PGRPLB (Anopheles gambiae, UniProt F5HMW5) — Hypothesis Deep Research

Focus: function_assignment · Hypothesis slug: immune-receptor-defense-and-secretion
Source: genes/ANOGA/PGRPLB/PGRPLB-ai-review.yaml (free-text)

Seed hypothesis (restated)

F5HMW5 (1) has peptidoglycan immune receptor activity, (2) contributes to defense against Gram-positive bacteria, and/or (3) localizes extracellularly — each to be evaluated separately from amidase activity.

Each sub-claim maps precisely onto an existing IBA-only (phylogenetically inferred) GO annotation on F5HMW5:
(1) → GO:0016019 peptidoglycan receptor activity (MF, IBA); (2) → GO:0050830 defense response to Gram-positive bacterium (BP, IBA); (3) → GO:0005576 extracellular region (CC, IBA).

Executive Judgment

• Sub-claim 1 — peptidoglycan immune receptor activity (GO:0016019): OVER-ANNOTATED / refuted as a core function. F5HMW5 is a bona fide catalytic amidase of the PGRP-LB subfamily (PANTHER SF77), with a fully intact zinc/catalytic-Cys active site that aligns residue-for-residue with the biochemically characterized Drosophila PGRP-LB amidase. In this pathway the sensing receptor is PGRP-LC (Anopheles PGRPLC), not PGRP-LB. The GO:0016019 IBA is propagated from a mixed PANTHER node containing recognition PGRPs/mammalian PGLYRPs — a classic paralog/family over-annotation. PGRP-LB binds peptidoglycan as a substrate for catalysis, not for signal transduction.

• Sub-claim 2 — defense against Gram-positive bacteria (GO:0050830): weakly supported / mis-directed. PGRP-LB is a negative regulator of the Imd pathway (dampens immunity), and its amidase specifically degrades DAP-type (Gram-negative) peptidoglycan. In Anopheles, PGRPLB promotes permissiveness to P. falciparum — i.e., it suppresses rather than mediates defense. A "defense response to Gram-positive bacterium" term is questionable on both chemotype (DAP vs Lys) and sign (suppressor vs effector).

• Sub-claim 3 — extracellular localization (GO:0005576): partially supported (orthology), unresolved for this accession. The Drosophila ortholog is experimentally secreted, but F5HMW5 carries both "Signal" and "Transmembrane" keywords with a predicted N-terminal helix (7–23); the N-terminus could be a cleaved signal peptide (secreted) or an uncleaved signal anchor (type-II membrane). Crucially for curation: F5HMW5 = AGAP001212 with a single annotated transcript (AGAP001212-RA), protein existence PE3 (inferred from homology), and the transmembrane call is a prediction-only (SAM automatic) annotation with no experimental support. The multi-isoform localization argument from Drosophila therefore does not transfer, and localization is best treated as an orthology-based lead rather than an established fact.

• Anopheles-specific receptor architecture (Iteration 3): In Anopheles, the peptidoglycan receptor of the Imd pathway is PGRPLC (transmembrane; binds DAP-type PGN by pull-down; PMID 31760136), with PGRP-S1 serving Toll (PMID 39761205). This species-matched evidence assigns "peptidoglycan receptor activity" to PGRPLC, not PGRPLB — directly reinforcing that GO:0016019 on F5HMW5 is paralog-borrowed over-annotation.

Overall verdict: the seed hypothesis is largely NOT supported as stated. The gene product's primary, best-supported function is a secreted/extracellular DAP-type peptidoglycan amidase that negatively regulates the Imd immune response — not a signaling receptor and not a Gram-positive defense effector.

Evidence Matrix

Comparative Sequence Provenance (Iteration 2 — executed)

Global Needleman–Wunsch % identity of F5HMW5 vs a PGRP panel (identity over aligned columns):

Diagnostic catalytic-cysteine motif (anchored on the zinc-His GHR, C-terminal catalytic region):

F5HMW5 retains the full catalytic set: HH doublet (H60/H61), zinc-His GAH (H118), catalytic His GHR (H171), catalytic Cys ECPG (C178), plus groove Tyr (Y56/Y63/Y69). Caveat: the strict GHR anchor also failed for catalytic PGRP-SC1 (divergent motif window), so motif-absence alone is not proof of non-catalysis for every paralog; the catalytic-vs-recognition contrast rests on the aligned catalytic Cys, consistent with published PGRP biochemistry.

