Eukaryotic expression system (Insect expression system)
Insect expression systems that areincreasingly applied for various proteins provide a quick and easy way toproduce proteins at the milligram levels. The host used for the expression ofproteins in insect cells is mainly Spodoptera Frugiperda, and the most commonvector system employed for the expression of proteins in insect cells isbaculovirus. The expression system of baculoviruses in the insect culturesystem is a powerful system, one component of which is the culture medium, andanother is the baculoviruses that are pathogenic viruses of insects and as theappropriate vector for the production of recombinant proteins in Insect cellswith high yields. These viruses are well replicated and expressed in cellsderived from insect larvae. The basis of the production of recombinant proteinsin the expression system of baculoviruses is the isolation of target genesexpressing under polyhedron promoter, which is expressed at a large scale inthe final stages of virus replication in the host.
The attractive features of insect expressionsystems are the ability to process and fold proteins such as creating disulfidebridges. Also, this expression system can make some modifications after thetranslation process, such as N- and O- glycosylation, phosphorylation,acetylation, amination, carboxymethylation, isoprenylation, nuclear transfer,peptide signal cleavage, proteolytic cleavage, as well as exon and intronprocessing. Hence, this system is widely used to produce recombinantglycoproteins. These systems also can produce protein on a large scale at arelatively short time (unlike mammalian systems).
The main disadvantage of this system is thatthe expression of the target protein is usually mediated by late viral genepromoters such as polyhydrin, so the maximum protein production is at the pointwhere the cells undergo apoptosis as a result of viral infection and in thecase of death in the host cell, the post-translational modifications that mustoccur on the expressed protein would be interrupted. Due to the cell lysis thatoccurs in this system, the target protein is highly contaminated with othercellular proteins such as proteases, so the time of isolation and purificationof the desired protein would be vital. Also, since the baculovirus genome isnot inserted into the host genome, the synthesis of recombinant protein in thenew cell requires re-infection, and since the host cells eventually die, theheterologous gene cannot be expressed continuously. Each cycle of protein synthesisneeds a new infection of the insect cell, so the system is lower in terms ofits capacity for continuous fermentation than prokaryotic and yeast systems.Besides, insect cells and mammalian cells differ in their glycosylatedpatterns, such as the mode of oligosaccharide formation and the mannosecontents.