An enzymatic method for providing a proteinaceous substance comprising a polypeptide of interest and a cyclic affinity tag, comprising the steps of: a) providing at least one precursor proteinaceous substance, the precursor comprising said protein of interest and at least one motif of the general formula X1-Tag-X2 wherein X1 and X2 represent amino acids whose side chains can be linked by a lantibiotic enzyme capable of forming a thioether bridge between residues X1 and X2, wherein X1 is selected from the group consisting of Dhb, Dha, Thr, and Ser and wherein X2 is Cys or Lys; or wherein X1 is Cys or Lys and X2 is selected from the group consisting of Dhb, Dha, Thr and Ser; Tag is an amino acid sequence serving as affinity tag when cyclised, said affinity tag allowing for capture of the proteinaceous substance to a specific binding partner of the tag; and wherein said motif is preceded N-terminally by a lantibiotic leader sequence; b) contacting said precursor with lantibiotic enzyme LanM, cyclase LanC in the case of a combination of a dehydroresidue and a cysteine, or a combination of a lantibiotic dehydratase LanB and cyclase LanC in case of X1 or X2 is Thr or Ser, allowing for the formation of a thioether bridge between X1 and X2, thereby introducing an intramolecular ring structure comprising the Tag sequence; and c) isolating the resulting cyclized proteinaceous substance, provided that said method does not involve a method for treatment of the human or animal body or a diagnostic method practiced on the human or animal body. a) providing at least one precursor proteinaceous substance, the precursor comprising said protein of interest and at least one motif of the general formula X1-Tag-X2 wherein X1 and X2 represent amino acids whose side chains can be linked by a lantibiotic enzyme capable of forming a thioether bridge between residues X1 and X2, wherein X1 is selected from the group consisting of Dhb, Dha, Thr, and Ser and wherein X2 is Cys or Lys; or wherein X1 is Cys or Lys and X2 is selected from the group consisting of Dhb, Dha, Thr and Ser; Tag is an amino acid sequence serving as affinity tag when cyclised, said affinity tag allowing for capture of the proteinaceous substance to a specific binding partner of the tag; and wherein said motif is preceded N-terminally by a lantibiotic leader sequence; wherein X1 and X2 represent amino acids whose side chains can be linked by a lantibiotic enzyme capable of forming a thioether bridge between residues X1 and X2, wherein X1 is selected from the group consisting of Dhb, Dha, Thr, and Ser and wherein X2 is Cys or Lys; or wherein X1 is Cys or Lys and X2 is selected from the group consisting of Dhb, Dha, Thr and Ser; Tag is an amino acid sequence serving as affinity tag when cyclised, said affinity tag allowing for capture of the proteinaceous substance to a specific binding partner of the tag; and wherein said motif is preceded N-terminally by a lantibiotic leader sequence; b) contacting said precursor with lantibiotic enzyme LanM, cyclase LanC in the case of a combination of a dehydroresidue and a cysteine, or a combination of a lantibiotic dehydratase LanB and cyclase LanC in case of X1 or X2 is Thr or Ser, allowing for the formation of a thioether bridge between X1 and X2, thereby introducing an intramolecular ring structure comprising the Tag sequence; and c) isolating the resulting cyclized proteinaceous substance,