Deep Research
Falcon
(PGRPLC-deep-research-falcon.md)
Falcon
(PGRPLC-deep-research-falcon.md)The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.
You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.
We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.
We are interested in where in or outside the cell the gene product carries out its function.
We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.
Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.
Research plan status: All objectives completed; composing the final report.
Gene/protein identity verification
PGRPLC is the Anopheles gambiae ortholog of Drosophila PGRP-LC and corresponds to AGAP005203; it encodes a transmembrane peptidoglycan-recognition receptor with a PGRP/amidase_2-fold domain. Alternative splicing yields three principal isoforms (PGRPLC1/2/3). This mapping and isoform architecture are documented for An. gambiae/coluzzii midgut and link directly to IMD/REL2 signaling (Insect Biochemistry and Molecular Biology, 2020; DOI:10.1016/j.ibmb.2019.103288) (rodgers2020functionalanalysisof pages 1-3, rodgers2020functionalanalysisof pages 3-6, rodgers2020functionalanalysisof pages 10-12). Foundational functional work in An. gambiae established PGRP-LC as the IMD-pathway receptor controlling antibacterial defense, gut bacterial homeostasis, and Plasmodium infection intensity (PLoS Pathogens, 2009; DOI:10.1371/journal.ppat.1000542) (meister2009anophelesgambiaepgrplcmediated pages 1-2, meister2009anophelesgambiaepgrplcmediated pages 2-4, meister2009anophelesgambiaepgrplcmediated pages 9-9).
1) Key concepts and definitions
- Molecular identity and domain architecture: PGRPLC (AGAP005203; UniProt A7UTA1) encodes a membrane receptor bearing an extracellular PGRP domain of the amidase_2 superfamily that recognizes peptidoglycan (PGN). Despite the amidase fold, PGRP-LC functions as a non-catalytic pattern-recognition receptor in mosquitoes (structural alignment/modeling; no amidase activity reported) (PLoS Pathogens, 2009; DOI:10.1371/journal.ppat.1000542) (meister2009anophelesgambiaepgrplcmediated pages 11-12, meister2009anophelesgambiaepgrplcmediated pages 10-11).
- Isoforms and receptor assembly: Three main isoforms (LC1/LC2/LC3) arise by alternative splicing and differ in their PGRP ectodomains. LC3 contains a two-residue insertion predicted to prevent it from initiating dimerization; modeling supports a role for LC3 in sequestering monomeric PGN under low-dose conditions and stabilizing signaling heterodimers under high-dose conditions (PLoS Pathogens, 2009; DOI:10.1371/journal.ppat.1000542) (meister2009anophelesgambiaepgrplcmediated pages 9-9, meister2009anophelesgambiaepgrplcmediated pages 11-12).
- Ligand specificity: PGRPLC discriminates between polymeric DAP-type PGN and monomeric muropeptides (TCT). In pull-down assays, LC1 and LC3 bind insoluble polymeric DAP-PGN, while LC2 forms complexes with LC3 in the presence of TCT monomer, indicating isoform-dependent sensing of distinct PGN forms (Insect Biochemistry and Molecular Biology, 2020; DOI:10.1016/j.ibmb.2019.103288) (rodgers2020functionalanalysisof pages 10-12, rodgers2020functionalanalysisof pages 6-8).
- Signaling pathway: PGRPLC is the principal transmembrane receptor that activates the mosquito IMD/REL2 NF-κB pathway, leading to induction of antimicrobial peptide (AMP) genes and bacterial control. Anopheles lacks PGRP-LE; thus, PGRP-LC-driven sensing in the gut is particularly important (IBMB 2020; PLoS Pathog 2009) (rodgers2020functionalanalysisof pages 1-3, rodgers2020functionalanalysisof pages 3-6, meister2009anophelesgambiaepgrplcmediated pages 1-2).
- Cellular/tissue localization: Functional analyses localize PGRPLC activity to the midgut epithelium with regionalized expression—PGRPLC1 enriched in cardia/anterior midgut; PGRPLC2/3 more uniform; REL2-responsive AMPs (GAM1, CEC1, LYSC1) show distinct regional patterns (IBMB 2020; DOI above) (rodgers2020functionalanalysisof pages 10-12, rodgers2020functionalanalysisof pages 12-17).
2) Recent developments and latest research (2023–2024 priority)
- Contemporary expert reviews emphasize PGRP-LC as the canonical IMD receptor in mosquitoes, integrating microbiota control with gut compartmentalization of AMPs and tolerance mechanisms (Philosophical Transactions B, 2024; DOI:10.1098/rstb.2023.0063) (hixson2024innateimmunityin pages 9-10). Parasites & Vectors (2024) reviews position cecropins, gambicins, and lysozymes under IMD/Toll control and outline their roles against bacteria and Plasmodium (DOI:10.1186/s13071-024-06161-4) (li2024responseofthe pages 10-11). A 2024 IMD/AMP review also reaffirms PGRP-LC/LE sensing of DAP-PGN to drive NF-κB–dependent AMPs (IJMS, 2024; DOI:10.3390/ijms25073835) (zhou2024insectantimicrobialpeptides pages 9-10).
- New mechanistic links to peritrophic matrix (PM): Recent work shows bacterial cell wall components (e.g., DAP-PGN, Lys-PGN, LPS) can stimulate PM formation through IMD activation; knockdown of PGRP-LC or REL2 reduces Per1 transcription and PM integrity in mosquitoes, implicating PGRP-LC–IMD in PM homeostasis and, by extension, parasite invasion barriers (PLOS Biology, 2025; DOI:10.1371/journal.pbio.3002967) (song2025cellwallcomponents pages 18-19).
3) Current applications and real-world implementations
- Vector competence modulation: RNAi silencing of PGRPLC increases Plasmodium infection intensities and prevalence (P. berghei and P. falciparum), demonstrating that modulating PGRP-LC–IMD signaling can alter malaria transmission potential. This positions PGRP-LC pathway components as candidate targets in genetic or microbial interventions to reduce vector competence by rebalancing gut microbiota and strengthening epithelial defenses (PLoS Pathog, 2009; DOI:10.1371/journal.ppat.1000542) (meister2009anophelesgambiaepgrplcmediated pages 4-5).
- Gut microbiota homeostasis/PM engineering: Given PGRP-LC’s role in controlling post-bloodmeal bacterial expansion and in promoting PM formation via IMD, manipulating PGRP-LC signaling could reinforce the PM barrier or microbiota composition to impair Plasmodium development (PLOS Biology, 2025; DOI:10.1371/journal.pbio.3002967) (song2025cellwallcomponents pages 18-19) and is consistent with 2024 reviews on gut immune zonation (Philosophical Transactions B, 2024; DOI:10.1098/rstb.2023.0063) (hixson2024innateimmunityin pages 9-10).
4) Expert opinions and analysis
- Consensus across 2024 reviews underscores PGRP-LC as the central IMD receptor in mosquitoes, with compartmentalized expression and downstream AMP programs shaping symbiont control and pathogen resistance (Philosophical Transactions B, 2024; Parasites & Vectors, 2024; IJMS, 2024) (hixson2024innateimmunityin pages 9-10, li2024responseofthe pages 10-11, zhou2024insectantimicrobialpeptides pages 9-10). These analyses align with functional and biochemical isoform specificity established in Anopheles (IBMB 2020) and foundational An. gambiae genetics (PLoS Pathog 2009) (rodgers2020functionalanalysisof pages 10-12, meister2009anophelesgambiaepgrplcmediated pages 1-2).
