Conjugation reaction of different proteins is of great interest, since it enables to combine the functions of each protein and open up the alternative use of proteins in many fields.We have recently demonstrated that by tethering a peptide containing tyrosine (Y-tag) at the C-terminus of F.coli alkaline phosphatase (BAP), the tyrosine residues in the peptide tag were efficiently recognized by horseradish peroxidase (HRP) and site-specifically cross-linked each other to yield BAP conjugates.In this study, we applied the peroxidase-catalyzed tyrosine cross-linking reaction to protein heteroconjugation.Streptavidin (SA) is a tetrameric protein and its each subunit binds to one biotin with extremely strong affinity.Y-tag was genetically introduced to C-terminus of SA and SA-BAP conjugate was prepared by co-cross-linking reaction of the Y-tagged SA and Y-tagged BAP to demonstrate protein heteroconjugation by the HRP-catalyzed tyrosine-coupling reaction.