ENSA (alpha-endosulfine, O43768) - curation notes

Identity and paralog caveat

• ENSA (ARPP-19e) and ARPP19 are separate endosulfine-family genes; the conserved central region
  (ENSA Ser67 / ARPP19 Ser62 = Greatwall site; ENSA Ser109 / ARPP19 Ser104 = PKA site) is nearly
  identical and many antibodies/siRNA designs hit both, so "ENSA/ARPP19" statements cannot be
  assigned to ENSA alone PMID:32085646.
• Human ENSA is intrinsically disordered with three transient helices PMID:34346186. No signal peptide PMID:9653196.

Core function: Greatwall-gated PP2A-B55 inhibitor

• Discovery in Xenopus egg extracts: Gwl-phosphorylated Ensa is the mitotic PP2A-B55δ inhibitor
  PMID:21164013; the companion paper found both Arpp19 and Ensa are Gwl substrates but Arpp19 is limiting for mitotic entry in extracts PMID:21164014.
• Mechanism: inhibition by unfair competition - pEndos is a high-affinity, slowly processed PP2A-B55 substrate PMID:24618897. The Ser67 region docks at the B55/C-subunit interface over the catalytic centre PMID:24354984.
• Human cells: in HeLa, ENSA rather than ARPP19 is the major MASTL substrate PMID:24120663; PP2A-B55 is the ENSA Ser67 phosphatase PMID:24120663; the ENSA/Greatwall timer delays PP2A-B55 reactivation so cytokinesis follows chromosome separation PMID:24120663. Fcp1 also contributes to Ensa/ARPP19 Ser67 dephosphorylation at mitotic exit PMID:24391510.
• Only the MASTL-site phosphoform inhibits PP2A; the PKA/PKG Ser109 form is a substrate but not an inhibitor PMID:32085646. Present at ~7,800 copies in anucleate platelets (deep research summary of the same paper).
• Structural note: in vitro, human ENSA binds the PP2A A subunit more strongly than the B56 subunits tested; B55 was not among the tested B subunits PMID:34346186.

S-phase role (human)

• Ensa knockdown in HeLa/U2OS dramatically extends S phase with fewer active origins; Treslin protein falls and Treslin overexpression rescues PMID:28785014. Nuclear ENSA increases in S phase PMID:28785014. Mechanism proposed: phospho-ENSA prevents PP2A-mediated Treslin dephosphorylation and hence Cullin-dependent degradation.
• Not annotated in GOA. Raised as a suggested question rather than a NEW term (single study; holoenzyme not pinned down).

Meiosis (mouse)

• Ensa RNAi in mouse oocytes blocks exit from prophase I, rescued by okadaic acid or Ppp2r2d co-depletion PMID:24675883. Not in human GOA; consistent with the core function.

Historical K-ATP channel / insulin-secretion story

• Cloning paper: recombinant human alpha-endosulfine displaces [3H]glibenclamide from MIN6 membranes (ED50 ~1 μM), reduces Kir6.2/SUR1 currents (~75% at 2 μM; no effect on Kir1.1) and stimulates insulin release at 1 mM glucose PMID:9653196. Caveats stated by the authors: interacting subunit unknown PMID:9653196; effect from both sides of the membrane PMID:9653196.
• Later assessment: no evidence ENSA is extracellular PMID:28167675. Mouse brain work links Ensa to a SUR1/Kir6.2 K-ATP-dependent regulation of neprilysin downstream of somatostatin PMID:34737456, keeping the channel connection alive without a biochemical binding demonstration.
• Grading: GO:0019870 potassium channel inhibitor activity IDA -> KEEP_AS_NON_CORE (direct assay, channel-selective, but micromolar and mechanistically unresolved). GO:0050796 regulation of insulin secretion IDA -> MARK_AS_OVER_ANNOTATED (exogenous extracellular protein on a cell line; no loss-of-function support). GO:0005102 signaling receptor binding (IDA, TAS) and GO:0008200 ion channel inhibitor activity (TAS) -> MODIFY to GO:0019870 (displacement assay is indirect; SUR1 is a drug-receptor ABC subunit, not a signalling receptor; the direct readout is channel inhibition). GO:0007584 response to nutrient (TAS) -> REMOVE (full text has no nutrient-response experiment; derived from speculative discussion).

Other annotations

• GO:0005515 protein binding IPI with ACD/TPP1 PMID:21044950: genome-wide BiFC screen hit, ENSA absent from main text, no follow-up -> REMOVE as uninformative (not asserting the signal is false).
• Xenopus ISS transfers (Q7ZXH9): GO:0000086 ACCEPT; GO:0000278 ACCEPT (legitimately broad: G2/M, mitotic exit timing, S phase); GO:0051721 PP2A binding ACCEPT (specific, mechanistic); GO:0019212 and GO:0019888 MODIFY -> GO:0004864.
• IBA GO:0004864 (PTN001309504; Endos, Igo1, Igo, ARPP19) ACCEPT; IBA/IEA cytoplasm ACCEPT; Reactome nucleoplasm TAS ACCEPT (nuclear pool shown in human cells).
• NEW: GO:0010923 negative regulation of phosphatase activity (IDA, PMID:24120663) - the BP counterpart of the inhibitor activity; ARPP19 carries it by IDA and the ARPP19 GO-CAM (gocams/69729a3800000481) uses it.
• SNCA interaction (UniProt SUBUNIT, PMID:17893145): NMR on SDS micelles only; not in GOA, not annotated here.

Publications fetched for this review (new cache files)

PMID:28785014, 32085646, 34346186, 24391510, 24120663, 24675883, 34686773, 17893145, 24618897,
34737456, 24354984 (via ai-gene-review fetch-pmid). 34346186, 34686773, 17893145 and 24354984 are
abstract-only.