Week 9: Cell-Free Protein Expression

Part A: General Questions

Q1. Explain the main advantages of cell-free protein synthesis over traditional in vivo methods, specifically in terms of flexibility and control over experimental variables. Name at least two cases where cell-free expression is more beneficial than cell production.

Cell-free systems can be freeze-dried and reactivated through hydration, making them deployable outside of a wet lab without refrigeration or live cell maintenance or biosafety issues. This opens up applications that in vivo systems simply can’t reach: (1) toxic proteins that would kill a living host can be expressed safely; (2) materials embedded with biological sensing or production capacity like textiles or wound dressings where living cells are not practical.

Q2. Describe the main components of a cell-free expression and role of each component.

Cell lysate provides the ribosomes, translation factors, RNA polymerase, and other endogenous machinery needed for transcription and translation. DNA template is the gene of interest to be expressed. An energy regeneration system (like phosphocreatine + creatine kinase) continuously regenerates ATP to power the reaction. Amino acids are the building blocks for the protein. Salts and buffers maintain optimal pH and ionic conditions to keep conditions stable for expression.

Q3. Why is energy provision regeneration critical in cell-free systems? Describe a method you could use to ensure continuous ATP supply in your cell-free experiment.

Cell-free reactions consume ATP rapidly and have no metabolism to regenerate it, so the reaction stalls quickly without a continuous supply. A common method is the phosphocreatine/creatine kinase system: creatine kinase transfers a phosphate group from phosphocreatine to ADP, regenerating ATP. An alternative is the maltose/maltose binding protein system which uses sugar metabolism to drive ATP regeneration over longer timescales.

Q4. Compare prokaryotic versus eukaryotic cell-free expression systems. Choose a protein to produce in each system and explain why.

Prokaryotic systems (like E. coli lysate) are cheaper, faster to prepare, and have higher yields which means they are good for simple soluble proteins. Eukaryotic systems (like wheat germ or HeLa cell lysate) support post-translational modifications like glycosylation and disulfide bond formation. A bacterial enzyme like Cas9 can be produced in a prokaryotic system since it doesn’t require post-translational modifications and benefits from high yield. Human antibodies can be produced in a eukaryotic system since proper glycosylation is critical for its function and stability.

Q5. How would you design a cell-free experiment to optimize the expression of a membrane protein? Discuss the challenges and how you would address them in your setup.

Membrane proteins are challenging because they are hydrophobic and aggregate easily without a lipid environment. I would supplement the cell-free reaction with nanodiscs or liposomes to provide a membrane-like environment for the protein to fold into as it’s being synthesized. Detergents can also be added at sub-CMC concentrations to stabilize the protein without denaturing it. Yield would be optimized by titrating Mg2+ and K+ concentrations, and the protein would be assessed by SDS-PAGE and a functional binding assay rather than just fluorescence, since aggregation can give a false positive for expression.

Q6. Imagine you observe a low yield of your target protein in a cell-free system. Describe three possible reasons for this and suggest a troubleshooting strategy for each.

First, the DNA template concentration may be too low or the template may be degraded which would need to run a gel to check template integrity and titrate DNA concentration across a range to find the optimum. Second, the Mg2+ concentration may be off (critical cofactor for ribosomes and polymerases) so I would run a Mg2+ optimization grid. Third, the protein may be expressed but insoluble and aggregating, I would run both soluble and total fractions on SDS-PAGE separately to check whether the protein is being made but crashing out, and if so, add chaperones or adjust the reaction temperature to slow translation and allow proper folding.