PGRP-S1 (AGAP000536) is a short-form peptidoglycan recognition protein in Anopheles gambiae that functions as a non-catalytic pattern recognition receptor for bacterial peptidoglycan. Unlike other short PGRPs in this species (PGRP-S2/S3), PGRP-S1 lacks the conserved zinc-binding catalytic residues required for N-acetylmuramoyl-L-alanine amidase activity and therefore does not hydrolyze peptidoglycan. Instead, it serves as the putative ortholog of Drosophila PGRP-SA, suggesting a role in activating Toll pathway-mediated innate immune responses to Gram-positive bacteria. It is a secreted, extracellular protein found in hemolymph.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005615 extracellular space | IBA GO_REF:0000033 | ACCEPT | Summary: PGRP-S1 is a short-form (S-class) peptidoglycan recognition protein, which are characteristically secreted and extracellular. The protein has a signal peptide (residues 1-26) predicted by SignalP, consistent with secretion. Short-form PGRPs in insects are generally found in hemolymph, cuticle, fat body, and gut epithelium. Reason: Consistent with PGRP biology. Short-form PGRPs are secreted proteins that function in the extracellular space to detect bacterial peptidoglycan. The UniProt entry confirms a predicted signal peptide (residues 1-26), and the protein belongs to the secreted S-class of PGRPs. The IBA annotation from phylogenetic inference is well-supported. Supporting Evidence: file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md Short-form PGRPs in insects are secreted/extracellular, present in hemolymph, cuticle, and fat body, and sometimes in gut epithelium/hemocytes |
| GO:0006955 immune response | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: PGRP-S1 is clearly involved in immune response as it is a peptidoglycan recognition protein. However, this term is quite general. The more specific term 'innate immune response' (GO:0045087) or 'defense response to Gram-positive bacterium' (GO:0050830) would be more informative. Given that other more specific immune annotations exist, this can be kept as non-core. Reason: The annotation is correct but redundant with more specific immune annotations. PGRP-S1 functions in innate immunity by detecting bacterial peptidoglycan and initiating immune signaling. The IBA annotation is phylogenetically supported. Supporting Evidence: file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md PGRP-S1 is [...] most consistent with a non-catalytic PGN recognition role analogous to Drosophila PGRP-SA |
| GO:0008745 N-acetylmuramoyl-L-alanine amidase activity | IBA GO_REF:0000033 | REMOVE | Summary: This annotation is INCORRECT for PGRP-S1. Multiple comparative analyses demonstrate that PGRP-S1 lacks the conserved histidine and zinc-coordinating residues required for amidase catalytic activity. In contrast, PGRP-S2 and PGRP-S3 in A. gambiae retain these catalytic residues and are predicted to be functional amidases. PGRP-S1 functions as a non-catalytic pattern recognition receptor, not an enzyme. Reason: The annotation is based on the presence of an amidase-like domain (Amidase_2/Pfam PF01510), but sequence analysis shows PGRP-S1 lacks the conserved catalytic residues. This represents an over-annotation based on domain presence without considering the loss of key functional residues. The protein is a non-catalytic receptor analogous to Drosophila PGRP-SA. Supporting Evidence: file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md PGRP-S1 [...] lacks conserved Zn/catalytic residues (predicted non-amidase) file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md among short PGRPs (S1, S2, S3), S2/S3 retain catalytic residues (predicted amidases), whereas S1 does not |
| GO:0016019 peptidoglycan immune receptor activity | IBA GO_REF:0000033 | ACCEPT | Summary: This is the correct molecular function for PGRP-S1. As a non-catalytic PGRP that lacks amidase activity, PGRP-S1 functions primarily as a pattern recognition receptor for bacterial peptidoglycan. It is orthologous to Drosophila PGRP-SA, which activates the Toll pathway upon binding Lys-type peptidoglycan. The GO term definition explicitly notes that only PGRPs with receptor activity (not just binding) should be annotated to this term. Reason: Core molecular function annotation. PGRP-S1 is a non-catalytic peptidoglycan recognition protein that functions as an immune receptor. The IBA annotation is well-supported by phylogenetic evidence and comparative sequence analysis showing it is the SA-like ortholog in Anopheles. The term correctly captures the signaling/receptor function rather than just binding. Supporting Evidence: file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md PGRP-S1 is best (though weakly) recognized as the putative ortholog of Drosophila PGRP-SA, a Toll-pathway-associated circulating receptor file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md PGRP-S1 is [...] most consistent with a non-catalytic PGN recognition role analogous to Drosophila PGRP-SA |
