Groups proteins from peptide identifications and controls protein-level FDR, framing inference as a chosen explanation (parsimony or a probability model) of underdetermined peptide evidence rather than a measurement. Reports protein GROUPS (proteins indistinguishable by observed peptides) with a leading protein, not flat lists. Covers shared-vs-unique peptides, indistinguishable/subsumable proteins, parsimony vs probabilistic (ProteinProphet, EPIFANY) vs razor inference, picked-protein and picked-group FDR, and why the two-peptide rule is wrong. Use when resolving which proteins are present from a peptide list, building protein groups, or estimating protein-level FDR. PSM/peptide FDR and search engines are peptide-identification; razor-vs-unique quant consequences are quantification; isoform/proteoform resolution is top-down and out of scope.
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Groups proteins from peptide identifications and controls protein-level FDR, framing inference as a chosen explanation (parsimony or a probability model) of underdetermined peptide evidence rather than a measurement. Reports protein GROUPS (proteins indistinguishable by observed peptides) with a leading protein, not flat lists. Covers shared-vs-unique peptides, indistinguishable/subsumable proteins, parsimony vs probabilistic (ProteinProphet, EPIFANY) vs razor inference, picked-protein and picked-group FDR, and why the two-peptide rule is wrong. Use when resolving which proteins are present from a peptide list, building protein groups, or estimating protein-level FDR. PSM/peptide FDR and search engines are peptide-identification; razor-vs-unique quant consequences are quantification; isoform/proteoform resolution is top-down and out of scope.
Before using code patterns, verify installed versions match. If versions differ:
Python: pip show <package> then help(module.function) to check signatures
R: packageVersion('<pkg>') then ?function_name to verify parameters
If code throws ImportError, AttributeError, or TypeError, introspect the installed
package and adapt the example to match the actual API rather than retrying.
The pyOpenMS protein-inference class names have varied across releases. Confirm the exact spelling at the installed version with and before relying on the reference code.
help(pyopenms.EpifanyAlgorithm)
help(pyopenms.BasicProteinInferenceAlgorithm)
Protein Inference -- A Chosen Explanation of Peptide Evidence, Reported as Groups
"Tell me which proteins are present from my identified peptides" -> Assign the observed peptides to a minimal or probability-weighted set of proteins, reported as groups of indistinguishable proteins with a leading accession -- because bottom-up MS measures peptides, and the protein set behind them is inferred, not observed.
Python: pyopenms.BasicProteinInferenceAlgorithm().run(peptide_ids, protein_ids) for parsimony grouping
Python: pyopenms.EpifanyAlgorithm (TOPP tool Epifany) for Bayesian belief-propagation inference
CLI: ProteinProphet (TPP) for EM-based probabilistic inference; Philosopher filter for FragPipe FDR
Scope: this skill OWNS peptide-to-protein grouping, the indistinguishable/subsumable distinction, the leading-protein convention, inference-method choice, and protein/protein-group FDR. PSM-level and peptide-level FDR plus the search engines that produce the peptide list -> peptide-identification. The quantitative fallout of razor vs unique peptides on protein abundance -> quantification. OUT OF SCOPE: resolving splice isoforms, single-AA variants, or PTM-defined proteoforms (bottom-up groups cannot separate them; that is top-down / proteoform work).
The Single Most Important Modern Insight -- Protein Inference Is Underdetermined, So the Honest Unit Is a Group, Not a List
The protein set is not uniquely recoverable from peptides, so a protein group -- not a flat protein list -- is the only honest reporting unit. Many peptides are shared across paralogs, gene families, and isoforms, so distinct protein sets can explain the same peptide evidence equally well. The inference picks ONE explanation under an assumption (parsimony, or a probability model); proteins that the observed peptides cannot tell apart (indistinguishable) MUST be reported as one group with a designated leading protein. A flat list double-counts indistinguishable proteins and breaks target/decoy symmetry at the protein level, silently corrupting FDR.
Protein FDR is its own estimation problem that INFLATES on large data; the fix is PICKED FDR, not the PSM formula reused. Controlling PSM-FDR at 1% does not give 1% protein-FDR. A deep run has many false PSMs in absolute terms, and each can nucleate a one-hit-wonder false protein; because true proteins accumulate many peptides while false proteins are hit once, the naive protein-FDR balloons to 10-30% on deep datasets. Savitski 2015 picked-protein FDR pairs each target protein with its decoy and keeps only the higher-scoring of the pair before counting, removing the target/decoy asymmetry; The & Kall 2016 extends this to the group level (picked-group FDR), which is required because parsimony grouping is anticonservative otherwise.
