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Biophys J. 2008 Apr 1;94(7):2859-68. Epub 2007 Dec 7.

Conformation of the c-Fos/c-Jun complex in vivo: a combined FRET, FCCS, and MD-modeling study.

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Cell Biology and Signaling Research Group of Hungarian Academy of Sciences, Department of Biophysics and Cell Biology, Medical and Health Science Center, University of Debrecen, Debrecen, Hungary.


The activator protein-1 transcription factor is a heterodimer containing one of each of the Fos and Jun subfamilies of basic-region leucine-zipper proteins. We have previously shown by fluorescence cross-correlation spectroscopy (FCCS) that the fluorescent fusion proteins Fos-EGFP and Jun-mRFP1, cotransfected in HeLa cells, formed stable complexes in situ. Here we studied the relative position of the C-terminal domains via fluorescence resonance energy transfer (FRET) measured by flow cytometry and confocal microscopy. To get a more detailed insight into the conformation of the C-terminal domains of the complex we constructed C-terminal labeled full-length and truncated forms of Fos. We developed a novel iterative evaluation method to determine accurate FRET efficiencies regardless of relative protein expression levels, using a spectral- or intensity-based approach. The full-length C-terminal-labeled Jun and Fos proteins displayed a FRET-measured average distance of 8 +/- 1 nm. Deletion of the last 164 amino acids at the C-terminus of Fos resulted in a distance of 6.1 +/- 1 nm between the labels. FCCS shows that Jun-mRFP1 and the truncated Fos-EGFP also interact stably in the nucleus, although they bind to nuclear components with lower affinity. Thus, the C-terminal end of Fos may play a role in the stabilization of the interaction between activator protein-1 and DNA. Molecular dynamics simulations predict a dye-to-dye distance of 6.7 +/- 0.1 nm for the dimer between Jun-mRFP1 and the truncated Fos-EGFP, in good agreement with our FRET data. A wide variety of models could be developed for the full-length dimer, with possible dye-to-dye distances varying largely between 6 and 20 nm. However, from our FRET results we can conclude that more than half of the occurring dye-to-dye distances are between 6 and 10 nm.

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