A novel affinity gene fusion system allowing protein A-based recovery of non-immunoglobulin gene products.

An expression vector system has been developed, taking advantage of a novel, Staphylococcus aureus protein A (SPA)-binding affinity tag Z(SPA-1), enabling straightforward affinity blotting procedures and efficient recovery by affinity purification of expressed gene products on readily available reagents and chromatography media. The 58 amino acid SPA-binding affinity tag Z(SPA-1), was previously selected from a library constructed by combinatorial mutagenesis of a protein domain from SPA. An Escherichia coli expression vector for intracellular T7 promoter (P(T7)) driven production was constructed with an N-terminal dual affinity tag, consisting of a hexahistidyl (His(6)) tag in frame with the Z(SPA-1) tag, thus allowing alternative affinity recovery methods. To evaluate the system, five cDNA clones from a mouse testis cDNA library were expressed, and two alternative blotting procedures were developed for convenient screening of expression efficiencies. The five produced fusion proteins were recovered on both immobilized metal-ion affinity chromatography (IMAC) columns and on Protein A-based chromatography media, to allow comparative studies. It was found that the Protein A-based recovery resulted in the highest degree of purity, and furthermore, gene products that were produced as inclusion bodies could after denaturation be efficiently affinity purified on Protein A-Sepharose in the presence of 0.5 M guanidine hydrochloride. The convenience and robustness of the presented expression system should make it highly suitable for various high-throughput protein expression efforts.