PGRP-LD is a long-form peptidoglycan recognition protein (PGRP) that functions as a non-catalytic, negative regulator of innate immune responses in the mosquito gut. Despite containing an Amidase_2/PGRP domain, PGRP-LD lacks the conserved residues required for peptidoglycan binding and amidase catalytic activity. Instead, it acts to restrain immune hyper-activation, thereby preserving gut commensal microbiota and maintaining peritrophic matrix integrity. This microbiota-PM barrier axis indirectly limits Plasmodium infection. PGRP-LD is a transmembrane protein localized at the gut epithelial barrier.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0002376 immune system process | IEA GO_REF:0000043 | ACCEPT | Summary: PGRP-LD is involved in immune system processes. Deep research synthesis indicates that PGRP-LD modulates gut immunity by restraining hyper-activation of immune effectors, thereby maintaining microbiota homeostasis and peritrophic matrix integrity. This is supported by functional studies in the closely related Anopheles stephensi ortholog. Reason: The annotation is correct but very broad. PGRP-LD plays a regulatory role in innate immunity at the gut barrier. The term is appropriately general for an IEA annotation derived from UniProt keyword mapping. Supporting Evidence: file:ANOGA/PGRPLD/PGRPLD-deep-research-falcon.md PGRP-LD restrains hyper-activation of innate immune effectors (e.g., AMPs, DUOX), thereby protecting commensal bacteria |
| GO:0008270 zinc ion binding | IEA GO_REF:0000002 | UNDECIDED | Summary: The PGRP/Amidase_2 domain family typically coordinates a zinc ion at the active site. However, PGRP-LD lacks the conserved residues required for catalytic activity, raising the question of whether the zinc-binding site is functional. Sequence analysis shows that PGRP-LD lacks most conserved residues for PGN binding and catalytic activity. Reason: While catalytic PGRPs bind zinc at the active site, the absence of key catalytic residues in PGRP-LD raises uncertainty about whether zinc binding is retained. No direct experimental evidence confirms zinc binding in this non-catalytic PGRP. The annotation may be a case of domain-based over-annotation. Supporting Evidence: file:ANOGA/PGRPLD/PGRPLD-deep-research-falcon.md The A. stephensi PGRP-LD lacks most conserved residues required for PGN binding and amidase catalytic activity |
| GO:0008745 N-acetylmuramoyl-L-alanine amidase activity | IEA GO_REF:0000002 | REMOVE | Summary: This annotation is INCORRECT. Despite belonging to the Amidase_2/PGRP superfamily, PGRP-LD is a non-catalytic PGRP that lacks the conserved residues required for peptidoglycan binding and amidase catalytic activity. Sequence analysis of the Anopheles stephensi ortholog demonstrated the absence of key catalytic residues. PGRP-LD functions as a regulatory PGRP rather than an enzymatic amidase. Reason: This is a classic example of domain-based over-annotation. The presence of an Amidase_2 domain (IPR002502) led to automatic annotation of amidase activity, but detailed sequence analysis shows PGRP-LD lacks the conserved residues required for catalysis. Not all PGRP family members are enzymatically active; many function as pattern recognition receptors or immune regulators without catalytic activity. The functional evidence clearly indicates PGRP-LD is non-catalytic. Supporting Evidence: file:ANOGA/PGRPLD/PGRPLD-deep-research-falcon.md The A. stephensi PGRP-LD lacks most conserved residues required for PGN binding and amidase catalytic activity file:ANOGA/PGRPLD/PGRPLD-deep-research-falcon.md Current evidence supports a regulatory PGRP model for PGRP-LD, not an enzymatic PGN hydrolase |
| GO:0009253 peptidoglycan catabolic process | IEA GO_REF:0000002 | REMOVE | Summary: This annotation is INCORRECT. Peptidoglycan catabolic process implies enzymatic degradation of peptidoglycan, which requires amidase activity. Since PGRP-LD lacks the conserved catalytic residues and does not function as an active amidase, it cannot be involved in peptidoglycan catabolism. The protein may recognize peptidoglycan as a pattern recognition molecule, but does not degrade it. Reason: This annotation logically follows from the incorrect amidase activity annotation. Since PGRP-LD lacks amidase catalytic activity, it cannot participate in peptidoglycan catabolic processes. The annotation should be removed along with the amidase activity annotation. Supporting Evidence: file:ANOGA/PGRPLD/PGRPLD-deep-research-falcon.md No direct amidase activity or substrate specificity has been demonstrated for PGRP-LD; rather, sequence features argue against catalysis |
