id: Q9LKW9
gene_symbol: SOS1
product_type: PROTEIN
status: DRAFT
taxon:
  id: NCBITaxon:3702
  label: Arabidopsis thaliana
description: >-
  SOS1 (also called NHX7) is a plasma-membrane sodium/proton (Na+/H+) antiporter
  of the monovalent cation:proton antiporter-1 (CPA1) family. The protein has
  roughly twelve N-terminal transmembrane helices that form the ion-translocation
  domain and a large (~700 residue) cytoplasmic C-terminal tail that mediates
  regulation. SOS1 catalyzes electroneutral exchange of intracellular Na+ (or Li+)
  for extracellular H+, driven by the inwardly directed proton-motive force
  established by plasma-membrane H+-ATPases, thereby extruding toxic Na+ from the
  cytosol to the apoplast. SOS1 is expressed most strongly in root tip epidermal
  cells and in parenchyma cells at the xylem/symplast boundary of roots, stems and
  leaves, where it controls the Na+ load of the xylem sap and mediates
  long-distance Na+ transport between root and shoot. It is the effector of the
  Salt Overly Sensitive (SOS) signaling pathway, in which the calcium sensor
  CBL4/SOS3 activates the protein kinase CIPK24/SOS2, which phosphorylates the
  C-terminal autoinhibitory domain of SOS1 to relieve autoinhibition and activate
  transport. By exporting Na+ and helping maintain cytosolic ion and pH
  homeostasis, SOS1 is a principal determinant of plant salt tolerance; its
  activity also modulates apoplastic pH and reactive-oxygen-species signaling
  under stress.
existing_annotations:
- term:
    id: GO:0051453
    label: regulation of intracellular pH
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: >-
      As an electroneutral Na+/H+ antiporter, SOS1 couples cytosolic Na+ efflux to
      H+ influx and so influences cytosolic and apoplastic pH. This is a real
      consequence of antiport activity but is secondary to the core Na+
      detoxification role.
    action: KEEP_AS_NON_CORE
    reason: >-
      The phylogenetically inferred (IBA) regulation of intracellular pH is
      consistent with the mechanism of a CPA1-family Na+/H+ exchanger, and SOS1 has
      been reported to affect H+ transport and apoplastic alkalinization even
      without salt stress. However, pH regulation is a downstream effect of the
      antiport reaction rather than the central biological role, which is Na+
      detoxification and salt tolerance.
    supported_by:
    - reference_id: PMID:17996020
      supporting_text: >-
        The plasma membrane Na + /H + antiporter SOS1 has also been shown to affect
        H + transport even in the absence of salt stress
- term:
    id: GO:0098719
    label: sodium ion import across plasma membrane
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: >-
      SOS1 transports Na+ across the plasma membrane. Its core physiological mode is
      Na+ efflux (export) from the cytosol for detoxification; the import direction
      captured by this term is a secondary, condition-dependent mode (xylem
      retrieval of Na+ under severe salt stress), reflecting the intrinsic
      reversibility of the antiporter.
    action: KEEP_AS_NON_CORE
    reason: >-
      The IBA term captures plasma-membrane Na+ transport, but specifically in the
      import direction. The directionality (import vs export) of an electroneutral
      antiporter depends on the prevailing Na+ and H+ gradients; experimental and
      modeling work shows SOS1 can both load Na+ into and retrieve Na+ from the
      xylem. However, the defining, core physiological function of SOS1 is Na+
      EXPORT/efflux from the cytosol (and proposed_new_terms request an explicit
      export term as the missing core function). Na+ import is a secondary,
      condition-dependent xylem-retrieval mode under severe stress, so this import
      annotation is retained as non-core rather than as the core sodium-transport
      function.
    supported_by:
    - reference_id: PMID:11884687
      supporting_text: >-
        SOS1 functions in retrieving Na + from the xylem stream under severe salt
        stress, whereas under mild salt stress it may function in loading Na + into
        the xylem.
    - reference_id: file:ARATH/SOS1/SOS1-deep-research-falcon.md
      supporting_text: >-
        consistent with roles in both direct Na+ efflux at the root surface and
        regulation of long-distance Na+ transport via xylem loading/unloading
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  qualifier: located_in
  review:
    summary: >-
      SOS1 is a multi-pass plasma-membrane protein, confirmed experimentally by
      SOS1-GFP confocal imaging in transgenic Arabidopsis.
    action: ACCEPT
    reason: >-
      The UniProt subcellular-location mapping agrees with direct experimental
      evidence and with the protein's transmembrane topology. This is the core
      cellular location of SOS1.
    supported_by:
    - reference_id: PMID:11884687
      supporting_text: >-
        Confocal imaging of a SOS1-green fluorescent protein fusion protein in
        transgenic Arabidopsis plants indicated that SOS1 is localized in the plasma
        membrane.
