PGRP-S3 (AGAP006342) is a short-form peptidoglycan recognition protein in Anopheles gambiae that functions as a secreted, zinc-dependent N-acetylmuramoyl-L-alanine amidase. Unlike PGRP-S1 in this species, PGRP-S3 retains the conserved zinc-binding catalytic residues required for amidase activity and is predicted to hydrolyze bacterial peptidoglycan. PGRP-S3 is a tandem duplicate of PGRP-S2 on chromosome 2L with approximately 95% sequence identity. As an amidase-type PGRP, it likely functions to modulate IMD/REL2 pathway signaling by degrading peptidoglycan ligands, thereby negatively regulating innate immune activation and promoting microbiota homeostasis in the mosquito midgut.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0002376 immune system process | IEA GO_REF:0000043 | KEEP AS NON CORE | Summary: PGRP-S3 is a peptidoglycan recognition protein involved in the innate immune system. This annotation is derived from UniProtKB keyword mapping (Immunity KW-0391). The annotation is correct but very general. More specific immune-related terms such as 'innate immune response' (GO:0045087) are also annotated and provide more functional detail. Reason: Correct but too general. The IEA annotation from keyword mapping is accurate but redundant with more specific immune annotations (GO:0045087). Keep as non-core since more informative terms are available. Supporting Evidence: file:ANOGA/PGRPS3/PGRPS3-deep-research-falcon.md PGRPs in insects: Peptidoglycan-recognition proteins are pattern-recognition proteins that bind bacterial peptidoglycan (PGN) and either signal to activate immune pathways or enzymatically cleave PGN to modulate signaling. |
| GO:0008270 zinc ion binding | IEA GO_REF:0000002 | ACCEPT | Summary: This annotation is CORRECT for PGRP-S3. Comparative sequence analyses consistently show that PGRP-S3 (along with PGRP-S2) retains the conserved zinc-coordinating residues required for N-acetylmuramoyl-L-alanine amidase activity. This contrasts with PGRP-S1 in the same species, which lacks these residues and is non-catalytic. The zinc ion is essential for the amidase catalytic mechanism. Reason: Well-supported by comparative sequence analysis. Multiple studies confirm that among Anopheles short PGRPs, S2 and S3 retain the zinc-coordinating catalytic residues while S1 does not. The InterPro-based annotation correctly identifies PGRP-S3 as a zinc-binding protein due to its functional amidase domain. Supporting Evidence: 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 file:ANOGA/PGRPS3/PGRPS3-deep-research-falcon.md short PGRPs S2 and S3 retain the canonical amidase signature motif(s) characteristic of Zn2+-dependent N-acetylmuramoyl-L-alanine amidases, supporting predicted catalytic activity |
| GO:0008745 N-acetylmuramoyl-L-alanine amidase activity | IEA GO_REF:0000002 | ACCEPT | Summary: This annotation is CORRECT for PGRP-S3. Unlike PGRP-S1 which lacks catalytic residues, PGRP-S3 retains the conserved zinc-coordinating and catalytic residues required for N-acetylmuramoyl-L-alanine amidase activity. This enzyme hydrolyzes the lactyl-amide bond between MurNAc and L-Ala in bacterial peptidoglycan. PGRP-S3 is classified in the N-acetylmuramoyl-L-alanine amidase 2 family and its amidase domain is predicted to be catalytically active. Reason: Core molecular function annotation. Multiple comparative analyses confirm PGRP-S3 retains the zinc-coordinating catalytic residues characteristic of functional amidases. This distinguishes it from the non-catalytic PGRP-S1 in the same organism. The InterPro annotation is appropriate for this catalytically active PGRP. Supporting Evidence: file:ANOGA/PGRPS1/PGRPS1-deep-research-falcon.md PGRP-S2/S3 retain these residues and are predicted to have amidase activity file:ANOGA/PGRPS3/PGRPS3-deep-research-falcon.md PGRP-S3 is most likely a secreted Zn2+-amidase that hydrolyzes PGN to negatively regulate IMD signaling file:ANOGA/PGRPS3/PGRPS3-deep-research-falcon.md PGRP-S2 and PGRP-S3 are autosomal paralogs under purifying selection, with high haplotype diversity for PGRP-S3 but Ka/Ks < 1, consistent with conserved biochemical constraints |
| GO:0009253 peptidoglycan catabolic process | IEA GO_REF:0000002 | ACCEPT | Summary: This annotation is appropriate for PGRP-S3. As a functional N-acetylmuramoyl-L-alanine amidase that retains catalytic residues, PGRP-S3 is predicted to participate in peptidoglycan catabolism by hydrolyzing muropeptides. Amidase-type PGRPs degrade bacterial peptidoglycan fragments, thereby modulating the availability of immune-stimulatory ligands. Reason: Consistent with the molecular function annotation. Since PGRP-S3 has predicted amidase activity (unlike the non-catalytic PGRP-S1), it is expected to participate in peptidoglycan catabolism. This process-level annotation appropriately follows from the catalytic function. Supporting Evidence: file:ANOGA/PGRPS3/PGRPS3-deep-research-falcon.md This family includes Zn2+-dependent amidases that hydrolyze the lactyl-amide bond between MurNAc and L-Ala in PGN, thereby degrading muropeptides file:ANOGA/PGRPS3/PGRPS3-deep-research-falcon.md short amidase-type PGRPs act as modulators by scavenging PGN, thereby shaping the amplitude/duration of IMD activation |
