Bioluminescent immunoassay using a fusion protein of protein A and the photoprotein aequorin

Aequorin is a photoprotein that emits light in the presence of Ca2+ ions. To develop a bioluminescent immunoassay based on the light emission property of aequorin, we have expressed the apoaequorin fusion protein with S. aureus protein A in E. coli by recombinant DNA techniques. The fusion protein expressed was purified by IgG-Sepharose affinity chromatography, gel filtration and HPLC procedures. The purified protein A-apoaequorin fusion protein has both the luminescent activity of aequorin and the IgG-binding ability of protein A. We compared results obtained using the protein A-aequorin fusion protein with those obtained using a protein A conjugated horseradish peroxidase based immunoassay, and found them to yield similar results.