NCU01540 encodes an 844-residue integral membrane protein with an N-terminal SPX domain (residues 1-260) followed by a CitMHS/SLC13-type permease region carrying twelve predicted transmembrane helices. PANTHER places it in subfamily PTHR10283:SF92 together with budding-yeast Pho91 and fission-yeast Plt1, and it is the only member of the PTHR10283 family in the N. crassa reference proteome. By descent from these fungal phosphate transporters it is inferred to be a phosphate transmembrane transporter whose activity is controlled through the SPX domain; the inositol-pyrophosphate-binding lysine of Pho91 (K237) is conserved as K241. In budding yeast the SF92 member Pho91 sits in the vacuolar membrane and releases phosphate from the vacuole to the cytosol when inositol pyrophosphate levels fall, which buffers cytosolic phosphate and limits vacuolar polyphosphate accumulation. The protein has not been studied experimentally in Neurospora, so its resident membrane (vacuolar versus plasma membrane), transport direction and coupling ion remain unknown.
Summary: PAINT placed phosphate transmembrane transporter activity at PTN007430421, seeded by the three experimentally characterized S. cerevisiae Pho87/Pho90/Pho91 transporters. NCU01540 is classified in PTHR10283:SF92 with Pho91 and S. pombe Plt1 and retains the full SPX plus CitMHS permease architecture, so it lies inside the clade that inherited this activity. No evidence of loss of function in the Neurospora branch exists. This is the core activity.
Reason: The target is inside the phosphate-transporter clade on which the IBD was placed (SF92 subfamily membership, conserved domain architecture, higher identity to SF92 members than to the SF110 Pho87/Pho90 pair). Yeast Pho91 is a functionally characterized vacuolar phosphate transporter. Direction and coupling are not specified by this term.
Supporting Evidence:
file:NEUCR/NCU01540/NCU01540-uniprot.txt
DR PANTHER; PTHR10283:SF92; LOW-AFFINITY PHOSPHATE TRANSPORTER PHO91; 1.
Collectively, our results are consistent with the notion that loss of InsPP binding activates Pho91 to export Pi from vacuoles to the cytosol, enhancing repression of the PHO pathway.
the low-affinity transporter Pho91 limits poly P accumulation in a strain lacking PHO85. This phenotype was not caused by a regulatory effect on the PHO pathway, but can be attributed to the unexpected localization of Pho91 in the vacuolar membrane.
Summary: This IBA comes from the family-root node PTN000031793, seeded by plasma-membrane SLC13 carriers of animals and by yeast Pho87 (S000000633) and Pho90 (S000003734), which are in subfamily SF110. The SF92 seed Pho91 (S000005296) is not among the seeds for this location, and its characterized location is the vacuolar membrane. NCU01540 is in SF92 with Pho91. Neurospora has no SF110 paralog, so NCU01540 could be the cell's only member of this family and might be located at the plasma membrane. Equally, it could resemble Pho91 and sit in the vacuolar membrane. No localization data exist for the Neurospora protein or for its SF92 relatives in filamentous fungi, so the evidence cannot decide between the two.
Reason: The ancestral-node placement conflicts with the location of the characterized member of the target's own subfamily, and PAINT curators did not propagate a vacuolar location from Pho91. A localization experiment is needed rather than a choice between inferences.
Propagation Review
Root cause:UNRESOLVED
Failure modes:COMPARTMENT OR COMPLEX MISMATCH
Sources checked:
PANTHER:PTN000031793UNRESOLVED
Family-root node whose plasma-membrane seeds are animal SLC13 carriers and the SF110 Pho87/Pho90 pair; the SF92 member Pho91 is vacuolar. Whether the SF92 lineage containing NCU01540 retained the plasma-membrane state is not resolved.
the low-affinity transporter Pho91 limits poly P accumulation in a strain lacking PHO85. This phenotype was not caused by a regulatory effect on the PHO pathway, but can be attributed to the unexpected localization of Pho91 in the vacuolar membrane.
Summary: The node PTN000815044 is seeded by all three yeast Pho87/Pho90/Pho91 transporters, whose mutants change cellular polyphosphate levels. In the characterized SF92 member Pho91, the effect comes from moving phosphate across the vacuolar membrane. Polyphosphate is made by VTC and broken down by vacuolar polyphosphatases. The transporter does none of the synthesis or hydrolysis; it controls the size of the phosphate pool these enzymes use. The term is kept because the curator-placed node covers the target. It is non-core because it describes a consequence of the phosphate-transport activity, not a separate activity.
Reason: The transporter affects polyphosphate by controlling phosphate supply, which is an indirect contribution. The IBD placement is sound for the clade; the target-level role is peripheral to its core transport function.
the low-affinity transporter Pho91 limits poly P accumulation in a strain lacking PHO85. This phenotype was not caused by a regulatory effect on the PHO pathway, but can be attributed to the unexpected localization of Pho91 in the vacuolar membrane.
Summary: Phosphate transport is the conserved process of the eukaryotic Pho87/Pho90/Pho91 clade (PTN000815044), and NCU01540 is inside this clade. Because the protein is a multi-pass membrane transporter, the process it carries out is the movement of phosphate across a membrane. The more specific term phosphate ion transmembrane transport (GO:0035435), which is_a both phosphate ion transport and transmembrane transport, states this directly.
Reason: The claim is correct but less specific than the evidence supports. GO:0035435 is a descendant of GO:0006817 and matches the transmembrane transporter activity accepted above.
