Analysis of Ligand Binding by Two-Colour Fluorescence Cross-Correlation Spectroscopy

Two-Hybrid Fluorescence Cross-Correlation Spectroscopy Detects Protein-Protein Interactions In Vivo

Fluorescence cross-correlation spectroscopy (FCCS) uses the correlated motion of two distinct fluorophores to detect their interaction. Whereas FCCS has been used with chemically or genetically labeled interaction partners in vitro, FCCS has never been demonstrated in vivo between two autofluorescent proteins. At least one reaction partner was always chemically labeled. Fos and Jun, two components of the AP-1 transcription factor, are known to exert their function as a dimer and can therefore serve as a reference for dimer formation. Expressing fusion proteins between Fos and the enhanced green fluorescent protein (EGFP), as well as Jun and the monomeric red fluorescent protein 1 (mRFP1) in HeLa cells, we show here, for the first time, in vivo FCCS detection of protein-protein interactions. The mobility of the dimerized species is slow, indicating that DNA-binding might stabilize dimerization. The technique has rich potential applications for the rapid screening of protein-protein interactions in vivo, which are able to clarify events during the whole life of cells.

IL-2 and IL-15 receptor alpha-subunits are coexpressed in a supramolecular receptor cluster in lipid rafts of T cells

The private alpha-chains of IL-2 and IL-15 receptors (IL-2R and IL-15R) share the signaling beta- and gamma(c)-subunits, resulting in both common and contrasting roles of IL-2 and IL-15 in T cell function. Knowledge of the cytokine-dependent subunit assembly is indispensable for understanding the paradox of distinct signaling capacities. By using fluorescence resonance energy transfer and confocal microscopy, we have shown that IL-2R alpha, IL-15R alpha, IL-2/15R beta and gamma(c)-subunits, as well as MHC class I and II glycoproteins formed supramolecular receptor clusters in lipid rafts of the T lymphoma line Kit 225 FT7.10. Fluorescence crosscorrelation microscopy demonstrated the comobility of IL-15R alpha with IL-2R alpha and MHC class I. A model was generated for subunit switching between IL-2R alpha and IL-15R alpha upon the binding of the appropriate cytokine resulting in the formation of high-affinity heterotrimeric receptors. This model suggests a direct role for the alpha-subunits, to which no definite function has been assigned so far, in tuning cellular responses to IL-2 or IL-15. In addition, both alpha-chains were at least partially homodimerized/oligomerized, which could be the basis of distinct signaling pathways by the two cytokines.

Counting Nucleosomes in Living Cells with a Combination of Fluorescence Correlation Spectroscopy and Confocal Imaging

Although methods for light microscopy of chromatin are well established, there are no quantitative data for nucleosome concentrations in vivo. To establish such a method we used a HeLa clone expressing the core histone H2B fused to the enhanced yellow fluorescent protein (H2B-EYFP). Quantitative gel electrophoresis and fluorescence correlation spectroscopy (FCS) of isolated oligonucleosomes show that 5% of the total H2Bs carry the fluorescent tag and an increased nucleosome repeat length of 204 bp for the fluorescent cells. In vivo, the mobility and distribution of H2B-EYFP were studied with a combination of FCS and confocal imaging. With FCS, concentration and brightness of nascent molecules were measured in the cytoplasm, while in the nucleoplasm a background of mobile fluorescent histones was determined by continuous photobleaching. Combining these results allows converting confocal fluorescence images of nuclei into calibrated nucleosome density maps. Absolute nucleosome concentrations in interphase amount up to 250 microM locally, with mean values of 140(+/-28)microM, suggesting that a condensation-controlled regulation of site accessibility takes place at length scales well below 200 nm.

Ultrasensitive investigations of biological systems by fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) extracts information about molecular dynamics from the tiny fluctuations that can be observed in the emission of small ensembles of fluorescent molecules in thermodynamic equilibrium. Employing a confocal setup in conjunction with highly dilute samples, the average number of fluorescent particles simultaneously within the measurement volume (approximately 1 fl) is minimized. Among the multitude of chemical and physical parameters accessible by FCS are local concentrations, mobility coefficients, rate constants for association and dissociation processes, and even enzyme kinetics. As any reaction causing an alteration of the primary measurement parameters such as fluorescence brightness or mobility can be monitored, the application of this noninvasive method to unravel processes in living cells is straightforward. Due to the high spatial resolution of less than 0.5 microm, selective measurements in cellular compartments, e.g., to probe receptor-ligand interactions on cell membranes, are feasible. Moreover, the observation of local molecular dynamics provides access to environmental parameters such as local oxygen concentrations, pH, or viscosity. Thus, this versatile technique is of particular attractiveness for researchers striving for quantitative assessment of interactions and dynamics of small molecular quantities in biologically relevant systems.

A dynamic view of cellular processes by in vivo fluorescence auto- and cross-correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) is becoming increasingly popular as a technique that aims at complementing live cell images with biophysical information. This article provides both a short overview over recent intracellular FCS applications and a practical guide for investigators, who are seeking to integrate FCS into live cell imaging to obtain information on particle mobility, local concentrations, and molecular interactions. A brief introduction to the principles of FCS is provided, particularly emphasizing practical aspects such as the choice of appropriate dyes and positioning of the measurement volume in the sample. Possibilities and limitations in extracting parameters from autocorrelation curves are discussed, and attention is drawn to potential artifacts, such as photobleaching and probe aggregation. The principle of dual-color cross-correlation is reviewed along with considerations for proper setup and adjustment. Practical implications of nonideal conditions including incomplete focus overlap and spectral cross-talk are considered. Recent examples of both auto- and cross-correlation applications demonstrate the potential of FCS for cell biology.