The two-peptide rule is wrong -- it increases protein FDR and discards real proteins. Requiring >=2 peptides per protein (Gupta & Pevzner 2009, "A strike against the two-peptide rule") removes MORE target proteins than decoy proteins, so it raises protein-level FDR rather than lowering it, while throwing away legitimate low-abundance single-peptide IDs. Replace the blanket rule with: control protein-level (picked) FDR, then judge single-peptide IDs by their score, not their peptide count.
Vocabulary the Rest of This Depends On
Shared (degenerate) peptide: maps to >1 protein in the searched database. Cannot, alone, distinguish which protein is present.
Unique peptide: maps to exactly one protein -- the only direct evidence for a specific protein. "Unique" is DATABASE-RELATIVE: a peptide unique against SwissProt may be shared against TrEMBL+isoforms+contaminants. Always document the exact database (isoforms, contaminants, decoys included).
Indistinguishable proteins: explained by the SAME set of observed peptides -> one group, never two confident IDs.
Subset / subsumable protein: its observed peptides are a subset of another protein's -> parsimony drops it (the larger protein explains everything it would).
Leading / representative protein: the group's reported accession. Convention: most peptides, then highest score, then SwissProt canonical over TrEMBL. Downstream tables key on this accession but must retain group membership -- "protein P12345" usually means "the group led by P12345".
Protein group vs proteoform: a group is an inference artifact (proteins lumped because peptides cannot separate them); a proteoform is a real molecular species (one gene product with a specific sequence + PTM + cleavage state). Bottom-up groups DO NOT resolve proteoforms -- claiming "isoform X present" from a shared-peptide group is overreach.
Tool Taxonomy
Tool / method
Citation
Mechanism / role
When
Parsimony (Occam)
--
Greedy minimal protein set explaining all peptides
Fast default; ties broken arbitrarily; anticonservative group-FDR on large data unless picked
ProteinProphet
Nesvizhskii 2003
EM APPORTIONS shared peptides across candidate proteins, weighted by other evidence
TPP / FragPipe pipelines; the classic probabilistic standard
EPIFANY
Pfeuffer 2020
Bayesian network over the peptide-protein graph, loopy belief propagation + convolution trees
OpenMS-recommended modern inference; strong at controlled protein-group FDR
Fido
--
Bayesian generative model (Percolator --protein)
Percolator pipelines; superseded by picked-protein for FDR
Razor peptide
--
Shared peptide assigned winner-take-all to the group with most evidence (MaxQuant)
MaxQuant default; ID-fine but distorts QUANT (route to quantification)
Picked-protein FDR
Savitski 2015
Pair target with its decoy, keep the higher-scoring of the pair, then count decoys
Protein-level FDR on any non-trivial dataset
Picked-group FDR
The & Kall 2016
Picking applied at the protein-GROUP level
When the inference unit is the group (the correct unit on deep data)
All-proteins / inclusive
--
Report every protein any peptide could come from
Almost never; massive false-positive protein inflation
Decision Tree by Scenario
Scenario
Recommended
Why
Standard DDA run, OpenMS-based pipeline
EpifanyAlgorithm (or BasicProteinInferenceAlgorithm for parsimony) + picked-group FDR
Modern, group-FDR aware; well-calibrated on benchmarks
MaxQuant output (proteinGroups.txt)
Parse groups as-is; quantify on UNIQUE peptides
Groups already inferred; razor quant is the trap, not the inference
Naive protein-FDR inflates to 10-30% from one-hit-wonders
Sensitive differential abundance downstream
Quantify on unique peptides only -> quantification
Razor assignment can flip between conditions and fake DE
Want isoform-level answers
Stop -- route to top-down / proteoform methods
Bottom-up groups cannot resolve proteoforms
Few PSMs (single-protein pulldown)
Report evidence, do not trust a "0% protein FDR"
Target-decoy FDR is meaningless at tiny counts
Default when uncertain: run parsimony grouping (BasicProteinInferenceAlgorithm with annotate_indistinguishable_groups), report protein GROUPS with a leading accession, and control protein-GROUP FDR with picked-group FDR at 1%. Do NOT impose a two-peptide rule.