5) Relevant statistics and data from recent studies
- Systemic antibacterial challenge survival (RNAi PGRPLC): Whole-gene KD decreased survival—E. coli, 50% mortality by day 4; S. aureus, 50% by day 2 and near-complete by day 6 (both P<0.001). Isoform KD: LC3 KD led to 50% mortality by day 2 (E. coli) and day 1 (S. aureus); LC1/LC2 KDs gave 40–50% mortality by day 6 (P≤0.05) (PLoS Pathog, 2009; DOI:10.1371/journal.ppat.1000542) (meister2009anophelesgambiaepgrplcmediated pages 4-5, meister2009anophelesgambiaepgrplcmediated pages 2-4).
- AMP induction after infection: At 3 h, CEC1 increased 4–5-fold and DEF1 2–3-fold; induction required PGRPLC for S. aureus but not for E. coli (PLoS Pathog, 2009) (meister2009anophelesgambiaepgrplcmediated pages 4-5).
- Midgut microbiota expansion: After a bloodmeal, gut bacteria expand by up to several thousand-fold, coincident with robust PGRPLC-mediated AMP induction (PLoS Pathog, 2009) (meister2009anophelesgambiaepgrplcmediated pages 9-9).
- Plasmodium infection outcomes: Whole PGRPLC KD increased P. berghei median oocysts 4.4-fold (P<0.001) and elevated P. falciparum prevalence from 41% to 52% with higher median intensity (P<0.005) (PLoS Pathog, 2009) (meister2009anophelesgambiaepgrplcmediated pages 4-5).
- Gut regionalization and isoform-specific AMP control (midgut): REL2 KD reduced GAM1 across regions (p<0.05) and LYSC1 in cardia (p=0.05) and anterior (p<0.05). PGRPLC1 KD increased GAM1 in anterior (p<0.05). PGRPLC2 KD reduced GAM1 in cardia/anterior (p<0.05), reduced LYSC1 in posterior (p<0.05), and reduced CEC1 in cardia (p<0.05). PGRPLC3 KD reduced LYSC1 in cardia (p<0.05) and posterior (p<0.01) (IBMB, 2020; DOI:10.1016/j.ibmb.2019.103288) (rodgers2020functionalanalysisof pages 10-12, rodgers2020functionalanalysisof pages 6-8).
- Biochemical ligand interactions: Co-precipitation shows LC1 and LC3 bind insoluble polymeric DAP-type PGN; LC2 forms complexes with LC3 in the presence of the monomeric muropeptide TCT (IBMB, 2020) (rodgers2020functionalanalysisof pages 10-12, rodgers2020functionalanalysisof pages 6-8).
Functional annotation synthesis for PGRPLC (AGAP005203; UniProt A7UTA1)
- Primary function: Transmembrane pattern-recognition receptor of the IMD/REL2 pathway. Recognizes DAP-type PGN ligands in both polymeric (bacterial sacculus) and monomeric (TCT-like) forms via isoform assemblies. It is not an active amidase enzyme; rather, it uses a PGRP/amidase_2 structural fold for ligand binding and receptor dimerization to initiate signaling (PLoS Pathog, 2009; IBMB, 2020) (meister2009anophelesgambiaepgrplcmediated pages 11-12, rodgers2020functionalanalysisof pages 10-12).
- Substrate specificity: Discriminates DAP-type PGN. LC1/LC3 bind polymeric DAP-PGN; LC2 associates with LC3 in the presence of TCT, suggesting LC2 contributes to monomeric muropeptide sensing (IBMB, 2020) (rodgers2020functionalanalysisof pages 10-12, rodgers2020functionalanalysisof pages 6-8).
- Cellular localization and site of action: Epithelial plasma membrane of the midgut, with region-specific expression patterns and downstream regulation of REL2-responsive AMPs; systemic roles also evidenced by survival phenotypes after hemolymph bacterial challenge (IBMB, 2020; PLoS Pathog, 2009) (rodgers2020functionalanalysisof pages 10-12, meister2009anophelesgambiaepgrplcmediated pages 2-4).
- Pathway placement: Upstream PRR of the mosquito IMD pathway, activating REL2/NF-κB to induce AMPs (e.g., CECs, GAM1, LYSC1) and shape gut bacterial homeostasis and peritrophic matrix regulation, indirectly constraining Plasmodium development (Philosophical Transactions B, 2024; Parasites & Vectors, 2024; PLoS Pathog, 2009; PLOS Biology, 2025) (hixson2024innateimmunityin pages 9-10, li2024responseofthe pages 10-11, meister2009anophelesgambiaepgrplcmediated pages 4-5, song2025cellwallcomponents pages 18-19).
Notes on symbol ambiguity check
No conflicting gene symbol usage for PGRPLC in Anopheles gambiae was found in the collected literature; the locus consistently maps to AGAP005203 and encodes the PGRP-LC IMD receptor with the described isoforms (IBMB, 2020; PLoS Pathog, 2009) (rodgers2020functionalanalysisof pages 1-3, meister2009anophelesgambiaepgrplcmediated pages 2-4).
Data availability: URLs and dates
- Meister et al., 2009, PLoS Pathogens, “Anopheles gambiae PGRPLC-Mediated Defense against Bacteria Modulates Infections with Malaria Parasites,” published Aug 2009. URL: https://doi.org/10.1371/journal.ppat.1000542 (meister2009anophelesgambiaepgrplcmediated pages 1-2, meister2009anophelesgambiaepgrplcmediated pages 2-4, meister2009anophelesgambiaepgrplcmediated pages 9-9, meister2009anophelesgambiaepgrplcmediated pages 4-5, meister2009anophelesgambiaepgrplcmediated pages 9-10).
- Rodgers et al., 2020, Insect Biochemistry and Molecular Biology, “Functional analysis of the three major PGRPLC isoforms in the midgut of the malaria mosquito Anopheles coluzzii,” published Mar 2020. URL: https://doi.org/10.1016/j.ibmb.2019.103288 (rodgers2020functionalanalysisof pages 1-3, rodgers2020functionalanalysisof pages 3-6, rodgers2020functionalanalysisof pages 12-17, rodgers2020functionalanalysisof pages 10-12, rodgers2020functionalanalysisof pages 6-8).
- Hixson et al., 2024, Philosophical Transactions of the Royal Society B, “Innate immunity in Aedes mosquitoes: from pathogen resistance to shaping the microbiota,” published Mar 2024. URL: https://doi.org/10.1098/rstb.2023.0063 (hixson2024innateimmunityin pages 9-10).
- Li et al., 2024, Parasites & Vectors, “Response of the mosquito immune system and symbiotic bacteria to pathogen infection,” published Feb 2024. URL: https://doi.org/10.1186/s13071-024-06161-4 (li2024responseofthe pages 10-11).
- Zhou et al., 2024, International Journal of Molecular Sciences, “Insect Antimicrobial Peptides as Guardians of Immunity and Beyond: A Review,” published Mar 2024. URL: https://doi.org/10.3390/ijms25073835 (zhou2024insectantimicrobialpeptides pages 9-10).
- Song et al., 2025, PLOS Biology, “Cell wall components of gut commensal bacteria stimulate peritrophic matrix formation in malaria vector mosquitoes through activation of the IMD pathway,” published Jan 2025. URL: https://doi.org/10.1371/journal.pbio.3002967 (song2025cellwallcomponents pages 18-19).
References
-
(rodgers2020functionalanalysisof pages 1-3): Faye H. Rodgers, Julia A. Cai, Andre N. Pitaluga, Dominique Mengin-Lecreulx, Mathilde Gendrin, and George K. Christophides. Functional analysis of the three major pgrplc isoforms in the midgut of the malaria mosquito anopheles coluzzii. Insect Biochemistry and Molecular Biology, 118:103288, Mar 2020. URL: https://doi.org/10.1016/j.ibmb.2019.103288, doi:10.1016/j.ibmb.2019.103288. This article has 11 citations and is from a peer-reviewed journal.
