Agent skill

protein-qc

Quality control metrics and filtering thresholds for protein design. Use this skill when: (1) Evaluating design quality for binding, expression, or structure, (2) Setting filtering thresholds for pLDDT, ipTM, PAE, (3) Checking sequence liabilities (cysteines, deamidation, polybasic clusters), (4) Creating multi-stage filtering pipelines, (5) Computing PyRosetta interface metrics (dG, SC, dSASA), (6) Checking biophysical properties (instability, GRAVY, pI), (7) Ranking designs with composite scoring. This skill provides research-backed thresholds from binder design competitions and published benchmarks.

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Install this agent skill to your Project

npx add-skill https://github.com/FreedomIntelligence/OpenClaw-Medical-Skills/tree/main/skills/protein-qc

SKILL.md

Protein Design Quality Control

Critical Limitation

Individual metrics have weak predictive power for binding. Research shows:

  • Individual metric ROC AUC: 0.64-0.66 (slightly better than random)
  • Metrics are pre-screening filters, not affinity predictors
  • Composite scoring is essential for meaningful ranking

These thresholds filter out poor designs but do NOT predict binding affinity.

QC Organization

QC is organized by purpose and level:

Purpose What it assesses Key metrics
Binding Interface quality, binding geometry ipTM, PAE, SC, dG, dSASA
Expression Manufacturability, solubility Instability, GRAVY, pI, cysteines
Structural Fold confidence, consistency pLDDT, pTM, scRMSD

Each category has two levels:

  • Metric-level: Calculated values with thresholds (pLDDT > 0.85)
  • Design-level: Pattern/motif detection (odd cysteines, NG sites)

Quick Reference: All Thresholds

Category Metric Standard Stringent Source
Structural pLDDT > 0.85 > 0.90 AF2/Chai/Boltz
pTM > 0.70 > 0.80 AF2/Chai/Boltz
scRMSD < 2.0 Å < 1.5 Å Design vs pred
Binding ipTM > 0.50 > 0.60 AF2/Chai/Boltz
PAE_interaction < 12 Å < 10 Å AF2/Chai/Boltz
Shape Comp (SC) > 0.50 > 0.60 PyRosetta
interface_dG < -10 < -15 PyRosetta
Expression Instability < 40 < 30 BioPython
GRAVY < 0.4 < 0.2 BioPython
ESM2 PLL > 0.0 > 0.2 ESM2

Design-Level Checks (Expression)

Pattern Risk Action
Odd cysteine count Unpaired disulfides Redesign
NG/NS/NT motifs Deamidation Flag/avoid
K/R >= 3 consecutive Proteolysis Flag
>= 6 hydrophobic run Aggregation Redesign

See: references/binding-qc.md, references/expression-qc.md, references/structural-qc.md


Sequential Filtering Pipeline

python
import pandas as pd

designs = pd.read_csv('designs.csv')

# Stage 1: Structural confidence
designs = designs[designs['pLDDT'] > 0.85]

# Stage 2: Self-consistency
designs = designs[designs['scRMSD'] < 2.0]

# Stage 3: Binding quality
designs = designs[(designs['ipTM'] > 0.5) & (designs['PAE_interaction'] < 10)]

# Stage 4: Sequence plausibility
designs = designs[designs['esm2_pll_normalized'] > 0.0]

# Stage 5: Expression checks (design-level)
designs = designs[designs['cysteine_count'] % 2 == 0]  # Even cysteines
designs = designs[designs['instability_index'] < 40]

Composite Scoring (Required for Ranking)

Individual metrics alone are too weak. Use composite scoring:

python
def composite_score(row):
    return (
        0.30 * row['pLDDT'] +
        0.20 * row['ipTM'] +
        0.20 * (1 - row['PAE_interaction'] / 20) +
        0.15 * row['shape_complementarity'] +
        0.15 * row['esm2_pll_normalized']
    )

designs['score'] = designs.apply(composite_score, axis=1)
top_designs = designs.nlargest(100, 'score')

For advanced composite scoring, see references/composite-scoring.md.


Tool-Specific Filtering

BindCraft Filter Levels

Level Use Case Stringency
Default Standard design Most stringent
Relaxed Need more designs Higher failure rate
Peptide Designs < 30 AA ~5-10x lower success

BoltzGen Filtering

bash
boltzgen run ... \
  --budget 60 \
  --alpha 0.01 \
  --filter_biased true \
  --refolding_rmsd_threshold 2.0 \
  --additional_filters 'ALA_fraction<0.3'
  • alpha=0.0: Quality-only ranking
  • alpha=0.01: Default (slight diversity)
  • alpha=1.0: Diversity-only

Design-Level Severity Scoring

For pattern-based checks, use severity scoring:

Severity Level Score Action
LOW 0-15 Proceed
MODERATE 16-35 Review flagged issues
HIGH 36-60 Redesign recommended
CRITICAL 61+ Redesign required

Experimental Correlation

Metric AUC Use
ipTM ~0.64 Pre-screening
PAE ~0.65 Pre-screening
ESM2 PLL ~0.72 Best single metric
Composite ~0.75+ Always use

Key insight: Metrics work as filters (eliminating failures) not predictors (ranking successes).


