UniProt: P04284 (PR06_SOLLC). RecName "Pathogenesis-related leaf protein 6";
Short "P6"; AltNames "Ethylene-induced protein P1", "P14", "P14A", "P14a", "PR protein".
Gene name PR1B1. Solanum lycopersicum, NCBI:txid4081. 159 aa precursor, signal
peptide 1-24, mature chain 25-159, pyroglutamate at Gln-25. SCP/CAP domain 32-147,
three disulfide bonds. Member of the PR-1 / CAP (CRISP) superfamily.
The UniProt entry is explicit that P04284 (PR1B1) carries the historical names "P14"
and "P14A"/"P14a". The seminal NMR structure (PDB 1CFE) was solved for "P14a"
PMID:9067611. The cDNA clones for
PR1B1 (P6/P14 isomer P6) were reported by van Kan et al. PMID:1421154. Tornero et al. cloned PR1b1 (basic) and PR1a2 (acidic) tomato PR-1 genes
PMID:9204567.
So this gene = the canonical tomato leaf PR-1 protein, the original "pathogenesis-related
leaf protein p14" whose 1985 sequencing revealed a "new type of structurally unfamiliar
protein" (the founding CAP-superfamily sequence).
PR-1 was the first founding member of the CAP superfamily (Cysteine-rich secretory
proteins, Antigen 5, and Pathogenesis-related 1 proteins; also SCP/TAPS). CAP proteins
occur across plants, animals, fungi and pathogens. PR-1 proteins are small (~14-15 kDa
mature), secreted, cysteine-rich, and built around a single CAP/SCP domain
[PMID:36932700 review].
PR-1 genes are the canonical molecular markers of salicylic-acid-dependent
systemic acquired resistance (SAR). "Up-regulation of the PR-1 gene was considered to
be the main marker of SAR elicitation"; PR-1/PR-2/PR-5 are "considered as markers for
salicylic acid (SA)-dependent systemic acquired resistance". They are massively
transcriptionally induced upon pathogen challenge.
Expression of tomato PR1b1 specifically: it is NOT constitutively expressed; it is
transcriptionally activated by pathogen attack, locally in HR tissue, and is induced
by salicylic acid AND ethylene precursors PMID:9204567.
The UniProt INDUCTION line: "Upon infection by virulent and avirulent races of
pathogens, for example fungal pathogen C.fulvum. Also induced by ethylene."
The landmark demonstration of direct antimicrobial activity: Niderman et al. 1995
isolated three basic 14-kD tomato proteins P14a, P14b, P14c from P. infestans-infected
tomato leaves PMID:7784503. Crucially the paper found differential
activity: "The various tomato and tobacco PR-1 proteins were compared for their
biological activity and found to display differential fungicidal activity against
P. infestans... the most efficient being the newly characterized tomato P14c and
tobacco PR-1g." So among the tomato P14 proteins, P14a (= PR1B1) was the LESS efficient;
the strongest activity was P14c (a different, more basic PR-1). The 2023 review
attributes the founding antimicrobial demonstration to this study: "A strong
antimicrobial activity of PR1 was first established in 1995, when zoospores of
Phytophthora infestans were challenged with purified PR1 proteins from tobacco or
tomato (Niderman et al.)" PMID:36932700.
Note: P. infestans is an oomycete, not a true fungus. The historical literature
(and UniProt's "Antifungal"/"Fungicide" keywords, FUNCTION line "Has antifungal
activity") loosely calls oomycete-inhibitory activity "antifungal".
So: P14a/PR1B1 specifically WAS tested and DID show measurable in-vitro and in-planta
inhibitory activity against P. infestans, but it was the weakest of the tomato P14
proteins tested. This is genuine direct evidence of antimicrobial/antioomycete
activity for this protein, albeit modest.
The mechanistic basis of PR-1 antimicrobial action was clarified by Gamir et al. 2017:
PR-1 proteins bind sterols, and the inhibitory effect on pathogen growth is caused by
sequestration of sterol away from the pathogen PMID:27747953.
This explains why oomycetes (sterol auxotrophs that require exogenous sterol) are the
most sensitive targets. The CAP domain has a caveolin-binding motif and a flexible loop
with aromatic residues important for binding sterols and small hydrophobic compounds
PMID:36932700. CAP/PRY proteins are secreted sterol-binding proteins (PNAS 2009,
Choudhary & Schneiter). The Gamir 2017 work was done largely with Arabidopsis PR-1 and
recombinant CAP proteins; it provides the family-level molecular function rather than a
tomato-PR1B1-specific assay, but it is the accepted mode of action for the family.
