The erbB/HER family of transmembrane receptor tyrosine kinases (RTKs) mediate cellular responses to epidermal growth factor (EGF) and related ligands. We have imaged the early stages of RTK-dependent signaling in living cells using: (i) stable expression of erbB1/2/3 fused with visible fluorescent proteins (VFPs), (ii) fluorescent quantum dots (QDs) bearing epidermal growth factor (EGF-QD) and (iii) continuous confocal laser scanning microscopy and flow cytometry. Here we demonstrate that EGF-QDs are highly specific and potent in the binding and activation of the EGF receptor (erbB1), being rapidly internalized into endosomes that exhibit active trafficking and extensive fusion. EGF-QDs bound to erbB1 expressed on filopodia revealed a previously unreported mechanism of retrograde transport to the cell body. When erbB2-monomeric yellow fluorescent protein (mYFP) or erbB3-monomeric Citrine (mCitrine) were coexpressed with erbB1, the rates and extent of endocytosis of EGF-QD and the RTK-VFP demonstrated that erbB2 but not erbB3 heterodimerizes with erbB1 after EGF stimulation, thereby modulating EGF-induced signaling. QD-ligands will find widespread use in basic research and biotechnological developments.

The fluid mosaic membrane model proved to be a very useful hypothesis in explaining many, but certainly not all, phenomena taking place in biological membranes. New experimental data show that the compartmentalization of membrane components can be as important for effective signal transduction as is the fluidity of the membrane. In this work, we pay tribute to the Singer-Nicolson model, which is near its 30th anniversary, honoring its basic features, ''mosaicism'' and ''diffusion,'' which predict the interspersion of proteins and lipids and their ability to undergo dynamic rearrangement via Brownian motion. At the same time, modifications based on quantitative data are proposed, highlighting the often genetically predestined, yet flexible, multilevel structure implementing a vast complexity of cellular functions. This new ''dynamically structured mosaic model'' bears the following characteristics: emphasis is shifted from fluidity to mosaicism, which, in our interpretation, means nonrandom codistribution patterns of specific kinds of membrane proteins forming smallscale clusters at the molecular level and large-scale clusters (groups of clusters, islands) at the submicrometer level. The cohesive forces, which maintain these assemblies as principal elements of the membranes, originate from within a microdomain structure, where lipid-lipid, protein-protein, and protein-lipid interactions, as well as sub-and supramembrane (cytoskeletal, extracellular matrix, other cell) effectors, many of them genetically predestined, play equally important roles. The concept of fluidity in the original model now is interpreted as permissiveness of the architecture to continuous, dynamic restructuring of the molecular-and higherlevel clusters according to the needs of the cell and as evoked by the environment.

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Ligand-driven dimerizations of ErbB receptor subunits fulfill a fundamental role in their activation. We have used the number and brightness analysis technique to investigate the existence of preformed ligand-independent dimers and clusters and to characterize the initial steps in the activation of ErbB1 and ErbB2. In cells expressing 50,000-200,000 receptors, ErbB1 was monomeric in the absence of ligand stimulation, whereas in CHO cells with receptor levels >500,000 as much as 30% of ErbB1 was present as preformed dimers. EGF induced the formation of ErbB1 dimers as well as larger clusters (up to pentamers) that colocalized with clathrin-coated pits. The distribution of unstimulated ErbB2 in cells expressing 3·10 5 − 10 6 receptors was fundamentally different, in that this receptor was present in preformed homoassociated aggregates containing 5-10 molecules. These constitutive ErbB2 homoclusters colocalized with caveolae, increased in size at subphysiological temperatures, but decreased in size upon EGF stimulation. We conclude that these ErbB2 clusters are promoted primarily by membrane-mediated interactions and are dispersed upon ligand stimulation.EGFR | epidermal growth factor | ErbB proteins | receptor clusters | signal transduction E rbB proteins (ErbB1-4, HER1-4) constitute the best characterized family of receptor tyrosine kinases (1). Biochemical analysis has demonstrated that ErbB1, the prototypical member of the family (also known as the epidermal growth factor receptor, EGFR or HER1) undergoes ligand-induced homodimerization as a key step in its activation (2). More recent crystallographic studies reveal that ligand binding induces a transition from a closed conformation of ErbB1 to an extended configuration with the capacity to dimerize via intermolecular interactions mediated by domain II (3, 4). The ultrastructural data also confirm that dimerization of ErbB1 plays a fundamental role in activating the kinase domain by a mechanism resembling that of cyclin dependent kinases (5, 6). The coreceptor ErbB2 has no known ligand but upon transactivation expresses the most potent kinase activity of the ErbB family, thereby increasing the efficiency of signaling mediated by ErbB2-containing heterodimers (7). ErbB2 constitutively adopts an extended conformation potentiating the formation of heterodimers (8, 9) that can be inhibited by pertuzumab, a monoclonal antibody sterically blocking the heterodimerization arm of ErbB2 (10). Although the extracellular domain of ErbB2 has failed to form crystallographic homodimers, molecular biological and fluorescence resonance energy transfer (FRET) experiments have shown that full-length ErbB2 exists in dimers or higher-order aggregates in the plasma membrane (11,12). The implication is that the transmembrane, juxtamembrane and other intracellular domains (5, 13, 14) act in conjunction with the membrane environment (15) to mediate the dimerization and, thereby, functional states of ErbB proteins.Many investigators have sought to determine the distribution of ErbB1 in ...

