Expression and purification of recombinant proteins in Escherichia coli tagged with a small metal-binding protein from Nitrosomonas europaea

• The SmbP tag increases recombinant protein solubility in the bacteria E. coli.
• Highly pure recombinant proteins tagged with SmbP are obtained after IMAC.
• After cleavage, high yields are obtained, since SmbP is a small protein of 10 kDa.
• SmbP could be the fusion protein of choice for the production of small-size proteins.

Escherichia coli is still the preferred organism for large-scale production of recombinant proteins. The use of fusion proteins has helped considerably in enhancing the solubility of heterologous proteins and their purification with affinity chromatography. Here, the use of a small metal-binding protein (SmbP) from Nitrosomonas europaea is described as a new fusion protein for protein expression and purification in E. coli. Fluorescent proteins tagged at the N-terminal with SmbP showed high levels of solubility, compared with those of maltose-binding protein and glutathione S-transferase, and low formation of inclusion bodies. Using commercially available IMAC resins charged with Ni(II), highly pure recombinant proteins were obtained after just one chromatography step. Proteins may be purified from the periplasm of E. coli if SmbP contains the signal sequence at the N-terminal. After removal of the SmbP tag from the protein of interest, high-yields are obtained since SmbP is a protein of just 9.9 kDa. The results here obtained suggest that SmbP is a good alternative as a fusion protein/affinity tag for the production of soluble recombinant proteins in E. coli.