Abstract: The invention provides a calcium-binding photoprotein which has a high ability to bind to IgG and is capable of more sensitively detecting IgG. A fusion protein of the present invention composed of a ZZ domain having the ability to bind to the Fc region of IgG and the calcium-binding photoprotein apoaequorin is used for the detection and assay of calcium ions and the detection of IgG.

Abstract: A target protein is prepared as soluble protein using a recombinant protein expression system. An expression vector is used that includes (1) an expression-inducible promoter sequence; (2) a first coding sequence including a polynucleotide coding for a polypeptide that is represented by the formula (Z)n; and (3) a second coding sequence that includes a polynucleotide that codes for a target protein. A method of producing the target protein is also used that includes expressing protein using this expression vector.