-
(rodgers2020functionalanalysisof pages 3-6): Faye H. Rodgers, Julia A. Cai, Andre N. Pitaluga, Dominique Mengin-Lecreulx, Mathilde Gendrin, and George K. Christophides. Functional analysis of the three major pgrplc isoforms in the midgut of the malaria mosquito anopheles coluzzii. Insect Biochemistry and Molecular Biology, 118:103288, Mar 2020. URL: https://doi.org/10.1016/j.ibmb.2019.103288, doi:10.1016/j.ibmb.2019.103288. This article has 11 citations and is from a peer-reviewed journal.
-
(rodgers2020functionalanalysisof pages 10-12): Faye H. Rodgers, Julia A. Cai, Andre N. Pitaluga, Dominique Mengin-Lecreulx, Mathilde Gendrin, and George K. Christophides. Functional analysis of the three major pgrplc isoforms in the midgut of the malaria mosquito anopheles coluzzii. Insect Biochemistry and Molecular Biology, 118:103288, Mar 2020. URL: https://doi.org/10.1016/j.ibmb.2019.103288, doi:10.1016/j.ibmb.2019.103288. This article has 11 citations and is from a peer-reviewed journal.
-
(meister2009anophelesgambiaepgrplcmediated pages 1-2): Stephan Meister, Bogos Agianian, Fanny Turlure, Angela Relógio, Isabelle Morlais, Fotis C. Kafatos, and George K. Christophides. Anopheles gambiae pgrplc-mediated defense against bacteria modulates infections with malaria parasites. PLoS Pathogens, 5:e1000542, Aug 2009. URL: https://doi.org/10.1371/journal.ppat.1000542, doi:10.1371/journal.ppat.1000542. This article has 290 citations and is from a highest quality peer-reviewed journal.
-
(meister2009anophelesgambiaepgrplcmediated pages 2-4): Stephan Meister, Bogos Agianian, Fanny Turlure, Angela Relógio, Isabelle Morlais, Fotis C. Kafatos, and George K. Christophides. Anopheles gambiae pgrplc-mediated defense against bacteria modulates infections with malaria parasites. PLoS Pathogens, 5:e1000542, Aug 2009. URL: https://doi.org/10.1371/journal.ppat.1000542, doi:10.1371/journal.ppat.1000542. This article has 290 citations and is from a highest quality peer-reviewed journal.
-
(meister2009anophelesgambiaepgrplcmediated pages 9-9): Stephan Meister, Bogos Agianian, Fanny Turlure, Angela Relógio, Isabelle Morlais, Fotis C. Kafatos, and George K. Christophides. Anopheles gambiae pgrplc-mediated defense against bacteria modulates infections with malaria parasites. PLoS Pathogens, 5:e1000542, Aug 2009. URL: https://doi.org/10.1371/journal.ppat.1000542, doi:10.1371/journal.ppat.1000542. This article has 290 citations and is from a highest quality peer-reviewed journal.
-
(meister2009anophelesgambiaepgrplcmediated pages 11-12): Stephan Meister, Bogos Agianian, Fanny Turlure, Angela Relógio, Isabelle Morlais, Fotis C. Kafatos, and George K. Christophides. Anopheles gambiae pgrplc-mediated defense against bacteria modulates infections with malaria parasites. PLoS Pathogens, 5:e1000542, Aug 2009. URL: https://doi.org/10.1371/journal.ppat.1000542, doi:10.1371/journal.ppat.1000542. This article has 290 citations and is from a highest quality peer-reviewed journal.
-
(meister2009anophelesgambiaepgrplcmediated pages 10-11): Stephan Meister, Bogos Agianian, Fanny Turlure, Angela Relógio, Isabelle Morlais, Fotis C. Kafatos, and George K. Christophides. Anopheles gambiae pgrplc-mediated defense against bacteria modulates infections with malaria parasites. PLoS Pathogens, 5:e1000542, Aug 2009. URL: https://doi.org/10.1371/journal.ppat.1000542, doi:10.1371/journal.ppat.1000542. This article has 290 citations and is from a highest quality peer-reviewed journal.
-
(rodgers2020functionalanalysisof pages 6-8): Faye H. Rodgers, Julia A. Cai, Andre N. Pitaluga, Dominique Mengin-Lecreulx, Mathilde Gendrin, and George K. Christophides. Functional analysis of the three major pgrplc isoforms in the midgut of the malaria mosquito anopheles coluzzii. Insect Biochemistry and Molecular Biology, 118:103288, Mar 2020. URL: https://doi.org/10.1016/j.ibmb.2019.103288, doi:10.1016/j.ibmb.2019.103288. This article has 11 citations and is from a peer-reviewed journal.
-
(rodgers2020functionalanalysisof pages 12-17): Faye H. Rodgers, Julia A. Cai, Andre N. Pitaluga, Dominique Mengin-Lecreulx, Mathilde Gendrin, and George K. Christophides. Functional analysis of the three major pgrplc isoforms in the midgut of the malaria mosquito anopheles coluzzii. Insect Biochemistry and Molecular Biology, 118:103288, Mar 2020. URL: https://doi.org/10.1016/j.ibmb.2019.103288, doi:10.1016/j.ibmb.2019.103288. This article has 11 citations and is from a peer-reviewed journal.
-
(hixson2024innateimmunityin pages 9-10): Bretta Hixson, Robin Chen, and Nicolas Buchon. Innate immunity in aedes mosquitoes: from pathogen resistance to shaping the microbiota. Philosophical Transactions of the Royal Society B: Biological Sciences, Mar 2024. URL: https://doi.org/10.1098/rstb.2023.0063, doi:10.1098/rstb.2023.0063. This article has 14 citations and is from a domain leading peer-reviewed journal.
-
(li2024responseofthe pages 10-11): Manjin Li, Yang Zhou, Jin Cheng, Yiqing Wang, Cejie Lan, and Yuan Shen. Response of the mosquito immune system and symbiotic bacteria to pathogen infection. Parasites & Vectors, Feb 2024. URL: https://doi.org/10.1186/s13071-024-06161-4, doi:10.1186/s13071-024-06161-4. This article has 35 citations and is from a peer-reviewed journal.
-
(zhou2024insectantimicrobialpeptides pages 9-10): Lizhen Zhou, Guanliang Meng, Ling Zhu, Li Ma, and Kangkang Chen. Insect antimicrobial peptides as guardians of immunity and beyond: a review. International Journal of Molecular Sciences, Mar 2024. URL: https://doi.org/10.3390/ijms25073835, doi:10.3390/ijms25073835. This article has 53 citations and is from a poor quality or predatory journal.
-
(song2025cellwallcomponents pages 18-19): Xiumei Song, Han Zhou, and Jingwen Wang. Cell wall components of gut commensal bacteria stimulate peritrophic matrix formation in malaria vector mosquitoes through activation of the imd pathway. PLOS Biology, 23:e3002967, Jan 2025. URL: https://doi.org/10.1371/journal.pbio.3002967, doi:10.1371/journal.pbio.3002967. This article has 7 citations and is from a highest quality peer-reviewed journal.
-
(meister2009anophelesgambiaepgrplcmediated pages 4-5): Stephan Meister, Bogos Agianian, Fanny Turlure, Angela Relógio, Isabelle Morlais, Fotis C. Kafatos, and George K. Christophides. Anopheles gambiae pgrplc-mediated defense against bacteria modulates infections with malaria parasites. PLoS Pathogens, 5:e1000542, Aug 2009. URL: https://doi.org/10.1371/journal.ppat.1000542, doi:10.1371/journal.ppat.1000542. This article has 290 citations and is from a highest quality peer-reviewed journal.
-
(meister2009anophelesgambiaepgrplcmediated pages 9-10): Stephan Meister, Bogos Agianian, Fanny Turlure, Angela Relógio, Isabelle Morlais, Fotis C. Kafatos, and George K. Christophides. Anopheles gambiae pgrplc-mediated defense against bacteria modulates infections with malaria parasites. PLoS Pathogens, 5:e1000542, Aug 2009. URL: https://doi.org/10.1371/journal.ppat.1000542, doi:10.1371/journal.ppat.1000542. This article has 290 citations and is from a highest quality peer-reviewed journal.
