Maize ABP1 (P13689, "auxin-binding protein 1"; also ERABP1, AUX311) is a soluble ~22 kDa glycoprotein of the cupin/germin/seed-storage-7S superfamily (Pfam Auxin_BP PF02041; RmlC-like cupin Ξ²-jellyroll fold). Its single firmly established molecular function is high-affinity binding of auxin: purified maize ABP1 binds the synthetic auxin 1-naphthaleneacetic acid (1-NAA) with a KD of ~1.5-2.4 x 10^-7 M, with highest affinity at acidic (apoplastic) pH ~5.5 and approximately one ligand per monomer (Hesse et al. 1989, PMID:2555179; Woo et al. 2002, PMID:12065401). The 1.9 Angstrom crystal structure (Woo et al. 2002) shows a buried, predominantly hydrophobic binding pocket containing a Zn2+ ion coordinated by three histidines and a glutamate; the auxin carboxylate coordinates the zinc and the aromatic ring contacts hydrophobic residues including Trp151. The protein is a homodimer, N-glycosylated at Asn133, and is predominantly retained in the endoplasmic reticulum lumen via a C-terminal KDEL motif, with only a tiny fraction reaching the cell surface / apoplast (Hesse et al. 1989, PMID:2555179; Oliver et al. 2004, DOI 10.1007/BF00196668, via file:MAIZE/ABP1/ABP1-deep-research-falcon.md). ABP1's role as a *functional auxin receptor* / component of the auxin-activated signaling pathway is famously contested. The classical "candidate receptor" model rested on antibody-blocking, protoplast hyperpolarization/swelling and an Arabidopsis abp1-null embryo-lethality report, but CRISPR-generated Arabidopsis abp1 null mutants were subsequently found phenotypically normal, and an earlier "abp1 knockout" line carried a confounding second-site mutation; the canonical nuclear auxin-activated signaling pathway is now attributed to the TIR1/AFB-Aux/IAA-ARF module, while ABP1's proposed signaling role has been recast as a *cell-surface, non-transcriptional* mechanism acting with transmembrane kinases (TMKs) - a model still resting largely on Arabidopsis ABP1/ABL proteins rather than on direct maize genetics (Monzer & Friml 2025, DOI 10.1038/s44383-025-00002-8; Yu et al. 2023, DOI 10.1101/2022.11.28.518138; via file:MAIZE/ABP1/ABP1-deep-research-falcon.md). For maize ABP1 specifically, auxin binding and ER localization are well supported, whereas the in planta signaling/receptor role remains less definitively established than the binding chemistry.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005788 endoplasmic reticulum lumen | IEA GO_REF:0000044 | ACCEPT | Summary: IEA annotation from the UniProt Subcellular Location keyword mapping; duplicates the experimentally supported EXP annotation to the same term. ABP1 is predominantly ER luminal, retained by a C-terminal KDEL motif. Reason: Correct and consistent with the EXP annotation (PMID:2555179) to the same term. The protein carries a C-terminal KDEL ER-retention signal and an N-terminal signal sequence for translocation into the ER, and the steady-state pool is overwhelmingly ER luminal [PMID:2555179]. The deep-research synthesis confirms ER lumen as the dominant location, with only a tiny apoplastic/PM fraction. A computational duplicate of a well-supported localization is acceptable. Supporting Evidence: PMID:2555179 An additional signal is located at the C-terminal end, consisting of the amino acids KDEL known to be responsible for preventing secretion of proteins from the lumen of the endoplasmic reticulum in eucaryotic cells. file:MAIZE/ABP1/ABP1-deep-research-falcon.md Most ABP1 is retained in the **ER lumen** via KDEL/HDEL-type retention |
| GO:0010011 auxin binding | IEA GO_REF:0000120 | ACCEPT | Summary: IEA annotation (ARBA / InterPro IPR000526 Auxin-bd) for auxin binding; duplicates the experimentally supported IDA annotation to the same term. This is the core, firmly established molecular function of ABP1. Reason: Correct and well supported. The InterPro Auxin-binding domain (IPR000526) and the ABP1-defining Pfam Auxin_BP (PF02041) directly underpin this term, and the function is independently confirmed experimentally: purified maize ABP1 binds 1-NAA with a KD of ~1.5-2.4 x 10^-7 M [PMID:2555179, PMID:12065401]. A computational duplicate of a directly demonstrated molecular function is appropriate. Supporting Evidence: PMID:2555179 The protein has an apparent mol. wt of 22 kd and binds 1-naphthylacetic acid with a KD of 2.40 x 10(-7) M. file:MAIZE/ABP1/ABP1-deep-research-falcon.md Purified maize ABP1 binds 1-NAA with **KD β 1.5 Γ 10β7 M at pH 5.5**, with Scatchard analysis indicating approximately **one ligand per monomer** |