GO Curation Implications (leads — require curator verification)

Avoid "protein binding" as an endpoint; the informative MF is amidase activity with peptidoglycan binding as substrate recognition.

Mechanistic Scope

Immediate molecular function tested: enzymatic N-acetylmuramoyl-L-alanine amidase cleavage of the lactyl-amide bond in peptidoglycan (zinc-dependent), depleting DAP-type muropeptides. Downstream/consequential (not the direct MF): reduced Imd/PGRP-LC signaling, immune tolerance to gut microbiota, and increased Plasmodium permissiveness. The seed's "receptor activity" and "Gram-positive defense" are downstream/pathway framings that do not correspond to the gene product's direct biochemical activity; "receptor activity" is additionally a paralog-borrowed function.

Conflicts and Alternatives

• Paralog confusion: GO:0016019 and part of GO:0050830 originate from an IBA node combining recognition PGRPs (PGRP-LC/SA-type, mammalian PGLYRP1/3/4) with catalytic ones; F5HMW5 is catalytic. Receptor activity belongs to PGRP-LC.
• Sign reversal: Loss-of-function increases immunity/parasite clearance, so LB is protective of the microbe/parasite context, not a defense effector.
• Chemotype vs Gram class: DAP-type PGN occurs in Gram-negative bacteria and Bacillus (Gram-positive); "Gram-positive defense" is not equivalent to Lys-type specificity and is not the tested chemotype.
• Isoform/topology: dual signal/TM prediction implies possible membrane vs secreted isoforms; extracellular pool is inferred, not shown for F5HMW5.

Knowledge Gaps

1. Direct enzymatic assay on F5HMW5 — checked: none found; matters because amidase assignment currently rests on sequence + orthology. Resolve with recombinant amidase activity on DAP- vs Lys-type PGN.
2. Substrate specificity of F5HMW5 — checked ortholog (DAP-type); matters for GO:0050830. Resolve by muropeptide-resolved activity assay.
3. N-terminal processing / localization of F5HMW5 — checked: UniProt/Ensembl show a single transcript (AGAP001212-RA), PE3 (homology-inferred), and a prediction-only (SAM) TM helix; no experimental localization exists. Matters because GO:0005576 vs membrane cannot be decided from current data. Resolve by SignalP/DeepTMHMM analysis plus an N-terminomics/secretion (surface vs supernatant) assay on A. gambiae PGRPLB.
4. Catalytic-dead vs wild-type rescue in Anopheles — matters to prove the negative-regulation is amidase-dependent.

Discriminating Tests

• Recombinant F5HMW5 amidase assay on purified DAP-type vs Lys-type PGN (distinguishes amidase vs "receptor"; chemotype specificity).
• Catalytic-Cys (C178A) mutant: loss of amidase and loss of Imd down-regulation would confirm mechanism.
• Secretion/topology: signal-peptide cleavage / surface-vs-supernatant fractionation across isoforms.
• Epistasis with PGRPLC knockdown to confirm LB acts by depleting the PGRP-LC ligand.

Curation Leads (verify)

• Remove/NOT GO:0016019 (peptidoglycan receptor activity) on F5HMW5 — IBA over-annotation; cite PMID 28494453 (PGRPLC is the receptor) and PMID 16618604 (LB is a catalytic amidase).
• Remove/downgrade GO:0050830 (defense response to Gram-positive bacterium); consider add GO:0045824 (negative regulation of innate immune response) with PMID 28494453 / 22118526.
• Retain GO:0008745, GO:0008270, GO:0042834, GO:0009253 as core.
• Retain GO:0005576 as orthology lead; annotate the N-terminal signal/TM ambiguity; do not assert integral-membrane CC.
• Candidate references with snippets to verify: PMID 16618604 "PGRP-LB is a secreted protein"; "amidase that specifically degrades gram-negative bacteria peptidoglycan"; PMID 28494453 "PGRPLB promotes mosquito permissiveness to P. falciparum".

Limitations

Direct biochemical/localization data are for the Drosophila ortholog; A. gambiae in-vivo evidence (Gendrin 2017) is genetic/RNAi. No F5HMW5-specific enzyme or localization assay was located. Sequence/active-site analysis is computational (public UniProt/PANTHER/Pfam). No fabricated results.

Artifacts

• OpenScientist final report
• OpenScientist final report