Assigns GO terms, Pfam/InterPro domains, KEGG orthologs, EC numbers, and product names to predicted proteins using eggNOG-mapper (orthology), InterProScan (domain signatures), and KofamScan (KEGG), routing specialized functions to dbCAN/antiSMASH/AMRFinderPlus/SignalP. Covers the orthology-vs-domain-vs-homology paradigms, the annotation-error percolation cascade, domain-presence-is-not-function, GO IEA circularity in enrichment, evidence tiering, and bit-score/coverage thresholds. Use when adding functional annotation to predicted genes, choosing between eggNOG-mapper and InterProScan, or judging how much to trust a functional label.
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Assigns GO terms, Pfam/InterPro domains, KEGG orthologs, EC numbers, and product names to predicted proteins using eggNOG-mapper (orthology), InterProScan (domain signatures), and KofamScan (KEGG), routing specialized functions to dbCAN/antiSMASH/AMRFinderPlus/SignalP. Covers the orthology-vs-domain-vs-homology paradigms, the annotation-error percolation cascade, domain-presence-is-not-function, GO IEA circularity in enrichment, evidence tiering, and bit-score/coverage thresholds. Use when adding functional annotation to predicted genes, choosing between eggNOG-mapper and InterProScan, or judging how much to trust a functional label.
Before using code patterns, verify installed versions match. If versions differ:
CLI: <tool> --version then <tool> --help to confirm flags
Python: pip show <package> then help(module.function) to check signatures
Annotation content tracks database release: record the eggNOG DB version, InterPro/Pfam release, and KEGG/KofamScan profile date, and note whether InterProScan used the EBI precalculated lookup service. eggNOG-mapper v3 is under testing (not production) - pin v2.1.15. If code throws an error, introspect the installed tool and adapt rather than retrying.
Functional Annotation
"Functionally annotate my predicted proteins" -> Transfer GO/KEGG/Pfam/EC/product labels from characterized proteins by orthology and domain signatures, attaching a confidence tier and provenance to each.
The Single Most Important Modern Insight -- Annotation Is a Propagated Hypothesis, Not a Measurement
Almost every label on a new genome is transferred by homology/orthology/ML from a small island of experimentally characterized proteins. The transfer chain is lossy and self-reinforcing - it behaves like a percolation cascade (Gilks 2002 Bioinformatics 18:1641): an over-specific name assigned in year 0, deposited with no record that it was transferred, becomes the nearest hit for the next genome, whose label becomes evidence for the next. By the time a query reaches NR, "number of hits agreeing" measures how far an error spread, not correctness. Schnoes 2009 (PLoS Comput Biol 5:e1000605) found misannotation reaching ~80% in bulk databases (TrEMBL/NR) and near-zero in curated Swiss-Prot - the gap is the curation. Three load-bearing consequences:
The goal is not "maximally annotated" - it is "honestly tiered." A genome that is 40% "hypothetical protein" with the rest correctly tiered by evidence is a better scientific object than one 95% named with half the names wrong. Prefer curated/orthology donors (Swiss-Prot, eggNOG OG consensus) over best-hits, and demote specificity as identity/coverage fall (full EC -> partial 1.1.1.-; specific name -> superfamily; whole-protein -> per-domain). PI/reviewer pressure to "annotate everything" manufactures the next genome's percolating error.
"Domain present" and "function known" are different claims. A Pfam hit reports architecture, not activity - ~10% of the human kinome are catalytically dead pseudokinases that carry a confident "protein kinase" domain. Moonlighting (GAPDH), promiscuity, and mechanistically-diverse superfamilies (enolase, amidohydrolase, HAD, TIM-barrel: shared fold, divergent substrate) make this a first-order effect. Fold conservation != function conservation any more than sequence does - so structure-based transfer (Foldseek) inherits the same trap with higher false confidence.
Record provenance on every label (method, donor, donor evidence code, identity/coverage/bitscore, DB version). That is the only thing that stops the provenance-amnesia step that turns a transfer into a "fact."
Tool Taxonomy
Paradigm
Tool
Mechanism
Failure mode
Orthology
eggNOG-mapper
seed-ortholog -> orthologous group -> consensus transfer
tax-scope sensitive; HGT/xenologs break the orthology assumption
low precision; ontology terms not products; reaches twilight zone only
Default workhorse pair: eggNOG-mapper + InterProScan (orthogonal evidence: orthology vs signatures), reconciled afterward. Add KofamScan if KEGG pathway reconstruction is the goal (its adaptive per-KO thresholds are stricter than eggNOG's KEGG_ko). DIAMOND-vs-Swiss-Prot is the cheap product-name layer; never use it alone for GO.
