The present invention relates to a method of determining an analyte in a sample, said method comprising (a) contacting said sample with (i) a binding compound binding to said analyte, said binding compound comprising a binding agent and a first partner of an affinity pair (first affinity partner); and (ii) a second partner of the affinity pair (second affinity partner) coupled to a solid surface, to an indicator, and/or to a second binding agent; and (b) determining said analyte based on complexes formed in step (a); wherein one of said first affinity partner and said second affinity partner is a polypeptide comprising the amino acid sequence of SEQ ID NO:1 or a sequence at least 50% identical thereto, and wherein the other of said first affinity partner and said second affinity partner is a polypeptide comprising the amino acid sequence of SEQ ID NO:2 or a sequence at least 50% identical thereto. The present invention also relates to a polypeptide comprising an amino acid sequence as specified in SEQ ID NO:1 or a sequence at least 50% identical thereto, wherein the amino acid at the position corresponding to position 77 in SEQ ID NO:1 is not a histidine; and to a polypeptide comprising an amino acid sequence as specified in SEQ ID NO:2 or a sequence at least 50% identical thereto, wherein (i) the amino acid at the position corresponding to position 17 in SEQ ID NO:2 is not a cysteine, in an embodiment is alanine, serine, leucine, isoleucine, or glycine and/or (ii) said polypeptide further comprises at least one functional peptide; and to fusion polypeptides, polypeptide complexes, polynucleotides and kit related to the aforesaid.