- term:
    id: GO:0006812
    label: monoatomic cation transport
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: involved_in
  review:
    summary: >-
      SOS1 transports the monovalent cations Na+ (and Li+) across the plasma
      membrane. This is a correct but generic parent of the specific
      sodium-transport annotations.
    action: ACCEPT
    reason: >-
      InterPro-based generic cation-transport term is accurate; it is broader than
      the specific sodium transport terms but not misleading, so it is retained.
    supported_by:
    - reference_id: PMID:10823923
      supporting_text: >-
        The transmembrane region of SOS1 has significant sequence similarities to
        plasma membrane Na + /H + antiporters from bacteria and fungi.
- term:
    id: GO:0015297
    label: antiporter activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      SOS1 is a secondary active antiporter that exchanges Na+ for H+. This generic
      antiporter term is correct but less informative than the specific
      sodium:proton antiporter activity.
    action: ACCEPT
    reason: >-
      The InterPro-derived term is a correct parent of GO:0015385 (sodium:proton
      antiporter activity); the more specific term is also annotated, so this
      broader term is acceptable as-is.
    supported_by:
    - reference_id: file:ARATH/SOS1/SOS1-uniprot.txt
      supporting_text: >-
        Acts in electroneutral exchange of protons for cations such as Na(+) or
        Li(+) across plasma membrane.
- term:
    id: GO:0015385
    label: sodium:proton antiporter activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      This is the core molecular function of SOS1: electroneutral exchange of
      intracellular Na+ for extracellular H+ across the plasma membrane, verified by
      a curated catalytic activity (RHEA:29419) and by complementation of a yeast
      Na+-transport mutant.
    action: ACCEPT
    reason: >-
      Strong convergent evidence (InterPro/IBA, UniProt catalytic activity, and
      functional yeast complementation showing Na+-specific transport) supports
      GO:0015385 as the central molecular function. It is the most informative MF
      term available and represents the core activity.
    supported_by:
    - reference_id: PMID:11884687
      supporting_text: >-
        SOS1 activity was specific for Na + because the plant protein was
        inefficient for K + efflux or uptake in vivo
    - reference_id: file:ARATH/SOS1/SOS1-uniprot.txt
      supporting_text: >-
        Acts in electroneutral exchange of protons for cations such as Na(+) or
        Li(+) across plasma membrane.
    - reference_id: file:ARATH/SOS1/SOS1-deep-research-falcon.md
      supporting_text: >-
        SOS1/NHX7 mediates active Na+ efflux from the cytosol in exchange for H+,
        lowering cytosolic Na+ during salt stress.
- term:
    id: GO:0016020
    label: membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: located_in
  review:
    summary: >-
      SOS1 is an integral membrane protein with ~12 transmembrane helices. The term
      is a correct but generic parent of the specific plasma-membrane location.
    action: ACCEPT
    reason: >-
      InterPro-derived generic membrane localization is accurate; the specific
      plasma-membrane term is also annotated, so this broader term is retained
      as-is.
    supported_by:
    - reference_id: PMID:10823923
      supporting_text: >-
        predicted to encode a 127-kDa protein with 12 transmembrane domains in the
        N-terminal part and a long hydrophilic cytoplasmic tail in the C-terminal
        part
- term:
    id: GO:0055085
    label: transmembrane transport
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: involved_in
  review:
    summary: >-
      SOS1 mediates transmembrane transport of Na+ and H+ across the plasma
      membrane. Correct but generic parent of the specific sodium/proton transport
      terms.
    action: ACCEPT
    reason: >-
      InterPro-based generic transmembrane-transport term is accurate and consistent
      with the antiporter mechanism; retained as a non-misleading parent.
    supported_by:
    - reference_id: file:ARATH/SOS1/SOS1-uniprot.txt
      supporting_text: >-
        Acts in electroneutral exchange of protons for cations such as Na(+) or
        Li(+) across plasma membrane.
