Slp1 (Secreted LysM Protein 1, MGG_10097) is a small (162 aa) secreted, N-glycosylated effector protein of the rice blast fungus Magnaporthe oryzae (Pyricularia oryzae) consisting of a signal peptide followed by two LysM carbohydrate-binding domains. Its expression is induced during plant infection, and the protein is secreted via the conventional ER-Golgi route into the extra-invasive hyphal membrane compartment, where it accumulates at the interface between the fungal cell wall and the rice plasma membrane; it is an apoplastic effector rather than one delivered into host cells. Slp1 forms homodimers and binds chitin and chitin oligosaccharides with high (nanomolar) affinity, but does not protect fungal hyphae from chitinases. By sequestering chitin oligosaccharides released from the fungal cell wall, Slp1 outcompetes the rice chitin-binding pattern recognition receptor OsCEBiP, preventing chitin perception and the downstream pattern-triggered immune responses (reactive oxygen burst and defense gene expression). Alg3-dependent N-glycosylation at three asparagine residues is required for Slp1 stability and chitin binding. Deletion of slp1 reduces fungal proliferation in rice tissue and lesion expansion, a defect that is abolished when CEBiP is silenced in the host.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005576 extracellular region | EXP PMID:22267486 Effector-mediated suppression of chitin-triggered immunity b... | ACCEPT | Summary: Slp1 is a signal-peptide-bearing secreted protein that accumulates at the interface between the fungal cell wall and the rice plasma membrane during infection (Mentlak et al. 2012), and is retained in the extra-invasive hyphal membrane (EIHM) compartment as an apoplastic effector (Qian et al. 2022). Reason: Direct localization evidence (Slp1-GFP live-cell imaging) supports extracellular localization. The apoplastic site of action is the location where its chitin-sequestering function is exerted, so this is the core location. A more specific host-interface term could be considered, but extracellular region is accurate and appropriately conservative. Supporting Evidence: PMID:22267486 We demonstrate that Slp1 accumulates at the interface between the fungal cell wall and the rice plasma membrane PMID:34761375 Apoplastic effectors are dispersed within the EIHM compartment and do not enter host cells |
| GO:0005576 extracellular region | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic mapping from the UniProt subcellular location "Secreted" (itself experimentally supported by PMID:22267486) to extracellular region. Reason: Consistent with the experimental annotation and the experimentally determined apoplastic localization of Slp1; the mapping is correct. Supporting Evidence: PMID:22267486 by secreting an effector protein, Secreted LysM Protein1 (Slp1), during invasion of new rice cells |
| GO:0140403 effector-mediated suppression of host innate immune response | TAS PMID:34761375 MoErv29 promotes apoplastic effector secretion contributing ... | MODIFY | Summary: Slp1 suppresses rice chitin-triggered immunity by sequestering chitin oligosaccharides and outcompeting the pattern recognition receptor CEBiP, thereby blocking chitin-induced ROS generation and defense gene expression (Mentlak et al. 2012; Chen et al. 2014). The cited PHI-base TAS source (Qian et al. 2022) is a study of the COPII cargo receptor MoErv29, which shows that Slp1 secretion depends on MoErv29, but does not itself test Slp1's immunosuppressive activity. Reason: The process is correct and symbiont-side (appropriate for an effector), but it is more general than the experimental evidence warrants. Slp1 acts specifically upstream of PTI, by preventing CEBiP-mediated perception of the chitin MAMP and hence the chitin-triggered signaling that leads to ROS and defense gene induction. GO:0140423 effector-mediated suppression of host pattern-triggered immunity signaling (is_a GO:0052034 effector-mediated suppression of host pattern-triggered immunity, is_a GO:0140403) captures this precisely, and is the term used experimentally for the orthologous-in-role Cladosporium fulvum LysM effector Ecp6. The evidence for the specific term comes from Mentlak et al. 2012, not from the MoErv29 paper cited for the TAS row. Proposed replacements: effector-mediated suppression of host pattern-triggered immunity signaling Supporting Evidence: PMID:22267486 is able to suppress chitin-induced plant immune responses, including generation of reactive oxygen species and plant defense gene expression PMID:22267486 We propose that Slp1 sequesters chitin oligosaccharides to prevent PAMP-triggered immunity in rice PMID:24642938 Slp1 sequesters chitin oligosaccharides to avoid their recognition by the rice (Oryza sativa) chitin elicitor binding protein CEBiP and the induction of innate immune responses PMID:34761375 These results indicated that MoErv29 was required for the efficient secretion of MoSlp1. |
| GO:0008061 chitin binding | IDA PMID:22267486 Effector-mediated suppression of chitin-triggered immunity b... | NEW | Summary: Purified Slp1 binds chitin and chitin oligosaccharides (high-affinity binding to (GlcNAc)8 measured by SPR) and competes with CEBiP for chitin oligosaccharide binding; N-glycosylation is required for this chitin-binding activity. Reason: Chitin binding is the molecular activity through which Slp1 executes its effector function and is directly demonstrated on the purified protein, but it is absent from GOA. It passes the participation test (the binding is the step itself - ligand sequestration) and the comparator check: the Cladosporium fulvum LysM effector Ecp6 carries GO:0008061 chitin binding by IDA, as does the competing receptor OsCEBiP. This is the activity term, not a perception/signaling term, so it does not leak receptor semantics onto the effector. Supporting Evidence: PMID:22267486 can bind to chitin PMID:22267486 Slp1 competes with CEBiP for binding of chitin oligosaccharides PMID:24642938 required to maintain protein stability and the chitin binding activity of Slp1 file:PYRO7/slp1/slp1-deep-research-falcon.md No catalytic reaction or transported substrate has been demonstrated for Slp1. |
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Download this section (compressed HTML)Q: GO:0140423 (suppression of host PTI signaling) vs its parent GO:0052034 (suppression of host PTI) - Slp1 acts by ligand sequestration upstream of receptor activation rather than on a signaling component. Is "signaling" the right level for MAMP-scavenging effectors such as Slp1 and Ecp6, or should GO provide a dedicated child such as "effector-mediated suppression of host MAMP recognition"?
Q: Slp1 does not protect hyphae from chitinases (unlike Avr4). Does Slp1 also bind deacetylated or shorter chitin fragments, and does its homodimer bind chitin via intrachain or interchain LysM pairing as described for Ecp6?
Experiment: Crystal structure of Slp1 bound to (GlcNAc)n oligomers, to determine whether chitin is bound by an intramolecular LysM groove (Ecp6-like) or across the homodimer, and to design binding-dead mutants.
Experiment: Complement the slp1 deletion with structure-guided chitin-binding-dead LysM mutants and test virulence and host ROS on wild-type versus CEBiP-silenced rice, separating chitin binding from protein stability effects of glycosylation-site mutants.
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