural-fate two-hybrid network: PMID:9371806.

Hairy additionally contacts the CtBP corepressor through a weak C-terminal PLSLV motif:
[PMID:9524128 "dCtBP interacts" specifically and directly with a small, previously uncharacterized
C-terminal region of Hairy], and dCtBP is PMID:9524128. However, in vivo the WRPW/Groucho pathway is dominant and the two corepressors can
act antagonistically: [PMID:9892668 "recruit a different corepressor" and dCtBP/Gro "mediate
separate pathways of transcriptional repression"]. The Groucho/WRPW paradigm generalises to other
FRPW/WRPW repressors (e.g. Huckebein) [PMID:10433905 "WRPW Groucho-recruitment domain found in"
Hairy-related repressor proteins].

Both CtBP and Groucho are transcription corepressors; therefore several GOA "protein
binding" (GO:0005515) IPI annotations whose actual partner is Groucho or CtBP are more informatively
captured as transcription corepressor binding (GO:0001222).

4. Post-translational regulation (E3 ligases bind Hairy)

Two RING E3 ubiquitin ligases bind Hairy and antagonise its repression:
- Topors (dTopors): PMID:14871887 and
PMID:14871887, targeting Hairy for regulated
proteolysis required for correct segmentation.
- Degringolade (Dgrn), a SUMO-targeted ubiquitin ligase: PMID:21343912, where PMID:21343912.

Because both partners are ubiquitin ligases that bind Hairy, the corresponding "protein binding"
IPIs are more informatively ubiquitin protein ligase binding (GO:0031625).

5. Biological processes

5a. Pair-rule segmentation (CORE)

Hairy is a primary pair-rule / segmentation gene: [PMID:9524128 "hairy is a Drosophila
pair-rule segmentation gene that functions genetically as" a repressor]. It represses fushi
tarazu (ftz): [PMID:7768186 "runt and hairy are required for the proper" transcriptional
regulation of ftz], acting through an fDE1 element where ftz expression depends on "activation by
runt and repression by hairy" PMID:7768186. In the segmentation-evolution
literature Hairy is grouped among the PMID:15382142.
Supports: periodic partitioning by pair rule gene (GO:0007366), trunk segmentation (GO:0035290),
posterior head segmentation (GO:0035289), anterior/posterior pattern specification (GO:0009952).

5b. Peripheral nervous system / bristle (SOP) patterning (CORE)

Hairy patterns the adult PNS by repressing proneural achaete-scute, controlling sensory
bristle/chaeta spacing: [PMID:7958929 "hairy is a direct" transcriptional repressor of achaete],
acting as a [PMID:7958929 "negative regulator in both embryonic segmentation and adult" PNS
development]. In the neural-fate network Hairy is one of the proneural antagonists
[PMID:9371806 "antagonized by" the products of E(spl), hairy, and extramacrochaetae]. Supports:
regulation of neurogenesis (GO:0050767), nervous system development (GO:0007399).

5c. Pleiotropic / peripheral roles (NON-CORE)

6. Localization

Nucleus (UniProt PROSITE-derived; bHLH DNA-binding TF). GOA carries nucleus by IBA, IEA, and IC
(GO:0005634) — all consistent and accepted.

7. Over-annotation flags

8. Core function summary

  1. Sequence-specific DNA-binding transcriptional repressor (bHLH-O; N-box/class-C sites) —
    GO:0001227 / GO:0000978.
  2. Groucho corepressor recruitment via the C-terminal WRPW motif — GO:0001222, driving
    long-range repression.
  3. Deployed in pair-rule segmentation (GO:0007366) and PNS/bristle patterning via
    achaete-scute repression     (GO:0050767) in the nucleus (GO:0005634).

9. Reference-block sweep (round 2 review follow-up)

When the five bare-protein binding IPIs were switched to MODIFY → GO:0001222, the matching
reference_review blocks in the references: list were not swept and still asserted the
superseded "left UNDECIDED" verdicts. Corrected here for PMID:17898168 and PMID:19805071:
correctness: UNVERIFIED is retained, because it is a claim about what the cached text
supports, but the prose now records that the partner comes from the GOA WITH/FROM column
(UniProtKB:P16371-2 and UniProtKB:P16371, both gro) rather than from the paper.

Also added UniProtKB:P14003 as a reference so the two Groucho MODIFY rows carry a
supported_by anchor. The SUBUNIT block is the right source, since the partner is curated
input data rather than something asserted by either cited paper
[UniProtKB:P14003 "Interacts with gro (via WPRW motif)"].