Citations
- hixson2024innateimmunityin pages 9-10
- li2024responseofthe pages 10-11
- zhou2024insectantimicrobialpeptides pages 9-10
- song2025cellwallcomponents pages 18-19
- meister2009anophelesgambiaepgrplcmediated pages 4-5
- meister2009anophelesgambiaepgrplcmediated pages 9-9
- rodgers2020functionalanalysisof pages 1-3
- rodgers2020functionalanalysisof pages 3-6
- rodgers2020functionalanalysisof pages 10-12
- meister2009anophelesgambiaepgrplcmediated pages 1-2
- meister2009anophelesgambiaepgrplcmediated pages 2-4
- meister2009anophelesgambiaepgrplcmediated pages 11-12
- meister2009anophelesgambiaepgrplcmediated pages 10-11
- rodgers2020functionalanalysisof pages 6-8
- rodgers2020functionalanalysisof pages 12-17
- meister2009anophelesgambiaepgrplcmediated pages 9-10
- https://doi.org/10.1371/journal.ppat.1000542
- https://doi.org/10.1016/j.ibmb.2019.103288
- https://doi.org/10.1098/rstb.2023.0063
- https://doi.org/10.1186/s13071-024-06161-4
- https://doi.org/10.3390/ijms25073835
- https://doi.org/10.1371/journal.pbio.3002967
- https://doi.org/10.1016/j.ibmb.2019.103288,
- https://doi.org/10.1371/journal.ppat.1000542,
- https://doi.org/10.1098/rstb.2023.0063,
- https://doi.org/10.1186/s13071-024-06161-4,
- https://doi.org/10.3390/ijms25073835,
- https://doi.org/10.1371/journal.pbio.3002967,
OpenScientist
(PGRPLC-hypotheses/function-hypothesis-go-0008745/openscientist.md)
OpenScientist
(PGRPLC-hypotheses/function-hypothesis-go-0008745/openscientist.md)Deep Research Report: PGRPLC N-acetylmuramoyl-L-alanine Amidase Activity Hypothesis (GO:0008745)
Gene: PGRPLC (Anopheles gambiae, UniProt A7UTA1)
Hypothesis: PGRPLC has N-acetylmuramoyl-L-alanine amidase activity (GO:0008745)
Evidence type: IBA (Inferred from Biological Aspect of Ancestor), GO_REF:0000033
Verdict: Over-annotated — annotation should be removed
Executive Judgment
Verdict: Over-annotated. The annotation of N-acetylmuramoyl-L-alanine amidase activity (GO:0008745) on Anopheles gambiae PGRPLC (UniProt A7UTA1) is not supported by sequence, structural, or functional evidence and should be removed. This annotation was propagated via phylogenetic inference (IBA, GO_REF:0000033) but is contradicted by three independent and convergent lines of evidence: (1) sequence analysis demonstrating loss of two of three essential zinc-coordinating catalytic residues, (2) high-confidence AlphaFold structural modeling confirming a disrupted active site incapable of zinc coordination, and (3) primary literature unambiguously establishing PGRP-LC orthologs as non-catalytic pattern recognition receptors that activate innate immune signaling. The IBA annotation represents a case of phylogenetic over-propagation from the ancestral PGRP amidase fold to a receptor subfamily that universally lost catalytic capacity early in its evolutionary divergence.
Key caveat: No direct biochemical assay of Ag PGRPLC amidase activity has been published. The verdict rests on strong negative computational evidence (missing catalytic residues in sequence and 3D structure, confirmed across all six PGRP-LC orthologs examined) combined with positive evidence for an alternative function (receptor activity, confirmed in the direct organism). The computational evidence alone is sufficient for a curation decision given its completeness and internal consistency.
Summary
The seed hypothesis proposes that Anopheles gambiae PGRPLC (A7UTA1) possesses N-acetylmuramoyl-L-alanine amidase activity (GO:0008745), an enzymatic function that cleaves the amide bond between N-acetylmuramic acid and L-alanine in bacterial peptidoglycan. This annotation was assigned via Inferred by Biological Aspect of Ancestor (IBA) evidence, meaning it was computationally propagated based on phylogenetic relatedness to other PGRP family members — some of which are indeed bona fide amidases (e.g., PGRP-LB, PGRP-SC1a/b, PGRP-SB1).
Our investigation systematically tested this hypothesis through catalytic residue analysis, AlphaFold structural modeling, cross-species conservation surveys across the PGRP-LC subfamily, and comprehensive literature review spanning 14 publications. We found that PGRPLC lacks two of the three zinc-binding residues absolutely required for amidase catalysis: the first zinc-ligand histidine is replaced by alanine (H→A at position 310), and the third zinc-ligand cysteine is replaced by serine (C→S at position 429). The AlphaFold model AF-A7UTA1-F1 (version 6) confirms these substitutions at very high confidence (pLDDT 96–99) and shows the spatial geometry is consistent with a vestigial, non-functional zinc-binding pocket — the fold is preserved for peptidoglycan binding, but the catalytic machinery is disrupted. This catalytic residue loss is universal across all six PGRP-LC orthologs examined from three insect species (Drosophila melanogaster, Anopheles gambiae, and Bombus species), representing two insect orders (Diptera and Hymenoptera), confirming that PGRP-LC is a dedicated non-catalytic receptor subfamily.
The primary literature unambiguously characterizes PGRP-LC as a transmembrane pattern recognition receptor that activates the Imd/NF-κB innate immune signaling pathway upon binding DAP-type peptidoglycan — it senses the ligand and transduces a signal rather than cleaving the ligand. Multiple studies explicitly distinguish "recognition PGRPs" (including PGRP-LC) from "catalytic PGRPs" (such as PGRP-LB). Notably, the well-characterized Drosophila ortholog (Q9GNK5) correctly lacks the GO:0008745 annotation in UniProt/FlyBase and instead carries experimentally derived annotations for peptidoglycan binding (GO:0042834, IDA) and peptidoglycan immune receptor activity (GO:0016019, IMP). The annotation on A7UTA1 is therefore inconsistent with the ortholog curation.
Key Findings
Finding 1: PGRPLC Lacks Two of Three Critical Zinc-Binding Residues for Amidase Activity
N-acetylmuramoyl-L-alanine amidase activity in the PGRP family requires a zinc-dependent catalytic mechanism with three essential zinc-coordinating residues: two histidines and one cysteine, forming the zinc triad that positions the catalytic water for amide bond hydrolysis. Multiple sequence alignment of Ag PGRPLC (A7UTA1) against five confirmed catalytic PGRPs revealed that PGRPLC has lost two of these three residues:
First zinc ligand His → Ala: At the conserved HH motif position, Ag PGRPLC has the sequence "VIIIAHT" where catalytic PGRPs have "YVIIHH", "YAIIHH", or "FLYVHH". The first histidine, which contributes an imidazole nitrogen for zinc coordination, is replaced by alanine — a small hydrophobic residue with no metal-coordinating capacity whatsoever.
Third zinc ligand Cys → Ser: The cysteine residue that completes the zinc coordination triad, consistently found in the CPG motif (ECPG or SCPG) of catalytic PGRPs upstream of the conserved WPH signature, is replaced by serine. No cysteine was found anywhere in the 25-residue region upstream of the WPH motif in PGRPLC. Cysteine's thiol group is critical for zinc coordination; serine's hydroxyl group is a far weaker zinc ligand (log K_a difference of ~3–4 orders of magnitude).
