Fluorogen-activating proteins: beyond classical fluorescent proteins

Xu, Shengnan; Hu, Hai-Yu

FAPs proved to be beneficial for the design of functional biomaterials and biosensors, as well. For instance, combined with FAP, a genetically encoded pH sensor was developed by coupling modified TO with a pH sensitive Cy5 analog. It was successfully used to track surface proteins through endocytosis, which undergoes a significant change in FRET efficiency in response to environmental pH change (Fig. 11) 54 . Saunders et al. 55 developed a membrane materials in which dL5 FAP and AEAEAKAK, an amphiphilic peptide, are combined to form a solid-phase fluorescence detection platform. It is envisioned that dL5 FAP membranes can be established in diseased locales to monitor infiltration and migration of inflammatory cells marked with antibodies conjugated to MG. In combination with polymeric materials, targeted biosensor have been developed to distinct subcellular structures within living cells 56 . More recently, Meng group 57 reported an injectable film by which antibodies can be localized in vivo. Their system builds upon a bifunctional polypeptide consisting of a FAP and a β-fibrillizing peptide (βFP). The FAP domain generates fluorescence that reflects IgG binding sites conferred by protein A/G (pAG) conjugated with the fluorogen MG. When injected into the subcutaneous space of mouse footpads, film-embedded IgG were retained locally, with distribution through the lymphatics impeded. Further, a targetable near-infrared photosensitizer (TAP)-FAP that has been described by Bruchez et al. 58 , which allows researchers to study protein inactivation, targeted-damage introduction and cellular ablation with unprecedented precision (Fig. 12) .