- term:
    id: GO:1902600
    label: proton transmembrane transport
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: involved_in
  review:
    summary: >-
      As a Na+/H+ antiporter, SOS1 translocates H+ across the plasma membrane
      coupled to Na+ movement. Proton transport is an integral part of the antiport
      cycle.
    action: ACCEPT
    reason: >-
      The curated catalytic activity explicitly involves H+ counter-transport
      (RHEA:29419), so proton transmembrane transport is a correct InterPro-derived
      annotation, consistent with the electroneutral antiport reaction.
    supported_by:
    - reference_id: file:ARATH/SOS1/SOS1-uniprot.txt
      supporting_text: >-
        Acts in electroneutral exchange of protons for cations such as Na(+) or
        Li(+) across plasma membrane.
    - reference_id: file:ARATH/SOS1/SOS1-deep-research-falcon.md
      supporting_text: >-
        Transport is driven by the proton electrochemical gradient generated by
        the plasma-membrane H+-ATPase.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:17023541
  qualifier: enables
  review:
    summary: >-
      This IPI annotation records the interaction between the SOS1 cytoplasmic tail
      and RCD1 (a regulator of oxidative-stress responses). The interaction is
      biologically meaningful but the generic 'protein binding' term conveys no
      specific molecular function.
    action: REMOVE
    reason: >-
      Per curation guidelines, the uninformative GO:0005515 term should not be
      retained as it does not describe SOS1 molecular function. The underlying RCD1
      interaction is captured biologically by the oxidative-stress process
      annotations.
    supported_by:
    - reference_id: PMID:17023541
      supporting_text: >-
        SOS1 interacts through its predicted cytoplasmic tail with RCD1, a regulator
        of oxidative-stress responses.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:21262798
  qualifier: enables
  review:
    summary: >-
      This IPI annotation records interaction with CIPK24/SOS2, the kinase that
      phosphorylates and activates SOS1. Biologically important for regulation, but
      the generic 'protein binding' term is uninformative about molecular function.
    action: REMOVE
    reason: >-
      The generic protein-binding term should not be kept. The functionally relevant
      SOS2-SOS3 regulatory interaction is described by the protein's regulation and
      phosphorylation, and a kinase-binding/regulatory relationship is better
      captured elsewhere than by bare GO:0005515.
    supported_by:
    - reference_id: PMID:21262798
      supporting_text: >-
        SOS1 is relieved from auto-inhibition upon phosphorylation of the
        auto-inhibitory domain by SOS2-SOS3.
- term:
    id: GO:0015386
    label: potassium:proton antiporter activity
  evidence_type: EXP
  original_reference_id: PMID:12239394
  qualifier: enables
  review:
    summary: >-
      A K+/H+ catalytic activity is curated for SOS1, but direct functional testing
      in yeast showed SOS1 is specific for Na+ and inefficient at K+ transport in
      vivo. The sos1 potassium-acquisition phenotype is an indirect, genetic
      consequence of disrupted Na+/H+ exchange and linked Na+/K+ exchange at the
      xylem/symplast boundary, not direct K+ transport by SOS1.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      PMID:12239394 (Wu et al. 1996) is a genetic-mapping study reporting that sos1
      mutants are defective in high-affinity K+ uptake; it does not directly
      demonstrate SOS1-catalyzed K+/H+ antiport. Direct transport assays
      (PMID:11884687) show SOS1 is Na+-specific and inefficient for K+, and the K+
      phenotype is best explained by coupled antiport activities at the
      xylem/symplast boundary. The K+/H+ antiporter MF therefore over-annotates the
      protein's intrinsic activity.
    supported_by:
    - reference_id: PMID:11884687
      supporting_text: >-
        SOS1 activity was specific for Na + because the plant protein was
        inefficient for K + efflux or uptake in vivo
    - reference_id: PMID:12239394
      supporting_text: sos1 mutants are defective in high-affinity potassium uptake
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: HDA
  original_reference_id: PMID:14506206
  qualifier: located_in
  review:
    summary: >-
      A large-scale plasma-membrane phosphoproteomics study identified SOS1 among
      plasma-membrane phosphoproteins, consistent with its plasma-membrane
      localization and its regulation by phosphorylation.
    action: ACCEPT
    reason: >-
      High-throughput direct-assay evidence places SOS1 in the plasma membrane, in
      full agreement with experimental GFP localization and the protein's topology.