Conserved but insufficient residues: The catalytic tyrosine (Y347) and the second zinc-binding histidine (H421) are conserved in PGRPLC, but these alone cannot support zinc coordination or catalysis. This pattern — partial conservation of the catalytic apparatus — is the hallmark of a protein that retains the PGRP structural fold for ligand recognition while having lost enzymatic function. The catalytic residue substitution pattern in PGRPLC matches exactly the pattern in other confirmed non-catalytic receptor PGRPs (Dm PGRP-LC, Dm PGRP-SA, Dm PGRP-LE).
Mutagenesis studies on the related zinc-dependent amidase AmpD from Citrobacter freundii (PMID: 14507260) confirmed that mutation of zinc-ligand histidine to alanine (H34A) abolishes both enzymatic activity and zinc binding, establishing the biochemical framework for interpreting the natural H→A substitution in PGRPLC.
{{figure:pgrp_catalytic_residues.png|caption=Catalytic residue conservation analysis across PGRP family members. Ag PGRPLC (A7UTA1) lacks two of three zinc-coordinating residues (His→Ala and Cys→Ser substitutions) compared to confirmed catalytic PGRPs (PGRP-LB, PGRP-SC1a/b, PGRP-SB1, PGLYRP2). The conserved Tyr and second His are necessary but not sufficient for catalysis.}}
Finding 2: Drosophila PGRP-LC Ortholog Correctly Lacks GO:0008745
As a critical validation check, we examined the GO annotations of the best-characterized PGRP-LC ortholog, Drosophila melanogaster PGRP-LC (UniProt Q9GNK5). This protein has multiple experimentally-derived GO annotations including:
- GO:0042834 — peptidoglycan binding (IDA evidence, FlyBase)
- GO:0016019 — peptidoglycan immune receptor activity (IMP evidence, FlyBase)
- Multiple immune response biological process annotations (IMP/IDA)
Critically, GO:0008745 (amidase activity) is NOT annotated for Dm PGRP-LC, despite this being the most extensively studied PGRP-LC ortholog with decades of experimental characterization. Computational verification confirmed that Q9GNK5 also lacks the HH motif and the catalytic Cys. This creates an inconsistency in the GO database: the IBA pipeline assigned amidase activity to the Anopheles ortholog while the Drosophila ortholog — which has far superior experimental annotation — correctly lacks this term. This inconsistency strongly supports the conclusion that the IBA annotation on A7UTA1 is erroneous.
Finding 3: AlphaFold Structure Confirms Disrupted Zinc-Binding Site in 3D
Analysis of the AlphaFold model AF-A7UTA1-F1 (version 6) provided three-dimensional structural confirmation of the disrupted active site. All residues at the putative active-site positions were modeled with very high confidence (pLDDT >96), meaning the structural predictions are reliable:
| Position | Residue | Expected (catalytic) | pLDDT | Status |
|---|---|---|---|---|
| 310 | ALA | His (Zn ligand 1) | 98.5 | Substituted — no imidazole nitrogen |
| 311 | HIS | His (structural) | 98.6 | Present |
| 347 | TYR | Tyr (catalytic) | 98.6 | Present |
| 421 | HIS | His (Zn ligand 2) | 96.0 | Present |
| 429 | SER | Cys (Zn ligand 3) | 97.2 | Substituted — hydroxyl too weak |
Inter-residue distance measurements at the vestigial zinc-binding pocket:
| Atom Pair | Distance (Å) | Interpretation |
|---|---|---|
| A310.CB — S429.OG | 3.6 | Substituted ligands in vestigial pocket geometry |
| H311.NE2 — S429.OG | 4.1 | Near expected zinc coordination distance |
| H421.NE2 — S429.OG | 5.7 | Consistent with vestigial zinc site |
| H311.NE2 — H421.NE2 | 9.4 | Typical for zinc amidase fold |
The spatial arrangement is consistent with a vestigial zinc-binding pocket — the overall fold is maintained (important for peptidoglycan recognition), but the critical chemical groups for zinc coordination are absent. Alanine lacks an imidazole nitrogen entirely, and serine's hydroxyl is thermodynamically and kinetically inadequate to substitute for cysteine's thiol in zinc coordination. The combination makes zinc binding — and therefore zinc-dependent amidase catalysis — impossible.
Finding 4: PGRP-LC Subfamily Universally Lacks Catalytic Amidase Residues Across Insects
A cross-species survey of PGRP-LC orthologs from three insect species demonstrated that the loss of catalytic residues is a universal, subfamily-defining feature — not an organism-specific anomaly in Anopheles:
| Protein | Species | Order | HH Motif | CPG Cys | Classification |
|---|---|---|---|---|---|
| PGRPLC (A7UTA1) | A. gambiae | Diptera | AH (no HH) | Ser (no Cys) | Non-catalytic |
| PGRP-LC-x (Q9GNK5) | D. melanogaster | Diptera | xH (no HH) | absent | Non-catalytic |
| PGRP-LC-a (Q9GNK5) | D. melanogaster | Diptera | xH (no HH) | absent | Non-catalytic |
| PGRP-LC-y (Q9GNK5) | D. melanogaster | Diptera | SH (no HH) | absent | Non-catalytic |
| PGRP-LC-1 | Bombus sp. | Hymenoptera | xH (no HH) | absent | Non-catalytic |
| PGRP-LC-2 | Bombus sp. | Hymenoptera | xH (no HH) | absent | Non-catalytic |
In contrast, all four confirmed catalytic PGRPs tested retained both diagnostic features:
| Protein | Species | HH Motif | CPG Cys | Classification |
|---|---|---|---|---|
| PGRP-LB (Q8INK6) | D. melanogaster | IIHHSY | ECPG | Catalytic |
| PGRP-SC1a (C0HK98) | D. melanogaster | AIIHHT | SCPG | Catalytic |
| PGRP-SC1b (C0HK99) | D. melanogaster | VIIHHSD | SCPG | Catalytic |
| PGRP-SB1 (Q70PY2) | D. melanogaster | present | present | Catalytic |
The 0/6 vs 4/4 split (non-catalytic PGRP-LC vs catalytic PGRPs) is perfectly concordant and spans two insect orders, providing strong evidence that catalytic residue loss occurred early in the evolutionary divergence of the PGRP-LC subfamily as a dedicated signaling receptor. The amidase annotation on any PGRP-LC ortholog is therefore a systematic over-annotation.
