What are the types of protein expression systems that are used for protein production and purification?

The types of protein expression systems that are used for protein production and purification are insect, mammalian, bacterial, yeast, plant, and cell-free expression systems. 

• Mammalian cells serve as an excellent platform for expressing proteins that undergo multiple post-translational modifications for proper functionality. In mammalian expression designs, viral promoters such as CMV, SV40, and RSV are commonly used for expression following transfection. Mammalian systems offer the flexibility of expressing proteins either transiently or through stable cell lines, with successful transfections yielding high protein yields. Certain mammalian systems also provide the ability to regulate the timing of protein expression using inducible promoters.
• E. coli is one of the most widely used bacteria for bacterial protein expression systems. The process of these systems is straightforward. First, DNA encoding the desired protein is inserted into a plasmid expression vector, which is then introduced into a bacterial cell. Transformed cells multiply, undergo induction to produce the desired protein, and are subsequently lysed. The protein can then be purified from the cellular debris. Various DNA vectors are popular for generating substantial protein amounts in bacterial cells, including the pET, pRSET, and pBAD vectors. Each vector utilizes a different promoter, leading to varying levels of protein expression. Adjusting expression levels may be necessary if the protein is toxic to E. coli. Among these vectors, pET (driven by the T7 lac promoter and induced by lactose) provides the highest level of protein expression.
• There are two types of insect expression systems: 
• Baculovirus expression systems are efficient in achieving high-level expression of recombinant proteins. They are particularly effective for producing glycosylated proteins that may be challenging in E. coli or yeast cells. The drawback of baculovirus systems is the lysis of the infected host cell, stopping protein formation.
• Non-lytic insect cell expression systems (e.g. sf9, Sf21 cells) overcome this limitation, permitting continuous expression of genes into the insect cell genome. Both types of insect expression systems are capable of generating large quantities of proteins.
• In cell-free expression systems, proteins are created in vitro by combining purified components of transcription and translation. These components include ribosomes, tRNAs, RNA polymerase, amino acids and ribonucleotides.These systems are advantageous for quickly organizing multiple proteins in a single reaction. An important feature is their ability to construct proteins with labeled or modified amino acids, which are crucial for diverse downstream applications.
• S. cerevisiae is the most widely utilized yeast in yeast expression systems. In S. cerevisiae, researchers often use the galactose-inducible promoter (GAL) to regulate the expression of recombinant proteins. Additionally, widely used promoters include the phosphate-inducible PHO5 promoter and the copper-inducible CUP1 promoter. Yeast cells are cultivated in precisely defined media and can be readily adapted to fermentation processes, enabling the large-scale generation of proteins.
• Various plant cells, like those from maize, rice tobacco, and sugarcane, are used for plant protein expression. Plant systems exhibit similarities to mammalian cell expression systems, encompassing post-translational modifications. However, extracting and purifying recombinant proteins from plants can be a labor-intensive and costly process due to the complexity of plant tissue.