Group Proteins by Parsimony with pyOpenMS
Goal: Turn an FDR-filtered peptide identification list into protein groups with a leading protein, resolving shared-peptide ambiguity.
Approach: Load the idXML from peptide identification, run the parsimony algorithm with indistinguishable-group annotation on, then read the inferred groups off the protein identification run.
from pyopenms import IdXMLFile, BasicProteinInferenceAlgorithm
protein_ids = []
peptide_ids = []
# protein_ids is FIRST in both load() and store() for IdXMLFile
IdXMLFile().load('peptides_1pct_fdr.idXML', protein_ids, peptide_ids)
inference = BasicProteinInferenceAlgorithm()
params = inference.getParameters()
# annotate_indistinguishable_groups reports indistinguishable proteins as ONE group
params.setValue('annotate_indistinguishable_groups', 'true')
inference.setParameters(params)
inference.run(peptide_ids, protein_ids)
# indistinguishable groups live on the protein identification runfor prot_id in protein_ids:
for group in prot_id.getIndistinguishableProteins():
leading = group.accessions[0] # convention: highest-evidence accession firstprint(leading, group.probability, list(group.accessions))
Bayesian Inference + Group FDR with EPIFANY
Goal: Assign calibrated protein/group posteriors and control protein-group FDR with a probability model rather than greedy parsimony.
Approach: EPIFANY consumes idXML whose PSMs already carry posterior error probabilities (from Percolator or IDPosteriorErrorProbability), then propagates belief over the peptide-protein graph. The TOPP tool is reliably named Epifany; the pyOpenMS class spelling has varied across releases, so introspect first.
import pyopenms
from pyopenms import IdXMLFile
# CONFIRM the class name at the installed version before use:# help(pyopenms.EpifanyAlgorithm)
algo_cls = getattr(pyopenms, 'EpifanyAlgorithm')
protein_ids = []
peptide_ids = []
IdXMLFile().load('peptides_with_pep.idXML', protein_ids, peptide_ids)
algo = algo_cls()
# EPIFANY expects PSM posteriors as input; greedy_group_resolution controls# whether shared peptides are razor-resolved after inference
algo.inferPosteriorProbabilities(protein_ids, peptide_ids, False)
for prot_id in protein_ids:
for group in prot_id.getIndistinguishableProteins():
print(group.accessions[0], group.probability)
Picked Protein-Group FDR
Goal: Estimate protein-group FDR without the inflation that the reused PSM formula causes on large data.
Approach: For each target group, find its decoy counterpart (same accessions with the decoy prefix); keep only the higher-scoring member of each target/decoy PAIR; rank the picked set and count decoys as the FDR estimate. This is the operation the reference example demonstrates end to end.
defpicked_group_fdr(groups, decoy_prefix='DECOY_'):
# groups: list of dicts with 'accessions', 'score', 'is_decoy'
by_base = {}
for g in groups:
base = frozenset(a.replace(decoy_prefix, '') for a in g['accessions'])
# keep only the higher-scoring of the target/decoy pair (the 'pick')if base notin by_base or g['score'] > by_base[base]['score']:
by_base[base] = g
picked = sorted(by_base.values(), key=lambda g: g['score'], reverse=True)
targets = decoys = 0for g in picked:
if g['is_decoy']:
decoys += 1else:
targets += 1
g['fdr'] = decoys / targets if targets else1.0
running_min = 1.0for g inreversed(picked): # monotone q-values from the bottom up
running_min = min(running_min, g['fdr'])
g['qvalue'] = running_min
return [g for g in picked ifnot g['is_decoy'] and g['qvalue'] <= 0.01]
Per-Method Failure Modes
Naive (non-picked) protein/group FDR
Trigger: Reusing the PSM-level decoys/targets formula at the protein level on a deep dataset.
Mechanism: False target proteins (one-hit-wonders) and decoy proteins are not symmetric once peptides are mapped to proteins; true proteins absorb many peptides, false ones do not.
Symptom: Reported 1% protein FDR, actual 10-30%; reviewer or entrapment check exposes it.
Fix: Picked-protein FDR (Savitski 2015) or picked-group FDR (The & Kall 2016); validate with a two-species or entrapment search.
Two-peptide rule
Trigger: Filtering to proteins with >=2 (unique) peptides "for confidence".
Mechanism: The rule removes more target proteins than decoy proteins, inverting the FDR effect, and deletes real low-abundance single-peptide proteins.
