PIN2 (PIN-FORMED 2; also EIR1, AGR1, WAV6) is a root-expressed member of the long PIN auxin efflux carrier family: a ten-transmembrane secondary transporter that exports indole-3-acetic acid (IAA) across the plasma membrane. PIN2 is expressed in the lateral root cap, epidermis and cortex of the root meristem and transition/elongation zones. In epidermal and lateral root cap cells it localizes to the apical (shootward) plasma membrane, and in young cortex cells to the basal (rootward) side, so it carries auxin from the root tip shootward through the outer tissues. This shootward stream, together with the AUX1 influx carrier, redistributes auxin to the lower flank of gravistimulated roots, and pin2 mutants are therefore root-agravitropic, wavy and ethylene-insensitive in the root. PIN2 polarity is set by phosphorylation of its central hydrophilic loop by PINOID/WAG AGC kinases, opposed by PP2A/PP6 phosphatases, and stabilized by NPH3-family MAB4/MEL proteins that limit lateral diffusion. Its abundance is controlled by endocytic recycling, ubiquitination and retromer-dependent vacuolar degradation, which is modulated asymmetrically during gravitropism.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000323 lytic vacuole | IDA PMID:19004783 Differential degradation of PIN2 auxin efflux carrier by ret... | KEEP AS NON CORE | Summary: PIN2 is trafficked to the lytic vacuole for degradation, a route that tunes PIN2 abundance (e.g. differentially during gravistimulation). Reason: Vacuole is a degradation destination, not the site of PIN2 transport activity; retained as non-core. Supporting Evidence: PMID:19004783 In vivo visualization of PIN2 vacuolar targeting revealed its differential degradation in response to environmental signals, such as gravity. |
| GO:0001666 response to hypoxia | IEP PMID:28698356 Root Bending Is Antagonistically Affected by Hypoxia and ERF... | KEEP AS NON CORE | Summary: PIN2 transport contributes to hypoxia-dependent changes in root growth direction. Reason: Full PMID:28698356 goes beyond expression: pin2-1 roots fail to further increase their growth angle under hypoxia or hypoxia plus auxin, whereas pin1, pin3 and pin4 retain responses. PIN2-mediated transport therefore contributes to the contextual root-growth response, without being an oxygen sensor. Supporting Evidence: file:publications/url_https___pmc.ncbi.nlm.nih.gov_articles_PMC5580755__report_xml.md neither hypoxia nor hypoxia in the presence of Ξ±-NAA further increased the angle at which pin2-1 roots grew |
| GO:0005515 protein binding | IPI PMID:22715043 A PP6-type phosphatase holoenzyme directly regulates PIN pho... | REMOVE | Summary: Physical interaction of PIN2 with FyPP3, catalytic subunit of the PP6 phosphatase that dephosphorylates PINs. The interaction is real and relevant to PIN2 polarity/phosphorylation control, but GO:0005515 protein binding conveys no molecular function of PIN2. Reason: Removal does not mean the interaction is false. PIN2 is the substrate/cargo in this interaction and has no informative binding MF to substitute; the relationship is better captured on the regulator side (kinase/phosphatase activity with PIN2 as input) and in the auxin polar transport module. Supporting Evidence: PMID:22715043 We demonstrate that FyPP1 and FyPP3 interact with a subset of PIN proteins and regulate PIN protein phosphorylation and targeting in vivo. |
| GO:0005515 protein binding | IPI PMID:22715043 A PP6-type phosphatase holoenzyme directly regulates PIN pho... | REMOVE | Summary: Physical interaction of PIN2 with FyPP1, catalytic subunit of the PP6 phosphatase that dephosphorylates PINs. The interaction is real and relevant to PIN2 polarity/phosphorylation control, but GO:0005515 protein binding conveys no molecular function of PIN2. Reason: Removal does not mean the interaction is false. PIN2 is the substrate/cargo in this interaction and has no informative binding MF to substitute; the relationship is better captured on the regulator side (kinase/phosphatase activity with PIN2 as input) and in the auxin polar transport module. Supporting Evidence: PMID:22715043 We demonstrate that FyPP1 and FyPP3 interact with a subset of PIN proteins and regulate PIN protein phosphorylation and targeting in vivo. |
