Fusion polypeptides are disclosed which are substrates for Kutzneria albida transglutaminase. The fusion polypeptides comprise one or more FKBP chaperone(s) and a target polypeptide. Each of these elements is separated from the neighboring element by a linker amino acid sequence. It was found that inserting glutamic acid containing transglutaminase recognition motifs into the linker amino acid chains is advantageous. Subsequent labeling reactions catalyzed by the transglutaminase surprisingly provide labeled fusion polypeptides have superior properties when compared with chemically random-labeled fusion polypeptides of similar design. Assays and kits are provided for in vitro detection of target antibodies in samples.