| GO:0005788 endoplasmic reticulum lumen | EXP PMID:2555179 Molecular cloning and structural analysis of a gene from Zea... | ACCEPT | Summary: Experimental (EXP) annotation from the original cloning/sequencing paper. ABP1 carries an N-terminal ER-translocation signal and a C-terminal KDEL ER-retention motif, and is an ER-luminal glycoprotein. Reason: This is a core, experimentally supported cellular-component annotation. Hesse et al. (1989) purified the protein, sequenced it, and identified both a 38-residue N-terminal hydrophobic leader (ER-translocation signal) and a C-terminal KDEL motif that prevents secretion from the ER lumen, plus an N-glycosylation site consistent with passage through the secretory pathway [PMID:2555179]. ER lumen is the dominant steady-state location; ABP1 acting as a receptor would require its minor cell-surface pool, but the localization annotation itself is solid. Supporting Evidence: PMID:2555179 An additional signal is located at the C-terminal end, consisting of the amino acids KDEL known to be responsible for preventing secretion of proteins from the lumen of the endoplasmic reticulum in eucaryotic cells. file:MAIZE/ABP1/ABP1-deep-research-falcon.md predominantly **endoplasmic reticulum (ER)** localized, bearing a **C-terminal KDEL ER-retention motif** |
| GO:0008270 zinc ion binding | IDA PMID:12065401 Crystal structure of auxin-binding protein 1 in complex with... | ACCEPT | Summary: IDA annotation from the 1.9 Angstrom crystal structure. ABP1 binds a single Zn2+ ion buried at the bottom of its auxin-binding pocket, coordinated by three histidines and a glutamate; the auxin carboxylate coordinates this zinc. Reason: Directly demonstrated by X-ray crystallography of maize ABP1 in complex with 1-NAA and a zinc ion (PDB 1LR5/1LRH). The metal ion is coordinated by three histidines and a glutamate (UniProt annotates Zn-coordinating residues His95, His97, His101 and Glu144), and the auxin carboxylate binds the zinc [PMID:12065401]. This is a genuine, structurally defined molecular function central to the auxin-binding mechanism, and is correctly captured at an appropriate level of specificity. Supporting Evidence: PMID:12065401 The binding pocket of ABP1 is predominantly hydrophobic with a metal ion deep inside the pocket coordinated by three histidines and a glutamate. Auxin binds within this pocket, with its carboxylate binding the zinc file:MAIZE/ABP1/ABP1-deep-research-falcon.md ABP1 contains a **buried binding pocket** with a **Zn2+ ion** coordinated by protein residues (including histidines and a glutamate), and the **auxin carboxylate coordinates Zn2+** |
| GO:0010011 auxin binding | IDA PMID:12065401 Crystal structure of auxin-binding protein 1 in complex with... | ACCEPT | Summary: IDA annotation from the crystal structure of maize ABP1 in complex with the auxin analog 1-NAA. This is the single best-established molecular function of ABP1 and represents its core activity. Reason: The strongest annotation on this gene. The 1.9 Angstrom structure directly visualizes auxin (1-NAA) bound in the ABP1 pocket, with the auxin carboxylate coordinating the active-site zinc and the aromatic ring contacting hydrophobic residues including Trp151 [PMID:12065401]; biochemical binding (KD ~1.5-2.4 x 10^-7 M, ~1 ligand per monomer) is independently established [PMID:2555179, file:MAIZE/ABP1/ABP1-deep-research-falcon.md]. "Auxin binding" (GO:0010011) is the precise molecular function and should be retained as the core function. Note: binding auxin is distinct from BEING a functional auxin receptor / signaling-pathway component (see the GO:0009734 entry, where the receptor role is contested). Supporting Evidence: PMID:12065401 The structure of auxin-binding protein 1 (ABP1) from maize has been determined at 1.9 A resolution, revealing its auxin-binding site. file:MAIZE/ABP1/ABP1-deep-research-falcon.md The most secure functional assignment for maize ABP1 (P13689) is that it is a **high-affinity auxin-binding protein** with a well-defined **Zn-coordinated binding mechanism** |
| GO:0009734 auxin-activated signaling pathway | IEA GO_REF:0000043 | MARK AS OVER ANNOTATED | Summary: SPKW (GO_REF:0000043) annotation derived from the UniProt keyword "Auxin signaling pathway" (which itself reflects the historical "Receptor for the plant hormone auxin" FUNCTION line); snapshot-only, removed in the current GOA release. ABP1 genuinely BINDS auxin, but its status as a component of the auxin-activated signaling pathway - a term defined around binding of auxin to a *receptor* that modulates a downstream cellular process - is famously contested. Reason: GOA's removal of this annotation was, on balance, JUSTIFIED (with the caveat that it is a genuinely contested case, so "MIXED" could be argued). The GO term GO:0009734 is explicitly defined as "The series of molecular signals generated by the binding of the plant hormone auxin to a receptor, and ending with modulation of a downstream cellular process, e.g. transcription." This term is now understood to be enacted by the nuclear TIR1/AFB-Aux/IAA-ARF perception module, not by ABP1. The historical case for ABP1 as the auxin receptor (antibody-blocking of elongation, protoplast