A major modern reinterpretation of PR-1 function: tomato PR-1b is the precursor of
the CAPE1 immune-signalling peptide. Chen et al. 2014 used quantitative peptidomics
and found "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" — "The third peptide
(designated as CAPE1) was derived from PR-1b, a protein of unclear function"
PMID:25361956. CAPE1 is the C-terminal ~11 residues of PR-1b, released proteolytically
(in Arabidopsis by the cysteine protease XCP1). CAPE1 acts as a DAMP elicitor: "CAPE1
was shown to be a DAMP elicitor in this study as it was induced by wounding and
activated defense responses"; "CAPE1 significantly induced several pathogen-related
marker genes, including PR-2, PR-7, Chi2;1, and the precursor of CAPE1 (PR-1b)"; and it
activates SA- and JA-dependent defense and SAR-related responses. "This study
highlights a role for PR-1 in immune signaling." The 2023 review confirms: "plant PR1
is proteolytically cleaved to release a C-terminal CAPE1 peptide, which is sufficient
to activate an immune response. The release of this signalling peptide is blocked by
pathogenic effectors to evade immune defence" PMID:36932700.
So PR1B1 has (at least) TWO defense-related roles: (1) a modest direct
antimicrobial/sterol-sequestering CAP protein, and (2) the proprotein source of the
CAPE1 immune-signalling peptide. Both are "defense response", broadly.
PR1B1 has an N-terminal signal peptide (1-24) and is a classic extracellular /
apoplastic PR protein. UniProt classifies it as one of the extracellular PR
proteins; van Kan et al. studied "extracellular and intracellular PR proteins" and P6
is in the extracellular group PMID:1421154. The InterPro IPR018244-derived GO
annotation "extracellular region" is well supported. PR1/CAPE1 biology occurs in the
apoplastic space.
PR-1 is the textbook marker of SAR — its transcript abundance reports SA signalling
status. But it is also a genuine defense participant/effector: (a) it has direct
(if modest, isoform-variable) antimicrobial activity demonstrated for P14a/PR1B1
itself PMID:7784503; (b) it is the precursor of the CAPE1 immune-signalling peptide
PMID:25361956; (c) overexpression of PR-1 increases pathogen resistance
PMID:36932700. So PR1B1 is both a marker and an effector — it is not "merely
co-induced".
This term implies a direct cytotoxic/cell-killing activity. For PR1B1/P14a specifically,
Niderman et al. 1995 showed direct inhibition of P. infestans zoospore germination and
reduction of infected leaf surface — i.e., direct antioomycete activity. UniProt carries
"Antimicrobial", "Fungicide" keywords and FUNCTION "Has antifungal activity". The CAP
sterol-sequestration mechanism (Gamir 2017) provides a plausible direct
growth-inhibitory/killing mechanism. So this annotation is NOT baseless — it traces to
the UniProt Antimicrobial/Fungicide keywords which are themselves grounded in
experimental work on this protein. However, "killing of cells of another organism" is
a fairly strong/specific BP term; the demonstrated activity for P14a is modest
(weakest of the P14 set) and is better described as growth inhibition / antimicrobial
than outright cytotoxic "killing". GOA's removal of the SPKW annotation removed a
correct-in-spirit annotation; this is a case where the SPKW removal was only PARTLY
justified — the activity is real but the keyword pipeline produced a term that is
arguably stronger than the literature warrants for this specific protein. A better
annotation would be a "defense response" / "response to oomycete" BP plus a sterol-
binding MF, rather than the cytotoxic-sounding "killing of cells of another organism".
PR1B1 is induced by and tested against pathogens. The direct in-vitro/in-planta target
in Niderman 1995 is Phytophthora infestans, an oomycete, not a true fungus.
UniProt INDUCTION cites the fungus Cladosporium fulvum as an inducer. So PR1B1 is
genuinely a participant in antimicrobial defense, and is induced during fungal
infection — "defense response to fungus" is essentially correct as a process the
protein participates in (it is more than mere co-induction: it has antimicrobial
activity and yields CAPE1). The most precisely demonstrated direct target is an
oomycete, so "defense response to oomycetes" (GO:0002229) would be the most accurate
specific term; "defense response to fungus" is reasonable but slightly imprecise.
GOA's removal of this SPKW annotation removed a substantially correct annotation.
This is the broad parent that the ARBA model assigned. It is unambiguously correct and
well supported (PR1B1 participates in defense against pathogens; antimicrobial activity
+ CAPE1 signalling). It is broad but accurate, and it appropriately covers both the
oomycete and fungal contexts. This is a reasonable replacement / retention.
PR1B1 is the canonical extracellular (apoplastic) tomato PR-1 / CAP-superfamily defense
protein, strongly transcriptionally induced by pathogen attack, salicylic acid and
ethylene. It functions in plant defense against pathogens in two ways: as a sterol-
binding CAP protein with direct (modest) antimicrobial/antioomycete activity, and as
the proprotein precursor of the CAPE1 C-terminal immune-signalling peptide. It is the
textbook molecular marker of systemic acquired resistance.