Background: In the process of recombinant protein biosynthesis affinity tags are efficient tools to achieve the expected purity and yield during the purification steps. Nonetheless these tags might alter enzyme specificity and activity, therefore in functional assays it is recommended to use authentic or native proteins. Several ubiquitin fusion systems have been developed for E. coli-based recombinant protein expression that provide high levels of expression, with simple purification, and allow the production of various proteins with authentic N-terminus for subsequent applications.

Results: In the present research, we describe an ubiquitin fused bacterial biosynthetic system (pUbiq) for the production of the native Second mitochondria-derived activator of caspases (SMAC) recombinant protein. Using this system, the recombinant protein is expressed with an ubiquitin-decahistidine fusion partner, then purified from the cell-forming proteins by affinity chromatography. The fusion partner is then removed by proteolytic digestion, resulting the native structure of the recombinant protein without unnecessary amino acid residues. Following proteolysis, another affinity chromatography method is used to separate the native protein from the fusion partner and the proteolytic enzyme. The folding of the protein of interest was verified by a pull-down assay.

Conclusions: Based on our results, the presented pUbiq system was successfully applied in the production of native SMAC recombinant protein, where the affinity tag required for purification was completely removed. Our study suggests that the ubiquitin-fusion technology will be useful for enhancing expression and purification of native and authentic proteins for structural and functional studies as well as for therapeutic uses.