| GO:0005515 protein binding | IPI PMID:33705718 AGC kinases and MAB4/MEL proteins maintain PIN polarity by l... | REMOVE | Summary: Physical interaction of PIN2 with PINOID (PID), the AGC kinase that phosphorylates the PIN2 hydrophilic loop. The interaction is real and relevant to PIN2 polarity/phosphorylation control, but GO:0005515 protein binding conveys no molecular function of PIN2. Reason: Removal does not mean the interaction is false. PIN2 is the substrate/cargo in this interaction and has no informative binding MF to substitute; the relationship is better captured on the regulator side (kinase/phosphatase activity with PIN2 as input) and in the auxin polar transport module. Supporting Evidence: PMID:33705718 GST-PID/WAGs and HIS-PIN2HL |
| GO:0005515 protein binding | IPI PMID:33705718 AGC kinases and MAB4/MEL proteins maintain PIN polarity by l... | REMOVE | Summary: Physical interaction of PIN2 with MEL1/NPY5, an NPH3-family polarity adaptor. The interaction is real and relevant to PIN2 polarity/phosphorylation control, but GO:0005515 protein binding conveys no molecular function of PIN2. Reason: Removal does not mean the interaction is false. PIN2 is the substrate/cargo in this interaction and has no informative binding MF to substitute; the relationship is better captured on the regulator side (kinase/phosphatase activity with PIN2 as input) and in the auxin polar transport module. Supporting Evidence: PMID:33705718 confirming that MEL1 and PIN2 interact in vivo and in planta |
| GO:0005515 protein binding | IPI PMID:33705718 AGC kinases and MAB4/MEL proteins maintain PIN polarity by l... | REMOVE | Summary: Physical interaction of PIN2 with MAB4/NPY1, an NPH3-family polarity adaptor. The interaction is real and relevant to PIN2 polarity/phosphorylation control, but GO:0005515 protein binding conveys no molecular function of PIN2. Reason: Removal does not mean the interaction is false. PIN2 is the substrate/cargo in this interaction and has no informative binding MF to substitute; the relationship is better captured on the regulator side (kinase/phosphatase activity with PIN2 as input) and in the auxin polar transport module. Supporting Evidence: PMID:33705718 These data show that MAB4 physically interacts with the PIN2HL |
| GO:0005515 protein binding | IPI PMID:35274300 SAV4 is required for ethylene-induced root hair growth throu... | REMOVE | Summary: Physical interaction of PIN2 with SAV4, which stabilizes PIN2 at the shootward plasma membrane. The interaction is real and relevant to PIN2 polarity/phosphorylation control, but GO:0005515 protein binding conveys no molecular function of PIN2. Reason: Removal does not mean the interaction is false. PIN2 is the substrate/cargo in this interaction and has no informative binding MF to substitute; the relationship is better captured on the regulator side (kinase/phosphatase activity with PIN2 as input) and in the auxin polar transport module. Supporting Evidence: PMID:35274300 SAV4 directly interacts with the PIN2 hydrophilic region (PIN2HL) and regulates PIN2 abundance on the PM. |
| GO:0005783 endoplasmic reticulum | IBA GO_REF:0000033 | UNDECIDED | Summary: The ancestral ER localization inference is unresolved for PIN2. Reason: The actual PTHR31752 PAINT table places ER and plasma-membrane assertions at PTN001589272, with ER experimental seeds PIN5/6/8. That broad ancestral placement warrants scrutiny for PIN2, but a long-PIN plasma-membrane role does not demonstrate loss of every ER pool. No target-specific ER exclusion or tested negative was recovered. Leave the location unresolved rather than infer a false assertion from compartment preference. Propagation Review Root cause: UNRESOLVED Sources checked: PANTHER:PTN001589272 UNRESOLVED The cached PTHR31752 PAINT table confirms the ER IBD and PIN5/6/8 experimental seeds at this node. Predominant PIN2 plasma-membrane localization does not establish exclusion of every ER pool; target inheritance or loss remains unresolved. Supporting Evidence: PMID:20439545 those PINs were localized in the plasma membrane, where they likely export auxin to the apoplast |
| GO:0005886 plasma membrane | EXP PMID:20439545 Differential auxin-transporting activities of PIN-FORMED pro... | ACCEPT | Summary: PIN2 expressed in root hair cells localized to the plasma membrane and reduced root-hair growth by exporting auxin. Reason: Experimental support for the core location. Supporting Evidence: PMID:20439545 those PINs were localized in the plasma membrane, where they likely export auxin to the apoplast |
