Expressing recombinant protein can be done in all kinds of systems. Many different cell lines can be used for expressing protein, including mammalian cells like HEK293 or CHO, or even yeast, plant or insect cell lines. Each expression system has its own advantages in different applications and purposes.

Protein Expression in Bacteria

The most common bacterial protein expression systems in use are E. coli cell lines. E. coliculture grows extremely quickly, and are easy to keep healthy and use because of it. The bacteria are easily transfected using methods like heat shock. Unlike eukaryotes, their hearty cell walls and simple anatomy can withstand the drastic temperature change and survive the heat shocking process, while eukaryotes may suffer acute permanent damage that may prevent proper expression or cell replication, or result in cell death.

The advantages of E. coli are most easily observed for certain types of proteins. Specifically, cytosolic and excreted proteins with few post translational modifications can be easily developed in E. coli. Unfortunately, trans-membrane proteins, glycoproteins and other membrane bound proteins are not so easily expressed because there is no endoplasmic reticulum to facilitate proper translation of the structure. This lack of organelles is where bacteria begin to lose their advantage, especially when it comes to studying mammalian proteins.

Protein Expression in Mammalian Cells

Mammalian cells are generally more difficult to work with than bacteria since they are much more fragile, often produce less product overall, and can be much more expensive. However, using mammalian cells for protein expression has the great advantage of being able to produce mammalian protein with all the correct post translational modifications that provide the native structure.

Most post translational modifications occur during or shortly after translation. Excreted proteins are made in the endoplasmic reticulum where there are enzymes that facilitate proper folding, disulfide bonding, and N-linked glycosylation. Additionally, before it is excreted, a protein would pass through the golgi apparatus, where O-linked glycosylation may occur as well. All 4 of these modifications occur in many proteins expressed by mammalian cells, including antibodies.

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