Cell-Free Protein Synthesis (CFPS) Skill

Cell-Free Protein Synthesis (CFPS)

System Selection Guide

| System | Best For | Yield | PTMs | Disulfides | Cost | |--------|----------|-------|------|------------|------| | E. coli extract | Rapid prototyping, prokaryotic proteins | High (100-400 μg/mL) | None | Poor (reducing) | Low | | E. coli PURE | Defined conditions, unnatural AAs | Medium (50-150 μg/mL) | None | Controllable | High | | Wheat germ | Eukaryotic proteins, membrane proteins | High (100-500 μg/mL) | Limited | Moderate | Medium | | Rabbit reticulocyte | Mammalian proteins, post-translational studies | Low (10-50 μg/mL) | Some | Poor | High | | Insect (Sf21) | Glycoproteins, complex folds | Medium (50-100 μg/mL) | Glycosylation | Good | High | | HeLa/CHO | Native mammalian proteins | Low (10-50 μg/mL) | Full mammalian | Good | Very High |

CFPS Troubleshooting Matrix

| Problem | Likely Causes | Design Fix | Reagent Fix | |---------|---------------|------------|-------------| | No expression | Rare codons at N-terminus, poor RBS | Codon optimize first 30 codons | Use BL21-CodonPlus extract | | Low yield | Strong mRNA secondary structure, template issues | Optimize 5' UTR (ΔG > -5 kcal/mol) | Increase Mg²⁺ (10-18 mM), ATP | | Aggregation | Hydrophobic protein, fast translation | Add solubility tags (MBP, SUMO) | Add 0.1% Tween-20, chaperones | | Inactive protein | Misfolding, missing cofactors | Slow translation (use rare codons!) | Add GroEL/ES, DnaK/J | | Truncation | Rare codon clusters, mRNA instability | Remove AGG/AGA/CUA clusters | Supplement rare tRNAs | | Degradation | Proteolysis | N-terminal Met-Ala | Add protease inhibitors |

Codon Optimization for CFPS

Codons to Avoid in E. coli CFPS

| Codon | Amino Acid | Issue | tRNA Abundance | |-------|------------|-------|----------------| | AGG | Arg | Very rare, stalling | 0.2% | | AGA | Arg | Very rare, stalling | 0.4% | | CUA | Leu | Low abundance | 0.4% | | AUA | Ile | Rare | 0.5% | | CGA | Arg | Inefficient decoding | 0.6% | | CCC | Pro | Can cause pausing | 0.5% | | GGA | Gly | Moderate | 1.1% |

Design Rules

First 30 codons: Most critical - use only high-frequency codons
Rare codon clusters: Avoid 2+ rare codons within 10 nt
Rare codon content: Keep overall <5% of coding sequence
GC content: Target 40-60% for balanced expression
Avoid runs: No >6 consecutive G or C residues (secondary structure)
Strategic slow codons: Place rare codons between domains (aids folding!)

When to Use Rare Codons

Domain boundaries (allow cotranslational folding)
Before complex structural elements
When protein is prone to misfolding

mRNA Template Design

5' UTR Optimization

| Element | Optimal Design | Impact | |---------|----------------|--------| | RBS (SD sequence) | AGGAGG, 7-9 nt from start | Ribosome binding | | Spacing | 7 nt between SD and AUG | Translation initiation | | Secondary structure | ΔG > -5 kcal/mol | Accessibility | | Upstream AUG | Avoid (causes false starts) | Reduces truncations |

Secondary Structure Targets

| Region | Ideal ΔG | Impact | |--------|----------|--------| | -30 to +30 around AUG | > -5 kcal/mol | Translation initiation | | Full 5' UTR | > -10 kcal/mol | Ribosome loading | | RBS accessibility | Unpaired | Critical |

Template Format

| Format | Advantages | Disadvantages | |--------|------------|---------------| | Plasmid | Stable, high yield | Requires cloning | | Linear PCR | Fast, no cloning | May need stabilization | | mRNA | Direct translation | Unstable, expensive |

Disulfide Bond Formation

System Capabilities

| System | Native Disulfide Support | Additives Needed | |--------|--------------------------|------------------| | Standard E. coli extract | Poor (DTT present) | IAM, PDI, GSSG/GSH | | Oxidizing E. coli extract | Good | Pre-oxidized glutathione | | Wheat germ | Moderate | Lower DTT, add PDI | | PURE system | Minimal | Full oxidative system | | Insect/Mammalian | Good | Microsome membranes |