      Core location.
    supported_by:
    - reference_id: PMID:14506206
      supporting_text: identification of plasma membrane phosphoproteins of Arabidopsis
- term:
    id: GO:0009941
    label: chloroplast envelope
  evidence_type: HDA
  original_reference_id: PMID:12938931
  qualifier: located_in
  review:
    summary: >-
      A single large-scale proteomic survey of a 'mixed' chloroplast envelope
      preparation listed SOS1 among 392 nonredundant proteins. This is inconsistent
      with the well-established plasma-membrane localization and biology of SOS1 and
      most likely reflects plasma-membrane contamination of the envelope fraction.
    action: REMOVE
    reason: >-
      The chloroplast-envelope assignment comes from an untargeted proteomics dataset
      prone to contamination, with no functional rationale; SOS1 has no role in
      chloroplast biology and is robustly localized to the plasma membrane by GFP
      imaging and topology. This annotation is judged incorrect.
    supported_by:
    - reference_id: PMID:12938931
      supporting_text: >-
        "mixed" envelopes were subsequently isolated using sucrose step gradients
    - reference_id: PMID:11884687
      supporting_text: >-
        Confocal imaging of a SOS1-green fluorescent protein fusion protein in
        transgenic Arabidopsis plants indicated that SOS1 is localized in the plasma
        membrane.
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: ISM
  original_reference_id: GO_REF:0000122
  qualifier: located_in
  review:
    summary: >-
      Sequence-based (AtSubP) prediction of plasma-membrane localization, consistent
      with experimental evidence and topology.
    action: ACCEPT
    reason: >-
      The predicted plasma-membrane location agrees with multiple lines of
      experimental evidence (GFP imaging, phosphoproteomics) and the multi-pass
      topology. Core location.
    supported_by:
    - reference_id: PMID:11884687
      supporting_text: >-
        Confocal imaging of a SOS1-green fluorescent protein fusion protein in
        transgenic Arabidopsis plants indicated that SOS1 is localized in the plasma
        membrane.
- term:
    id: GO:0071805
    label: potassium ion transmembrane transport
  evidence_type: IMP
  original_reference_id: PMID:12239394
  qualifier: acts_upstream_of_or_within
  review:
    summary: >-
      sos1 mutants are defective in high-affinity K+ uptake and become K+-deficient
      under NaCl, but SOS1 does not directly transport K+; this is an indirect
      consequence of disrupted Na+/H+ exchange and the tight coupling of Na+ and K+
      fluxes at the xylem/symplast boundary.
    action: KEEP_AS_NON_CORE
    reason: >-
      The acts_upstream_of_or_within qualifier appropriately reflects an indirect
      genetic effect on K+ transport rather than direct catalysis. Direct assays show
      SOS1 is Na+-specific (PMID:11884687), so this should not be treated as a core
      function but is retained as a genuine, non-core physiological phenotype.
    supported_by:
    - reference_id: PMID:12239394
      supporting_text: sos1 mutants are defective in high-affinity potassium uptake
    - reference_id: PMID:11884687
      supporting_text: >-
        the proposed coupling between Na + and K + exchange at the xylem/symplast
        boundary also could provide an explanation for why sos1 mutant plants are not
        capable of growing on low K + culture medium
- term:
    id: GO:2000377
    label: regulation of reactive oxygen species metabolic process
  evidence_type: IMP
  original_reference_id: PMID:17023541
  qualifier: acts_upstream_of_or_within
  review:
    summary: >-
      Through interaction of its cytoplasmic tail with RCD1 and effects on apoplastic
      pH/NADPH-oxidase activity, SOS1 influences ROS-related gene expression and ROS
      accumulation under stress. This is a secondary regulatory role distinct from
      ion transport.
    action: KEEP_AS_NON_CORE
    reason: >-
      Experimental genetic evidence supports a role for SOS1 in modulating ROS
      metabolism (sos1 mutants over-accumulate ROS under salt; SOS1 and RCD1 jointly
      control ROS-scavenging genes), but this is downstream of and secondary to the
      core Na+/H+ antiport function.
    supported_by:
    - reference_id: PMID:17023541
      supporting_text: >-
        Several genes related to oxidative-stress tolerance were found to be
        regulated by both RCD1 and SOS1.
