{"id":"5bc347a2-b784-45f9-9dc1-dda5b9d409a0","shortId":"ZujMs8","kind":"skill","title":"cell-free-expression","tagline":"Guidance for cell-free protein synthesis (CFPS) optimization. Use when: (1) Planning CFPS experiments, (2) Troubleshooting low yield or aggregation, (3) Optimizing DNA template design for CFPS, (4) Expressing difficult proteins (disulfide-rich, toxic, membrane).","description":"# Cell-Free Protein Synthesis (CFPS)\n\n## System Selection Guide\n\n| System | Best For | Yield | PTMs | Disulfides | Cost |\n|--------|----------|-------|------|------------|------|\n| **E. coli extract** | Rapid prototyping, prokaryotic proteins | High (100-400 μg/mL) | None | Poor (reducing) | Low |\n| **E. coli PURE** | Defined conditions, unnatural AAs | Medium (50-150 μg/mL) | None | Controllable | High |\n| **Wheat germ** | Eukaryotic proteins, membrane proteins | High (100-500 μg/mL) | Limited | Moderate | Medium |\n| **Rabbit reticulocyte** | Mammalian proteins, post-translational studies | Low (10-50 μg/mL) | Some | Poor | High |\n| **Insect (Sf21)** | Glycoproteins, complex folds | Medium (50-100 μg/mL) | Glycosylation | Good | High |\n| **HeLa/CHO** | Native mammalian proteins | Low (10-50 μg/mL) | Full mammalian | Good | Very High |\n\n---\n\n## CFPS Troubleshooting Matrix\n\n| Problem | Likely Causes | Design Fix | Reagent Fix |\n|---------|---------------|------------|-------------|\n| **No expression** | Rare codons at N-terminus, poor RBS | Codon optimize first 30 codons | Use BL21-CodonPlus extract |\n| **Low yield** | Strong mRNA secondary structure, template issues | Optimize 5' UTR (ΔG > -5 kcal/mol) | Increase Mg²⁺ (10-18 mM), ATP |\n| **Aggregation** | Hydrophobic protein, fast translation | Add solubility tags (MBP, SUMO) | Add 0.1% Tween-20, chaperones |\n| **Inactive protein** | Misfolding, missing cofactors | Slow translation (use rare codons!) | Add GroEL/ES, DnaK/J |\n| **Truncation** | Rare codon clusters, mRNA instability | Remove AGG/AGA/CUA clusters | Supplement rare tRNAs |\n| **Degradation** | Proteolysis | N-terminal Met-Ala | Add protease inhibitors |\n\n---\n\n## Codon Optimization for CFPS\n\n### Codons to Avoid in E. coli CFPS\n\n| Codon | Amino Acid | Issue | tRNA Abundance |\n|-------|------------|-------|----------------|\n| AGG | Arg | Very rare, stalling | 0.2% |\n| AGA | Arg | Very rare, stalling | 0.4% |\n| CUA | Leu | Low abundance | 0.4% |\n| AUA | Ile | Rare | 0.5% |\n| CGA | Arg | Inefficient decoding | 0.6% |\n| CCC | Pro | Can cause pausing | 0.5% |\n| GGA | Gly | Moderate | 1.1% |\n\n### Design Rules\n\n1. **First 30 codons**: Most critical - use only high-frequency codons\n2. **Rare codon clusters**: Avoid 2+ rare codons within 10 nt\n3. **Rare codon content**: Keep overall <5% of coding sequence\n4. **GC content**: Target 40-60% for balanced expression\n5. **Avoid runs**: No >6 consecutive G or C residues (secondary structure)\n6. **Strategic slow codons**: Place rare codons between domains (aids folding!)\n\n### When to Use Rare Codons\n- Domain boundaries (allow cotranslational folding)\n- Before complex structural elements\n- When protein is prone to misfolding\n\n---\n\n## mRNA Template Design\n\n### 5' UTR Optimization\n\n| Element | Optimal Design | Impact |\n|---------|----------------|--------|\n| **RBS (SD sequence)** | AGGAGG, 7-9 nt from start | Ribosome binding |\n| **Spacing** | 7 nt between SD and AUG | Translation initiation |\n| **Secondary structure** | ΔG > -5 kcal/mol | Accessibility |\n| **Upstream AUG** | Avoid (causes false starts) | Reduces truncations |\n\n### Secondary Structure Targets\n\n| Region | Ideal ΔG | Impact |\n|--------|----------|--------|\n| -30 to +30 around AUG | > -5 kcal/mol | Translation initiation |\n| Full 5' UTR | > -10 kcal/mol | Ribosome loading |\n| RBS accessibility | Unpaired | Critical |\n\n### Template Format\n\n| Format | Advantages | Disadvantages |\n|--------|------------|---------------|\n| **Plasmid** | Stable, high yield | Requires cloning |\n| **Linear PCR** | Fast, no cloning | May need stabilization |\n| **mRNA** | Direct translation | Unstable, expensive |\n\n---\n\n## Disulfide Bond Formation\n\n### System Capabilities\n\n| System | Native Disulfide Support | Additives Needed |\n|--------|--------------------------|------------------|\n| Standard E. coli extract | Poor (DTT present) | IAM, PDI, GSSG/GSH |\n| Oxidizing E. coli extract | Good | Pre-oxidized glutathione |\n| Wheat germ | Moderate | Lower DTT, add PDI |\n| PURE system | Minimal | Full oxidative system |\n| Insect/Mammalian | Good | Microsome membranes |\n\n### Oxidative Folding Protocol (E. coli extract)\n\n```\n1. Deplete DTT from extract (dialysis or treatment with IAM 5 mM)\n2. Add oxidized/reduced glutathione: 4 mM GSSG, 1 mM GSH (4:1 ratio)\n3. Add 10 μM PDI (protein disulfide isomerase)\n4. Optional: Add 5 μM DsbC (disulfide isomerase)\n5. Express at 25°C (not 37°C) for better folding\n6. Incubation time: 4-6 hours\n```\n\n### Disulfide-Rich Protein Tips\n- Start with wheat germ or oxidizing extract\n- Use PURE system for precise control\n- Consider co-expression of PDI/DsbC\n- Verify by non-reducing SDS-PAGE\n\n---\n\n## Expression Prediction from Sequence\n\n| Feature | Good | Marginal | Bad |\n|---------|------|----------|-----|\n| **Rare codon content** | <3% | 3-8% | >10% |\n| **First 30 codons rare** | 0 | 1-2 | >2 |\n| **GC content** | 45-55% | 35-45% or 55-65% | <30% or >70% |\n| **5' UTR ΔG** | > -3 kcal/mol | -3 to -8 | < -10 kcal/mol |\n| **Hydrophobic stretches** | <5 consecutive | 5-7 | >8 consecutive |\n| **N-terminal residue** | Met-Ala, Met-Ser, Met-Gly | Met-Val, Met-Thr | Met-Arg, Met-Lys |\n| **Cysteine pairs** | Paired (even number) | Mixed | Odd number (free thiols) |\n\n---\n\n## Solubility Enhancement Strategies\n\n### Fusion Tags (ranked by effectiveness)\n\n| Tag | Size | Solubility Enhancement | Cleavage | Notes |\n|-----|------|------------------------|----------|-------|\n| **MBP** | 40 kDa | Excellent | TEV, Factor Xa | Best overall |\n| **SUMO** | 11 kDa | Very Good | SUMO protease | Native N-terminus after cleavage |\n| **NusA** | 55 kDa | Excellent | - | Large size |\n| **Trx** | 12 kDa | Good | Enterokinase | For disulfide proteins |\n| **GST** | 26 kDa | Moderate | - | Dimeric |\n| **His₆** | 1 kDa | Minimal | - | Mainly for purification |\n\n### Buffer Additives for Solubility\n\n| Additive | Concentration | Mechanism |\n|----------|---------------|-----------|\n| Trehalose | 50-100 mM | Chemical chaperone |\n| Glycerol | 5-10% | Reduces hydrophobic aggregation |\n| L-Arginine | 50-100 mM | Suppresses aggregation |\n| Tween-20 | 0.05-0.1% | Prevents surface adsorption |\n| Proline | 50 mM | Osmolyte stabilization |\n\n### Chaperone Supplementation\n\n| Chaperone System | Target Problem | Concentration |\n|------------------|----------------|---------------|\n| GroEL/GroES | General folding | 1-2 μM |\n| DnaK/DnaJ/GrpE | Aggregation-prone | 1 μM each |\n| Trigger Factor | Nascent chain | 1-2 μM |\n| ClpB | Aggregate resolubilization | 0.5 μM |\n\n---\n\n## Temperature Optimization\n\n| Temperature | Use Case | Trade-offs |\n|-------------|----------|------------|\n| **37°C** | Fast expression, stable proteins | Higher aggregation risk |\n| **30°C** | Balanced (default) | Good compromise |\n| **25°C** | Disulfide proteins, complex folds | Slower, better folding |\n| **18-20°C** | Aggregation-prone proteins | Much slower, best folding |\n| **16°C** | Cold-shock proteins | Very slow, specialized |\n\n---\n\n## E. coli Extract Preparation (Key Variables)\n\n| Variable | Impact | Optimal Range |\n|----------|--------|---------------|\n| **Cell density at harvest** | Ribosome content | OD₆₀₀ 2.5-3.5 |\n| **Lysis method** | Extract activity | Sonication, bead beating |\n| **Run-off reaction** | Removes endogenous mRNA | 20-80 min at 37°C |\n| **Mg²⁺ concentration** | Translation fidelity | 10-18 mM |\n| **K⁺ concentration** | Translation rate | 150-200 mM |\n| **Energy system** | Sustained synthesis | ATP/GTP, creatine phosphate |\n\n---\n\n## PURE System Specifics\n\n### Advantages\n- Defined composition (no proteases/nucleases)\n- Linear DNA templates work well\n- Unnatural amino acid incorporation\n- Reproducible between