Campaign Health Assessment

Quick assessment of your design campaign:

Pass Rate Status Interpretation
> 15% Excellent Above average, proceed
10-15% Good Normal, proceed
5-10% Marginal Below average, review issues
< 5% Poor Significant problems, diagnose

Failure Recovery Trees

Too Few Pass pLDDT Filter (< 5% with pLDDT > 0.85)

Low pLDDT across campaign
├── Check scRMSD distribution
│   ├── High scRMSD (>2.5Å): Backbone issue
│   │   └── Fix: Regenerate backbones with lower noise_scale (0.5-0.8)
│   └── Low scRMSD but low pLDDT: Disordered regions
│       └── Fix: Check design length, simplify topology
├── Try more sequences per backbone
│   └── modal run modal_proteinmpnn.py --num-seq-per-target 32 --sampling-temp 0.1
├── Use SolubleMPNN instead of ProteinMPNN
│   └── Better for expression-optimized sequences
└── Consider different design tool
    └── BindCraft (integrated design) may work better

Too Few Pass ipTM Filter (< 5% with ipTM > 0.5)

Low ipTM across campaign
├── Review hotspot selection
│   ├── Are hotspots surface-exposed? (SASA > 20Ų)
│   ├── Are hotspots conserved? (check MSA)
│   └── Try 3-6 different hotspot combinations
├── Increase binder length (more contact area)
│   └── Try 80-100 AA instead of 60-80 AA
├── Check interface geometry
│   ├── Is target flat? → Try helical binders
│   └── Is target concave? → Try smaller binders
└── Try all-atom design tool
    └── BoltzGen (all-atom, better packing)

High scRMSD (> 50% with scRMSD > 2.0Å)

Sequences don't specify intended structure
├── ProteinMPNN issue
│   ├── Lower temperature: --sampling-temp 0.1
│   ├── Increase sequences: --num-seq-per-target 32
│   └── Check fixed_positions aren't over-constraining
├── Backbone geometry issue
│   ├── Backbones may be unusual/strained
│   ├── Regenerate with lower noise_scale (0.5-0.8)
│   └── Reduce diffuser.T to 30-40
└── Try different sequence design
    └── ColabDesign (AF2 gradient-based) may work better

Everything Passes But No Experimental Hits

In silico metrics don't predict affinity
├── Generate MORE designs (10x current)
│   └── Computational metrics have high false positive rate
├── Increase diversity
│   ├── Higher ProteinMPNN temperature (0.2-0.3)
│   ├── Different backbone topologies
│   └── Different hotspot combinations
├── Try different design approach
│   ├── BindCraft (different algorithm)
│   ├── ColabDesign (AF2 hallucination)
│   └── BoltzGen (all-atom diffusion)
└── Check if target is druggable
    └── Some targets are inherently difficult

Too Many Designs Pass (> 50%)

Suspiciously high pass rate
├── Check if thresholds are too lenient
│   └── Use stringent thresholds: pLDDT > 0.90, ipTM > 0.60
├── Verify prediction quality
│   ├── Are predictions actually running? Check output files
│   └── Are complexes being predicted, not just monomers?
├── Check for data issues
│   ├── Same sequence being predicted multiple times?
│   └── Wrong FASTA format (missing chain separator)?
└── Apply diversity filter
    └── Cluster at 70% identity, take top per cluster

Diagnostic Commands

Quick Campaign Assessment

python
import pandas as pd

df = pd.read_csv('designs.csv')

# Pass rates at each stage
print(f"Total designs: {len(df)}")
print(f"pLDDT > 0.85: {(df['pLDDT'] > 0.85).mean():.1%}")
print(f"ipTM > 0.50: {(df['ipTM'] > 0.50).mean():.1%}")
print(f"scRMSD < 2.0: {(df['scRMSD'] < 2.0).mean():.1%}")
print(f"All filters: {((df['pLDDT'] > 0.85) & (df['ipTM'] > 0.5) & (df['scRMSD'] < 2.0)).mean():.1%}")

# Identify top issue
if (df['pLDDT'] > 0.85).mean() < 0.1:
    print("ISSUE: Low pLDDT - check backbone or sequence quality")
elif (df['ipTM'] > 0.50).mean() < 0.1:
    print("ISSUE: Low ipTM - check hotspots or interface geometry")
elif (df['scRMSD'] < 2.0).mean() < 0.5:
    print("ISSUE: High scRMSD - sequences don't specify backbone")

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