- term:
    id: GO:0000302
    label: response to reactive oxygen species
  evidence_type: IEP
  original_reference_id: PMID:17996020
  qualifier: acts_upstream_of_or_within
  review:
    summary: >-
      SOS1 mRNA is stabilized by ROS (H2O2) and SOS1 activity feeds back into
      apoplastic ROS production via pH changes and NADPH oxidase, linking SOS1 to the
      ROS response. A secondary, signaling-level role.
    action: KEEP_AS_NON_CORE
    reason: >-
      Expression/phenotype evidence supports involvement of SOS1 in ROS responses,
      but this is an indirect signaling role downstream of its transport activity
      rather than a core function.
    supported_by:
    - reference_id: PMID:17996020
      supporting_text: >-
        Stress-induced SOS1 mRNA stability is mediated by reactive oxygen species
        (ROS).
- term:
    id: GO:0006979
    label: response to oxidative stress
  evidence_type: IMP
  original_reference_id: PMID:17996020
  qualifier: acts_upstream_of_or_within
  review:
    summary: >-
      sos1 mutants show altered oxidative-stress sensitivity (more tolerant to
      paraquat/methyl viologen), indicating SOS1 participates in oxidative-stress
      responses, with a proposed negative role mediated by apoplastic pH and NADPH
      oxidase.
    action: KEEP_AS_NON_CORE
    reason: >-
      Mutant-phenotype evidence supports involvement in oxidative-stress responses,
      but as an indirect, secondary effect of SOS1 antiport activity on apoplastic pH
      and ROS production, not a core function.
    supported_by:
    - reference_id: PMID:17996020
      supporting_text: >-
        mutations in the SOS1 gene render sos1 mutants more tolerant to paraquat, a
        non-selective herbicide causing oxidative stress, indicating that SOS1 plays
        negative roles in tolerance of oxidative stress
- term:
    id: GO:0009651
    label: response to salt stress
  evidence_type: IEP
  original_reference_id: PMID:17996020
  qualifier: acts_upstream_of_or_within
  review:
    summary: >-
      SOS1 is a salt-tolerance determinant; its mRNA is stabilized under salt stress
      and the protein extrudes toxic Na+, making salt-stress response a core
      biological process for SOS1.
    action: ACCEPT
    reason: >-
      Multiple independent lines of evidence (expression induction by Na+, severe
      salt hypersensitivity of sos1 mutants, Na+ efflux activity) establish response
      to salt stress as a core process of SOS1.
    supported_by:
    - reference_id: PMID:17996020
      supporting_text: >-
        Salt Overly Sensitive 1 (SOS1), a plasma membrane Na+/H+ antiporter in
        Arabidopsis, is a salt tolerance determinant crucial for the maintenance of
        ion homeostasis in saline stress conditions.
- term:
    id: GO:0042542
    label: response to hydrogen peroxide
  evidence_type: IEP
  original_reference_id: PMID:17996020
  qualifier: acts_upstream_of_or_within
  review:
    summary: >-
      H2O2 treatment increases SOS1 mRNA stability, and SOS1 activity affects
      apoplastic H2O2 production, linking SOS1 to the hydrogen-peroxide response as a
      secondary signaling role.
    action: KEEP_AS_NON_CORE
    reason: >-
      Expression evidence supports a connection between SOS1 and H2O2 responses, but
      this is downstream signaling/ROS biology, secondary to the core ion-transport
      function.
    supported_by:
    - reference_id: PMID:17996020
      supporting_text: H2O2 treatment increases the stability of SOS1 mRNA.
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: IDA
  original_reference_id: PMID:10823923
  qualifier: located_in
  review:
    summary: >-
      Direct experimental evidence (TAIR IDA) supports plasma-membrane localization,
      consistent with the predicted multi-pass topology and similarity to
      bacterial/fungal plasma-membrane Na+/H+ antiporters.
    action: ACCEPT
    reason: >-
      Direct-assay plasma-membrane localization agrees with all other location
      evidence; core cellular location of SOS1.
    supported_by:
    - reference_id: PMID:10823923
      supporting_text: >-
        Phylogenetic analysis showed that SOS1 is more closely related to plasma
        membrane Na + /H + antiporters from microorganisms than to the vacuolar
        antiporters
- term:
    id: GO:0042542
    label: response to hydrogen peroxide
  evidence_type: IMP
  original_reference_id: PMID:17023541
  qualifier: acts_upstream_of_or_within
  review:
    summary: >-
      sos1 mutants show altered sensitivity to H2O2/apoplastic ROS, supporting
      involvement of SOS1 in the hydrogen-peroxide response via its role in
      oxidative-stress tolerance with RCD1.
    action: KEEP_AS_NON_CORE
    reason: >-
      Mutant-phenotype evidence supports involvement in H2O2 responses, but as a
      secondary, indirect role downstream of SOS1 antiport activity and the SOS1-RCD1
      module, not a core function.
    supported_by:
    - reference_id: PMID:17023541
      supporting_text: >-
        Like rcd1 mutants, sos1 mutant plants show an altered sensitivity to
        oxidative stresses.
