FRET microscopy: Basics, issues and advantages of FLIM-FRET imaging

Förster resonance energy transfer (FRET) is an effective and high resolution method to investigate protein–protein interaction in live or fixed specimens. The FRET technique is increasingly employed to evaluate the molecular mechanisms governing diverse cellular processes such as vesicular transport, signal transduction and the regulation of gene expression. For FRET to occur, protein moieties should be close together within 10 nm, the dipole moment of the fluorophore targeted to the proteins should have an appropriate orientation, and the spectral overlap of the donor emission with the acceptor absorption should be >30 %. FRET can be used to estimate the distance between interacting protein molecules in vivo or in vitro using light microscopy systems. Visible fluorescent proteins (VFPs) have been widely used as a FRET pair in addition to organic dyes. Light microscopy techniques including wide-field, confocal and multiphoton microscopy systems provide spatial information of the interacting proteins with nanometer resolution. For better interpretation and quantitation of the FRET signal the contaminations—also called spectral bleedthrough (SBT)—have to be removed.