batches\n\n### Limitations\n- No chaperones (add separately)\n- No post-translational modifications\n- Lower yields than crude extracts\n- Higher cost\n\n### When to Use PURE\n- Unnatural amino acid incorporation\n- Studying translation mechanisms\n- \"Clean\" proteins needed\n- Protease-sensitive targets\n- Linear template expression\n\n---\n\n## Common Artifacts and Solutions\n\n### Low Molecular Weight Bands\n**Causes**: Premature termination, proteolysis, internal initiation\n**Solutions**:\n- Optimize rare codon clusters\n- Add protease inhibitors\n- Check for internal AUG codons\n- Use PURE system\n\n### Higher MW Bands\n**Causes**: Incomplete termination, read-through, aggregation\n**Solutions**:\n- Ensure strong stop codon (UAA preferred)\n- Check template 3' end\n- Add release factors (RF1/RF2)\n- Reduce protein concentration\n\n### No Soluble Protein\n**Causes**: Aggregation during synthesis\n**Solutions**:\n- Lower temperature (25°C → 18°C)\n- Add chaperones\n- Use solubility tag\n- Optimize translation rate\n\n---\n\n## References\n\n### CFPS Overview\n- [User's Guide to CFPS - PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC6481089/)\n- [Optimising Protein Synthesis in Cell-Free Systems - PMC](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9996726/)\n- [CFPS Systems Comparison - PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC8258279/)\n\n### Extract Preparation\n- [Crude Extract Preparation - MDPI Methods](https://www.mdpi.com/2409-9279/2/3/68)\n- [Simple Rapid Cell-Free Lysate - PLOS One](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0165137)\n- [High-Throughput Extract Preparation - Nature Scientific Reports](https://www.nature.com/articles/srep08663)\n\n### PURE System\n- [PURE System Evolution - PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC10521753/)\n- [PURE System for Membrane Proteins - Nature Protocols](https://www.nature.com/articles/nprot.2015.082)\n\n### Wheat Germ\n- [Wheat Germ Systems Review - FEBS Letters](https://febs.onlinelibrary.wiley.com/doi/10.1016/j.febslet.2014.05.061)\n- [Wheat Germ for Structural Biology - PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC8027086/)\n\n### Codon Optimization\n- [Rare Codons and Solubility - PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC2723077/)\n- [Codon Influence on Expression - Nature](https://www.nature.com/articles/nature16509)\n- [Synonymous Codon Substitutions Perturb Folding - PNAS](https://www.pnas.org/doi/10.1073/pnas.1907126117)\n\n### Disulfide Formation\n- [Oxidative Protein Folding in ER - PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4312752/)\n- [PDI-Regulated Disulfide Formation - PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC7794689/)\n\n### Solubility Tags\n- [SUMO Fusion for Difficult Proteins - PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC7129290/)\n- [Fusion Tags Review - Frontiers Microbiol](https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2014.00063/full)\n\n### Temperature Effects\n- [Cold Shock Promoters - PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC9800685/)\n- [Strategies to Optimize E. coli Expression - PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC7162232/)","tags":["cell","free","expression","protein","design","skills","adaptyvbio","agent-skills","claude-code","protein-design","protein-engineering"],"capabilities":["skill","source-adaptyvbio","skill-cell-free-expression","topic-agent-skills","topic-claude-code","topic-protein-design","topic-protein-engineering"],"categories":["protein-design-skills"],"synonyms":[],"warnings":[],"endpointUrl":"https://skills.sh/adaptyvbio/protein-design-skills/cell-free-expression","protocol":"skill","transport":"skills-sh","auth":{"type":"none","details":{"cli":"npx skills add adaptyvbio/protein-design-skills","source_repo":"https://github.com/adaptyvbio/protein-design-skills","install_from":"skills.sh"}},"qualityScore":"0.513","qualityRationale":"deterministic score 0.51 from registry signals: · indexed on github topic:agent-skills · 126 github stars · SKILL.md body (10,760 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