The heterologous expression of recombinant proteins is a valuable tool in the study of gene expression, and has resulted in the development of many systems to express and purify hybrid proteins. Most of these systems are based on the fusion of the protein of interest with a naturally occurring protein (glutathioneS-transferase, maltose binding protein, or protein A) and using their natural affinity to substrates (glutathione, amylose, or immunoglobulins) coupled to columns in the purification step. Among the main drawbacks with these systems are that the affinity tag may affect protein structure and function, and that it is not possible to purify insoluble proteins.