| GO:0042834 peptidoglycan binding | IEA GO_REF:0000002 | ACCEPT | Summary: PGRP-S3 contains a PGRP domain (IPR017331) that enables binding to bacterial peptidoglycan. All PGRPs share this binding function; in the case of catalytic PGRPs like PGRP-S3, binding precedes enzymatic hydrolysis of the substrate. Reason: Correct annotation. The PGRP domain mediates peptidoglycan binding in all members of this protein family. For amidase-type PGRPs, this binding function is prerequisite to the catalytic activity. The IEA annotation from InterPro is appropriate. Supporting Evidence: file:ANOGA/PGRPS3/PGRPS3-deep-research-falcon.md Peptidoglycan-recognition proteins are pattern-recognition proteins that bind bacterial peptidoglycan (PGN) |
| GO:0045087 innate immune response | IEA GO_REF:0000120 | ACCEPT | Summary: PGRP-S3 functions in the mosquito innate immune system as a modulator of IMD/REL2 pathway signaling. Amidase-type PGRPs like PGRP-S3 negatively regulate innate immune activation by degrading peptidoglycan ligands that would otherwise stimulate the IMD pathway. This maintains microbiota homeostasis and prevents immune overactivation. Reason: Appropriate biological process annotation. PGRP-S3 is part of the innate immune system in mosquitoes, functioning to modulate immune responses by enzymatically degrading immunostimulatory peptidoglycan fragments. The IEA annotation accurately reflects this role. Supporting Evidence: file:ANOGA/PGRPS3/PGRPS3-deep-research-falcon.md short PGRPs are described as small extracellular (secreted) PGRPs in insects [...] amidase-type PGRPs act as modulators by scavenging PGN, thereby shaping the amplitude/duration of IMD activation file:ANOGA/PGRPS3/PGRPS3-deep-research-falcon.md PGRP-S3 likely functions as an extracellular PGN amidase (negative regulator) rather than a transmembrane signaling receptor |
| GO:0005576 extracellular region | ISS file:ANOGA/PGRPS3/PGRPS3-deep-research-falcon.md | NEW | Summary: PGRP-S3 is a short-form (S-class) PGRP with a signal peptide (residues 1-20), indicating secretion. Short PGRPs are characteristically secreted and extracellular, found in hemolymph, cuticle, and gut lumen. The signal peptide prediction from SignalP and the classification as an S-class PGRP both support extracellular localization. Reason: This cellular component annotation should be added. PGRP-S3 has a predicted signal peptide and belongs to the secreted short-form class of PGRPs. Short PGRPs are characteristically extracellular, functioning in hemolymph and gut lumen. This is consistent with its role as a secreted amidase that modulates extracellular peptidoglycan availability. Supporting Evidence: file:ANOGA/PGRPS3/PGRPS3-deep-research-falcon.md Short PGRPs are described as small extracellular (secreted) PGRPs in insects, distinguishing them from transmembrane/cytosolic long PGRPs file:ANOGA/PGRPS3/PGRPS3-deep-research-falcon.md PGRP-S3, as a short PGRP, is therefore most likely secreted into the hemolymph or local extracellular spaces |
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Download this section (compressed HTML)Q: Does PGRP-S3 have any substrate preference for DAP-type versus Lys-type peptidoglycan?
Suggested experts: George K. Christophides, Elena A. Levashina
Q: What is the relative contribution of PGRP-S3 versus PGRP-S2 to IMD pathway modulation, given their near-identical sequences as tandem duplicates?
Suggested experts: George K. Christophides
Experiment: Express and purify recombinant PGRP-S3 and test for hydrolytic activity against purified peptidoglycan substrates. Use HPLC or mass spectrometry to detect muropeptide products. Compare activity to a catalytically inactive mutant (e.g., zinc-binding residue mutation).
Hypothesis: PGRP-S3 has N-acetylmuramoyl-L-alanine amidase activity in vitro
Type: Biochemical enzyme assay
Experiment: Perform RNAi knockdown of PGRP-S3 (ideally with S2-specific controls given sequence similarity) and challenge mosquitoes with bacteria. Measure expression of IMD-regulated antimicrobial peptides and assess bacterial load changes.
Hypothesis: PGRP-S3 negatively regulates IMD pathway activation
Type: RNAi knockdown with immune challenge
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