{{figure:final_evidence_figure.png|caption=Comprehensive evidence for PGRPLC non-catalytic classification. Left: disrupted active-site residues (Ala replaces zinc-ligand His, Ser replaces zinc-ligand Cys). Center: AlphaFold 3D geometry of the vestigial zinc pocket (AF-A7UTA1-F1, pLDDT >96). Right: cross-species classification showing all 6 PGRP-LC orthologs are universally non-catalytic while all 4 catalytic PGRPs retain the complete zinc triad.}}
Evidence Matrix
| # | Citation | Evidence Type | Direction | Claim Tested | Key Finding | Context | Confidence |
|---|---|---|---|---|---|---|---|
| 1 | This study | Computational (active-site residue analysis) | Refutes amidase | PGRPLC has catalytic zinc triad | First zinc ligand His→Ala (pos 310); third zinc ligand Cys→Ser (pos 429); 2/3 zinc ligands lost | MSA of 7 PGRPs | High |
| 2 | This study | Computational (AlphaFold structure) | Refutes amidase | Active site is structurally intact | AF-A7UTA1-F1 shows vestigial zinc pocket with chemically incompetent ligands; all pLDDT >96 | 3D structural analysis | High |
| 3 | This study | Computational (cross-species survey) | Refutes amidase | Loss is PGRPLC-specific | All 6/6 PGRP-LC orthologs lack HH + CPG; 4/4 catalytic PGRPs retain both; universal subfamily feature | UniProt survey, 3 species, 2 orders | High |
| 4 | PMID: 19662170 | Mutant phenotype / functional | Supports receptor function | PGRPLC function in A. gambiae | "the transmembrane PGN Recognition Protein LC (PGRP-LC) is a receptor of the Imd signaling pathway that is activated after infection with bacteria" | A. gambiae, in vivo | High — direct study of this gene |
| 5 | PMID: 15657141 | Functional assay | Supports receptor function | Dm PGRP-LC molecular function | "PGRP-LC, a transmembrane protein required for the response to bacterial infection, acts at the top of a cytoplasmic signaling cascade" | D. melanogaster, in vivo | High — direct ortholog |
| 6 | PMID: 16556841 | Structural (crystal, 2.1 Å) | Supports binding, refutes catalysis | PGRP-LC binding mode | Crystal structure of TCT-PGRP-LCa/LCx complex shows ligand binding and receptor dimerization, no cleavage | D. melanogaster, in vitro | High — atomic resolution |
| 7 | PMID: 22118526 | Genetic/functional | Qualifies (distinguishes classes) | Catalytic vs. recognition PGRPs | "recognition PGRPs, which activate the Toll and Imd pathways" vs "six catalytic PGRPs with the capacity to scavenge peptidoglycan" | D. melanogaster, systematic analysis | High |
| 8 | PMID: 16618604 | Direct assay, comparison | Qualifies (contrasts functions) | PGRP-LB vs PGRP-LC | "host defense against gram-negative bacteria is mediated by the Imd pathway upon sensing of peptidoglycan by PGRP-LC. Here we report ... PGRP-LB, a catalytic member of the PGRP family" | D. melanogaster, biochemical assay | High |
| 9 | PMID: 17363965 | Review (synthesized evidence) | Qualifies | PGRP family diversity | "only some PGRPs have the catalytic activity...most PGRPs have diversified to carry out other host-defence functions" | Cross-species review | High — authoritative |
| 10 | PMID: 34066955 | Structural/review | Qualifies | PGRP catalytic mechanism | "Non-catalytic PGRPs are involved in the activation of immune pathways by binding to the PGN, whereas amidase PGRPs are capable of cleaving the PGN" | General, structural | High |
| 11 | PMID: 14507260 | Mutagenesis, enzymology | Supports residue framework | Zinc ligand requirements | H34A mutation in AmpD (a bona fide amidase) abolishes activity and zinc binding | C. freundii, in vitro | High |
| 12 | UniProt Q9GNK5 | Database (expert-curated) | Supports removal | Ortholog annotation state | Dm PGRP-LC has GO:0042834 (IDA) and GO:0016019 (IMP) but NOT GO:0008745 | FlyBase/UniProt curation | High |
GO Curation Implications
Primary Recommendation: REMOVE GO:0008745
The current annotation of GO:0008745 (N-acetylmuramoyl-L-alanine amidase activity) with IBA evidence should be removed from A7UTA1. This is a high-confidence lead based on convergent computational and literature evidence.
Rationale:
1. The protein lacks 2/3 zinc-binding residues essential for amidase catalysis (His→Ala, Cys→Ser)
2. The Drosophila ortholog Q9GNK5, with superior experimental characterization, does NOT carry this annotation
3. Multiple primary publications classify PGRP-LC explicitly as a non-catalytic pattern recognition receptor
4. The loss is universal across the PGRP-LC subfamily (6/6 orthologs from 2 insect orders)
5. AlphaFold structural analysis confirms the active site is chemically incapable of zinc coordination
Recommended Replacement Annotations (Curator Leads)
| GO Term | Label | Ontology | Suggested Evidence | Justification |
|---|---|---|---|---|
| GO:0042834 | peptidoglycan binding | MF | IBA or ISS (from Q9GNK5) | Conserved PGRP fold retains PGN-binding capacity; Dm ortholog has IDA |
| GO:0016019 | peptidoglycan immune receptor activity | MF | IBA or ISS (from Q9GNK5), or IMP citing PMID:19662170 | Confirmed for Ag PGRPLC (PMID: 19662170) and Dm PGRP-LC (IMP) |
| GO:0004888 | transmembrane signaling receptor activity | MF | ISS | Supported by transmembrane topology and signal transduction function |
Terms to Review
- GO:0008270 (zinc ion binding, IEA:InterPro): Should be reviewed since 2/3 zinc ligands are substituted. The IEA annotation from InterPro's Amidase_2 domain entry (PF01510) does not distinguish catalytic from non-catalytic family members.
Evidence Code Considerations
The IBA evidence code is appropriate for propagating conserved functions across orthologs, but it requires that the function being propagated is actually conserved — including conservation of the mechanistic basis for that function. In this case, the catalytic residues are not conserved, so IBA propagation of the catalytic activity is incorrect even though the overall PGRP domain fold is conserved. This is an instance of a known limitation of phylogenetic annotation transfer that occurs when binding and catalytic functions have diverged within a protein family.
Mechanistic Scope
Direct Molecular Function of PGRPLC
PGRPLC functions as a transmembrane pattern recognition receptor for bacterial peptidoglycan, specifically DAP-type peptidoglycan found in Gram-negative bacteria. Its PGRP domain retains the structural fold necessary to bind peptidoglycan but has lost the zinc-dependent catalytic apparatus required to cleave it. Upon binding peptidoglycan, PGRPLC undergoes conformational changes (likely involving homo- or hetero-dimerization, as demonstrated for the Drosophila ortholog in the crystal structure study by Chang et al.) that activate the intracellular Imd signaling cascade, ultimately leading to NF-κB/Relish-dependent transcription of antimicrobial peptide genes.
Separation of Binding from Catalysis
This distinction is critical for GO annotation. The PGRP family has diverged into two functionally distinct groups that perform opposite biological roles:
PGRP Family (common ancestor: zinc-dependent amidase)
│
├── Catalytic PGRPs (amidases): PGRP-LB, PGRP-SC1a/b, PGRP-SB1, PGLYRP2
│ ├── Retain HH + CPG zinc triad → bind zinc → cleave PGN
│ ├── Cleave PGN → non-immunogenic fragments
│ └── Function: IMMUNE DAMPENING / negative regulation
│
└── Non-catalytic PGRPs (receptors): PGRP-LC, PGRP-LE, PGRP-SA, PGRP-SD
├── Lost 2/3 zinc ligands (H→A/S, C→S) → no zinc → no cleavage
├── Bind PGN intact → receptor dimerization → signal transduction
└── Function: IMMUNE ACTIVATION via Toll or Imd pathways
The amidase annotation conflates these two fundamentally different biological roles. Catalytic PGRPs act as negative regulators of immunity by degrading the immunostimulatory ligand, while receptor PGRPs like PGRPLC act as positive activators by sensing the ligand and transducing a signal. Assigning amidase activity to PGRPLC therefore not only misrepresents the molecular function but inverts the biological logic of its immune role.
Downstream Effects (Not Direct Gene-Product Activity)
The following are downstream consequences of PGRPLC's receptor function, not its direct molecular activity — they should not be confused with the MF annotation:
- Activation of the Imd signaling pathway
- Nuclear translocation of Relish/Rel2 (NF-κB homolog)
- Transcription of antimicrobial peptide genes (Diptericin, Cecropin, Defensin, etc.)
- Anti-Plasmodium defense in mosquitoes (PMID: 19662170)
- Regulation of gut microbiota homeostasis
Conflicts and Alternatives
Why the IBA Annotation Was Propagated (Root Cause)
The IBA evidence code, assigned via GO_REF:0000033 (phylogenetic annotation by GO_Central using PANTHER), infers function from evolutionary relationships. All PGRPs share the Amidase_2 domain fold (Pfam PF01510) and are homologous to T7 lysozyme and bacterial amidases. The phylogenetic inference correctly identifies PGRPLC as a member of the PGRP family but incorrectly infers that the ancestral amidase activity is retained. This is a classic case of subfamily over-annotation where a shared domain fold does not equate to shared enzymatic function — analogous to pseudokinases that retain the kinase fold but lack phosphotransferase activity.
