PR1B1 (Pathogenesis-related leaf protein 6 / P6 / P14 / P14a; UniProt P04284) is the canonical tomato PR-1 protein and the founding member of the CAP/SCP superfamily (cysteine-rich secretory protein, antigen 5, pathogenesis-related 1). It is a small (~15 kDa mature) secreted, cysteine-rich protein consisting of an N-terminal signal peptide, a single CAP/SCP domain stabilized by three disulfide bonds, and an extended C-terminus. PR1B1 is one of the most strongly pathogen-inducible proteins of tomato leaves and the textbook molecular marker of salicylic-acid-dependent systemic acquired resistance (SAR). Its transcription is induced locally by pathogen attack (e.g. the fungus Cladosporium fulvum, tobacco mosaic virus during the hypersensitive response) and by salicylic acid and ethylene. Beyond serving as a defense marker, PR1B1 is a genuine defense participant: it is a secreted apoplastic protein with direct (modest, isoform-variable) antimicrobial/antioomycete activity, and the CAP superfamily acts mechanistically through sterol binding and sequestration of sterol away from sterol-auxotrophic pathogens such as Phytophthora. PR1B1 is additionally the proprotein precursor of the C-terminal immune-signalling peptide CAPE1, a damage-associated molecular pattern that activates SA/JA-dependent defense gene expression and pathogen resistance. Its NMR structure (PDB 1CFE) defined the alpha-beta-alpha sandwich CAP fold.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0031640 killing of cells of another organism | IEA GO_REF:0000043 | MARK AS OVER ANNOTATED | Summary: SPKW (GO_REF:0000043) annotation derived from the UniProt "Antimicrobial" and "Fungicide" keywords; present in the Sept 2025 snapshot, removed in the current GOA release. PR1B1/P14a was directly tested by Niderman et al. (1995), who showed that the purified tomato P14 proteins inhibit Phytophthora infestans zoospore germination in vitro and reduce infected leaf surface in vivo. UniProt records "Has antifungal activity". The CAP-superfamily mechanism is sterol binding and sequestration of sterol from sterol-auxotrophic pathogens (Gamir et al. 2017). Reason: The annotation is not baseless: the UniProt Antimicrobial/Fungicide keywords are grounded in genuine experimental work, and P14a (= PR1B1) itself showed direct antioomycete activity. However "killing of cells of another organism" implies a strong, direct cytotoxic cell-killing activity. The demonstrated activity for this specific protein is growth inhibition / antimicrobial activity that is modest and isoform-variable (Niderman et al. found P14a to be the WEAKEST of the tomato P14 proteins tested; P14c was the most efficient). The accepted mechanism (sterol sequestration) is growth-inhibitory rather than overtly lytic/cytotoxic. The keyword2GO pipeline produced a term stronger than the literature warrants for this protein. GOA's removal of the raw SPKW annotation was therefore PARTLY JUSTIFIED - a cytotoxic "killing" term is an over-annotation - but a correct underlying biology (direct antimicrobial CAP/sterol-binding activity, defense response) was lost and should be re-expressed with better terms (sterol binding MF and a defense-response BP), not simply deleted. Proposed replacements: sterol binding defense response to oomycetes Supporting Evidence: PMID:7784503 Three distinct basic 14-kD proteins, P14a, P14b, and P14c, were isolated from tomato (Lycopersicon esculentum Mill. cv Baby) leaves infected with Phytophthora infestans. They exhibited antifungal activity against P. infestans both in vitro (inhibition of zoospore germination) and in vivo with a tomato leaf disc assay (decrease in infected leaf surface). PMID:7784503 The various tomato and tobacco PR-1 proteins were compared for their biological activity and found to display differential fungicidal activity against P. infestans in both the in vitro and in vivo assays, the most efficient being the newly characterized tomato P14c and tobacco PR-1g. PMID:27747953 we provide genetic and biochemical evidence for the capacity of PR-1 proteins to bind sterols, and demonstrate that the inhibitory effect on pathogen growth is caused by the sequestration of sterol from pathogens. PMID:27747953 In support of our findings, sterol-auxotroph pathogens such as the oomycete Phytophthora are particularly sensitive to PR-1, whereas sterol-prototroph fungal pathogens become highly sensitive only when sterol biosynthesis is compromised. file:SOLLC/PR1B1/PR1B1-deep-research-falcon.md While these assays are not reproduced in the retrieved excerpts for P04284 specifically, they strongly support the working functional model that tomato PR1B1-like proteins contribute to defense as **extracellular antimicrobial proteins**, especially against oomycetes. |
