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FRET is a well established method for cellular and subcellular imaging of protein interactions. However, FRET obligatorily necessitates fluorescence excitation with its concomitant problems of photobleaching, autofluorescence, phototoxicity, and undesirable stimulation of photobiological processes. A sister technique, bioluminescence resonance energy transfer (BRET), avoids these problems because it uses enzyme-catalyzed luminescence; however, BRET signals usually have been too dim to image effectively in the past. Using a new generation electron bombardment-charge-coupled device camera coupled to an image splitter, we demonstrate that BRET can be used to image protein interactions in plant and animal cells and in tissues; even subcellular imaging is possible. We have applied this technology to image two different protein interactions: (i) dimerization of the developmental regulator, COP1, in plant seedlings; and (ii) CCAAT/ enhancer binding protein [alpha] (C/EBP[alpha]) in the mammalian nucleus. This advance heralds a host of applications for imaging without fluorescent excitation and its consequent limitations. C/EBP | COP1 | FRET | luminescence | fluorescence

Bioluminescence resonance energy transfer (BRET) between Renilla luciferase and yellow fluorescent protein has been adapted to serve as a real-time reporter on protein-protein interactions in live plant cells by using the Arabidopsis Constitutive photomorphogenesis 1 (COP1) protein as a model system. COP1 is a repressor of light signal transduction that functions as part of a nuclear E3 ubiquitin ligase. COP1 possesses a leucine-rich nuclear-exclusion signal that resides in a domain implicated in COP1 dimerization. BRET was applied in conjunction with site-directed mutagenesis to explore the respective contributions of the nuclear-exclusion and dimerization motifs to the regulation of COP1 activity in vivo. One specific mutant protein, [COP1.sup.L105A], showed increased nuclear accumulation but retained the ability to dimerize, as monitored by BRET, whereas other mutations inhibited both nuclear exclusion and COP1 dimerization. Mutant rescue and overexpression experiments indicated that nuclear exclusion of COP1 protein is a rate-limiting step in light signal transduction. dimerization | photomorphogenesis | nuclear export

Isolation and analysis of an Arabidopsis cDNA clone encoding the protein CIP1 suggests that it interacts with the coiled-coil region of the light-regulatory protein COP1 in vitro. COP1 protein is involved in light control of plant development, and mutations result in deviant seedling characteristics, such as short hypocotyls and open and enlarged cotyledons. The CIP1 protein has been found in hypocotyl and cotyledon cells, but not in roots.

FRET is a well established method for cellular and subcellular imaging of protein interactions. However, FRET obligatorily necessitates fluorescence excitation with its concomitant problems of photobleaching, autofluorescence, phototoxicity, and undesirable stimulation of photobiological processes. A sister technique, bioluminescence resonance energy transfer (BRET), avoids these problems because it uses enzyme-catalyzed luminescence; however, BRET signals usually have been too dim to image effectively in the past. Using a new generation electron bombardment-charge-coupled device camera coupled to an image splitter, we demonstrate that BRET can be used to image protein interactions in plant and animal cells and in tissues; even subcellular imaging is possible. We have applied this technology to image two different protein interactions: (11 dimerization of the developmental regulator, COP1, in plant seedlings; and (ii) CCAAT/enhancer binding protein α (C/EBPα) in the mammalian nucleus. This advance heralds a host of applications for imaging without fluorescent excitation and its consequent limitations. [ABSTRACT FROM AUTHOR]