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: IDA
  original_reference_id: PMID:11884687
  qualifier: located_in
  review:
    summary: >-
      Direct experimental localization by SOS1-GFP confocal imaging in transgenic
      Arabidopsis demonstrates plasma-membrane localization.
    action: ACCEPT
    reason: >-
      This is the strongest direct evidence for the core plasma-membrane location of
      SOS1.
    supported_by:
    - reference_id: PMID:11884687
      supporting_text: >-
        Confocal imaging of a SOS1-green fluorescent protein fusion protein in
        transgenic Arabidopsis plants indicated that SOS1 is localized in the plasma
        membrane.
    - reference_id: file:ARATH/SOS1/SOS1-deep-research-falcon.md
      supporting_text: >-
        SOS1:GFP becomes increasingly recruited to the plasma membrane over
- term:
    id: GO:0006814
    label: sodium ion transport
  evidence_type: IMP
  original_reference_id: PMID:11874577
  qualifier: acts_upstream_of_or_within
  review:
    summary: >-
      SOS1 mediates Na+ transport across the plasma membrane and is required to limit
      cytosolic Na+ accumulation; sodium ion transport is a core process for this
      antiporter.
    action: ACCEPT
    reason: >-
      Although PMID:11874577 uses the sos1 mutant only as a salt-sensitive genetic
      background (salt-induced programmed cell death), sodium ion transport is firmly
      established as a core SOS1 function by direct transport assays (PMID:11884687)
      and UniProt. The annotation is correct; supporting evidence is drawn from the
      directly relevant transport literature.
    supported_by:
    - reference_id: PMID:11884687
      supporting_text: >-
        SOS1 is a plasma membrane Na + transporter essential for controlling
        long-distance Na + movement in plants.
    - reference_id: file:ARATH/SOS1/SOS1-deep-research-falcon.md
      supporting_text: >-
        SOS1/NHX7 mediates active Na+ efflux from the cytosol in exchange for H+,
        lowering cytosolic Na+ during salt stress.
    - reference_id: PMID:11874577
      supporting_text: >-
        salt-sensitive mutants of yeast (cnb1Delta) and Arabidopsis (sos1) exhibit
        substantially more profound PCD symptoms, indicating that salt-induced PCD is
        mediated by ion disequilibrium
- term:
    id: GO:0009651
    label: response to salt stress
  evidence_type: IMP
  original_reference_id: PMID:12239394
  qualifier: acts_upstream_of_or_within
  review:
    summary: >-
      sos1 mutants are >20-fold more sensitive to NaCl, establishing SOS1 as
      essential for the response to salt stress.
    action: ACCEPT
    reason: >-
      Loss-of-function mutant phenotype (extreme NaCl hypersensitivity) directly
      supports the core role of SOS1 in salt-stress response.
    supported_by:
    - reference_id: PMID:12239394
      supporting_text: their growth was >20 times more sensitive to inhibition by NaCl
    - reference_id: file:ARATH/SOS1/SOS1-deep-research-falcon.md
      supporting_text: >-
        Loss-of-function sos mutants (sos1, sos2, sos3) exhibit strong salt
        sensitivity, supporting the conclusion that the SOS pathway is essential
        for salt tolerance through ion homeostasis maintenance
- term:
    id: GO:0009651
    label: response to salt stress
  evidence_type: IMP
  original_reference_id: PMID:10823923
  qualifier: acts_upstream_of_or_within
  review:
    summary: >-
      The SOS1 locus is essential for salt tolerance; sos1 mutations render plants
      extremely sensitive to high Na+ and SOS1 expression is induced by NaCl. Core
      biological process.
    action: ACCEPT
    reason: >-
      Cloning and mutant analysis establish SOS1 as required for the salt-stress
      response, a core function. Duplicate IMP support across independent studies is
      appropriate.