No Competing Evidence for Amidase Activity
We found no evidence in the primary literature or any database suggesting that any PGRP-LC ortholog has amidase activity. No biochemical assay demonstrating peptidoglycan cleavage by any PGRP-LC has ever been published. Every characterization of PGRP-LC molecular function in the literature describes receptor and binding activities exclusively.
Residual Zinc Binding Through an Alternative Mechanism?
One theoretical alternative is that PGRPLC might bind zinc through a non-canonical coordination mechanism. However, this is unlikely because: (1) alanine has no metal-coordinating side chain, (2) serine is thermodynamically inadequate as a zinc ligand compared to cysteine, and (3) no alternative zinc-coordinating residues are positioned appropriately in the AlphaFold model. Even if trace zinc binding occurred, it would not be sufficient for catalysis without the complete coordination geometry.
Organism-Specific Considerations
The primary literature on PGRPLC function specifically in Anopheles gambiae (PMID: 19662170) directly confirms the receptor function and provides no evidence for amidase activity. There is no organism-specific divergence that would rescue catalytic activity lost at the sequence level. The mosquito and fly PGRP-LC orthologs share the same substitution pattern and functional characterization.
InterPro Domain vs GO Function Distinction
InterPro annotates A7UTA1 with IPR002502 (N-acetylmuramoyl-L-alanine amidase domain). This describes the structural fold, not the enzymatic activity. The distinction between a domain annotation (structural classification) and a GO molecular function annotation (biochemical activity) is important: many protein families retain domain folds while losing specific catalytic activities. The InterPro domain annotation is technically correct (PGRPLC has the amidase fold); the GO function annotation is not (PGRPLC does not perform the amidase reaction).
Knowledge Gaps
| # | Gap | What Was Checked | Why It Matters | What Would Resolve It |
|---|---|---|---|---|
| 1 | No direct biochemical amidase assay for A7UTA1 | PubMed literature search; no published enzymatic assay found | A negative enzymatic result would provide definitive IDA-level evidence for removal | Recombinant expression of PGRPLC ectodomain + PGN hydrolysis assay (HPLC muropeptide profiling) |
| 2 | No experimental crystal structure of Ag PGRPLC | AlphaFold model analyzed (AF-A7UTA1-F1, pLDDT >96 at all active-site positions) | Experimental structure would be definitive; AlphaFold prediction is high-confidence but not experimental | X-ray crystallography of PGRPLC ectodomain ± PGN fragment |
| 3 | Zinc binding not directly tested | 2/3 zinc ligands missing by sequence; AlphaFold distances measured | Even if amidase activity is absent, zinc-binding status affects GO:0008270 annotation | ICP-MS, PAR assay, or anomalous diffraction on purified protein |
| 4 | IBA pipeline propagation logic not traced | PANTHER tree node not examined | Understanding the source of the error could prevent similar over-annotations | Review PTHR11022 PANTHER tree ancestral reconstruction for amidase activity |
| 5 | Peptidoglycan binding confirmed only by homology for Ag PGRPLC | Crystal structure exists for Dm ortholog (PMID:16556841); no direct binding data for Ag | Replacement annotation (GO:0042834) should ideally have experimental support in this organism | SPR or pull-down assay with DAP-type PGN |
Discriminating Tests
Biochemical (Highest Priority)
-
Peptidoglycan hydrolysis assay (definitive): Express and purify the PGRPLC ectodomain (approximately residues 242–464), and test for N-acetylmuramoyl-L-alanine amidase activity using HPLC-based muropeptide profiling with DAP-type PGN as substrate. Include Dm PGRP-LB as a positive control and heat-inactivated PGRP-LB as a negative control. A negative result would provide definitive IDA evidence for annotation removal.
-
Zinc-binding assay: Use ICP-MS or a colorimetric zinc assay (PAR assay) on purified PGRPLC PGRP domain to determine whether the disrupted triad retains any zinc-binding capacity. This directly tests the structural prediction.
-
Gain-of-function mutagenesis: Introduce the two missing zinc ligands (A310H, S429C) into PGRPLC and test whether this restores amidase activity. A positive result would definitively prove these substitutions are responsible for loss of catalysis.
Computational (Supporting)
-
Expanded PGRP-LC subfamily survey: Extend the cross-species analysis to additional insect orders (Lepidoptera, Coleoptera, Hemiptera) to confirm universal catalytic residue loss in the PGRP-LC clade and identify the evolutionary branch point where loss occurred.
-
Systematic IBA audit: Search the GO database for all proteins annotated with GO:0008745 via IBA and check whether each has the complete zinc triad. This would identify other potential over-annotations in the PGRP family.
-
Molecular dynamics simulation: Simulate the PGRPLC active site with and without zinc ion placement to assess whether the disrupted triad can transiently coordinate zinc under physiological conditions.
Curation Leads
Lead 1: Remove GO:0008745 from A7UTA1 (High Confidence)
- Action: Remove N-acetylmuramoyl-L-alanine amidase activity (GO:0008745) with IBA evidence
- Reason: Two of three zinc-coordinating catalytic residues are substituted (H→A, C→S); entire PGRP-LC subfamily is non-catalytic; ortholog Dm PGRP-LC (Q9GNK5) correctly lacks this annotation
- Key references to verify:
- PMID: 17363965 — Snippet: "only some PGRPs have the catalytic activity that protects the host from excessive inflammation, and most PGRPs have diversified to carry out other host-defence functions"
- PMID: 22118526 — Snippet: "In addition to recognition PGRPs, which activate the Toll and Imd pathways, the Drosophila genome encodes six catalytic PGRPs with the capacity to scavenge peptidoglycan"
- PMID: 16618604 — Snippet: "host defense against gram-negative bacteria is mediated by the Imd pathway upon sensing of peptidoglycan by PGRP-LC. Here we report a functional analysis of PGRP-LB, a catalytic member of the PGRP family"
Lead 2: Add GO:0016019 — peptidoglycan immune receptor activity (High Confidence)
- Action: Add GO:0016019 with ISS or IBA evidence referencing Dm PGRP-LC (Q9GNK5), or IMP citing PMID:19662170
- Reference to verify: PMID: 19662170 — Snippet: "the transmembrane PGN Recognition Protein LC (PGRP-LC) is a receptor of the Imd signaling pathway that is activated after infection with bacteria"
Lead 3: Add GO:0042834 — peptidoglycan binding (Moderate-High Confidence)
- Action: Add GO:0042834 with ISS evidence referencing Dm PGRP-LC (Q9GNK5, which has IDA for this term)
- Reference to verify: PMID: 16556841 — Snippet: "Tracheal cytotoxin (TCT), a naturally occurring fragment of Gram-negative peptidoglycan, is a potent elicitor of innate immune responses in Drosophila. It induces the heterodimerization of its recognition receptors, the peptidoglycan recognition proteins (PGRPs) LCa and LCx"
Lead 4: Review GO:0008270 — zinc ion binding (Moderate Confidence)
- Action: Review whether zinc ion binding (IEA:InterPro) is appropriate given 2/3 zinc ligands are substituted
- Reason: The IEA from InterPro's Amidase_2 domain does not distinguish catalytic from non-catalytic family members; zinc binding is likely abolished
Lead 5: Flag PANTHER IBA Pipeline for PGRP Family (Systemic)
- Action: Flag PTHR11022 (PGRP family) for review of amidase activity propagation
- Reason: The phylogenetic inference should distinguish between the conserved peptidoglycan-binding function (appropriate for IBA propagation to all PGRPs) and the catalytic amidase function (appropriate only for the PGRP-LB/SC/SB catalytic subclade)
- Scope: May affect multiple non-catalytic PGRP family members across insect genomes
Evidence Base: Key Literature
Direct Studies of PGRP-LC Function
Meister et al. (2009) (PMID: 19662170) is the most directly relevant publication, examining PGRPLC function specifically in Anopheles gambiae. The authors describe PGRP-LC as "a receptor of the Imd signaling pathway that is activated after infection with bacteria," establishing its role as a sensor/receptor in the exact organism under review. The study demonstrates that PGRPLC-mediated defense modulates Plasmodium infection, connecting innate immune signaling to malaria vector competence.
