Animal Biotechnology

EUKARYOTIC PROTEIN EXPRESSION SYSTEMS

• Eukaryotic genes are not really “at home” in prokaryotic cells, even when they are expressed under the control of the prokaryotic vectors. 
  One reason is that E. coli cells frequently recognize the protein products of cloned eukaryotic genes as outsiders and destroy them. 
• Another is that prokaryotes do not carry out the same kinds of posttranslational modification as eukaryotes do.  For example, a protein that would ordinarily be coupled to sugars in a eukaryotic cell will be expressed as a bare protein when cloned in bacteria.  This can affect a protein’s activity or stability, or at least its response to antibodies. 
• Another problem is that the interior of a bacterial cell is not as conducive to proper folding of eukaryotic proteins as the interior of a eukaryotic cell.  Frequently, the result is improperly folded, inactive products of cloned genes.
• Eukaryotic systems for the expression of protein include:
  • Yeast
  • Mammalian cells
  • Baculovirus cells (insect)
• All these systems are great eukaryotic systems for the expression of recombinant proteins.
• Advantages of eukaryotic protein expression systems include the fact that very high levels of expression. The proteins are easy to purify using special tags which are included into the vectors including His, and other tags.
• The disadvantages of eukaryotic protein expression systems include the fact that eukaryotic cells do grow slower than prokaryotic cells.