| GO:0005886 plasma membrane | IBA GO_REF:0000033 | ACCEPT | Summary: PIN2 is an integral plasma-membrane auxin efflux carrier; PAINT inference consistent with extensive experimental data on PIN2 itself. Reason: Core functional location. Supporting Evidence: PMID:20439545 those PINs were localized in the plasma membrane, where they likely export auxin to the apoplast |
| GO:0005886 plasma membrane | IDA PMID:19004783 Differential degradation of PIN2 auxin efflux carrier by ret... | ACCEPT | Summary: PIN2 recycles to the polar plasma membrane domain. Reason: Core location. Supporting Evidence: PMID:19004783 PIN2 vacuolar trafficking and its polar recycling to the plasma membrane are interdependent |
| GO:0005886 plasma membrane | IDA PMID:19223973 Glucose and auxin signaling interaction in controlling Arabi... | ACCEPT | Summary: PIN2::GFP at the plasma membrane of root tip cells. Reason: Core location. Supporting Evidence: PMID:19223973 Increasing concentrations of glucose promotes more PIN2::GFP accumulation in the plasma membrane. |
| GO:0005886 plasma membrane | IDA PMID:33705718 AGC kinases and MAB4/MEL proteins maintain PIN polarity by l... | ACCEPT | Summary: PIN2-GFP imaged at the plasma membrane of root epidermal cells. Reason: Core location. Supporting Evidence: PMID:33705718 we thus replaced the apical PIN2 with a predominantly basally localized PIN1-GFP2 |
| GO:0005886 plasma membrane | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location mapping to plasma membrane. Reason: Consistent with experimental localization; core location. Supporting Evidence: PMID:20439545 those PINs were localized in the plasma membrane, where they likely export auxin to the apoplast |
| GO:0009507 chloroplast | ISM GO_REF:0000122 | MARK AS OVER ANNOTATED | Summary: The AtSubP chloroplast assignment overextends sequence prediction beyond the supported PIN2 membrane localization. Reason: AtSubP is an ISM compartment prediction, not a curator-read localization experiment. Native PIN2 imaging supports polar root-cell plasma-membrane localization (PMID:9843496), and the experimentally curated cell-membrane assignment is consistent with its multipass carrier architecture. The sequence-based UniProt topology places the first transmembrane helix at residues 8β28, providing an N-terminal membrane anchor rather than evidence for a conventional cleavable chloroplast-import presequence. Together these favor the established membrane-trafficking interpretation over this specific chloroplast prediction. The prediction is over-annotated, not experimentally disproved: the missing TRANSIT feature alone is not decisive, chloroplast-envelope proteins need not all use a cleavable transit peptide, and neither root expression nor plasma-membrane imaging excludes every plastid pool. Supporting Evidence: PMID:9843496 The AtPIN2 protein was localized in membranes of root cortical and epidermal cells in the meristematic and elongation zones revealing a polar localization. file:ARATH/PIN2/PIN2-uniprot.txt FT TRANSMEM 8..28 |
| GO:0009723 response to ethylene | IMP PMID:9679062 EIR1, a root-specific protein involved in auxin transport, i... | KEEP AS NON CORE | Summary: eir1 (pin2) roots are ethylene-insensitive because ethylene inhibition of root growth requires shootward auxin transport through PIN2. Reason: Downstream physiological consequence of the transport role, not an ethylene-signaling function; non-core. Supporting Evidence: PMID:9679062 are agravitropic and have a reduced sensitivity to ethylene |
| GO:0009733 response to auxin | IMP PMID:9843496 AtPIN2 defines a locus of Arabidopsis for root gravitropism ... | KEEP AS NON CORE | Summary: pin2 roots show altered auxin sensitivity. Reason: Indirect consequence of altered auxin distribution; non-core. Supporting Evidence: PMID:9843496 Roots of the Atpin2::En701 null-mutant were agravitropic and showed altered auxin sensitivity |
| GO:0009749 response to glucose | IEP PMID:19223973 Glucose and auxin signaling interaction in controlling Arabi... | KEEP AS NON CORE | Summary: PIN2 transcript and PIN2::GFP protein levels increase with glucose. Reason: Expression response; PIN2 is not a glucose-signaling component. Non-core. Supporting Evidence: PMID:19223973 On increasing concentration of glucose, the level of PIN2::GFP was also found to increase |
