Herein is reported a fusion polypeptide according to formula I NH2—S2—X1—S1—COOH  (formula I) wherein X1 comprises either a random amino acid sequence or an amino acid sequence derived from a first polypeptide, S2 and S1 are non-overlapping amino acid sequences derived from a second polypeptide, and — denotes a peptide bond, wherein the second polypeptide is a polypeptide with peptidyl-prolyl cis/trans-isomerase activity (PPIase activity) or is derived from the FKBP-fold domain family, wherein X1 is inserted in place of the insert-in-flap-domain of the second polypeptide.