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Investigating state restriction in fluorescent protein FRET using time-resolved fluorescence and anisotropy

Blacker TS, Chen WY, Avezov E, Marsh RJ, Duchen MR, Kaminski CF, Bain AJ, "Investigating state restriction in fluorescent protein FRET using time-resolved fluorescence and anisotropy"J. Phys. Chem. C (2017), 121(3), 1507-1514.

DOI: 10.1021/acs.jpcc.6b11235 | pdf


Abstract

Most fluorescent proteins exhibit multi-exponential fluorescence decays, indicating a heterogeneous excited state population. FRET between fluorescent proteins should therefore involve multiple energy transfer pathways. We recently demonstrated the FRET pathways between EGFP and mCherry (mC), upon the dimerisation of 3-phosphoinositide dependent protein kinase 1 (PDK1), to be highly restricted. A mechanism for FRET restriction based on a highly unfavourable κ2 orientation factor arising from differences in donor–acceptor transition dipole moment angles in a far from co-planar and near static interaction geometry was proposed. Here this is tested via FRET to mC arising from the association of glutathione (GSH) and glutathione S-transferase (GST) with an intrinsically homogeneous and more mobile donor Oregon Green 488 (OG). A new analysis of the acceptor window intensity, based on the turnover point of the sensitized fluorescence, is combined with donor window intensity and anisotropy measurements which show that unrestricted FRET to mC takes place. However, a long lived anisotropy decay component in the donor window reveals a GST-GSH population in which FRET does not occur, explaining previous discrepancies between quantitative FRET measurements of GST-GSH association and their accepted values. This reinforces the importance of the local donor-acceptor environment in mediating energy transfer and the need to perform spectrally-resolved intensity and anisotropy decay measurements in the accurate quantification of fluorescent protein FRET.