The epidermal growth factor receptor (EGFR) is a member of the erbB tyrosine kinase family of receptors. For many years it has been believed that receptor activation occurs via a monomer-dimer transition that is associated with a conformational change to activate the kinase. However, little is known about the quaternary state of the receptor at normal levels of expression (<10 5 receptors/cell). We employed multidimensional microscopy techniques to gain insight into the state of association of the human EGFR, in the absence and presence of ligand, on the surface of intact BaF/3 cells (50,000 receptors/cell). Image correlation microscopy of an EGFR-enhanced green fluorescent protein chimera was used to establish an average degree of aggregation on the submicron scale of 2.2 receptors/cluster in the absence of ligand increasing to 3.7 receptors/cluster in the presence of ligand. Energy transfer measurements between mixtures of fluorescein isothiocyanate-EGF and Alexa 555-EGF were performed using fluorescence lifetime imaging microscopy as a function of the donor: acceptor labeling ratio to gain insight into the spatial disposition of EGFR ligand binding sites on the nanometer scale. In the context of a two-state Fö rster resonance energy transfer (FRET)/non-FRET model, the data are consistent with a minimum transfer efficiency of 75% in the FRET population. The microscopy data are related to biophysical data on the EGFR in the A431 cell line and the three-dimensional structure of the ligated EGFR extracellular domain. In the context of a monomer-dimer-oligomer model, the biophysical data are consistent with a significant fraction of ligated EGFR tetramers comprising two dimers juxtaposed in a sideby-side (or slightly staggered) arrangement. Our data are consistent with a specific higher order association of the ligand-bound EGFR on the nanometer scale and indicate the existence of distinct signaling entities beyond the level of the EGFR dimer which could play an important role in receptor transactivation. The epidermal growth factor receptor (EGFR)1 is a member of the epidermal growth factor (EGF) receptor tyrosine kinase family (1, 2). Aberrant signaling from the EGFR network contributes to a number of processes important to cancer development and progression, including cell proliferation, apoptosis, angiogenesis, and metastatic spread. EGFR overexpression and truncation (1) have both been observed in common cancers including brain, lung, breast, colon, and prostate, giving credence to the notion that a molecular understanding of EGFR activation will yield opportunities for developing new anticancer drugs (1).Ever since Yarden and Schlessinger's report (3) on ligandinduced aggregation of the EGFR, it has been considered that activation of the EGFR involves the formation of ligand-induced receptor oligomers resulting in kinase activation, transautophosphorylation, and a cascade of intracellular signaling (3-5). However, although experimental evidence for soluble EGFRs in solution is consistent with a ligand-induce...

Key words. Epidermal growth factor receptor, fluorescence lifetime imaging microscopy (FLIM), fluorescence resonance energy transfer (FRET), green fluorescent protein (GFP), phosphorylation, protein interactions, two-component analysis. SummaryGraphical representation of fluorescence lifetime imaging microscopy data demonstrates that a mixture of two components with single exponential decays can be resolved by single frequency measurements. We derive a method based on linear fitting that allows the calculation of the fluorescence lifetimes of the two components. We show that introduction of proper error-weighting results in a non-linear method that is mathematically identical to a global analysis algorithm that was recently derived. The graphical approach was applied to cellular data obtained from a lifetime-based phosphorylation assay for the epidermal growth factor receptor and yielded results similar to those obtained by a global analysis algorithm.

Epidermal growth factor receptor (EGFR) signalling is activated by ligand-induced receptor dimerization. Notably, ligand binding also induces EGFR oligomerization, but the structures and functions of the oligomers are poorly understood. Here, we use fluorophore localization imaging with photobleaching to probe the structure of EGFR oligomers. We find that at physiological epidermal growth factor (EGF) concentrations, EGFR assembles into oligomers, as indicated by pairwise distances of receptor-bound fluorophore-conjugated EGF ligands. The pairwise ligand distances correspond well with the predictions of our structural model of the oligomers constructed from molecular dynamics simulations. The model suggests that oligomerization is mediated extracellularly by unoccupied ligand-binding sites and that oligomerization organizes kinase-active dimers in ways optimal for auto-phosphorylation in trans between neighbouring dimers. We argue that ligand-induced oligomerization is essential to the regulation of EGFR signalling.