| GO:0009925 basal plasma membrane | IDA PMID:33705718 AGC kinases and MAB4/MEL proteins maintain PIN polarity by l... | ACCEPT | Summary: PIN2 has a basal cortical plasma-membrane pool. Reason: Basal PIN2 in young cortical cells is a genuine component of the native root auxin-distribution system. Its different orientation from apical epidermal/lateral-root-cap PIN2 is tissue-specific organization of the same core transporter, not a peripheral activity. Supporting Evidence: PMID:33705718 following the rapid BFA-induced basal-to-apical transcytosis of PIN2 in these cells |
| GO:0009925 basal plasma membrane | IDA PMID:9843496 AtPIN2 defines a locus of Arabidopsis for root gravitropism ... | ACCEPT | Summary: PIN2 has a basal cortical plasma-membrane pool. Reason: Basal PIN2 in young cortical cells is a genuine component of the native root auxin-distribution system. Its different orientation from apical epidermal/lateral-root-cap PIN2 is tissue-specific organization of the same core transporter, not a peripheral activity. Supporting Evidence: PMID:9843496 The AtPIN2 protein was localized in membranes of root cortical and epidermal cells in the meristematic and elongation zones revealing a polar localization. |
| GO:0009926 auxin polar transport | IBA GO_REF:0000033 | ACCEPT | Summary: Polar auxin transport inferred from the PIN family node with PIN2 (AT5G57090) itself among the experimental donors. Reason: PIN2 mediates shootward (basipetal) auxin flow in the root epidermis; core process. Supporting Evidence: PMID:9843496 Roots of the Atpin2::En701 null-mutant were agravitropic and showed altered auxin sensitivity PMID:16601150 PINs mediate auxin efflux from mammalian and yeast cells without needing additional plant-specific factors |
| GO:0009926 auxin polar transport | IMP PMID:16601150 PIN proteins perform a rate-limiting function in cellular au... | ACCEPT | Summary: PIN proteins, including PIN2, catalyze rate-limiting cellular auxin efflux, the basis of polar auxin transport. Reason: Core process. (Abstract-only cache; curator saw full text; consistent with all other PIN2 data.) Supporting Evidence: PMID:16601150 PINs mediate auxin efflux from mammalian and yeast cells without needing additional plant-specific factors |
| GO:0009958 positive gravitropism | IMP PMID:9843496 AtPIN2 defines a locus of Arabidopsis for root gravitropism ... | ACCEPT | Summary: pin2 null roots are agravitropic; PIN2-mediated shootward auxin transport in the epidermis redistributes auxin to the lower flank of gravistimulated roots. Reason: Root gravitropism is the defining physiological role of PIN2 (AGR1/EIR1/WAV6 alleles were isolated as agravitropic). Supporting Evidence: PMID:9843496 Roots of the Atpin2::En701 null-mutant were agravitropic and showed altered auxin sensitivity PMID:9844024 we demonstrate that the Arabidopsis thaliana AGR1 gene encodes a component of the auxin efflux carrier that mediates the root gravitropic response |
| GO:0010315 auxin export across the plasma membrane | IEA GO_REF:0000108 | ACCEPT | Summary: Inferred from the auxin efflux transporter MF annotation; PIN2 exports auxin across the plasma membrane. Reason: Correct process for the core activity. Supporting Evidence: PMID:9844024 AGR1 promotes an increased efflux of radiolabeled IAA from the cells |
| GO:0010329 auxin efflux transmembrane transporter activity | IBA GO_REF:0000033 | ACCEPT | Summary: Auxin efflux transporter activity inferred for the PIN clade; PIN2 itself is an experimental donor. Reason: Core molecular function, experimentally shown for PIN2 in yeast and mammalian/plant cells. Supporting Evidence: PMID:9844024 AGR1 promotes an increased efflux of radiolabeled IAA from the cells PMID:16601150 PINs mediate auxin efflux from mammalian and yeast cells without needing additional plant-specific factors |
| GO:0010329 auxin efflux transmembrane transporter activity | IDA PMID:9844024 The arabidopsis thaliana AGRAVITROPIC 1 gene encodes a compo... | ACCEPT | Summary: AGR1/PIN2 expression in yeast increased efflux of radiolabelled IAA. Reason: Direct transport evidence for the core molecular function. Supporting Evidence: PMID:9844024 AGR1 promotes an increased efflux of radiolabeled IAA from the cells |
| GO:0016020 membrane | IEA GO_REF:0000120 | MODIFY | Summary: Generic membrane localization from ARBA/InterPro. Reason: The integral membrane claim is correct; direct imaging establishes plasma membrane as the principal transport compartment. Refine the broad membrane term without denying intracellular trafficking pools. Proposed replacements: plasma membrane Supporting Evidence: PMID:9843496 The AtPIN2 protein was localized in membranes of root cortical and epidermal cells in the meristematic and elongation zones revealing a polar localization. |