Symptom: Fewer proteins AND higher true FDR than picked FDR at the same nominal cutoff.
Fix: Drop the rule; control picked protein-level FDR and score single-peptide IDs individually.
Razor-peptide quantification
Trigger: Quantifying on MaxQuant's default unique+razor peptides for a sensitive comparison.
Mechanism: A shared peptide's full intensity is credited to one group; that razor assignment can flip between conditions when peptide counts shift, so a protein's quantity changes for inference reasons, not biology.
Symptom: Spurious differential abundance concentrated on proteins sharing peptides with paralogs.
Fix: Quantify on unique peptides only for sensitive comparisons -> quantification.
Parsimony tie-breaking
Trigger: Multiple minimal protein sets explain the peptides equally well.
Mechanism: Greedy parsimony breaks ties arbitrarily; minimality is a heuristic, not truth, and a real protein with only shared peptides is silently dropped.
Symptom: Reported lead protein differs run-to-run or pipeline-to-pipeline on the same data.
Fix: Prefer a probabilistic method (EPIFANY/ProteinProphet) that apportions shared evidence; retain group membership.
Proteoform overreach
Trigger: Reporting "isoform X is present" from a group whose evidence is shared peptides.
Mechanism: Splice isoforms, variants, and PTM forms collapse into groups in bottom-up data; the group cannot separate them.
Symptom: Isoform-specific claim with no isoform-unique peptide behind it.
Fix: Require an isoform-unique peptide for any isoform claim, or use top-down / proteoform methods.
Quantitative Thresholds
Threshold
Source
Rationale
Protein / protein-group FDR 1% (sometimes 5% for discovery)
community standard
SEPARATE estimation from PSM FDR; never assume 1% PSM implies 1% protein
Standard null; unequal ratios require formula correction
Min PSMs for trustworthy protein FDR
hundreds+
Below ~100s of items decoy counts are too noisy; "0% FDR" from zero decoys is luck, not control
Two-peptide rule
DO NOT USE (Gupta & Pevzner 2009)
Increases protein FDR and drops real proteins; replaced by picked FDR + per-ID score
Single-peptide IDs
judge by score, not count
A high-confidence unique peptide can be a legitimate ID
Common Errors
Error / symptom
Cause
Solution
Protein FDR much higher than nominal on deep data
Naive decoy/target reused from PSM level
Picked-protein or picked-group FDR
Real low-abundance proteins missing
Two-peptide rule applied
Remove the rule; control picked FDR
AttributeError on EpifanyAlgorithm / infer_proteins
Class name varies by version; the R ProteinInference::infer_proteins could not be confirmed to exist
help(pyopenms.EpifanyAlgorithm) to find the real name; use pyOpenMS, not an unverified R package
Indistinguishable proteins reported as separate IDs
Flat protein list instead of groups
Enable annotate_indistinguishable_groups; report groups with a leading protein
Spurious DE on paralog-sharing proteins
Razor-peptide quant flipped between conditions
Quantify on unique peptides -> quantification
"Unique" peptide count changed when DB changed
Uniqueness is database-relative
Fix and document the database (isoforms, contaminants, decoys)
References
Nesvizhskii, A.I., Keller, A., Kolker, E. & Aebersold, R. (2003). A statistical model for identifying proteins by tandem mass spectrometry. Analytical Chemistry 75(17):4646-4658.
Gupta, N. & Pevzner, P.A. (2009). False discovery rates of protein identifications: a strike against the two-peptide rule. Journal of Proteome Research 8(9):4173-4181.
Savitski, M.M., Wilhelm, M., Hahne, H., Kuster, B. & Bantscheff, M. (2015). A scalable approach for protein false discovery rate estimation in large proteomic data sets. Molecular & Cellular Proteomics 14(9):2394-2404.
The, M., Tasnim, A. & Kall, L. (2016). How to talk about protein-level false discovery rates in shotgun proteomics. Proteomics 16(18):2461-2469.
Pfeuffer, J., Sachsenberg, T., Dijkstra, T.M.H., Serang, O., Reinert, K. & Kohlbacher, O. (2020). EPIFANY: a method for efficient high-confidence protein inference. Journal of Proteome Research 19(3):1060-1072.
Related Skills
peptide-identification - Produces the FDR-filtered peptide list that feeds inference and shares the target-decoy machinery
quantification - Consumes inferred groups; razor-vs-unique peptide choice lives here