Oxidative Folding Protocol (E. coli extract)

1. Deplete DTT from extract (dialysis or treatment with IAM 5 mM)
2. Add oxidized/reduced glutathione: 4 mM GSSG, 1 mM GSH (4:1 ratio)
3. Add 10 μM PDI (protein disulfide isomerase)
4. Optional: Add 5 μM DsbC (disulfide isomerase)
5. Express at 25°C (not 37°C) for better folding
6. Incubation time: 4-6 hours

Disulfide-Rich Protein Tips

Start with wheat germ or oxidizing extract
Use PURE system for precise control
Consider co-expression of PDI/DsbC
Verify by non-reducing SDS-PAGE

Expression Prediction from Sequence

| Feature | Good | Marginal | Bad | |---------|------|----------|-----| | Rare codon content | <3% | 3-8% | >10% | | First 30 codons rare | 0 | 1-2 | >2 | | GC content | 45-55% | 35-45% or 55-65% | <30% or >70% | | 5' UTR ΔG | > -3 kcal/mol | -3 to -8 | < -10 kcal/mol | | Hydrophobic stretches | <5 consecutive | 5-7 | >8 consecutive | | N-terminal residue | Met-Ala, Met-Ser, Met-Gly | Met-Val, Met-Thr | Met-Arg, Met-Lys | | Cysteine pairs | Paired (even number) | Mixed | Odd number (free thiols) |

Solubility Enhancement Strategies

Fusion Tags (ranked by effectiveness)

| Tag | Size | Solubility Enhancement | Cleavage | Notes | |-----|------|------------------------|----------|-------| | MBP | 40 kDa | Excellent | TEV, Factor Xa | Best overall | | SUMO | 11 kDa | Very Good | SUMO protease | Native N-terminus after cleavage | | NusA | 55 kDa | Excellent | - | Large size | | Trx | 12 kDa | Good | Enterokinase | For disulfide proteins | | GST | 26 kDa | Moderate | - | Dimeric | | His₆ | 1 kDa | Minimal | - | Mainly for purification |

Buffer Additives for Solubility

| Additive | Concentration | Mechanism | |----------|---------------|-----------| | Trehalose | 50-100 mM | Chemical chaperone | | Glycerol | 5-10% | Reduces hydrophobic aggregation | | L-Arginine | 50-100 mM | Suppresses aggregation | | Tween-20 | 0.05-0.1% | Prevents surface adsorption | | Proline | 50 mM | Osmolyte stabilization |

Chaperone Supplementation

| Chaperone System | Target Problem | Concentration | |------------------|----------------|---------------| | GroEL/GroES | General folding | 1-2 μM | | DnaK/DnaJ/GrpE | Aggregation-prone | 1 μM each | | Trigger Factor | Nascent chain | 1-2 μM | | ClpB | Aggregate resolubilization | 0.5 μM |

Temperature Optimization

| Temperature | Use Case | Trade-offs | |-------------|----------|------------| | 37°C | Fast expression, stable proteins | Higher aggregation risk | | 30°C | Balanced (default) | Good compromise | | 25°C | Disulfide proteins, complex folds | Slower, better folding | | 18-20°C | Aggregation-prone proteins | Much slower, best folding | | 16°C | Cold-shock proteins | Very slow, specialized |

E. coli Extract Preparation (Key Variables)

| Variable | Impact | Optimal Range | |----------|--------|---------------| | Cell density at harvest | Ribosome content | OD₆₀₀ 2.5-3.5 | | Lysis method | Extract activity | Sonication, bead beating | | Run-off reaction | Removes endogenous mRNA | 20-80 min at 37°C | | Mg²⁺ concentration | Translation fidelity | 10-18 mM | | K⁺ concentration | Translation rate | 150-200 mM | | Energy system | Sustained synthesis | ATP/GTP, creatine phosphate |

PURE System Specifics

Advantages

Defined composition (no proteases/nucleases)
Linear DNA templates work well
Unnatural amino acid incorporation
Reproducible between batches

Limitations

No chaperones (add separately)
No post-translational modifications
Lower yields than crude extracts
Higher cost

When to Use PURE

Unnatural amino acid incorporation
Studying translation mechanisms
"Clean" proteins needed
Protease-sensitive targets
Linear template expression

Common Artifacts and Solutions

Low Molecular Weight Bands

Causes: Premature termination, proteolysis, internal initiation Solutions:

Agent Skills: Cell-Free Protein Synthesis (CFPS)

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