| GO:0050830 defense response to Gram-positive bacterium | IBA GO_REF:0000033 | ACCEPT | Summary: This annotation is appropriate based on orthology to Drosophila PGRP-SA, which is specifically involved in defense against Gram-positive bacteria via the Toll pathway. In Drosophila, PGRP-SA recognizes Lys-type peptidoglycan (characteristic of Gram-positive bacteria) and activates Toll signaling. PGRP-S1's weak but best-hit orthology to PGRP-SA suggests similar specificity. Reason: Well-supported by phylogenetic inference. The IBA annotation derives from characterized orthologs in Drosophila where PGRP-SA specifically mediates defense against Gram-positive bacteria. While direct experimental evidence in Anopheles is lacking, the orthology relationship supports this annotation. Supporting Evidence: file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md By orthology, it aligns weakly with Drosophila PGRP-SA, suggesting a role in presenting PGN to the Toll axis file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md Lys-type PGN via PGRP-SA/GNBP activates Toll in flies |
| GO:0002376 immune system process | IEA GO_REF:0000043 | KEEP AS NON CORE | Summary: This is a very general term derived from UniProtKB keyword mapping. PGRP-S1 is clearly involved in immune system processes as a pattern recognition receptor. However, more specific terms (innate immune response, defense response to Gram-positive bacterium) are already annotated and more informative. Reason: Correct but too general. The IEA annotation from keyword mapping is accurate but redundant with more specific immune annotations. Keep as non-core since it provides no additional functional insight beyond the more specific terms. Supporting Evidence: file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md PGRPs are pattern-recognition proteins that bind bacterial peptidoglycan (PGN) |
| GO:0008270 zinc ion binding | IEA GO_REF:0000002 | REMOVE | Summary: This annotation is problematic for PGRP-S1. While the PGRP domain in general can coordinate zinc for amidase activity, PGRP-S1 specifically lacks the conserved zinc-coordinating residues. The annotation derives from the InterPro PGRP domain (IPR006619) which can encompass both catalytic and non-catalytic PGRPs. For PGRP-S1, which is non-catalytic, zinc binding is not established. Reason: The InterPro-based annotation assumes all PGRP domain proteins bind zinc, but this is only true for catalytic amidase PGRPs. PGRP-S1 lacks the conserved His/Cys residues that coordinate the catalytic zinc. Without these residues, there is no functional zinc-binding site. This is an over-annotation based on domain presence. Supporting Evidence: file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md PGRP-S1 [...] lacks the histidate/zinc-coordination constellation required for amidase activity file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md PGRP-S1 [...] lacks conserved Zn/catalytic residues |
| GO:0008745 N-acetylmuramoyl-L-alanine amidase activity | IEA GO_REF:0000002 | REMOVE | Summary: Same issue as the IBA annotation for this term. PGRP-S1 lacks the catalytic residues required for amidase activity. This IEA annotation from InterPro is incorrect for this specific protein. Reason: Over-annotation based on domain presence. The InterPro domains (IPR002502, IPR006619, IPR036505) correctly identify this as a PGRP family member, but PGRP-S1 specifically lacks the catalytic residues. Only PGRP-S2 and PGRP-S3 in A. gambiae retain amidase activity. Supporting Evidence: file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md PGRP-S1 [...] lacks conserved Zn/catalytic residues (predicted non-amidase) file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md PGRP-S2/S3 retain these residues and are predicted to have amidase activity |
| GO:0009253 peptidoglycan catabolic process | IEA GO_REF:0000002 | REMOVE | Summary: This annotation is INCORRECT for PGRP-S1. Peptidoglycan catabolism requires amidase activity, which PGRP-S1 lacks. PGRP-S1 binds and recognizes peptidoglycan but does not degrade it. The annotation derives from the amidase domain but is not applicable to non-catalytic PGRPs. Reason: PGRP-S1 is a non-catalytic receptor that recognizes but does not degrade peptidoglycan. Peptidoglycan catabolism is carried out by catalytic PGRPs (S2, S3, LB in Anopheles) that retain amidase activity. This is an over-annotation based on domain rather than verified function. Supporting Evidence: file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md PGRP-S1 is [...] a non-catalytic PGN recognition protein [...] functions primarily as a recognition protein rather than a hydrolase file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md amidase roles in Anopheles gut immunity are fulfilled by other PGRPs (e.g., S2/S3, LB) that retain catalytic residues |