Three stages: (1) seed-ortholog search (DIAMOND/MMseqs2/HMMER) anchors the query - this is a best-hit and is not the annotation; (2) orthology assignment retrieves the seed's fine-grained orthologs within the chosen taxonomic scope; (3) functional transfer pools terms across the set of orthologs (which damps single-entry misannotation - this is why eggNOG-mapper beats raw DIAMOND-vs-NR). --tax_scope is the single most consequential parameter: too broad gathers distant orthologs and over-generalizes function; auto lets each seed take its most-informative phylogenetic ceiling. --itype {proteins,CDS,genome,metagenome} (genome/metagenome runs Prodigal first). Output .emapper.annotations columns include seed_ortholog, eggNOG_OGs, COG_category, Description, Preferred_name, GOs, EC, KEGG_ko, PFAMs (read the actual header; - = empty).
Runs member-database scanners (Pfam, PANTHER, NCBIfam, SUPERFAMILY, CDD, SMART, Gene3D, Hamap, PROSITE, ...) and integrates overlapping signatures into InterPro entries (stable IPRxxxxxx, with a type: Family/Domain/Repeat/Site/Homologous Superfamily). Report at the InterPro-entry level - it is the consensus that survives one member DB being wrong. -goterms adds the interpro2go mapping (these GO are IEA/electronic); -pa maps Reactome/MetaCyc. By default it queries the EBI precalculated lookup service (fast, MD5-keyed); -dp forces local compute (novel/confidential sequences, reproducibility). Java 11+ and a tens-of-GB data bundle required; for millions of proteins, chunk the FASTA into array jobs.
Reconciling Multi-Tool Output with Python
Goal: Merge eggNOG and InterProScan per protein while preserving provenance, so a curated name is never silently overwritten by a generic domain.
Approach: Parse each tool's table, keep source namespaces separate, union GO with source tags, and prefer the orthology Preferred_name/Description for the human-readable product.
import pandas as pd
defparse_eggnog(path):
df = pd.read_csv(path, sep='\t', comment='#', header=None)
cols = ['query', 'seed_ortholog', 'evalue', 'score', 'eggNOG_OGs', 'max_annot_lvl',
'COG_category', 'Description', 'Preferred_name', 'GOs', 'EC', 'KEGG_ko']
df.columns = (cols + [f'c{i}'for i inrange(len(df.columns) - len(cols))])[:len(df.columns)]
return df
defbest_product_name(row):
name = row.get('Preferred_name', '-')
return name if name notin ('-', '', None) else'hypothetical protein'# honest default, not a forced guess
Use AGAT (agat_sp_manage_functional_annotation.pl) to graft BLAST/InterProScan results onto a GFF3 (it handles the spec edge cases). For GO deliverables use GAF (carries the evidence code); keep each tool in its own Dbxref namespace.
Ontology Rigor and the IEA Circularity
GO MF vs BP transfer with different reliability. Molecular Function ("DNA helicase activity") is local/chemical and transfers with the fold; Biological Process ("DNA replication") is systemic context and does not transfer reliably - CAFA confirmed BLAST beats naive baselines for MF but not BP (Radivojac 2013 Nat Methods 10:221). Weight MF over BP when evidence is limited; never let a transferred BP term drive a conclusion alone.
Essentially all genome-derived GO is IEA (Inferred from Electronic Annotation, never curator-reviewed). Running GO enrichment on IEA against an IEA background partly tests the pipeline against itself - if interpro2go maps a common domain to a term, every genome with that domain looks "enriched." Compounded by annotation bias (58% of human GO covers 16% of genes; Haynes 2018 Sci Rep 8:1362) and True-Path-Rule inflation of shallow terms. Pin versions, match background to foreground pipeline, prefer non-IEA where it exists, and state that the result is annotation-derived.
EC numbers are not stable. Deleted/transferred numbers are tombstones (never reused); a partial EC 1.1.1.- is a valid statement of ignorance (the EC equivalent of "hypothetical"). Demand orthology or a curated rule before asserting a full four-level EC.
KEGG bulk access is paywalled. Free routes: KofamScan/KofamKOALA (local HMMs + adaptive per-KO thresholds; an * marks above-threshold hits) or eggNOG's KEGG_ko. A "complete module" is a reconstruction (a gap can be non-orthologous gene displacement; a filled step can be a paralog doing something else), not proof of flux.
