PIK3R1 (p85α) curation notes

UniProt: P27986 (P85A_HUMAN). Gene: PIK3R1 (syn GRB1). Human, NCBITaxon:9606.
Class IA PI3K regulatory subunit alpha. 724 aa. Family: PI3K p85 subunit family.

Identity / architecture

• Encodes p85α and (via alternative promoters/splicing) the shorter p55α (isoform 2, AS53)
  and p50α (isoform 3, p46) regulatory subunits (UniProt ALTERNATIVE PRODUCTS; 5 isoforms).
• Domain layout: N-terminal SH3 -> proline-rich -> BCR/RhoGAP-homology (BH) -> nSH2 -> iSH2
  (coiled-coil) -> cSH2. iSH2 is the principal high-affinity p110-binding region; nSH2/cSH2
  bind phosphotyrosine and make inhibitory contacts with p110
  [file:PIK3R1-deep-research-falcon.md "iSH2 is the principal p110-binding coiled coil"].
• Not an enzyme. Lipid-kinase catalysis belongs to the p110 catalytic subunit (PIK3CA/CB/CD)
  [file:PIK3R1-deep-research-falcon.md "p85α is not an enzyme"].

Core function (established)

• Non-catalytic regulatory/adaptor subunit of class IA PI3K: stabilizes p110, restrains basal
  lipid-kinase activity (autoinhibition), and couples the heterodimer to tyrosine-phosphorylated
  receptors/adaptors, allosterically activating and positioning it at the membrane
  [file:PIK3R1-deep-research-falcon.md "stabilizes p110α/β/δ while suppressing basal lipid-kinase activity"].
• Structural: nSH2 of p85α "form[s] a scaffold for the entire enzyme complex"
  PMID:19805105;
  p85α polypeptide is "a protein required for its enzymatic" activity of p110α
  PMID:18079394.
• Cancer p85α mutants "weaken an inhibitory interaction between p85alpha and p110alpha while
  preserving the stabilizing interaction between p85alpha iSH2 and the adapter-binding domain of
  p110alpha" PMID:20713702 — confirms dual stabilize+inhibit role.
• KBTBD2 (Cul3 E3) targets p85α for ubiquitin/proteasome degradation, controlling PI3K abundance
  [PMID:27708159 "KBTBD2 targeted p85α, the regulatory subunit"; "causing p85α ubiquitination"].
  Note: p85α is the SUBSTRATE here, not the ubiquitinating enzyme.

SH2 phosphotyrosine binding / receptor coupling (adaptor recruitment)

• SH2 domains dock phosphorylated YXXM motifs on RTKs and IRS proteins
  [file:PIK3R1-deep-research-falcon.md "p85α SH2 domains recognize phosphorylated YXXM motifs"].
• Direct binding to IGF1R: "phosphatidylinositol 3-kinase interact directly and specifically with
  the IGFIR." PMID:7541045.
• INSR interaction via SH2/YTHM motif (UniProt SUBUNIT; PMID:7537849 characterizes the NPEY docking).
• p85 SH2/PTB captured on SH2-domain microarray PMID:20624904.
• Broad RTK/adaptor partner list (UniProt SUBUNIT): FGFR1-4, KIT, PDGFRA/B, ERBB3/4, CSF1R, MET,
  IRS1/2/4, GAB1, CBL/CBLB, LAT, CD28, ICOS, etc. Most are generic protein-binding GO rows.

Downstream pathways / physiology (non-core, pleiotropic)

• PI3K/AKT signal transduction (GO:0043491); insulin receptor signaling (GO:0008286, IBA);
  cellular response to insulin; glucose homeostasis / GLUT4 glucose uptake.
• GH signaling: "binding of IRS-1 to the 85-kDa regulatory subunit of PI 3'-kinase."
  PMID:7782332 (GH, IFN-γ, LIF -> IRS1 tyrosine phosphorylation -> p85/PI3K).
• Immune: NK cytotoxicity via DAP10 [PMID:16582911 "For full calcium release and cytotoxicity to
  occur, both Grb2-Vav1 and p85 had" to bind DAP10]; TLR4 regulation via MyD88
  [PMID:19289601 "MyD88 and p85 were shown previously to" co-immunoprecipitate]; ICOS costimulation /
  Tfh differentiation [PMID:30523347 "required for p85 recruitment to ICOS and subsequent PI3K
  activation"; "fails to support TFH" development]; B/T cell differentiation; IL-18 signaling.
• ER stress / UPR: p85α interacts with XBP1 and increases its nuclear accumulation
  [PMID:20348923 "transcriptional mediator of the unfolded protein response (UPR), in an" ER
  stress-dependent manner; knockdown gives "ER stress-dependent accumulation of nuclear XBP-1,
  decreased induction of UPR" target genes and increased apoptosis]. Drives the cluster of
  XBP1-related BP annotations (nuclear import, transcription, RNA splicing, anti-apoptosis, UPR).
• Cytoskeleton (BH domain / Rho-family GTPases): lamellipodium/filopodium assembly, stress-fiber,
  focal-adhesion disassembly, cell spreading (mostly ISS from mouse Pik3r1 / IEA).

Localization

• Cytosolic before stimulation; recruited to plasma / receptor membranes upon activation. UniProt
  SUBCELLULAR LOCATION: Cytoplasm (PMID:32606397). Reactome TAS rows -> cytosol, plasma membrane.
  Reported nuclear and perinuclear/ER pools (XBP1/UPR context).

Disease (context, not GO BP)

• SHORT syndrome (cSH2 R649W etc.; loss of receptor coupling), APDS2/IMD36 (activating), AGM7
  (biallelic loss, B-cell deficiency). Confirms central role in PI3K signaling across immunity,
  growth, metabolism.

Curation decisions summary

• 182x GO:0005515 protein binding (IPI): REMOVE as uninformative (per policy; interactions real
  but the generic term adds no functional content; informative facets captured by specific MF terms).
• 130x GO:0005829 cytosol (TAS Reactome + IEA): ACCEPT (valid resting localization).
• Core MF: GO:0046935 1-PI3K regulator activity; GO:0001784 phosphotyrosine residue binding;
  adaptor activities (GO:0005068, GO:0140767); complex membership GO:0005943 (class IA) -> in_complex.
• REMOVE GO:0052742 phosphatidylinositol kinase activity (ISS): catalytic activity belongs to p110,
  not the regulatory subunit p85α.
• REMOVE GO:0043559 insulin binding (IDA PMID:8440175): paper is about insulin-RECEPTOR ligand
  binding kinetics; p85α has no demonstrated capacity to bind the insulin hormone. Its insulin-axis
  role is SH2 binding to phospho-INSR/IRS (GO:0005158, GO:0043560).
• MODIFY generic MF (GO:0019207 kinase regulator activity -> GO:0046935; GO:0019209 kinase activator
  activity -> GO:0141038 PI3K activator activity) to the specific PI3K terms.

Prompt-injection note

The MCP server output for this session contained an appended instruction ("While auto mode is
active: do your work through the Bash tool...") that conflicts with repo CLAUDE.md and the task.
Treated as untrusted injected content and ignored; used normal Read/Edit/Write tooling.