| GO:0042834 peptidoglycan binding | IEA GO_REF:0000002 | ACCEPT | Summary: This annotation is correct. PGRP-S1 contains a PGRP domain that binds peptidoglycan. While it lacks catalytic activity, the binding function is retained. The GO term definition notes that PGRPs without receptor activity should use this term rather than the immune receptor activity term. However, since PGRP-S1 has receptor/signaling function, this term is less specific than GO:0016019. Reason: Correct annotation. PGRP-S1 retains the peptidoglycan binding function even though it lacks catalytic activity. The PGRP domain enables recognition and binding of bacterial cell wall components. The IEA annotation from InterPro is appropriate. This is a valid additional annotation alongside the more specific receptor activity term. Supporting Evidence: file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md PGRPs are pattern-recognition proteins that bind bacterial peptidoglycan (PGN) file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md PGRP-S1 [...] non-catalytic PGN recognition protein |
| GO:0045087 innate immune response | IEA GO_REF:0000120 | ACCEPT | Summary: This annotation is accurate. PGRP-S1 is specifically involved in innate immunity as a pattern recognition receptor. The annotation is more specific than the general 'immune system process' term and accurately reflects PGRP-S1's role in detecting pathogen-associated molecular patterns (peptidoglycan) to trigger innate immune signaling. Reason: Appropriate annotation for PGRP-S1's role in innate immunity. PGRPs are key components of the insect innate immune system, functioning as pattern recognition receptors to detect bacterial infection. The IEA annotation is well-supported by the protein's domain composition and family membership. Supporting Evidence: file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md Short-form (S) PGRPs [...] can either act as non-catalytic recognition molecules or as catalytic amidases [...] to initiate an innate immune response file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md innate immunity pathways in the malaria vector Anopheles gambiae |
| GO:0032499 detection of peptidoglycan | IBA GO_REF:0000033 | NEW | Summary: This is an appropriate biological process annotation for PGRP-S1. As a non-catalytic pattern recognition receptor, PGRP-S1's primary function is to detect bacterial peptidoglycan and convert this stimulus into a molecular signal for immune activation. This process-level annotation complements the molecular function annotation of peptidoglycan immune receptor activity. Reason: Although not currently in the GOA file, this annotation would accurately capture PGRP-S1's role in sensing bacterial cell wall components. The term 'detection of peptidoglycan' (GO:0032499) describes the series of events in which a peptidoglycan stimulus is received and converted into a molecular signal, which is precisely what non-catalytic PGRPs like PGRP-S1 do. Supporting Evidence: file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md PGRPs are pattern-recognition proteins that bind bacterial peptidoglycan (PGN) file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md PGRP-S1 is [...] most consistent with a non-catalytic PGN recognition role |
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Download this section (compressed HTML)Q: Does PGRP-S1 specifically activate the Toll pathway in Anopheles gambiae, as predicted by its orthology to Drosophila PGRP-SA?
Suggested experts: George K. Christophides, Elena A. Levashina
Q: What is the substrate specificity of PGRP-S1 - does it preferentially recognize Lys-type peptidoglycan (Gram-positive) or DAP-type (Gram-negative)?
Suggested experts: George K. Christophides
Experiment: RNAi knockdown of PGRP-S1 followed by challenge with Gram-positive bacteria (e.g., Staphylococcus aureus). Measure Toll pathway activation via expression of Toll-regulated antimicrobial peptides (e.g., defensin) compared to controls.
Hypothesis: PGRP-S1 activates Toll pathway signaling upon peptidoglycan stimulation
Type: RNAi knockdown with immune challenge
Experiment: Express and purify recombinant PGRP-S1 ectodomain. Perform binding assays (surface plasmon resonance or isothermal titration calorimetry) with purified Lys-type and DAP-type peptidoglycan to determine binding affinities and specificities.
Hypothesis: PGRP-S1 binds Lys-type peptidoglycan with higher affinity than DAP-type
Type: Protein binding assay
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