    supported_by:
    - reference_id: PMID:10823923
      supporting_text: >-
        sos1 mutations render plants more sensitive to growth inhibition by high Na +
        and low K + environments
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO
    terms
  findings: []
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000044
  title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location
    vocabulary mapping, accompanied by conservative changes to GO terms applied by
    UniProt
  findings: []
- id: GO_REF:0000122
  title: AtSubP analysis
  findings: []
- id: PMID:10823923
  title: The Arabidopsis thaliana salt tolerance gene SOS1 encodes a putative Na+/H+
    antiporter.
  findings:
  - reference_section_type: ABSTRACT
    supporting_text: >-
      SOS1 is cloned and predicted to encode a 127-kDa protein with 12 transmembrane
      domains in the N-terminal part and a long hydrophilic cytoplasmic tail in the
      C-terminal part.
  - reference_section_type: DISCUSSION
    supporting_text: >-
      SOS1 is expected to function in exporting Na + from the cytosol to the
      extracellular space to prevent rapid accumulation of Na + in the cytoplasm.
- id: PMID:11874577
  title: Salt causes ion disequilibrium-induced programmed cell death in yeast and
    plants.
  findings:
  - reference_section_type: ABSTRACT
    supporting_text: >-
      salt-sensitive mutants of yeast (cnb1Delta) and Arabidopsis (sos1) exhibit
      substantially more profound PCD symptoms, indicating that salt-induced PCD is
      mediated by ion disequilibrium
- id: PMID:11884687
  title: The putative plasma membrane Na(+)/H(+) antiporter SOS1 controls long-distance
    Na(+) transport in plants.
  findings:
  - reference_section_type: ABSTRACT
    supporting_text: >-
      These results suggest that SOS1 is critical for controlling long-distance Na +
      transport from root to shoot.
  - reference_section_type: DISCUSSION
    supporting_text: >-
      SOS1 activity was specific for Na + because the plant protein was inefficient
      for K + efflux or uptake in vivo
- id: PMID:12239394
  title: SOS1, a Genetic Locus Essential for Salt Tolerance and Potassium Acquisition.
  findings:
  - reference_section_type: ABSTRACT
    supporting_text: The sos1 mutants are specifically hypersensitive to Na+ and Li+.
  - reference_section_type: ABSTRACT
    supporting_text: >-
      Uptake experiments using 86Rb showed that sos1 mutants are defective in
      high-affinity potassium uptake.
- id: PMID:12938931
  title: Proteomic study of the Arabidopsis thaliana chloroplastic envelope membrane
    utilizing alternatives to traditional two-dimensional electrophoresis.
  findings:
  - reference_section_type: ABSTRACT
    supporting_text: This proteomic study enabled us to identify 392 nonredundant proteins.
- id: PMID:14506206
  title: Large-scale analysis of in vivo phosphorylated membrane proteins by immobilized
    metal ion affinity chromatography and mass spectrometry.
  findings:
  - reference_section_type: ABSTRACT
    supporting_text: identification of plasma membrane phosphoproteins of Arabidopsis
- id: PMID:17023541
  title: The plasma membrane Na+/H+ antiporter SOS1 interacts with RCD1 and functions
    in oxidative stress tolerance in Arabidopsis.
  findings:
  - reference_section_type: ABSTRACT
    supporting_text: >-
      SOS1 interacts through its predicted cytoplasmic tail with RCD1, a regulator of
      oxidative-stress responses.
- id: PMID:17996020
  title: Reactive oxygen species mediate Na+-induced SOS1 mRNA stability in Arabidopsis.
  findings:
  - reference_section_type: ABSTRACT
    supporting_text: >-
      Salt Overly Sensitive 1 (SOS1), a plasma membrane Na+/H+ antiporter in
      Arabidopsis, is a salt tolerance determinant crucial for the maintenance of ion
      homeostasis in saline stress conditions.
- id: PMID:21262798
  title: Activation of the plasma membrane Na/H antiporter Salt-Overly-Sensitive 1
    (SOS1) by phosphorylation of an auto-inhibitory C-terminal domain.
  findings:
  - reference_section_type: ABSTRACT
    supporting_text: >-
      SOS1 is maintained in a resting state by a C-terminal auto-inhibitory domain
      that is the target of SOS2-SOS3.
