Eukaryotic Expression System of yeast cells
Yeasts are monocellular eukaryoticfungi and are often used to produce recombinant proteins that are not usuallyproduced in the prokaryotic system due to the need for folding andglycosylation. Several expression systems of yeasts have been successfullyestablished to produce recombinant proteins. The two most known yeastexpression systems are Saccharomyces cerevisiae and Pichia pastoris. Thesestrains have many characteristics, such as resistance to heat or light, havehigh speeds to achieve high cell-density, and they consume abnormal sources ofcarbon.
These yeast strains are geneticallywell known, and their genetic manipulation has been easier to generate. Inaddition to the benefits of simple culture medium, fast growth, and low cost,similar to bacterial cells, yeast can produce and release soluble recombinantproteins in a correct folding state to undergo post-translational modificationssuch as glycosylation, phosphorylation, acetylation, and acylation. The yeastexpression system is the most cost-effective eukaryotic expression system forboth secreting and intracellular proteins. This system is ideal for producinglarge amounts of recombinant eukaryotic proteins. In yeast expression systems,strains that are capable of generating stable and durable recombinant proteinswould be selected, and this system produces large quantities of proteins. Theuse of this system also dramatically reduces costs and time.
However, there are somedisadvantages of using yeast as a host to express recombinant proteins. Severalproteins in this system require specific chaperones to fold correctly.Primitive eukaryotes are different from their mammalian counterparts in termsof O- and N-glycosylation. In mammals, oligosaccharides are covalently attachedwith various sugars such as N-acetyl-galactose-amine, galactose, and sialicacid. Instead, in primitive eukaryotes such as Pichia pastoris, only mannoseresidue is added to the O-oligosaccharides. N-glycosylation in Pichia pastorisis different from the advanced eukaryotic cells. The addition of excess mannoseresidue is a common characteristic in the yeast preventing proper proteinfolding, which, in turn, decreases protein activity and causes safety problems.Also, yeasts are not able to mediate gamma-carboxylation in newly-synthesizedproteins.