Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt
Combined Automated Annotation using Multiple IEA Methods
Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword mapping
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SwissProt keyword-derived (SPKW) annotation present in the Sept 2025 goa_uniprot_gcrp snapshot but removed from the current GOA release after GOA retired the keyword2GO pipeline for cellular organisms.
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For ABP1, the keyword "Auxin signaling pathway" mapped to GO:0009734 (auxin-activated signaling pathway) - a term defined around auxin binding to a receptor. ABP1's receptor/signaling-pathway role is contested, so this keyword-derived term is an over-annotation; "response to auxin" (GO:0009733) is the more defensible term.
Molecular cloning and structural analysis of a gene from Zea mays (L.) coding for a putative receptor for the plant hormone auxin.
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The major maize auxin-binding protein was purified to homogeneity; it is ~22 kDa and binds 1-naphthylacetic acid (1-NAA) with a KD of 2.40 x 10^-7 M.
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ABP1 has an N-terminal 38-residue hydrophobic ER-translocation signal and a C-terminal KDEL ER-retention motif, and is N-glycosylated - establishing ER-lumen localization.
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The paper titles the protein only a "putative receptor" for auxin, reflecting that receptor function was inferred rather than demonstrated.
Crystal structure of auxin-binding protein 1 in complex with auxin.
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The 1.9 Angstrom crystal structure of maize ABP1 in complex with 1-NAA reveals the auxin-binding site and places ABP1 in the germin/seed-storage-7S (cupin) superfamily.
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A single Zn2+ ion sits deep in a predominantly hydrophobic pocket, coordinated by three histidines and a glutamate; the auxin carboxylate coordinates the zinc and the aromatic ring contacts hydrophobic residues including Trp151. ABP1 is a homodimer with a single Cys2-Cys155 disulfide.
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No conformational rearrangement was observed on auxin binding in the crystal; the authors propose only a "possible mechanism of signal transduction," and cite an Arabidopsis abp1-null embryo-lethality report (Chen et al. 2001) that later work has challenged.
Deep-research report (falcon / Edison Scientific Literature) - functional annotation of maize ABP1 (P13689).
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The most secure assignment for maize ABP1 is that it is a high-affinity, Zn-dependent auxin-binding protein (cupin fold), binding 1-NAA with KD ~1.5 x 10^-7 M at pH 5.5, ~1 ligand per monomer, with highest affinity at acidic apoplastic pH.
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ABP1 is predominantly ER luminal via KDEL retention; only a tiny fraction reaches the plasma-membrane/apoplast (Oliver et al. 2004), and total ABP1 pools are long-lived and not strongly auxin-regulated.
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ABP1's in planta signaling/receptor role is contested - Arabidopsis abp1 null mutants can lack obvious phenotypes; the canonical receptor role is now attributed elsewhere, and the modern model recasts ABP1 in a cell-surface, non-transcriptional pathway with TMKs/ABL proteins (Monzer & Friml 2025; Yu et al. 2023), demonstrated mainly in Arabidopsis. For maize specifically, binding and localization are well supported but the primary signaling role remains less definitive than the chemistry/structure.