Choe et al. (2005) (PMID: 15657141) characterizes Drosophila PGRP-LC as a protein that "acts at the top of a cytoplasmic signaling cascade," functioning as a signal-transducing innate immune receptor. This foundational study establishes PGRP-LC as a receptor, not an enzyme.
Chang et al. (2006) (PMID: 16556841) provides the crystal structure of tracheal cytotoxin (a monomeric PGN fragment) in complex with the PGRP-LCa/LCx ectodomain heterodimer at 2.1 Å resolution. This atomic-level structural data shows PGRP-LC binds peptidoglycan and triggers receptor dimerization, with no evidence of substrate cleavage — consistent with receptor function.
Studies Distinguishing Catalytic from Non-Catalytic PGRPs
Zaidman-Rémy et al. (2006) (PMID: 16618604) directly contrasts PGRP-LC with PGRP-LB within a single study: "host defense against gram-negative bacteria is mediated by the Imd pathway upon sensing of peptidoglycan by PGRP-LC. Here we report a functional analysis of PGRP-LB, a catalytic member of the PGRP family." This explicit side-by-side comparison — PGRP-LC as sensor, PGRP-LB as catalyst — provides strong qualitative evidence.
Paredes et al. (2011) (PMID: 22118526) systematically categorizes PGRPs: "recognition PGRPs, which activate the Toll and Imd pathways" versus "six catalytic PGRPs with the capacity to scavenge peptidoglycan." PGRP-LC is explicitly placed in the recognition (non-catalytic) category.
Dziarski and Gupta (2006) (PMID: 17363965) provides an authoritative review: "only some PGRPs have the catalytic activity that protects the host from excessive inflammation, and most PGRPs have diversified to carry out other host-defence functions."
Catalytic Mechanism and Residue Requirements
Kerff et al. (2003) (PMID: 14507260) — Mutagenesis of Citrobacter freundii AmpD, a zinc-dependent amidase in the same structural family as PGRPs, demonstrated that mutation of zinc ligands (H34A, D164A) abolishes both enzymatic activity and zinc binding. This establishes the biochemical requirement for intact zinc-coordinating residues — the same residues that PGRPLC naturally lacks.
Brisset et al. (2021) (PMID: 34066955) — Structural analysis of PGRP-LB amidase mechanism confirms the functional dichotomy: "Non-catalytic PGRPs are involved in the activation of immune pathways by binding to the PGN, whereas amidase PGRPs are capable of cleaving the PGN into non-immunogenic compounds."
Additional PGRP Immune Function Studies
Paquette et al. (2017) (PMID: 29045898) describes PGRP-LC and PGRP-LE as receptors that activate the Imd pathway through amyloid-like aggregation of the adaptor protein Imd, providing additional mechanistic detail on the receptor signaling mechanism that is the true function of these proteins.
Lhocine et al. (2008) (PMID: 18688280) identifies Rudra as a negative regulator that binds and inhibits PGRP-LC, further confirming its role as a signaling receptor requiring regulation.
Limitations
-
No direct biochemical assay of Ag PGRPLC amidase activity: Our conclusion is based on computational analysis (sequence, structure, conservation) and analogy to the well-characterized Drosophila ortholog. While the convergent evidence is overwhelming, it remains formally indirect — no one has demonstrated in vitro that purified PGRPLC cannot cleave PGN.
-
AlphaFold model rather than experimental structure: The structural analysis relies on a predicted model. However, the very high pLDDT scores (>96 for all active-site residues) and complete consistency with the sequence analysis provide strong confidence. The Drosophila PGRP-LC crystal structure (PMID: 16556841) corroborates the predicted fold.
-
Cross-species survey limited to 3 insect species: The conservation survey covered 6 PGRP-LC sequences from Drosophila, Anopheles, and Bombus (2 insect orders). A broader survey across additional orders would strengthen the universality claim, though the complete concordance observed (0/6 catalytic) spanning 2 orders is already highly informative.
-
IBA pipeline internals not audited: We could not directly examine the PANTHER ancestral reconstruction to determine exactly how GO:0008745 was assigned to A7UTA1. The specific phylogenetic node and inference logic that led to this over-propagation remain unclear.
-
Replacement annotations based on ortholog inference: The recommended replacement terms (GO:0042834, GO:0016019) for Ag PGRPLC are based primarily on ISS/IBA from the Drosophila ortholog. Direct experimental evidence in A. gambiae for peptidoglycan binding (IDA-level) is limited to functional studies rather than direct binding assays.
Proposed Follow-up Experiments/Actions
Immediate Curation Actions (No Additional Data Needed)
- Remove GO:0008745 from A7UTA1 with documentation noting the missing zinc ligands and receptor function evidence
- Add GO:0042834 (peptidoglycan binding) with ISS evidence referencing Q9GNK5 if not already present
- Add GO:0016019 (peptidoglycan immune receptor activity) with ISS evidence referencing Q9GNK5 or IMP citing PMID:19662170
- Review GO:0008270 (zinc ion binding, IEA:InterPro) for appropriateness given the disrupted zinc site
- Flag PTHR11022 in PANTHER for review of amidase activity propagation to non-catalytic subfamily members
Experimental Priorities (If Resources Available)
- Peptidoglycan hydrolysis assay with purified Ag PGRPLC ectodomain — highest priority for definitive evidence
- Zinc-binding assay (ICP-MS or PAR) to confirm absence of metal coordination
- Gain-of-function mutagenesis (A310H + S429C) to test whether catalytic activity can be restored
- Direct PGN-binding assay (SPR or pull-down) for Ag PGRPLC to provide IDA-level support for the replacement GO:0042834 annotation
Computational Extensions
- Pan-insect PGRP-LC survey across Lepidoptera, Coleoptera, Hemiptera, and other orders
- Systematic audit of IBA-propagated GO:0008745 annotations across all PGRP family members to identify other over-annotations
- Structural overlay of AlphaFold PGRPLC model with experimental PGRP-LB crystal structure to visualize the active-site disruption for publication-quality figures
Methods Summary
Iteration 1: Sequence-Based Active-Site Analysis
- UniProt entries retrieved for A7UTA1 and 6 reference PGRPs (catalytic and non-catalytic)
- Multiple sequence alignment computed via EBI Clustal Omega API
- Catalytic residue positions mapped from annotated zinc-binding sites in catalytic PGRPs (H59/Y95/H169/C177 in PGRP-LB; H410/Y447/H522/C530 in PGLYRP2)
- Independent motif verification: HH motif search and CPG motif search upstream of WPH signature
- Literature search: 14 papers reviewed covering PGRP-LC function, PGRP family diversity, amidase mechanism
Iteration 2: Structural and Cross-Species Validation
- AlphaFold model AF-A7UTA1-F1 (v6) downloaded and analyzed: residue identities, pLDDT scores, inter-residue distances at active site
- Cross-species PGRP-LC survey: 50 insect PGRP entries from UniProt classified by HH motif and CPG motif presence
- 6/6 PGRP-LC orthologs confirmed non-catalytic; 4/4 catalytic PGRPs confirmed catalytic
- AlphaFold active-site distance measurements: A310.CB–S429.OG (3.6 Å), H311.NE2–S429.OG (4.1 Å), H421.NE2–S429.OG (5.7 Å)
{{figure:pgrp_evidence_summary.png|caption=Summary evidence figure integrating sequence analysis, structural modeling, and literature evidence supporting the over-annotation verdict for GO:0008745 on Ag PGRPLC}}