| GO:0050832 defense response to fungus | IEA GO_REF:0000043 | MODIFY | Summary: SPKW (GO_REF:0000043) annotation derived from the UniProt "Plant defense" / "Pathogenesis-related protein" / "Fungicide" keywords; present in the Sept 2025 snapshot, removed in the current GOA release. PR1B1 is strongly induced during fungal infection (Cladosporium fulvum is cited as an inducer by UniProt and by van Kan et al. 1992), it has direct antimicrobial activity, and it is the precursor of the CAPE1 immune-signalling peptide that activates anti-pathogen defense. Reason: PR1B1 is a genuine participant in antimicrobial defense, not merely co-induced: it is the founding marker of SAR, it has direct antimicrobial activity, and it yields the CAPE1 defense-signalling peptide. "Defense response to fungus" is therefore substantially correct in spirit. However, the best-characterized direct target of the tomato P14 proteins is Phytophthora infestans, which is an OOMYCETE, not a true fungus; the loosely-applied historical term "antifungal" conflates oomycetes with fungi. The most accurate specific term is "defense response to oomycetes" (GO:0002229), with the broad "defense response to other organism" (GO:0098542) also appropriate to cover both the oomycete and fungal contexts. GOA's removal of this SPKW annotation removed a substantially correct annotation; the removal is at best PARTLY JUSTIFIED, and the biology is better re-expressed via the current ARBA "defense response to other organism" annotation plus the more precise oomycete term. Proposed replacements: defense response to oomycetes Supporting Evidence: PMID:1421154 Tomato leaves infected by the fungal pathogen Cladosporium fulvum contain several types of intracellular and extracellular pathogenesis-related (PR) proteins. PMID:7784503 They exhibited antifungal activity against P. infestans both in vitro (inhibition of zoospore germination) and in vivo with a tomato leaf disc assay (decrease in infected leaf surface). PMID:25361956 A wounding or wounding plus MeJA-induced peptide derived from the pathogenesis-related protein 1 (PR-1) family was found to induce significant antipathogen and minor antiherbivore responses in tomato. PMID:9204567 the PR1b1 gene is strongly activated locally in tissues undergoing the hypersensitive response but not systemically in uninoculated tissues. Furthermore, its expression is induced by both salicylic acid and ethylene precursors PMID:27747953 In support of our findings, sterol-auxotroph pathogens such as the oomycete Phytophthora are particularly sensitive to PR-1, whereas sterol-prototroph fungal pathogens become highly sensitive only when sterol biosynthesis is compromised. |
| GO:0005576 extracellular region | IEA GO_REF:0000002 | ACCEPT | Summary: IEA annotation from InterPro2GO (InterPro:IPR018244, the V5/Tpx-1-related CAP conserved-site signature). PR1B1 has a cleaved N-terminal signal peptide (residues 1-24) and is a classic extracellular/apoplastic PR protein; van Kan et al. (1992) explicitly classify the P6/P14 protein among the extracellular PR proteins of tomato. Reason: Well supported. The signal peptide and the established apoplastic localization of tomato PR-1 proteins make extracellular region the correct compartment. PR-1 biology, including CAPE1 release and sterol sequestration from apoplastic pathogens, occurs in the extracellular space. The IBA annotation in UniProt (GO:0005615 extracellular space) is fully consistent and more specific; the broader extracellular region term is also correct. Supporting Evidence: PMID:1421154 Previously, we reported the purification and serological characterization of five extracellular PR proteins: P2, P4, P6, a chitinase and a beta-1,3-glucanase PMID:16453639 p14 represents not only the first completely sequenced PR plant protein but also a new type of structurally unfamiliar proteins whose biological function in the diseased plant remains to be elucidated. |
| GO:0098542 defense response to other organism | IEA GO_REF:0000117 | ACCEPT | Summary: IEA annotation from an ARBA machine-learning model (ARBA:ARBA00027395). This is the broad term that, in the 2026 GOA release, effectively replaces the two retired SPKW defense annotations. PR1B1 is a genuine participant in plant defense against pathogens: it has direct antimicrobial/antioomycete activity (Niderman et al. 1995), the CAP family acts through sterol binding/sequestration (Gamir et al. 2017), and PR1B1 is the precursor of the CAPE1 immune-signalling peptide (Chen et al. 2014). Reason: Broad but accurate and well supported. "Defense response to other organism" correctly captures PR1B1's role in defense against both fungal and oomycete pathogens without over-committing to a specific pathogen class or to a cytotoxic "killing" activity. It is an appropriate replacement for the retired SPKW annotations: it retains the correct biology while avoiding the over-strong "killing of cells of another organism" term. A more specific child term, "defense response to oomycetes" (GO:0002229), is also justified by the direct experimental evidence and is proposed as a new annotation below. Supporting Evidence: PMID:7784503 They exhibited antifungal activity against P. infestans both in vitro (inhibition of zoospore germination) and in vivo with a tomato leaf disc assay (decrease in infected leaf surface). PMID:25361956 This study highlights a role for PR-1 in immune signaling and suggests the potential application of plant endogenous peptides in efforts to defeat biological threats in crop production. PMID:36932700 The importance of these proteins in immune defence is illustrated by the fact that PR1 overexpression in plants results in increased resistance against pathogens. file:SOLLC/PR1B1/PR1B1-deep-research-falcon.md **Primary biological role:** **defense-associated secreted protein** induced by pathogens and by immune hormones (SA and, depending on context, ethylene-related signaling) and by abiotic stress (chilling). |
