Advantages and Development of Expression System for Animal Proteins
For expression of animal proteins, there must be a good expression system. Most commonly used hosts for heterogonous protein expression are bacteria, yeast, insect and mammalian and human cells. Advantages and development of these systems are briefly discussed in this section.
Bacterial host system offers fast development, high efficiency, inexpensive production as the main advantage. Recombinant protein accumulates intracellular or secreted into medium.
Post transcriptional processing of eukaryotic proteins such as glycocylation or disulphide bond formation does not occur in bacterial cells. Proteins expressed in bacteria differ from their natural forms.
Yeast and other fungi are eukaryotes which are cultivated on a large scale. Intracellular, recombinant and heterologous proteins are located inside the cells and accumulate to
about 10-100 mg/litre. Human proteins expressed inside yeast cells are correctly folded and disulphide bridged but glycocylation differs from the mammalian pattern.
(iii) Mammalian Cells:
A high level expression (10 or more than 100 pictogram per cell/day) of recombinant protein can be achieved using mammalian host cells e.g. CHO or BHK.
The proteins are secreted into fermentation medium in properly folded and active form. Glycocylation and other post-translational modifications occur in ‘human-like’ manner with some differences.
(iv) Human Cells:
Human cell lines are used as expression system to ensure the identity of the recombinant proteins. In this case gene is not manipulated but its promoter is manipulated to activate expression of endogenous human gene. Then ‘gene activation technology’ becomes the commercialised technology.