- Heterologous expression systems are experimental platforms used to express genes or proteins in a host organism that is different from the gene’s organism of origin. These systems enable researchers to produce proteins that are otherwise difficult to obtain in sufficient quantity or purity, to study protein function in a simplified or controlled environment, and to develop biotechnological and pharmaceutical products.
- Choosing an appropriate heterologous system depends on the biological properties of the protein being expressed, such as its need for proper folding, post-translational modifications (e.g., glycosylation, phosphorylation), disulfide bond formation, or membrane localization. The major types of heterologous expression systems include:
- Prokaryotic systems (e.g., Escherichia coli): E. coli is one of the most widely used hosts due to its rapid growth, ease of genetic manipulation, and low cost. It is ideal for the high-yield production of simple proteins that do not require extensive post-translational modifications. However, it often struggles to properly fold complex eukaryotic proteins or perform eukaryotic-specific modifications, which can lead to the formation of inclusion bodies.
- Yeast systems (e.g., Saccharomyces cerevisiae, Pichia pastoris): Yeasts combine some of the advantages of bacterial systems (fast growth, simple cultivation) with the ability to perform certain eukaryotic post-translational modifications. Pichia pastoris, in particular, is favored for high-level secretion of recombinant proteins and is commonly used for producing enzymes, antibodies, and vaccines.
- Insect cell systems (e.g., Sf9, Sf21 cells with baculovirus vectors): Insect cells can perform many post-translational modifications similar to those in higher eukaryotes and are capable of expressing large and complex proteins, including membrane proteins. The baculovirus expression vector system (BEVS) is highly efficient and is used for the production of vaccines (such as the HPV vaccine) and recombinant viral proteins for structural studies.
- Mammalian cell systems (e.g., HEK293, CHO cells): Mammalian cells are the gold standard for expressing proteins that require authentic human-like post-translational modifications, especially glycosylation patterns critical for therapeutic efficacy. Chinese Hamster Ovary (CHO) cells, for example, are widely used in the pharmaceutical industry for the production of monoclonal antibodies, growth factors, and other biologics.
- Cell-free systems (in vitro transcription/translation systems): These systems bypass living cells entirely and allow rapid protein production using extracts prepared from bacteria, yeast, insect, or mammalian cells. They are particularly useful for high-throughput screening, the synthesis of toxic proteins, or producing proteins with non-natural amino acids.