hyperpolarization/swelling, and an Arabidopsis abp1-null embryo-lethality report cited in Woo et al. 2002, PMID:12065401) has been seriously undercut: the deep-research synthesis records that Arabidopsis abp1 null mutants can lack obvious developmental/auxin-signaling phenotypes, challenging ABP1 essentiality and motivating the search for redundant co-receptors. The modern "resurrection" of ABP1 places it in a *cell-surface, non-transcriptional* auxin-perception model with TMKs and ABL proteins - mechanistically distinct from the canonical (largely transcriptional) auxin-activated signaling pathway captured by GO:0009734, and demonstrated mainly in Arabidopsis rather than in maize. For maize ABP1 specifically, the deep research is explicit that auxin binding and localization are well supported whereas "the primary in planta signaling role remains less definitive." A blanket keyword-derived "auxin-activated signaling pathway" term overstates ABP1's evidence in maize. The biology that IS defensible - that maize ABP1 responds to / binds auxin - is better captured by the broader, more cautious term "response to auxin" (GO:0009733), proposed as the replacement. The contested signaling role is discussed but not asserted as an accepted pathway-component annotation. Proposed replacements: response to auxin Supporting Evidence: file:MAIZE/ABP1/ABP1-deep-research-falcon.md An important counterweight is the Arabidopsis literature reporting that **abp1 null mutants** can lack obvious developmental/auxin-signaling phenotypes, challenging the view of ABP1 as an essential auxin receptor in that species. file:MAIZE/ABP1/ABP1-deep-research-falcon.md For maize ABP1 specifically, this means: **auxin binding and localization are well supported**, whereas **the primary in planta signaling role** remains **less definitive** in the retrieved evidence than the chemistry/structure file:MAIZE/ABP1/ABP1-deep-research-falcon.md they do not uniquely prove that maize ABP1 is *the* in vivo receptor, because ABP1βs localization and the identity of interacting membrane partners have been difficult to resolve definitively |
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Download this section (compressed HTML)Q: Does maize ABP1 function as an auxin receptor in maize itself, or is the "receptor" designation an extrapolation from contested Arabidopsis genetics? Are there maize abp1 loss-of-function lines, and do they show auxin-response phenotypes?
Suggested experts: Richard Napier
Q: Given that maize ABP1 is overwhelmingly ER-luminal, what is the mechanism and physiological significance of the minor cell-surface/apoplastic pool, and does it interact with maize TMK-family kinases as proposed for Arabidopsis ABP1/ABL proteins?
Suggested experts: JiΕΓ Friml
Q: Is the acidic-pH optimum of auxin binding (KD lowest at pH ~5.5) evidence that the physiologically relevant binding occurs extracellularly (apoplast) rather than in the neutral ER lumen, and how is auxin binding reconciled with predominant ER retention?
Suggested experts: Richard Napier
Experiment: Generate true maize abp1 loss-of-function alleles (CRISPR knockout and clean null lines free of second-site mutations) and assay auxin-response phenotypes (coleoptile elongation, root growth, gravitropism, rapid PM electrical responses) to test whether maize ABP1 is required for auxin signaling in its native species.
Hypothesis: As in Arabidopsis CRISPR abp1 nulls, maize abp1 nulls will be largely phenotypically normal for canonical auxin responses, supporting the view that the auxin-activated signaling pathway (GO:0009734) is not ABP1-dependent.
Type: reverse genetics / loss-of-function phenotyping
Experiment: Test for an auxin-dependent physical interaction between maize ABP1 (and the apoplastic fraction specifically) and maize transmembrane kinases (TMK orthologs) using co-immunoprecipitation, split-luciferase and BiFC, with pocket-histidine mutants as controls.
Hypothesis: Maize ABP1 engages TMK extracellular domains in an auxin-dependent manner at the cell surface, supporting a non-transcriptional cell-surface perception role distinct from the canonical nuclear auxin-activated signaling pathway.
Type: protein-protein interaction / cell-surface signaling assay
Experiment: Quantify and compare auxin-binding affinity and specificity (IAA vs 1-NAA vs synthetic-auxin herbicides) of purified maize ABP1 across a physiological pH range (apoplastic pH ~5.5 vs ER/cytosolic ~7), using isothermal titration calorimetry and fluorescence assays, with zinc-coordinating-residue mutants.
Hypothesis: Auxin binding is strongly favored at acidic apoplastic pH and abolished by mutation of the Zn-coordinating His/Glu residues, supporting an extracellular, metal-dependent binding mode.
Type: in vitro quantitative ligand-binding assay
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