Expression and purification of recombinant proteins in Escherichia coli tagged with the metal-binding protein CusF

• The fusion protein CusF increases solubility of target proteins expressed in E. coli.
• CusF-tagged proteins can be purified using immobilized Cu(II) affinity chromatography.
• Using cell-free expression systems, CusF increases protein production in vitro.
• Target proteins can be exported to the periplasm if CusF contains the signal sequence.
• After cleavage, high yields are obtained, since CusF is a small protein of 10 kDa.

Production of recombinant proteins in Escherichia coli has been improved considerably through the use of fusion proteins, because they increase protein solubility and facilitate purification via affinity chromatography. In this article, we propose the use of CusF as a new fusion partner for expression and purification of recombinant proteins in E. coli. Using a cell-free protein expression system, based on the E. coli S30 extract, Green Fluorescent Protein (GFP) was expressed with a series of different N-terminal tags, immobilized on self-assembled protein microarrays, and its fluorescence quantified. GFP tagged with CusF showed the highest fluorescence intensity, and this was greater than the intensities from corresponding GFP constructs that contained MBP or GST tags. Analysis of protein production in vivo showed that CusF produces large amounts of soluble protein with low levels of inclusion bodies. Furthermore, fusion proteins can be exported to the cellular periplasm, if CusF contains the signal sequence. Taking advantage of its ability to bind copper ions, recombinant proteins can be purified with readily available IMAC resins charged with this metal ion, producing pure proteins after purification and tag removal. We therefore recommend the use of CusF as a viable alternative to MBP or GST as a fusion protein/affinity tag for the production of soluble recombinant proteins in E. coli.