Does the spectral properties of GFP can be modulated by antibody-derivatives? To explore this hypothesis, Axel Kirchhofer and colleagues from Munich Center for Advanced Photonics have designed a number of NanoBodies (NBs) to bind to GFP. (Nanobodies? They are small, antigen-binding, single-domain polypeptides derived from some camelid antibodies). The authors found NBs could increase or decrease GFP fluorescence: in fact, co-chystallization of GFP-NB complexes revealed NBs inducing subtle changes in the chromophore environment.
The camel advantage is that, in contrast to conventional antibodies, small, stable Nanobodies are functional in living cells. Exploting this feature, a beautiful experiment was done: a NB 'Enhancer' able to shift the absorption spectra of GFP was first confined into nucleus by means of a lamin-B1 tag. This transformed it in a ratiometric biosensor for subsequent nuclear traslocation of GFP-tagged proteins. So, when a GFP-tagged protein translocates in the nucleus, the nanobody shifts its spectral emission. The ratio between the original GFP emission (cytoplasmatic) and shifted emission (nuclear) would be a proxy of nuclear translocation. In conclusion, the NB ability to modify the conformation of proteins was demonstrated in living cells anticipating new opportunities for bioimaging studies of cell signaling.
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Kirchhofer, A., Helma, J., Schmidthals, K., Frauer, C., Cui, S., Karcher, A., Pellis, M., Muyldermans, S., Casas-Delucchi, C., Cardoso, M., Leonhardt, H., Hopfner, K., & Rothbauer, U. (2009). Modulation of protein properties in living cells using nanobodies Nature Structural & Molecular Biology, 17 (1), 133-138 DOI: 10.1038/nsmb.1727