Table 1: Recombinant proteins – a selection guide for expression systems:

Escherichia coli (E.coli) expression system	Advantages	1. Clear genetic background; 2. Low cost and simple culture conditions; 3. Simple transformation operation; 4. Short protein expression period; 5. High rate of reproduction and expression level; 6. A variety of carriers and fusion tags to choose; 7. Many parameters can be altered to optimize expression. 8. Tag can be available at both N-terminal and C-terminal with an enzyme cutting site that is easy to cut.
	Disadvantages	1. Inefficient disulfide bonds formation and poor folding lead to inclusion body formation; 2. Poor post-translational modifications (such as glycosylation, alkylation, phosphorylation, specificity of proteolytic processing; 3. The success rate and recovery rate of inclusion body renaturation is not very high.
	Applications	1. Purified protein (structure, enzymology, drug discovery); 2. Protein therapeutics.
Pichia pastoris (Yeast) expression system	Advantages	1. Low cost and high expression level; 2. Self-produced Endotoxin-free; 3. Efficient protein folding and secretory expression; 4. Simple culture conditions and purification operation; 5. Extensive post-translational modifications: Glycosylation, phosphorylation, acyl lipid; 6. Can develop high density fermentation by using simple inorganic salt, large biomass; 7. The protein is more stable than the prokaryotic expression, and is especially suitable for the expression of eukaryotic genes and the preparation of functional expression proteins; 8. Simple laboratory and industrial operation.
	Disadvantages	1. Glycosylation modification is not as good as mammalian cells
	Applications	1. Purified protein (structure, enzymology, drug discovery).
Baculovirus-infected insect cells expression system	Advantages	1. Good expression levels and relatively rapid growth; 2. More advantages over the expression of viral proteins; 3. Glycosylation modification more like mammalian cells; 4. Relatively de-glycosylation modification (good for structure determination); 5. Large gene volume: because the baculovirus genome is larger, it can carry a larger gene fragment; 6. High security: the baculovirus has a strict species specificity, not infected with vertebrates and plants; 7. High expression efficiency of exogenous gene: compared with other eukaryotic expression systems, the baculovirus system can efficiently express exogenous protein from infected cells; 8. The expression product has high activity: baculovirus will proliferate in insect host cells, thus resulting in recombinant protein that will be similar to mammalian cells in post-translational modification, and the expression product has a strong biological activity; 9. Easy to amplify: baculovirus can mass produce recombinant protein products with biological activity.
	Disadvantages	1. Expensive culture media cost; 2. Inefficient processing of pro-peptides in secretory pathway; 3. Viral infection leads to cell lysis and potential degradation of expressed proteins; 4. Glycosylation modification is not as good as mammalian cells.
	Applications	1. Purified protein (structure, enzymology, drug discovery).
Mammalian cells expression system	Advantages	1. Self-produced Endotoxin-free; 2. Efficient protein folding and secretory expression; 3. All post-translational modifications provide a variety of complex N glycosylation and accurate O type glycosylation and other post-translational processing functions; 4. The expressed products are closest to the natural biological proteins in molecular structure, physical and chemical properties and biological functions.
	Disadvantages	1. Expensive culture media cost; 2. Complex growth requirements.
	Applications	1. Purified protein (structure, enzymology, drug discovery); 2. Protein therapeutics; 3. Cell-based studies.
vitro E.coil expression system	Advantages	1. High expression level: we provide the original and optimized pET carrier with a series of labels, so that the membrane protein yield can reach mg grades; 2. Protein synthesis conditions can be manipulated; 3. The target fusion protein with 6*His is beneficial to the purification.
	Disadvantages	1. Limited post-translational modifications; 2. Expensive for scale-up.
	Applications	1. Purified protein (structure, enzymology, drug discovery); 2. In vitro expression cloning; 3. Isotopic labelling of proteins for NMR; 4. Incorporation of non-natural amino acids.

Source: Cusabio https://www.cusabio.com/c-20621.html

Table 2: How to choose an expression system depending on the type of protein:

Expression System	System Benefits	Application	Features of CUSABIO
In vitro E.coli Expression System	Simple, take short time, high expression quantity, open and flexible, easy to express specific proteins, prepare protein complexes, parallel to synthesize a variety of different proteins, etc.	Toxic proteins, membrane proteins	Ony a few expression conditions fail, and problems are solved professionally, greatly reducing the experimental period and increasing espression quantity.
E. coli Expression System	High targeted gene expression quantity, low cost, simple culture conditions, rapid productions, strong scalability, simple conversion operation, easy to form disulfide bond	Prokaryotic proteins, simple eukaryotic proteins	Expression includes soluble protein, inclusion body, fusion proteins, etc., with wealthy experience and expertise, we can solve a variety of bottlenecks during the protein expression process
Yeast Expression System	Cost-effective, low-cost for amplifying medium, simple culture conditions, rapid production, strong scalability, good choice for secretory protein or intracellular protein expression, secrete proteins efficiently and allow simple purification, extensive post-translational modifications, no endotoxin	Industrial strain improvement, amplification	The combination of self-transformed efficient secretion vector and host can achieve the highest quality protein expression to the maximum extent; Patented Biobrick technology can be successfully used to the improvement and optimization of industrial strain
Insect baculovirus expression system	Large gene capacity, high efficiency of exogenous gene expression, effective cell fold, moderate scalability, extensive post-translational modifications, glycosylation similar to mammalian cells, is relatively easy enzymatic deglycosylation, no endotoxin	Virus vaccines, signal proteins, cytokines, kinases, etc.	Adopt AcNPV-sf9 cells and high5 cells two expression systems, the selectivity of multiple expression systems, multiple hosts, multi-carrier greatly improve the success rate of protein expression
Mammalian Cell Expression System	Higher expression levels, moderate scalability, cell suspension culture characteristic can do mass production, effective protein fold, suitable for protein secretion, full post-translational modifications, no endotoxin	Complex higher eukaryotes proteins	Adopt the specific combined methods of mammalian cell expression vector and a variety of transfections, optimize expression conditions, improve transfection efficiency, greatly shorten the experimental period, significantly increase the expression quantity