SIAH1 (seven in absentia homolog 1) review notes

UniProt: Q8IUQ4 (SIAH1_HUMAN), 282 aa. EC 2.3.2.27. RING-type E3 ubiquitin ligase.

Domain architecture

• RING-type zinc finger 41..76 (catalytic RING, recruits E2) [UniProt FT].
• Two additional zinc fingers (93..153) within the SIAH-type substrate-binding domain (SBD).
• Forms homodimers (and heterodimers with SIAH2). N-terminal RING; C-terminal SBD recognizes
  substrate degrons (often a VxP/PxAxVxP motif).

Core molecular function

• RING-type E3 ubiquitin-protein ligase / ubiquitin-protein transferase (EC 2.3.2.27).
  Accepts ubiquitin from an E2 (thioester) and transfers directly to substrate.
  [UniProt FUNCTION; EC 2.3.2.27 ECO:0000269|PubMed:19224863, PubMed:28546513]
• Zinc ion binding (RING + Zn fingers structurally require Zn) [UniProt; PMID:16085652 structural; PMID:11863358].
• Binds E2 ubiquitin-conjugating enzymes (UBE2D1, UBE2E2, UBE2I, UBE2L6) — "ubiquitin conjugating enzyme binding".
  PMID:9334332

Substrates / processes (the heart of SIAH1 biology)

• DCC (deleted in colorectal cancer) — first mammalian substrate; ubiquitin-proteasome degradation.
  PMID:9334332
• beta-catenin (CTNNB1) — Siah1 mediates a p53-inducible, GSK3beta/beta-TrCP-INDEPENDENT
  beta-catenin degradation pathway via APC; part of a multiprotein E3 complex with SIP/CACYBP,
  SKP1, Ebi (TBL1X). [PMID:11389840; PMID:16085652 "Siah1 is the central component of a multiprotein
  E3 ubiquitin ligase complex that targets beta-catenin for destruction in response to p53 activation"]
  -> beta-catenin destruction complex (GO:0030877) part_of PMID:16085652.
• AXIN1 — Wnt-induced degradation; SIAH1/2 mediate Wnt-induced Axin degradation, a feed-forward
  mechanism to sustain Wnt/beta-catenin signaling. NOTE: this is POSITIVE regulation of canonical Wnt
  (degrading Axin promotes signaling), distinct from the beta-catenin-degrading (negative) role above.
  PMID:28546513
• De novo SIAH1 variants -> developmental delay; also affect Wnt. PMID:32430360
• alpha-synuclein (SNCA) monoubiquitylation + synphilin-1 (SNCAIP) ubiquitination -> Lewy body /
  inclusion formation; relevant to Parkinson disease. SIAH activity inhibited by synphilin-1A.
  PMID:19224863. Also Reactome SIAH1 ubiquitinates SNCA/SNCAIP.
• XIAP — ARTS (SEPT4) bridges SIAH1 to XIAP for degradation -> promotes apoptosis (intrinsic).
  PMID:21185211
• HIPK2 — constitutive degradation; DAZAP2 promotes SIAH1-mediated HIPK2 ubiquitination/degradation;
  DNA-damage context (p53 response). PMID:33591310
• Jade-1 (PHF17) — ubiquitination regulated by polycystin-1; "Jade-1 ubiquitination was mediated by
  Siah-1". PMID:23001567
• BOB.1/OBF.1 (POU2AF1) stability regulated by SIAH. PMID:11483518
• HBx (hepatitis B X protein) poly-ubiquitylation/degradation. PMID:21878328
• EGLN2/EGLN3 (PHD prolyl hydroxylases) — degraded by SIAH1/2 under UPR/hypoxia, stabilizing ATF4
  (and indirectly HIF). [UniProt FUNCTION, By similarity] — hypoxia link.
• Many others: ELL2, MYB, PML, RBBP8, FLT3, KLF10/TIEG1, NUMB, BAG1, KIF22, SYP [UniProt].

Subcellular location

• Predominantly cytoplasmic; partially nuclear. [UniProt SUBCELLULAR LOCATION: Cytoplasm. Nucleus.]
• Cytoplasm/cytosol is the active compartment (is_active_in cytoplasm, IBA). Nuclear pool real
  (HIPK2, transcription-factor substrates). Nucleoplasm IDA from HPA (GO_REF:0000052).

Apoptosis / tumor suppression / nervous system

• p53-inducible; originally identified in apoptosis and tumor suppression [UniProt ref 8799150].
• Promotes apoptosis (intrinsic pathway via XIAP; POSH/JNK). [PMID:21185211; PMID:16230351 POSH-JNK]
• Nervous system development / axon guidance via DCC regulation. [PMID:9334332; PMID:9403064]

Annotation-call reasoning

• Core MF: ubiquitin protein ligase activity (GO:0061630) / ubiquitin-protein transferase
  (GO:0004842) — genuine RING E3, catalytic. ACCEPT.
• ubiquitin conjugating enzyme binding (GO:0031624) — ACCEPT (E2 binding is core to RING mechanism).
• zinc ion binding (GO:0008270) — ACCEPT (RING + Zn fingers; structurally demonstrated).
• proteasome-mediated ubiquitin-dependent protein catabolic process (GO:0043161) — ACCEPT core BP.
• cytoplasm/cytosol — ACCEPT (active site). nucleus/nucleoplasm — KEEP_AS_NON_CORE (real secondary).
• canonical Wnt signaling (GO:0060070) — KEEP_AS_NON_CORE (downstream pathway; AXIN1 substrate).
• apoptosis terms — KEEP_AS_NON_CORE (downstream processes of substrate degradation).
• amyloid fibril formation (GO:1990000) — Reactome/ARBA: alpha-synuclein context; KEEP_AS_NON_CORE
  (this is the substrate's aggregation behavior, not SIAH1's intrinsic function). Borderline
  over-annotation but Reactome TAS curates SIAH1 in the SNCA amyloid pathway.
• protein binding (GO:0005515) bare IPI — KEEP_AS_NON_CORE per curation guidelines.
• identical protein binding (GO:0042802) — ACCEPT/KEEP_AS_NON_CORE: SIAH1 homodimerizes (real),
  but it is a specific informative interaction; keep as non-core (dimerization supports function).
• nervous system development / axon guidance / morphogenesis — KEEP_AS_NON_CORE (pleiotropic dev roles).
• protein destabilization (GO:0031648) — KEEP_AS_NON_CORE (consequence of degradation activity).
• neuron apoptotic process — KEEP_AS_NON_CORE.

Citation issue (PMID:11863358)

• The GOA zinc ion binding (IDA) annotation cites PMID:11863358 "An anthropoid-specific
  locus of orphan C to U RNA-editing enzymes on chromosome 22." The cached (abstract-only)
  entry is entirely about APOBEC C-to-U RNA-editing enzymes and contains ZERO mention of
  SIAH1/seven-in-absentia. This looks like a wrong-identifier citation. Per project
  guardrails I do not REMOVE an experimental IDA on the basis of incomplete cached evidence;
  marked UNDECIDED and flagged correctness=WRONG_IDENTIFIER. SIAH1 zinc binding itself is
  well supported by the structural study PMID:16085652 (RING + SIAH-type Zn fingers).