Trastuzumab is a recombinant antibody drug that is widely used for the treatment of breast cancer.

The ErbB family of transmembrane receptor tyrosine kinases plays an important role in the pathogenesis of many cancers. The four members of the family, ErbB1-4, form various homo- and heterodimers during the course of signal transduction. A second hierarchical level of molecular associations involving 102-103 molecules, termed large-scale clustering, has also been identified, but the regulatory factors and biological consequences of such structures have not been systematically evaluated. In this report, we describe the states of association of ErbB2 and their relationship to local ErbB3 density and lipid rafts based on quantitative fluorescence microscopy of SKBR-3 breast cancer cells. Clusters of ErbB2 colocalized with lipid rafts identified by the GM1-binding B subunit of cholera toxin. Pixel-by-pixel analysis of fluorescence resonance energy transfer between labeled antibodies indicated that the homoassociation (homodimerization) of ErbB2 was proportional to the local density of ErbB2 and inversely proportional to that of ErbB3 and of the raft-specific lipid GM1. Crosslinking lipid rafts with the B subunit of cholera toxin caused dissociation of the rafts and ErbB2 clusters, an effect that was independent of the cytoskeletal anchoring of ErbB2. Crosslinking also decreased ErbB2-ErbB3 heteroassociation and the EGF- and heregulin-induced tyrosine phosphorylation of Shc. When cells were treated with the anti-ErbB2 monoclonal antibody 4D5 (parent murine version of Trastuzumab used in the immunotherapy of breast cancer), internalization of the antibody was inhibited by crosslinking of lipid rafts, but the antiproliferative activity of 4D5 was retained and even enhanced. We conclude that local densities of ErbB2 and ErbB3, as well as the lipid environment profoundly influence the association properties and biological function of ErbB2.

Communication of molecular species through dynamic association and/or dissociation at various cellular sites governs biological functions. Understanding these physiological processes require delineation of molecular events occurring at the level of individual complexes in a living cell. Among the few non-invasive approaches with nanometer resolution are methods based on Förster Resonance Energy Transfer (FRET). FRET is effective at a distance of 1–10 nm which is equivalent to the size of macromolecules, thus providing an unprecedented level of detail on molecular interactions. The emergence of fluorescent proteins and SNAP- and CLIP- tag proteins provided FRET with the capability to monitor changes in a molecular complex in real-time making it possible to establish the functional significance of the studied molecules in a native environment. Now, FRET is widely used in biological sciences, including the field of proteomics, signal transduction, diagnostics and drug development to address questions almost unimaginable with biochemical methods and conventional microscopies. However, the underlying physics of FRET often scares biologists. Therefore, in this review, our goal is to introduce FRET to non-physicists in a lucid manner. We will also discuss our contributions to various FRET methodologies based on microscopy and flow cytometry, while describing its application for determining the molecular heterogeneity of the plasma membrane in various cell types.

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.