PT4 (MtPT4, Q8GSG4) notes

2026-10-02 initial review

Sources: UniProt Q8GSG4, GOA (27 rows), falcon deep research, cached PMIDs 12368495,
17242358, 19692536 (full text), 20687807, 25841038, 26476189, 26511916. Most cached papers are
abstract-only. Added PMID:17242358 (Javot 2007 PNAS) and PMID:19692536 (Pumplin & Harrison
2009), verified by DOI lookup in NCBI eutils.

Molecular function

• Pht1 subfamily I (AM-specific) phosphate transporter, MFS fold, 12 TM.
• PMID:12368495
• UniProt: H+/Pi symport reaction (RHEA:29939), Km 500 uM, pH optimum 3-4.
• H+ coupling: PMID:19692536
• Deep research: HA1 needed for symbiotic 33P uptake (Krajinski 2014; not cached), but PT4 H+:Pi stoichiometry never measured.
• No existing annotation has an experimental MF. GO:0005315 IEA modified to GO:0015317 phosphate:proton symporter activity.

Location

• PMID:12368495
• PMID:19692536
• PT4 defines the arbuscule branch domain of the PAM (MtBcp1 marks the trunk domain).
• GO:0085042 periarbuscular membrane is not under plasma membrane in GO (ancestors: host cell part,
  other organism part). So both PM rows (IBA, UniProt SubCell IEA) were modified to periarbuscular
  membrane. IBA: the family-level node is fine; target shows lineage-specific divergence in targeting.

Genetics

• PMID:17242358
• PMID:25841038; PMID:25841038
• So "cellular response to nitrogen levels" (IMP) is a genetic-suppression effect that runs through
  AMT2;3, not a PT4 activity. IMP marked over-annotated; the ARBA IEA copy removed.
• Root-tip, AM-independent role: PMID:26476189
• "detection of phosphate ion" claims a transceptor role; the authors only "suggest" P-sensing.
  Marked over-annotated, not removed (the curator had the full text).

Expression (IEP) rows

• response to symbiotic fungus x4 IEP + 1 IEA: accurate expression readouts (PT4 is a classic AM marker;
  RAM1-dependent). Kept as non-core.

Project curation-question notes (PLANT_FUNGAL_INTERACTIONS)

• Necessity vs participation (Q3): PT4 passes participation for arbuscular mycorrhizal association.
  It carries out the plant-side Pi uptake step at the interface, so the process rows are accepted.
  The N-response and Pi-detection rows are the necessity-only cases.
• Symbiosis vs defense (Q4): no defense terms are present, which is correct.