| GO:0002229 defense response to oomycetes | IDA PMID:7784503 Pathogenesis-related PR-1 proteins are antifungal. Isolation... | NEW | Summary: The tomato P14 proteins, including P14a (= PR1B1), were directly purified and shown to inhibit the oomycete Phytophthora infestans both in vitro (zoospore germination) and in planta (leaf disc assay). Oomycetes are the most sensitive targets of PR-1 because they are sterol auxotrophs and the CAP/PR-1 mechanism is sterol sequestration. Reason: This is the most precise process term supported by direct experimental evidence for this specific protein. It is more accurate than the retired "defense response to fungus" SPKW annotation, because the demonstrated direct target (P. infestans) is an oomycete rather than a true fungus, and it is more conservative than the retired "killing of cells of another organism". It is a specific child of the retained ARBA "defense response to other organism" annotation. Supporting Evidence: PMID:7784503 Three distinct basic 14-kD proteins, P14a, P14b, and P14c, were isolated from tomato (Lycopersicon esculentum Mill. cv Baby) leaves infected with Phytophthora infestans. They exhibited antifungal activity against P. infestans both in vitro (inhibition of zoospore germination) and in vivo with a tomato leaf disc assay (decrease in infected leaf surface). PMID:27747953 In support of our findings, sterol-auxotroph pathogens such as the oomycete Phytophthora are particularly sensitive to PR-1, whereas sterol-prototroph fungal pathogens become highly sensitive only when sterol biosynthesis is compromised. file:SOLLC/PR1B1/PR1B1-deep-research-falcon.md A highly cited comparative review summarizes experimental evidence that tomato PR-1 proteins can show **direct antifungal activity**, including inhibition of **Phytophthora infestans** zoospore germination in vitro and reduction of lesion area on infected leaf discs in vivo; the review also notes that **basic** tomato PR-1 proteins show higher antifungal activity than acidic counterparts. |
| GO:0032934 sterol binding | ISS PMID:27747953 The sterol-binding activity of PATHOGENESIS-RELATED PROTEIN ... | NEW | Summary: The molecular function of PR-1 / CAP-superfamily proteins is sterol binding. Gamir et al. (2017) provided genetic and biochemical evidence that PR-1 proteins bind sterols, and that their antimicrobial action is caused by sequestration of sterol away from pathogens. The CAP domain contains a caveolin-binding motif and a flexible aromatic loop responsible for binding sterols and small hydrophobic ligands. Reason: PR-1 has long been described as "a protein of unknown biochemical function"; the sterol-binding activity of the CAP domain is the best-supported molecular function for the family and explains the observed antimicrobial/antioomycete activity of PR1B1. The evidence is at the family level (recombinant CAP proteins and Arabidopsis PR-1) rather than a PR1B1-specific binding assay, so ISS (inferred from sequence/structural similarity within the conserved CAP domain) is the appropriate evidence code. This MF should replace the uninformative, over-strong process term "killing of cells of another organism". Supporting Evidence: PMID:27747953 Here, we provide genetic and biochemical evidence for the capacity of PR-1 proteins to bind sterols, and demonstrate that the inhibitory effect on pathogen growth is caused by the sequestration of sterol from pathogens. PMID:36932700 The flexibility of this loop and the presence of aromatic residues in this motif are important for the ability of CAP proteins to bind sterols and related small hydrophobic compounds. file:SOLLC/PR1B1/PR1B1-deep-research-falcon.md A precise sterol-binding affinity for tomato PR1B1 specifically is not present in the retrieved excerpts; lipid-binding is discussed at CAP-superfamily level and in models/reviews. |
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Download this section (compressed HTML)Q: Does purified tomato PR1B1 (P14a) directly bind sterols in vitro, and is its antioomycete activity abolished by mutation of the caveolin-binding motif, as shown for other CAP proteins?
Q: Is the modest antimicrobial activity of P14a (relative to the more potent P14c) biologically meaningful in planta, or is PR1B1's principal defense contribution the release of the CAPE1 signalling peptide?
Q: What protease releases CAPE1 from PR1B1 in tomato, and is CAPE1 release modulated by pathogen effectors as reported for other plant PR1 proteins?
Experiment: Express and purify recombinant tomato PR1B1 and assay sterol binding (e.g. ligand competition / fluorescent sterol binding) and yeast-based sterol export, comparing wild type with caveolin-binding-motif mutants.
Hypothesis: PR1B1 binds sterols via its CAP domain, and this activity is required for its antioomycete activity.
Experiment: Generate tomato PR1B1 knockout/knockdown and overexpression lines and challenge with Phytophthora infestans and Cladosporium fulvum, scoring disease and CAPE1 accumulation.
Hypothesis: PR1B1 contributes quantitatively to pathogen resistance both through direct antimicrobial activity and through CAPE1-mediated defense signalling.
Experiment: Identify the tomato protease(s) that cleave PR1B1 to release CAPE1 and test whether pathogen effectors block this cleavage.
Hypothesis: CAPE1 release from PR1B1 is a regulated proteolytic step targeted by pathogen effectors to suppress host immunity.
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