Expression in E. coli Systems

Abstract

Owing to cost advantage, speed of production, and often high product yield (up to 50% of total cell protein), expression in Escherichia coli is generally the first choice when attempting to express a recombinant protein. Expression systems exist to produce recombinant protein intracellularly (soluble or in inclusion bodies), secreted to the periplasm, or to the surrounding medium. When deciding on a genetic design strategy, it is important to consider the nature of the recombinant protein. The mildest and thus the obvious first-choice expression strategy is to attempt to express the protein intracellularly in soluble form. In E. coli, proteins containing disulfide bonds are best produced by secretion because the disulfide forming foldases reside in the periplasm. Likewise, a correct N-terminus is more likely to be obtained upon secretion. Moreover, potentially toxic proteins are more likely to be produced in high yield if secreted from the cell. Secretion eases later purification of the product as the host secretes relatively few of its own proteins. Although tags exist that will direct the protein to the periplasm, only a few reports exist of successfully tagging the protein for extracellular secretion (1). As another strategy to avoid toxicity to the host or if the recombinant protein is very susceptible to cellular proteases, some protection is obtained by targeting the protein to light-refractile aggregates known as inclusion bodies (2,3). Several different strategies exist for subsequent recovery and folding of the protein, which notably must be able to withstand the denaturing-renaturing process.