Our current understanding of epidermal growth factor receptor (EGFR) autoinhibition is based on X-ray structural data of monomer and dimer receptor fragments and does not explain how mutations achieve ligand-independent phosphorylation. Using a repertoire of imaging technologies and simulations we reveal an extracellular head-to-head interaction through which ligand-free receptor polymer chains of various lengths assemble. The architecture of the head-to-head interaction prevents kinase-mediated dimerisation. The latter, afforded by mutation or intracellular treatments, splits the autoinhibited head-to-head polymers to form stalk-to-stalk flexible non-extended dimers structurally coupled across the plasma membrane to active asymmetric tyrosine kinase dimers, and extended dimers coupled to inactive symmetric kinase dimers. Contrary to the previously proposed main autoinhibitory function of the inactive symmetric kinase dimer, our data suggest that only dysregulated species bear populations of symmetric and asymmetric kinase dimers that coexist in equilibrium at the plasma membrane under the modulation of the C-terminal domain.

We report a simple, economical method for generating water-soluble, biocompatible nanocrystals that are colloidally robust and have a small hydrodynamic diameter. The nanocrystal phase transfer technique utilizes a low molecular weight amphiphilic polymer that is formed via maleic anhydride coupling of poly(styrene-co-maleic anhydride) with either ethanolamine or Jeffamine M-1000 polyetheramine. The polymer encapsulated water-soluble nanocrystals exhibit the same optical spectra as those formed initially in organic solvents, preserve photoluminescence intensities, are colloidally stable over a wide pH range (pH 3-13), have a small hydrodynamic diameter, and exhibit low levels of nonspecific binding to cells.

We describe a novel variant of fluorescence lifetime imaging microscopy (FLIM), denoted anisotropy-FLIM or rFLIM, which enables the wide-field measurement of the anisotropy decay of fluorophores on a pixel-by-pixel basis. We adapted existing frequency-domain FLIM technology for rFLIM by introducing linear polarizers in the excitation and emission paths. The phase delay and intensity ratios (AC and DC) between the polarized components of the fluorescence signal are recorded, leading to estimations of rotational correlation times and limiting anisotropies. Theory is developed that allows all the parameters of the hindered rotator model to be extracted from measurements carried out at a single modulation frequency. Two-dimensional image detection with a sensitive CCD camera provides wide-field imaging of dynamic depolarization with parallel interrogation of different compartments of a complex biological structure such as a cell. The concepts and technique of rFLIM are illustrated with a fluorophore-solvent (fluorescein-glycerol) system as a model for isotropic rotational dynamics and with bacteria expressing enhanced green fluorescent protein (EGFP) exhibiting depolarization due to homotransfer of electronic excitation energy (emFRET). The frequency-domain formalism was extended to cover the phenomenon of emFRET and yielded data consistent with a concentration depolarization mechanism resulting from the high intracellular concentration of EGFP. These investigations establish rFLIM as a powerful tool for cellular imaging based on rotational dynamics and molecular proximity.

We report the implementation and exploitation of fluorescence polarization measurements, in the form of anisotropy fluorescence lifetime imaging microscopy (rFLIM) and energy migration Förster resonance energy transfer (emFRET) modalities, for wide-field, confocal laser-scanning microscopy and flow cytometry of cells. These methods permit the assessment of rotational motion, association and proximity of cellular proteins in vivo. They are particularly applicable to probes generated by fusions of visible fluorescence proteins, as exemplified by studies of the erbB receptor tyrosine kinases involved in growth-factor-mediated signal transduction.

Ab initio and semiempirical calculations of transfer integrals for electron transfer and energy transfer have been carried out on rigidly linked norbornane-bridged naphthalene dimers to gain insight into the relative importance of through-bond and through-space interactions on photoinduced energy and electron transfer processes. In the absence of direct through-space orbital overlap between the naphthalene moieties, throughbond interaction involving the linking polynorbornane bridge is found to significantly enhance the transfer integrals for electron transfer and triplet-triplet energy transfer. For singlet-singlet energy transfer direct through-space Coulombic interaction between the naphthalene moieties is non-negligible at the separations considered and acts to reinforce the through-bond interaction. The relationship between electron and energy transfer processes and the application of these results to the interpretation of recent experimental data are discussed.