| GO:0042802 identical protein binding | ISS GO_REF:0000024 | MODIFY | Summary: Transferred from PIN1 (Q9C6B8), whose cryo-EM structure shows a homodimer. PIN2 likely dimerizes similarly, but "identical protein binding" is a redundant, less informative parent of the homodimerization annotation. Reason: The specific supported self-association is homodimerization. PIN1 has a directly resolved TM1/TM2/TM7 dimer interface; the PIN2 assertion is explicitly a homolog-based inference from PIN1, not a PIN2 structure. Identical-protein binding is refined rather than rejected as redundant. Proposed replacements: protein homodimerization activity Supporting Evidence: PMID:35917925 TM1, TM2 and TM7 constitute the dimer interface |
| GO:0042803 protein homodimerization activity | ISS GO_REF:0000024 | ACCEPT | Summary: Homodimerization transferred from the PIN1 structure; dimer interface residues are conserved among long PINs, so the transfer is plausible but not directly demonstrated for PIN2. Reason: The conserved long-PIN structure and the explicitly named PIN1 donor support the homodimerization inference. Retain it as a biochemical property, while distinguishing the direct PIN1 structure from an unmeasured PIN2 oligomeric equilibrium; structural properties are not non-core merely because they accompany transport. Supporting Evidence: PMID:35917925 TM1, TM2 and TM7 constitute the dimer interface |
| GO:0045178 basal part of cell | IDA PMID:33705718 AGC kinases and MAB4/MEL proteins maintain PIN polarity by l... | MODIFY | Summary: Basal part of cell, derived from the same basal (cortex) PIN2 observation. Reason: The basal signal is at the cortical-cell plasma membrane; refine the cellular region to the directly imaged membrane domain. Proposed replacements: basal plasma membrane Supporting Evidence: PMID:33705718 following the rapid BFA-induced basal-to-apical transcytosis of PIN2 in these cells |
| GO:0055085 transmembrane transport | IEA GO_REF:0000002 | MODIFY | Summary: Generic transmembrane transport from the auxin efflux carrier InterPro family (IPR004776). Reason: Direct cellular IAA efflux establishes the more informative process auxin export across the plasma membrane, already represented in this review. Proposed replacements: auxin export across the plasma membrane Supporting Evidence: PMID:9844024 AGR1 promotes an increased efflux of radiolabeled IAA from the cells |
| GO:0060918 auxin transport | IEA GO_REF:0000117 | ACCEPT | Summary: Auxin transport by ARBA rule. Reason: Correct; PIN2 is an auxin efflux carrier. The more specific auxin polar transport and auxin export terms are also present. Supporting Evidence: PMID:16601150 PINs mediate auxin efflux from mammalian and yeast cells without needing additional plant-specific factors |
| GO:0016324 apical plasma membrane | IDA PMID:33705718 AGC kinases and MAB4/MEL proteins maintain PIN polarity by l... | NEW | Summary: NEW annotation. In root epidermis and lateral root cap, the main PIN2 expression domain, PIN2 is localized to the apical (shootward) plasma membrane; this is the polarity that drives shootward auxin flow for gravitropism. GOA currently carries only basal plasma membrane (cortex) annotations. Reason: Defining polar localization of PIN2 in its principal functional domain, shown directly by PIN2-GFP imaging. Supporting Evidence: PMID:33705718 we thus replaced the apical PIN2 with a predominantly basally localized PIN1-GFP2 PMID:33705718 the PIN2-like apical PM localization of MEL1-GFP and MEL1-mCherry in the epidermis was restored |
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Download this section (compressed HTML)Q: Which features of the PIN2 hydrophilic loop (beyond the three AGC3-kinase phosphosites) encode epidermal apical versus cortical basal polarity in the same tissue?
Suggested experts: Plant cell polarity researchers
Q: How is asymmetric PIN2 degradation on the upper versus lower root flank coordinated with phosphorylation-dependent polarity during the gravitropic response?
Suggested experts: Root gravitropism researchers, Plant membrane trafficking specialists
Experiment: Measure IAA efflux of PIN2 phosphosite variants co-expressed with D6PK or PID in Xenopus oocytes or HEK cells, and compare with polarity of the same variants in planta.
Hypothesis: D6PK- and PID-dependent phosphorylation differentially activate PIN2 transport versus set its polarity.
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