Abstract: A kit for carrying out an immunoenzymological assay for protein, containing a diagnostic reagent consisting of a hybrid protein comprising, in order, a polypeptide P1 which is an N-terminal portion of mature alkaline phosphatase comprising from 6 to 28 of the N-terminal amino acids of said mature alkaline phosphatase, joined to a polypeptide P2, capable of interacting with an antibody or antigen joined to a polypeptide P3 which is selected from the group consisting of a) the C-terminal remaining amino acids of said mature alkaline phosphatase, b) the mature sequence of alkaline phosphatase, and c) enzymatically active fragments of b).

Abstract: The disclosed invention involves a DNA encoding a hybrid tripartite protein comprising an amino portion consisting of from 6 to 28 amino acid residues from the N-terminal of a mature E. coli alkaline phosphatase, a central portion corresponding to a heterologous polypeptide and a carboxyl portion which consists of the remainder of the mature E. coli alkaline phosphatase, a different alkaline phosphatase or an enzymatically active fragment of either. The tripartite proteins produced with this DNA can be used as a diagnostic reagent and in the screening of nucleic acid libraries or in the selection of recombinant clones.