Propagation Review
Root cause:TERM SCOPING PROBLEM
Failure modes:GRANULARITY MISMATCH
Sources checked:
PANTHER:PTN000815044SUPPORTS TRANSFER
The eukaryotic Pho87/Pho90/Pho91 node correctly carries phosphate transport and the target is inside it; the transmembrane refinement GO:0035435 follows from the multi-pass transporter architecture shared by the clade.
Collectively, our results are consistent with the notion that loss of InsPP binding activates Pho91 to export Pi from vacuoles to the cytosol, enhancing repression of the PHO pathway.
file:NEUCR/NCU01540/NCU01540-uniprot.txt
DR PANTHER; PTHR10283:SF92; LOW-AFFINITY PHOSPHATE TRANSPORTER PHO91; 1.
Summary: Twelve predicted transmembrane helices in the CitMHS permease region make this an integral membrane protein. Which membrane it resides in (vacuolar or plasma membrane) is not established (see the plasma membrane row), so the general term is as precise as the evidence allows.
Reason: Correct, and no more specific membrane is supported for this protein.
Summary: The ARBA rule gives only the generic transporter activity. The SF92 subfamily assignment and the phosphate-transport IBA give the substrate, so the specific descendant phosphate transmembrane transporter activity (GO:0005315), already carried by this gene, is the better term.
Reason: Correct but too general; the specific descendant GO:0005315 is supported by phylogenetic placement and is already annotated to the gene.
Collectively, our results are consistent with the notion that loss of InsPP binding activates Pho91 to export Pi from vacuoles to the cytosol, enhancing repression of the PHO pathway.
Summary: Transmembrane transport is correctly inherited from the family-root node PTN000031793, but the substrate is phosphate. The descendant phosphate ion transmembrane transport (GO:0035435) is supported by the SF92 placement and by the eukaryotic phosphate-transport node PTN000815044.
Reason: Correct but too general; GO:0035435 is_a GO:0055085.
Propagation Review
Root cause:TERM SCOPING PROBLEM
Failure modes:GRANULARITY MISMATCH
Sources checked:
PANTHER:PTN000031793SUPPORTS TRANSFER
Transmembrane transport is correctly inherited from this family-root node; the phosphate substrate is supported by the in-clade node PTN000815044 and the SF92 placement.
Collectively, our results are consistent with the notion that loss of InsPP binding activates Pho91 to export Pi from vacuoles to the cytosol, enhancing repression of the PHO pathway.
Summary: The InterPro2GO mapping from the CitMHS domain gives the generic process. As for the IBA row with the same term, the supported specific process is phosphate ion transmembrane transport (GO:0035435).
Reason: Correct but too general; GO:0035435 is_a GO:0055085.
DR PANTHER; PTHR10283:SF92; LOW-AFFINITY PHOSPHATE TRANSPORTER PHO91; 1.
Core Functions
SPX-regulated phosphate transmembrane transporter of the Pho91 subfamily, inferred to move inorganic phosphate across an internal or plasma membrane in a manner gated by inositol pyrophosphate binding to its SPX domain.
Collectively, our results are consistent with the notion that loss of InsPP binding activates Pho91 to export Pi from vacuoles to the cytosol, enhancing repression of the PHO pathway.
Q: Does NCU01540, the only PTHR10283 member in N. crassa, reside in the vacuolar membrane like its SF92 relative Pho91, or at the plasma membrane like the SF110 Pho87/Pho90 pair that Neurospora lacks?
Q: Does NCU01540 export phosphate from the vacuole under phosphate limitation, and is this gated by inositol pyrophosphate binding at SPX K241?
Suggested Experiments
Experiment: Tag NCU01540 with GFP at the endogenous locus and image it under high and low phosphate, co-staining the vacuole, to determine its resident membrane.
Experiment: Delete NCU01540 and introduce a K241A SPX allele; measure cytosolic and vacuolar phosphate and polyphosphate during phosphate starvation to test for SPX-regulated vacuolar phosphate export.
External Prediction Reviews
These computational predictions are reviewed separately from the GOA annotation set used for this review. The assessments below are from this project and do not constitute official GO annotations or endorsement by GO/UniProt. They are not included in the existing annotation review above.
The single GO prediction, transmembrane transporter activity, is correct for this SPX-domain CitMHS-family permease but less specific than the phosphate transmembrane transporter activity the gene carries by phylogenetic transfer from the PTHR10283:SF92 Pho91 subfamily. No experimental study of NCU01540 was found in PubMed.
Review rationale: The SPX-permease architecture and PANTHER Pho91 subfamily (PTHR10283:SF92) placement support phosphate transport by transfer from the characterized fungal Pho87/Pho90/Pho91 transporters; yeast Pho91 is a vacuolar phosphate transporter that exports phosphate to the cytosol (PMID:17804816, PMID:41324358). The gene already carries the same generic term from an ARBA rule, which the gene review refines to the more specific, IBA-supported phosphate transmembrane transporter activity (GO:0005315, a descendant of the predicted term). The prediction is therefore correct but less precise than an existing supported annotation. The unresolved plasma versus vacuolar location does not undermine generic transmembrane transport.
PMID:17804816: "the low-affinity transporter Pho91 limits poly P accumulation in a strain lacking PHO85. This phenotype was not caused by a regulatory effect on the PHO pathway, but can be attributed to the unexpected localization of Pho91 in the vacuolar membrane."
PMID:41324358: "Collectively, our results are consistent with the notion that loss of InsPP binding activates Pho91 to export Pi from vacuoles to the cytosol, enhancing repression of the PHO pathway."