Ecp6 (Extracellular protein 6) is a secreted LysM effector of the biotrophic tomato leaf mould fungus Cladosporium fulvum (Fulvia fulva). The precursor carries a signal peptide followed by three LysM carbohydrate-binding domains; the mature protein is abundantly secreted into the tomato leaf apoplast, with expression induced during infection. Ecp6 binds chitin oligosaccharides selectively. In the crystal structure, LysM1 and LysM3 of the same polypeptide come together (intrachain LysM dimerisation) to form a deeply buried composite groove that holds a chitin oligomer with ultra-high (picomolar) affinity, the highest chitin-binding affinity reported in nature, while LysM2 forms a separate low-micromolar chitin-binding site. Through this binding, Ecp6 sequesters chitin oligosaccharides released from the walls of invading hyphae and outcompetes host LysM pattern recognition receptors such as rice CEBiP, suppressing chitin-triggered immune responses (medium alkalinisation, reactive oxygen burst, defense gene induction) without affecting responses to unrelated elicitors. LysM2 can also suppress chitin-triggered immunity, possibly by perturbing host receptor complexes rather than by sequestration. Unlike the C. fulvum chitin-binding effector Avr4, Ecp6 does not protect fungal hyphae against plant chitinases. Silencing of Ecp6 reduces virulence on tomato, and Ecp6 orthologues are widespread in fungi.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005576 extracellular region | EXP PMID:18452583 The novel Cladosporium fulvum lysin motif effector Ecp6 is a... | ACCEPT | Summary: Ecp6 was discovered as a protein present in the tomato leaf apoplast during C. fulvum colonization (Bolton et al. 2008), and it is the extracellular site where it sequesters chitin oligosaccharides. Reason: Direct proteomic identification of the protein among fungal proteins secreted into the apoplast during infection supports extracellular localization; this is where the effector acts. Extracellular region is accurate and consistent with the orthologous-in-role LysM effector Slp1 (genes/PYRO7/slp1). Supporting Evidence: PMID:18452583 During tomato leaf colonization, the biotrophic fungus Cladosporium fulvum secretes several effector proteins into the apoplast PMID:18452583 Three novel C. fulvum proteins were identified: CfPhiA, Ecp6 and Ecp7 PMID:23840930 Considering also the high amount of Ecp6 effector protein that is secreted, especially during the initial stages of the infection |
| GO:0005576 extracellular region | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic mapping of the UniProt subcellular location "Secreted" (itself experimentally supported by PMID:18452583) to extracellular region. Reason: The mapping is correct and duplicates the experimental annotation; Ecp6 is a secreted apoplastic effector. Supporting Evidence: PMID:18452583 During tomato leaf colonization, the biotrophic fungus Cladosporium fulvum secretes several effector proteins into the apoplast |
| GO:0008061 chitin binding | IDA PMID:23840930 Fungal effector Ecp6 outcompetes host immune receptor for ch... | ACCEPT | Summary: Ecp6 binds chitin oligosaccharides. The crystal structure and ITC (Sanchez-Vallet et al. 2013) show a composite LysM1-LysM3 groove formed by intrachain dimerisation that binds one chitin oligomer with picomolar affinity (kd = 280 pM), plus a separate low-affinity LysM2 site; earlier work showed that Ecp6 sequesters chitin oligosaccharides during infection (de Jonge et al. 2010). Reason: Chitin binding is the core molecular activity through which Ecp6 performs its effector role (ligand sequestration), and it is directly measured on the purified protein. It is the activity term, not a perception or signaling term, so it does not carry receptor semantics onto the effector. Supporting Evidence: PMID:23840930 The composite LysM1–LysM3 binding site shows ultra-high chitin-binding affinity, thus explaining how LysM effectors outcompete plant host receptors for chitin binding. PMID:23840930 A first binding phase in which one chitin molecule was bound with ultra-high affinity (kd = 280 pM PMID:20724636 During infection, Ecp6 sequesters chitin oligosaccharides that are released from the cell walls of invading hyphae to prevent elicitation of host immunity. file:FULFL/ECP6/ECP6-deep-research-falcon.md Chitin binding does not make Ecp6 a demonstrated fungal-wall shield. |
| GO:0140423 effector-mediated suppression of host pattern-triggered immunity signaling | EXP PMID:23840930 Fungal effector Ecp6 outcompetes host immune receptor for ch... | ACCEPT | Summary: Ecp6 and its LysM-domain mutants suppress chitin-oligosaccharide-induced medium alkalinisation in tomato cell suspensions (Sanchez-Vallet et al. 2013), and Ecp6 mediates virulence by perturbing chitin-triggered host immunity (de Jonge et al. 2010). The LysM1-LysM3 groove acts by sequestering the chitin MAMP before receptor engagement; LysM2 suppresses by a mechanism unlikely to involve sequestration, possibly interference with the host receptor complex. Reason: This is the correct symbiont-side multi-organism process term, backed by experiments on the effector itself, and it matches the term chosen for the M. oryzae LysM effector Slp1 (genes/PYRO7/slp1). Ecp6 performs the work of suppression directly (binding the elicitor and possibly perturbing the receptor complex), so it passes the participation test. It is specific to PTI rather than general innate immunity, and Ecp6 does not suppress responses to unrelated elicitors. Supporting Evidence: PMID:23840930 Next, we assessed the ability of the various mutants to suppress chitin-triggered immunity in tomato cells. PMID:23840930 revealing that LysM2 contains a functional chitin-binding site that contributes to the suppression of chitin-triggered immunity in the cell suspension assay PMID:20724636 Ecp6 of the fungal plant pathogen Cladosporium fulvum mediates virulence through perturbation of chitin-triggered host immunity. |
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Download this section (compressed HTML)Q: GO:0140423 (suppression of host PTI signaling) vs its parent effector-mediated suppression of host pattern-triggered immunity - Ecp6's main mechanism is MAMP sequestration upstream of receptor activation. Should GO provide a dedicated child such as "effector-mediated suppression of host MAMP recognition" for chitin-scavenging LysM effectors (Ecp6, Slp1, Mg3LysM)?
Q: Does LysM2 of Ecp6 physically interact with host LysM receptors (e.g. tomato CERK1/LYK orthologues) to perturb receptor complex formation, as proposed, or does it act only through low-affinity chitin binding?
Experiment: Test direct binding of Ecp6 (wild type and LysM2 mutants T95R/D121K) to purified ectodomains of tomato chitin receptors (e.g. SlLYK1/SlCERK1 orthologues) in the absence and presence of chitin, to test the receptor-perturbation hypothesis for LysM2.
Experiment: Generate a clean ECP6 knockout in C. fulvum and complement with LysM1-LysM3 groove versus LysM2 binding-site mutants, measuring virulence and host chitin responses to separate the two suppression mechanisms in planta.
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