| GO:0045087 innate immune response | IEA GO_REF:0000043 | MODIFY | Summary: PGRP-LD is clearly involved in the innate immune response. However, it functions as a NEGATIVE regulator rather than a positive effector. RNAi knockdown of PGRP-LD upregulates multiple immune effectors (cecropin, gambicin, defensin, DUOX), indicating its normal function is to restrain immune activation. This immune tolerance function preserves commensal bacteria and maintains peritrophic matrix integrity. Reason: While PGRP-LD is involved in innate immune response, a more accurate annotation would capture its specific role as a negative regulator. The current annotation is not wrong but could be more informative about the direction of regulation. Proposed replacements: negative regulation of innate immune response Supporting Evidence: file:ANOGA/PGRPLD/PGRPLD-deep-research-falcon.md Knockdown of PGRP-LD upregulates multiple immune effectors (e.g., cecropin, gambicin, defensin, DUOX), consistent with a negative regulatory role constraining immune activation in response to microbiota file:ANOGA/PGRPLD/PGRPLD-deep-research-falcon.md Functions to restrain hyper-activation of immune effectors (negative regulator-like role), thereby limiting overactive Imd/Toll-like responses that deplete commensals |
| GO:0045824 negative regulation of innate immune response | ISS PMID:29489896 PGRP-LD mediates A. stephensi vector competency by regulatin... | NEW | Summary: Functional studies in Anopheles stephensi demonstrate that PGRP-LD negatively regulates innate immune responses. Knockdown leads to upregulation of antimicrobial peptides and DUOX, indicating PGRP-LD normally restrains immune activation. Reason: This annotation captures the core function of PGRP-LD as a negative regulator of innate immunity. Evidence is from the closely related A. stephensi ortholog (ISS - inferred from sequence similarity), as direct A. gambiae knockdown data is not available in the literature. Supporting Evidence: file:ANOGA/PGRPLD/PGRPLD-deep-research-falcon.md Knockdown of PGRP-LD upregulates multiple immune effectors (e.g., cecropin, gambicin, defensin, DUOX), consistent with a negative regulatory role constraining immune activation |
| GO:0061060 negative regulation of peptidoglycan recognition protein signaling pathway | ISS PMID:29489896 PGRP-LD mediates A. stephensi vector competency by regulatin... | NEW | Summary: PGRP-LD modulates the PGRP signaling pathway as a negative regulator. The protein restrains immune hyper-activation through the Imd/Toll pathways that are canonically activated by peptidoglycan recognition. Reason: This term accurately describes the molecular role of PGRP-LD in dampening PGRP-mediated immune signaling. The evidence is from orthologous studies in A. stephensi. Supporting Evidence: file:ANOGA/PGRPLD/PGRPLD-deep-research-falcon.md Functional data are most consistent with PGRP-LD acting upstream as a modulator that limits overactive responses commonly associated with the Imd/Toll axes in the gut |
| GO:0016020 membrane | IEA GO_REF:0000002 | NEW | Summary: UniProt annotation and Phobius prediction indicate PGRP-LD has a transmembrane helix (residues 70-94). The protein is described as a transmembrane PGRP in functional studies. Reason: The UniProt entry shows transmembrane domain prediction by Phobius. Functional studies describe PGRP-LD as a transmembrane PGRP at the gut barrier. Originally proposed as GO:0016021 integral component of membrane, which GO has merged into GO:0016020 membrane. Supporting Evidence: file:ANOGA/PGRPLD/PGRPLD-deep-research-falcon.md Described as a transmembrane PGRP |
| GO:0140678 molecular function inhibitor activity | ISS PMID:29489896 PGRP-LD mediates A. stephensi vector competency by regulatin... | NEW | Summary: PGRP-LD functions as an inhibitor of immune signaling pathways. By restraining the activity of Imd/Toll signaling, it prevents hyper-activation of downstream immune effectors. This inhibitory molecular function underlies its biological role in negative regulation of innate immunity. Reason: This molecular function term captures PGRP-LD's role as an inhibitor of immune signaling. While the precise molecular mechanism is not fully characterized, the functional evidence clearly demonstrates inhibitory activity on immune pathway components. Supporting Evidence: file:ANOGA/PGRPLD/PGRPLD-deep-research-falcon.md Functions to restrain hyper-activation of immune effectors (negative regulator-like role), thereby limiting overactive Imd/Toll-like responses that deplete commensals |
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Download this section (compressed HTML)Q: Does A. gambiae PGRP-LD retain any zinc-binding capacity despite lacking catalytic residues?
Q: What is the mechanism by which PGRP-LD inhibits immune signaling?
Experiment: Direct biochemical assay for amidase activity in recombinant A. gambiae PGRP-LD. While sequence analysis strongly suggests PGRP-LD is non-catalytic, direct enzymatic assays on the A. gambiae protein would definitively confirm the absence of amidase activity.
Experiment: RNAi knockdown of PGRP-LD in A. gambiae to confirm functional conservation. The functional characterization was performed in A. stephensi. Direct knockdown in A. gambiae would confirm the conserved immune regulatory function.
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