An insect expression system involves transposing the expression components from a transfer vector onto a baculovirus shuttle vector proliferating in E. coli, extracting the shuttle plasmid DNA, and transfecting insect cells with it; the resulting progeny virus is the recombinant virus. The recombinant protein is then obtained by infecting insect cells with the viral supernatant. By reviewing relevant literature, this paper summarizes common issues encountered during insect expression experiments, analyzes their possible causes, and proposes solutions.

1. Can the insect cell protein expression system co-express two genes?

A: Insect cells can express multiple genes simultaneously. The baculovirus capsid can accommodate relatively large exogenous DNA insert fragments. Two separate plasmids can be constructed and then co-transfected into insect cells. The advantage of this approach is that the expression levels of the two proteins can be optimized independently, and the balance between their expression can be controlled. Compared to co-expression using a single plasmid, the disadvantage is that the work involved—including plasmid construction, plasmid extraction, and protein-virus packaging—as well as the associated costs, are doubled; Another method of co-expression involves constructing two target genes on the pFastBac Dual vector, which allows for the simultaneous expression of both proteins. The pFastBac Dual vector contains two promoters: the PH promoter and the P10 promoter, with the PH promoter exhibiting higher protein expression efficiency than the P10 promoter.

2. How does the storage method of insect viruses affect the viruses?

A: Adding 2% cell culture-grade BSA or fetal bovine serum during virus preparation helps protect the virus. If you forget to add it, you can add it after harvesting the virus. Store the virus at 4°C or –80°C. For short-term use (generally about half a year), storage at 4°C is acceptable; for long-term use, storage at –80°C is recommended. Before reusing the virus, it is advisable to test the viral titer or conduct a small-scale expression trial to prevent a decrease in titer or loss of activity. For long-term or scale-up projects, you may also consider preparing BIIC for storage in liquid nitrogen.

3. Which types of proteins are best suited for insect expression systems?

A: Kinases, HDACs (histone deacetylases), viral/insect-derived proteins, glycoproteins, cytokines, capsid proteins, VLPs, envelope proteins, and membrane proteins. Some of these proteins are inherently difficult to express and can be attempted using an insect expression system.

4. Does insect protein expression involve using live insects? 

A: No, proteins are expressed by culturing insect cells. Commonly used insect expression host cell lines include Sf9, Sf21, Hi5, and S2. There are also methods that use actual insects to express proteins, such as the silkworm. This falls under the category of insect-infecting viruses, specifically the silkworm system (BmNPV). While the silkworm can express proteins in large quantities, purifying the proteins after expression is extremely difficult. Furthermore, our silkworm culture environment and techniques are limited; therefore, we do not recommend using silkworms for protein expression.

5. Is the target protein released into the extracellular space along with the virus after expression?

A: Whether the target protein is released into the extracellular space depends primarily on the nature of the protein itself and whether a signal peptide has been added. If a signal peptide was added during vector construction, the protein is generally released into the extracellular space. However, if the target protein is an intracellular protein—such as a cytoskeletal protein, nuclear protein, or cytoplasmic protein—it may not be released into the extracellular space even if a signal peptide is added. Therefore, viral release and the release of the target protein are two separate processes; the protein may be released into the extracellular space along with the virus, or it may remain intracellular.

6. Is the target gene integrated into the insect cell’s genome?

A: No. Exogenous genes generally do not readily integrate into the host genome; specific conditions are required. In insect systems, baculoviruses carry the target gene into the host and utilize the host environment for self-replication and gene expression; the target gene does not integrate into the host genome.

7. For insect protein expression, are there size requirements for the target protein?

A: Generally, proteins ranging from 10 kDa to 100 kDa are suitable for any expression system, while large proteins between 110 kDa and 250 kDa are best expressed using the insect expression system.

8. What is the difference between soluble proteins obtained by adding solubilizers and those obtained through low-temperature expression optimization? Do solubilizers affect protein activity?

A: Solubilizers are relatively mild surfactants that convert inherently insoluble proteins into soluble forms. Soluble proteins obtained through low-temperature expression optimization are produced by optimizing expression conditions to yield soluble proteins.

    In theory, solubilizers have minimal impact on protein activity; however, it cannot be ruled out that they have no effect at all. This largely depends on the specific objectives of the subsequent experiments and the required level of protein activity. If you are concerned about the impact of solubilizers on the protein, you can switch to a different expression system or attempt to optimize the expression conditions.

    In fact, there are many factors that influence protein activity. In terms of expression systems, E. coli < insect < mammalian; proteins expressed in E. coli are often inactive, activity is not guaranteed in insect systems, and most proteins expressed in mammalian systems are active. From a protein perspective, some proteins are inherently poorly soluble and have low hydrophilicity, making them prone to precipitation during expression; precipitated proteins are essentially inactive. Some proteins are inherently inactive upon expression; regardless of whether solubilizing agents are added, they will remain inactive. Therefore, the addition of solubilizing agents does not directly correlate with protein activity. However, if an active protein is required, it is best to express a soluble form that does not require the addition of solubilizing agents.

    KMD Bioscience possesses a comprehensive insect expression and purification system. Our insect cell protein expression system is well-suited for the recombinant expression of membrane proteins; transmembrane proteins with four or fewer transmembrane domains can be effectively expressed and purified in insect cells. The purification of membrane proteins is a relatively time-consuming and labor-intensive process; KMD Bioscience can provide custom full-length membrane protein products with a purity of up to 95%. We offer clients comprehensive protein expression solutions, including free sequence analysis and testing of customer-provided genes. Additionally, based on the codon preferences of Sf9, Sf21, Hi-5, and S2 insect cells, we provide free codon optimization to effectively increase recombinant protein yield, delivering a one-stop service.