- id: file:ARATH/SOS1/SOS1-uniprot.txt
  title: UniProt entry NHX7_ARATH (Q9LKW9)
  findings:
  - reference_section_type: OTHER
    supporting_text: >-
      Required for cytoplasmic Na(+) and Li(+) detoxification by secreting them from
      the cytoplasm to the extracellular space. Regulates Na(+) content of the xylem
      sap.
  - reference_section_type: OTHER
    supporting_text: >-
      Acts in electroneutral exchange of protons for cations such as Na(+) or
      Li(+) across plasma membrane.
- id: file:ARATH/SOS1/SOS1-deep-research-falcon.md
  title: Falcon (Edison Scientific) deep research report for SOS1
  findings:
  - reference_section_type: OTHER
    supporting_text: >-
      SOS1/NHX7 mediates active Na+ efflux from the cytosol in exchange for H+,
      lowering cytosolic Na+ during salt stress.
core_functions:
- description: >-
    Electroneutral plasma-membrane Na+/H+ antiporter that exchanges intracellular
    Na+ (or Li+) for extracellular H+, driven by the proton-motive force, to extrude
    toxic Na+ from the cytosol to the apoplast.
  molecular_function:
    id: GO:0015385
    label: sodium:proton antiporter activity
  directly_involved_in:
  - id: GO:0006814
    label: sodium ion transport
  - id: GO:0009651
    label: response to salt stress
  locations:
  - id: GO:0005886
    label: plasma membrane
  supported_by:
  - reference_id: PMID:11884687
    supporting_text: >-
      SOS1 activity was specific for Na + because the plant protein was inefficient
      for K + efflux or uptake in vivo
  - reference_id: file:ARATH/SOS1/SOS1-uniprot.txt
    supporting_text: >-
      Required for cytoplasmic Na(+) and Li(+) detoxification by secreting them from
      the cytoplasm to the extracellular space. Regulates Na(+) content of the xylem
      sap.
  - reference_id: file:ARATH/SOS1/SOS1-deep-research-falcon.md
    supporting_text: >-
      SOS1/NHX7 mediates active Na+ efflux from the cytosol in exchange for H+,
      lowering cytosolic Na+ during salt stress.
- description: >-
    Controls long-distance Na+ transport between root and shoot by operating at the
    xylem/symplast boundary, loading Na+ into or retrieving Na+ from the xylem sap
    depending on salinity, thereby regulating the Na+ load of the vascular system.
  molecular_function:
    id: GO:0015385
    label: sodium:proton antiporter activity
  directly_involved_in:
  - id: GO:0098719
    label: sodium ion import across plasma membrane
  locations:
  - id: GO:0005886
    label: plasma membrane
  supported_by:
  - reference_id: PMID:11884687
    supporting_text: >-
      SOS1 is critical for controlling long-distance Na + transport from root to
      shoot.
  - reference_id: file:ARATH/SOS1/SOS1-deep-research-falcon.md
    supporting_text: >-
      consistent with roles in both direct Na+ efflux at the root surface and
      regulation of long-distance Na+ transport via xylem loading/unloading
proposed_new_terms:
- proposed_name: sodium ion export across plasma membrane
  proposed_definition: >-
    The directed movement of sodium ions from the cytosol to the extracellular space
    across the plasma membrane, as carried out by a plasma-membrane sodium/proton
    antiporter.
  proposed_parent:
    id: GO:0006814
    label: sodium ion transport
  justification: >-
    The dominant physiological mode of SOS1 is Na+ efflux (export) from the cytosol
    to the apoplast for detoxification. The existing GOA term GO:0098719 captures Na+
    import across the plasma membrane, but no equally specific export term is
    annotated; an export-directed sodium transport term would more precisely
    represent the core detoxification function. Not added as a NEW annotation because
    a verified GO ID for plasma-membrane sodium export could not be confirmed in this
    session.
  supported_by:
  - reference_id: file:ARATH/SOS1/SOS1-uniprot.txt
    supporting_text: >-
      Required for cytoplasmic Na(+) and Li(+) detoxification by secreting them from
      the cytoplasm to the extracellular space.
suggested_questions:
- question: Is the apparent K+/H+ antiporter activity (GO:0015386) an intrinsic
    biochemical property of SOS1 or entirely an indirect consequence of coupled
    Na+/K+ fluxes at the xylem/symplast boundary?
- question: Does SOS1 directly bind 14-3-3 proteins at the SOS2-phosphorylated
    RIDSPSK motif, as for the related plasma-membrane H+-ATPase, and does this
    modulate activation?