Week 9 Cell-Free Systems

Homework Part A: General and Lecturer-Specific Questions

1. What advantages do IANNs have over traditional genetic circuits, whose input/output behaviors are Boolean functions?

Cell-free protein synthesis (CFPS) offers significant advantages over traditional in vivo methods by removing the constraints of cell viability.

  • Flexibility and Control: We can directly manipulate reaction variables like pH, temperature, and redox potential in an “open” system.
  • Non-Natural Amino Acids: It’s much easier to incorporate non-canonical amino acids since we don’t have to worry about cellular transport or toxicity.
  • Case 1 (Toxic Proteins): Synthesis of antimicrobial peptides or pore-forming toxins that would kill a living host cell.
  • Case 2 (Rapid Prototyping): High-throughput screening of genetic circuits where we can get results in hours instead of days required for cloning and cell culture.

2. Describe the main components of a cell-free expression system and explain the role of each component.

A standard CFPS system is a carefully balanced 　molecular soup　consisting of:

  • Cell Extract: Contains the core machinery like ribosomes, tRNAs, and aminoacyl-tRNA synthetases.
  • Energy Mix: ATP and GTP to power the transcription and translation machinery.
  • Amino Acids: The building blocks for the protein chain.
  • Cofactors and Salts: Magnesium and Potassium ions that are essential for ribosome stability and enzyme activity.
  • DNA Template: The genetic instructions (plasmid or linear PCR product).

3. 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.

Protein synthesis is extremely energy-expensive. ATP is rapidly converted to ADP during peptide bond formation and tRNA charging. If we don’t regenerate it, the reaction stalls very quickly.

  • Regeneration Method: I would use a secondary energy source like Phosphoenolpyruvate (PEP) combined with Pyruvate Kinase. This enzymatic pathway constantly recycles ADP back into ATP, allowing the reaction to run for much longer.

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

  • Prokaryotic (E. coli): High yield and low cost. Great for simple proteins.
    • Choice: GFP (Green Fluorescent Protein). It doesn’t need complex folding or sugar tags, so E. coli extracts can crank it out efficiently.
  • Eukaryotic (Wheat Germ or HeLa): Slower and more expensive, but capable of complex folding and post-translational modifications (PTMs).
    • Choice: Human Antibody (IgG). These require specific disulfide bond formation and glycosylation that only eukaryotic machinery can handle properly.

5. 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.

The biggest challenge is that membrane proteins are hydrophobic; they hate being in water and will aggregate (clump up) immediately.

  • Design Strategy: I would add Nanodiscs or Liposomes directly to the reaction. This provides a “synthetic home” (a lipid bilayer) for the protein to insert into as it’s being synthesized. This keeps the protein stable and functional.

6. 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.

  • Reason 1: Template Degradation. Nucleases in the extract might be eating the DNA.
    • Fix: Add RNase inhibitors or use a more purified DNA template.
  • Reason 2: Energy Depletion. The ATP ran out too fast.
    • Fix: Switch to a dialysis-based system to continuously supply fresh energy and remove waste.
  • Reason 3: Misfolding/Aggregation. The protein is being made but it’s just clumping up.
    • Fix: Lower the incubation temperature or add exogenous chaperones (like CCT4) to help with the folding process.

Homework Question from Kate Adamala: Design a Synthetic Minimal Cell

1. Pick a function and describe it.

a. What would your synthetic cell do? What is the input and what is the output? My synthetic cell functions as a “Neuro-Protective Sentinel.” It is designed to sense early-stage pathological markers of Alzheimer’s Disease and respond by releasing therapeutic chaperones.

  • Input: Extracellular Amyloid-beta (A$\beta$) oligomers or monomers.
  • Output: CCT4 (Chaperonin Containing TCP-1 subunit 4) protein.

b. Could this function be realized by cell-free Tx/Tl alone, without encapsulation? No. Without encapsulation, the transcription/translation (Tx/Tl) machinery would be diluted in the extracellular space and degraded by proteases. Encapsulation protects the PURE system and ensures the therapeutic protein is synthesized in a high local concentration before release.

c. Could this function be realized by genetically modified natural cell? Yes, but using a minimal cell is safer for neuro-therapeutic applications. Since minimal cells are non-replicative and have a finite lifespan, they eliminate the risk of uncontrolled proliferation or potential tumorigenesis in the brain tissue.

d. Describe the desired outcome of your synthetic cell operation. The local concentration of the CCT4 chaperone increases in the neuronal microenvironment, facilitating the refolding of misfolded proteins and preventing the formation of toxic tau aggregates or amyloid plaques.

2. Design all components that would need to be part of your synthetic cell.

a. What would the membrane made of? A mixture of DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine) and Cholesterol. Cholesterol is essential here to mimic the neuronal membrane environment and provide structural stability against mechanical stress.

b. What would you encapsulate inside? Enzymes, small molecules.

  • PURE system (cell-free Tx/Tl machinery).
  • DNA Template for the CCT4 gene under the control of an A$\beta$-sensitive riboswitch or aptamer.
  • Energy regeneration system (Creatine phosphate and Creatine kinase) to sustain protein synthesis.

c. Which organism your Tx/Tl system will come from? A mammalian cell extract (e.g., HeLa or CHO) is preferred over a bacterial system. Since CCT4 is a large, complex eukaryotic protein, mammalian machinery is more likely to ensure proper initial folding and post-translational stability of the output.

d. How will your synthetic cell communicate with the environment? The membrane will be embedded with alpha-hemolysin ($\alpha$HL) pores. These pores are large enough to allow the entry of A$\beta$ monomers (input) and the exit of the synthesized CCT4 protein (output) into the extracellular space.

3. Experimental details.

a. List all lipids and genes. (bonus: find the specific genes; for example, instead of just saying “small molecule membrane channel” pick the actual gene.)

  • Lipids: DOPC, Cholesterol.
  • Membrane Channel: $\alpha$-hemolysin (from Staphylococcus aureus).
  • Therapeutic Gene: CCT4 (Chaperonin Containing TCP1 Subunit 4, Human).
  • Sensor: A customized A$\beta$-binding DNA aptamer linked to a T7 promoter.

b. How will you measure the function of your system? I will use a Fluorescence Resonance Energy Transfer (FRET) assay. By encapsulating a FRET-labeled Tau protein reporter in nearby target vesicles, I can measure the reduction in Tau aggregation (indicated by a change in FRET signal) as a direct result of the CCT4 protein released by the minimal cells.

AI Disclosure Conceptual development and technical writing for this assignment were supported by AI assistance.