Review
The use of FRET imaging microscopy to detect protein–protein interactions and protein conformational changes in vivo

https://doi.org/10.1016/S0959-440X(00)00249-9Get rights and content

Abstract

Intermolecular and intramolecular FRET between two spectrally overlapping green fluorescent protein variants fused to two different host proteins or at two different sites within the same protein offers a unique opportunity to monitor real-time protein–protein interactions or protein conformational changes. By using fluorescence digital imaging microscopy, one can visualize the location of green fluorescent proteins within a living cell and follow the time course of the changes in FRET corresponding to cellular events at a millisecond time resolution. The observation of such dynamic molecular events in vivo provides vital insight into the action of biological molecules.

Introduction

In 1948, Förster formulated the principle of FRET 1., 2., a phenomenon that occurs when two different chromophores (donor and acceptor) with overlapping emission/absorption spectra are separated by a suitable orientation and a distance in the range 10–80Å (Fig. 1). In the early 1970s, after a long period of silence, groundbreaking work on FRET revealed the spatial proximity relationships of two fluorescence-labeled sites in biological macromolecules, thereby establishing the use of FRET as a spectroscopic ruler [3]. All of this early work used either fluorescent analogs of biomolecules or fluorescent reagents covalently or noncovalently attached to macromolecules as donors or acceptors of FRET [4].

Over the past decade, the use of FRET for structure elucidation became less significant, as atomic-resolution structural information on biological macromolecules was more effectively determined by X-ray crystallography or NMR spectroscopy. Recently, however, the introduction of the green fluorescent protein (GFP) to FRET-based imaging microscopy gave new life to its use as a sensitive probe of protein–protein interactions and protein conformational changes in vivo. This was the beginning of real-time in vivo imaging of dynamic molecular events, providing researchers with crucial insight into the biological mechanisms as well as the physiological functions of a cell 5., 6., 7., 8..

Section snippets

FRET meets GFP

GFP has a number of amazing properties that enable its use for in vivo imaging. Firstly, GFP can be expressed in a variety of cells, where it becomes spontaneously fluorescent without the aid of a cofactor [9]. Secondly, GFP can be fused to a host protein to create a fusion protein that usually retains both the fluorescence of the GFP and the biochemical function of the original host. Thirdly, fusion proteins can be targeted to specific organelles, such as the nucleus or endoplasmic reticulum,

Intramolecular FRET to monitor protease cleavage, calcium signaling and phosphorylation

Intramolecular FRET can be measured when both the GFP donor and the acceptor are fused to the same host molecule (Fig. 1a Fig. 2). One of the first demonstrations of this technique was performed by Mitra et al. [22], who fused a BFP and GFP in the same molecule, separated by a flexible polypeptide linker containing a Factor Xa protease cleavage site. When incubated with Factor Xa, cleavage of the linker was followed by a decrease in FRET. Subsequently, Heim and Tsien [16] demonstrated a similar

Intermolecular FRET to visualize protein–protein interactions

Intermolecular FRET can occur when the GFP donor and the acceptor are on different macromolecules (Fig. 1b and Fig. 2); however, this form of FRET is more difficult to observe because the stoichiometry of acceptors to donors can vary with transfection efficiencies, and also the donor and acceptor host proteins may not be constitutively bound in vivo. Optimal conditions occur when all the donors are paired with an acceptor, as any unpaired protein adds noise to the signal. Additionally, if the

Conclusions and future directions

Fluorescent imaging technology now offers numerous benefits to the expanding field of structural biology. High-resolution techniques such as X-ray crystallography and NMR spectroscopy determine three-dimensional structures of biological molecules, whereas fluorescence imaging technology, along with other imaging methods, unveils both temporal and spatial information on molecular structures in living cells. A combination of new and existing techniques provides a more comprehensive picture of

Acknowledgements

This work was supported by a grant to MI from the Howard Hughes Medical Institute (HHMI). MI is an HHMI International Scholar and a Canadian Institutes of Health Research Scientist. We are grateful to Atsushi Miyawaki, Klaus Hoeflich and Jane Gooding for critical comments on the manuscript.

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:• of special interest•• of outstanding interest

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