Toxic proteins defined here as proteins that cause cell death or severe cultivation and maintenance defects during the growth phase when their genes were introduced into E. coli strain.
2. Mechanisms of Protein Toxicity
The over-expressed recombinant protein will perform certain functions in the host cell if the soluble protein is expressed with proper folding. The function of the expressed recombinant protein may be detrimental to the proliferation and differentiation of the host cell. The phenotype of the host cells is slow growth rate and low cell density. In some cases, the recombinant protein causes death of the host cell.
3. Percentage of Protein Toxicity
Protein toxicity is a commonly observed phenomenon. It is estimated that about 80% soluble recombinant proteins have a certain degree of toxicity to their hosts. About 10% of these are highly toxic to host cells. The completely insoluble or dysfunctional proteins will not be toxic to the host cell, though they create metabolic burden for the host cell. Some low solubility or partially functional proteins may still be toxic to the host cell.
4. Phenotypes of Protein Toxicity
Protein toxicity adversely affects the cloning and expression. Protein toxicity is the most important reason for DNA cloning or subcloning problems. Most expression problems are also the result of protein toxicity.
4.1 Toxic Protein Cloning Problems
No colony in cloning
Fewer colonies than those in regular cloning experiments
Small percentage of positive clones
Mutations leading to defective products or no expression
4.2 Transformation Efficiency Problems
The transformation efficiency of the vector containing a toxic protein is lower or significantly lower than the control vectors or the vectors containing non-toxic proteins.
Transformation efficiency in a normal cell strain is significantly lower than that in a detoxification strain.
4.3 Host Cell Growth Rate and Cell Density Problems
Cell grows significantly slower and cannot reach its normal density in a given medium before induction.
Cell grows significantly slower and cannot reach its normal density after induction.
Growth rate and cell density are low in normal strain. In contrast, the growth rate and cell density are close to normal in a detoxification cell strain.
4.4 Toxic Protein Expression Problems
5. Strategies of Toxic Protein Cloning and Expression
Once the toxicity of a protein is determined, different strategies should be used.
5.1 Suppress Basal Expression
Use a more tightly regulated promoter, e.g. the arabinose promoter (PBAD).
Use a lower copy number plasmid to better regulate expression.
Use a strong terminator upstream of inducible promoter to prevent transcription read-through from upstream real and cryptic promoters.
Use a tightly regulated vector which contains multiple lac operators and transcription repressors.
Constitutive expression of phage T7 lysozyme from a compatible pLysS or pLysE plasmid. T7 lysozyme, a natural inhibitor of T7 RNA polymerase, can reduce basal activity from an inducible gene for T7 RNA polymerase and allow relatively toxic genes to be established in the same cell under control of a T7 promoter.
Use detoxification strains. Some host strains, e.g. C41 (DE3) and C43 (DE3), appear to be useful in some toxic proteins especially some membrane proteins.
Addition of 1% glucose to culture medium to repress induction of the lac promoter by lactose, which is present in most rich media (such as LB, 2xYT).
5.2 Optimization for Toxic Protein Expression
Induction conditions Induction time
Induction time should be closely monitored to produce a reasonable amount of protein while the host cells are not devastated (protein degradation or cell death). Inducer concentration
Cells induced using very low IPTG concentrations have more metabolic control over the toxic effect than those induced with standard IPTG concentrations (0.1- 1.0 mM). IPTG concentrations in the range 0 -10 μM are commonly used for induction of toxic proteins. Induction temperature
Protein toxicity may be reduced or eliminated at a lower temperature. Lower temperature also results in increased protein solubility. Longer induction time should always be associated with lower temperature because of lower protein synthesis rate. Temperatures of 25 and 15 degree Celsius are commonly used for induction of toxic proteins.
High concentration of antibiotics
Use of elevated levels of antibiotics (up to 200 mg/ml), which helps prevent losing the expression vector from the cells.
Cultures are inoculated by scraping off agar plates, which is a more stringent method to select plasmid-containing cells than in liquid culture media.
5.3 Other Approaches for Toxic Protein Expression
Other technologies may also be used in combination with above technologies to improve cloning efficiency and protein yield.
Large fusion partners such as GST or Trx will reduce protein toxicity in some cases. Small tags such as his-tag will not alter toxicity property significantly. In rare cases fusion partner can increase protein toxicity.
Express individual domains
Individual domains are almost always less toxic than the corresponding full-length proteins. Sometimes individual domains may lose toxicity completely and therefore are not functional.
Secretion of the target protein to the periplasm (or the medium) allows for the accumulation of proteins that are toxic in the cytoplasm.
Express in inclusion bodies
It is a good strategy to express toxic protein in inclusion bodies. In aggregates, the proteins are not toxic to the cell and they can be obtained by in vitro denaturation and refolding.
6. Vectors Suitable for Expression of Toxic Proteins
Table 1.Vectors Suitable for Expression of Toxic Proteins
Constitutive expression of lysozyme, a natural inhibitor of T7 RNAP
Expression of more lysozyme than pLysS
Tight regulation of gene expression
Blocking read-through transcription by upstream terminators
Arabinose-based control of copy number
7.E. coli Strains Suitable for Expression of Toxic Proteins
Table 2. E. coli Strains Suitable for Expression of Toxic Proteins
T7 RNAP gene under the control of PBAD promoter
The strain carries a temperature-sensitive allele of supF suppressor
CopyCutterTM and EPI400TM
Regulation of the copy number of ColE1-type plasmids using a chromosomal copy of the pcnB gene under the control of an inducible promoter
C41(DE3) and C43(DE3)
Empirically-selected strains with lower basal expression and slower induction rates compared to the original BL21(DE3)
The strain reduces 4-fold the copy number of ColE1-derived plasmids
The strain reduces 10-fold the copy number of ColE1-derived plasmids
We reviewed strategies for the expression of toxic proteins. The set of solutions summarized in this review highlight two important points: the need to characterize the level of toxicity of problematic genes and the need to tune the tightness of the inducible expression system to match the basal expression level that E. coli tolerates. By combining many rational strategies, the desirable tightness can be achieved.
Saïda F, Uzan M, Odaert B, Bontems F. Expression of Highly Toxic Genes in E. coli: Special Strategies and Genetic Tools. Curr Protein Pept Sci. 2006 Feb; 7(1):47-56.
Germán L. Rosano, Eduardo A. Ceccarelli. Recombinant Protein Expression in Escherichia Coli: Advances and Challenges. Front Microbiol. 2014; 5:172.