Per-Method Failure Modes
Best-hit-as-ortholog
Trigger: transferring a specific function from one DIAMOND/BLAST top hit (esp. TrEMBL/NR). Mechanism: best-hit != ortholog; the bulk-DB hit is likely itself an auto-annotation. Symptom: confident specific names with no provenance. Fix: orthology consensus + Swiss-Prot donors.
Over-specific transfer
Trigger: copying the exact substrate/EC of a characterized homolog onto a distant relative. Mechanism: mechanistically-diverse superfamilies share fold, not substrate. Symptom: a "muconate cycloisomerase" that does something else. Fix: demote to superfamily / partial EC as identity and coverage fall.
Wrong eggNOG tax_scope
Trigger: leaving scope too broad/narrow or unpinned. Mechanism: distant orthologs over-generalize, or no informative orthologs. Symptom: vague or missing function. Fix:auto, or pin the known clade.
Circular GO enrichment
Trigger: enriching IEA annotations against a mismatched background. Mechanism: measures the mapping table and study popularity, not biology. Symptom: "enriched" for whatever well-studied genes are annotated for. Fix: non-IEA where possible; matched background; pin versions; caveat the result.
Reading specialized function from a generic hit
Trigger: inferring CAZyme substrate / AMR phenotype / BGC product from a plain Pfam domain. Mechanism: the substrate/phenotype/cluster signal is not in a generic domain. Symptom: wrong substrate or phenotype call. Fix: route to dbCAN / AMRFinderPlus / antiSMASH.
Quantitative Thresholds
Threshold
Source
Rationale
Reason in bits-per-residue, not raw e-value
alignment statistics
e-value scales with DB size (a database-size artifact); bits/residue is density
Bidirectional coverage ≥50-70% query and subject
transfer practice
one-domain coverage justifies only a domain-level claim
~40% identity over full length (well-behaved families only)
soft floor
no safe identity in mechanistically-diverse superfamilies; demote specificity instead
Named fraction "too high for the taxon" (>90% on a novel isolate)
a single global e-value misfires across KO families
Common Errors
Error / symptom
Cause
Solution
Low annotation rate
fragmented ORFs / narrow scope
check protein quality; --tax_scope auto; run both tools and merge
Specific name on a distant homolog
over-specific transfer
demote to superfamily / partial EC; record identity
eggNOG DB errors
DB/version mismatch
re-download; pin emapper 2.1.15
InterProScan memory/time
full proteome at once
chunk FASTA; keep lookup service on; drop PANTHER/Gene3D if not needed
Enrichment "too clean"
IEA circularity / study bias
matched background; pin GO release; caveat
Multidomain protein mislabeled
named by first/best domain
report all domains with coordinates
References
Cantalapiedra CP, et al. 2021. eggNOG-mapper v2: functional annotation, orthology assignments, and domain prediction at the metagenomic scale. Mol Biol Evol 38:5825-5829.
Huerta-Cepas J, et al. 2019. eggNOG 5.0: a hierarchical, functionally and phylogenetically annotated orthology resource. Nucleic Acids Res 47:D309-D314.
Jones P, et al. 2014. InterProScan 5: genome-scale protein function classification. Bioinformatics 30:1236-1240.
Blum M, et al. 2025. InterPro: the protein sequence classification resource in 2025. Nucleic Acids Res 53:D444-D456.
Schnoes AM, et al. 2009. Annotation error in public databases: misannotation of molecular function in enzyme superfamilies. PLoS Comput Biol 5:e1000605.
Gilks WR, et al. 2002. Modeling the percolation of annotation errors in a database of protein sequences. Bioinformatics 18:1641-1649.
Aramaki T, et al. 2020. KofamKOALA: KEGG ortholog assignment based on profile HMM and adaptive score threshold. Bioinformatics 36:2251-2252.
Zheng J, et al. 2023. dbCAN3: automated carbohydrate-active enzyme and substrate annotation. Nucleic Acids Res 51:W115-W121.
Teufel F, et al. 2022. SignalP 6.0 predicts all five types of signal peptides using protein language models. Nat Biotechnol 40:1023-1025.
Feldgarden M, et al. 2021. AMRFinderPlus and the Reference Gene Catalog facilitate examination of the genomic links among antimicrobial resistance, stress response, and virulence. Sci Rep 11:12728.
Blin K, et al. 2023. antiSMASH 7.0: new and improved predictions for detection, regulation, chemical structures and visualisation. Nucleic Acids Res 51:W46-W50.
Radivojac P, et al. 2013. A large-scale evaluation of computational protein function prediction (CAFA). Nat Methods 10:221-227.
