FluoTags are camelid single domain antibodies consisting only of one antigen binding site of a Alpaka heavy chain antibody. With only ~15 kDa, these Tags are about 10-times smaller than conventional IgG antibody molecules.
Two variants of FluoTags are available, the -Q and -X series:
In FluoTag-Q each fluorophore is coupled to exactly one FluoTag, which in turn binds to its target molecule in a monovalent fashion. The high binding affinity and a coupling efficiency of > 95% guarantees a highly linear relation between the number of target molecules and the intensity of fluorescence. This enables a direct count of the target molecule of interest. The fluorophore is located exceptionally close to the recognized epitope (less than 1.5 nm), which is ideal for all microscopy techniques.
In FluoTag-X two fluorophore molecules are site-specifically coupled to each FluoTag molecule. Therefore, the reagent simultaneously targets up to four fluorophores to the protein of interest, which ensures extra-bright signals. Owing to the small size of the FluoTags, the distance between the target epitope and each fluorophore is below 4 nm.
In comparison to detection systems using conventional antibodies, FluoTag-X can thus improve the localization accuracy by 10-15 nm. Both features - superior brightness and precise fluorophore placement - render the FluoTag-X products excellent tools for all microscopy techniques.