Herein is reported a method for producing an enzymatic conjugation product of two polypeptides comprising incubating of a first polypeptide comprising the amino acid sequence LPXTG (SEQ ID NO: 20, wherein X can be any amino acid residue), a second polypeptide has an oligo-alanine Am (m=2 (SEQ ID NO: 26), or 3 (SEQ ID NO: 27), or 4 (SEQ ID NO: 28), or 5 (SEQ ID NO: 29)) amino acid sequence at its N-terminus, a third polypeptide with sortase activity which is derived from Staphylococcus aureus Sortase A, and recovering the conjugate from the reaction mixture and thereby producing the enzymatic conjugation product of two polypeptides.
