Chaperone-fusion expression plasmid vectors for improved solubility of recombinant proteins in Escherichia coli

The enteric bacterium Escherichia coli is the most extensively used prokaryotic organism for production of proteins of therapeutic or commercial interest. However, it is common that heterologous over-expressed recombinant proteins fail to properly fold resulting in formation of insoluble aggregates known as inclusion bodies. Complex systems have been developed that employ simultaneous over-expression of chaperone proteins to aid proper folding and solubility during bacterial expression. Here we describe a simple method whereby a protein of interest, when fused in frame to the E. coli chaperones DnaK or GroEL, is readily expressed in large amounts in a soluble form. This system was tested using expression of the mouse prion protein PrP, which is normally insoluble in bacteria. We show that while in trans over-expression of the chaperone DnaK failed to alter partitioning of PrP from the insoluble inclusion body fraction to the soluble cytosol, expression of a DnaK-PrP fusion protein yielded large amounts of soluble protein. Similar results were achieved with a fragment of insoluble Varicella Zoster virus protein ORF21p. In theory this approach could be applied to any protein that partitions with inclusion bodies to render it soluble for production in E. coli.