The fluorescence of single terrylene molecules in a crystalline host is investigated at room temperature by scanning confocal optical microscopy. Photon arrival times are analyzed in terms of interphoton time distributions, second order correlation functions, and the variance of the photon number probability distribution. Antibunching at short times and bunching behavior for longer times is observed, associated with sub- and super-Poissonian statistics, respectively. A rate-equation analysis of the molecular level populations indicates an accelerated reverse intersystem crossing.

The lateral organization of a prototypical G protein-coupled receptor, the neurokinin-1 receptor (NK1R), was investigated in living cells by fluorescence resonance energy transfer (FRET) microscopy, taking advantage of the recently developed acyl carrier protein (ACP) labeling technique. The NK1R was expressed as fusion protein with ACP to which small fluorophores were then covalently bound. Our approach allowed the recording of FRET images of receptors on living cells with unprecedented high signal-to-noise ratios and a subsequent unequivocal quantification of the FRET data owing to (i) the free choice of optimal fluorophores, (ii) the labeling of NK1Rs exclusively on the cell surface, and (iii) the precise control of the donor-acceptor molar ratio. Our single-cell FRET measurements exclude the presence of constitutive or ligandinduced homodimers or oligomers of NK1Rs. The strong dependence of FRET on the receptor concentration further reveals that NK1Rs tend to concentrate in microdomains, which are found to constitute Ϸ1% of the cell membrane and to be sensitive to cholesterol depletion.ACP labeling ͉ G protein-coupled receptor (GPCR) oligomerization G protein-coupled receptors (GPCRs) were for a long time presumed to be distributed in the plasma membrane exclusively in a monomeric form (1, 2), but recent reports have unveiled a more complex behavior; in particular, dimeric structures have been found for several GPCRs using biochemical and biophysical methods (3-9). Dimerization can occur between receptors of the same subtype (homodimerization) or of different subtypes (heterodimerization). Some GPCRs remain dimeric all of the time, whereas others cycle between monomeric and dimeric states in a ligand-regulated process (7). Although GPCR homodimerization seems to be important for receptor ontology and trafficking, heterodimerization might result in altered ligand selectivity and distinctive coupling to signal transduction pathways, providing an additional possibility for the fine tuning of cellular signaling.In addition to dimerization, the lateral distribution of GPCRs in cell membranes has been extensively debated recently. Several reports based on biochemical (10), plasmon-resonance spectroscopy (11), single-molecule microscopy (12), and fluorescence recovery after photobleaching experiments (13) propose that GPCRs are localized in microdomains, but a clear demonstration of the existence and nature of such microdomains in living cells remains elusive, in particular because the interpretation of biochemical data can be rather equivocal (14-17). Compartmentalization in form of microdomains was proposed to explain the efficiency of signal transduction at the low physiological surface concentrations of the signaling partners by their enrichment inside specialized signaling platforms (10, 18).Bioluminescence resonance energy transfer (BRET) and fluorescence resonance energy transfer (FRET) experiments have gained increasing interest to investigate these two central questions on GPCR signaling. (i) They can be p...

Measurements of the differential cross section for electron impact excitation of the b 3: continuum of molecular hydrogen are presented The data were taken at incident electron energies of 9.2, 10.2, 12.2. 15.2, 17.2 and 20.2eV. at scattering angles in the range of 20" to IZOO. Integral cross sections are derived from these differential cross sections and the data are compared to available experimentd and theoretical ROSS sections.

Despite the importance of trafficking for regulating G proteincoupled receptor signaling, for many members of the seven transmembrane helix protein family, such as odorant receptors, little is known about this process in live cells. Here, the complete life cycle of the human odorant receptor OR17-40 was directly monitored in living cells by ensemble and single-molecule imaging, using a double-labeling strategy. While the overall, intracellular trafficking of the receptor was visualized continuously by using a GFP tag, selective imaging of cell surface receptors was achieved by pulselabeling an acyl carrier protein tag. We found that OR17-40 efficiently translocated to the plasma membrane only at low expression, whereas at higher biosynthesis the receptor accumulated in intracellular compartments. Receptors in the plasma membrane showed high turnover resulting from constitutive internalization along the clathrin pathway, even in the absence of ligand. Singlemolecule microscopy allowed monitoring of the early, dynamic processes in odorant receptor signaling. Although mobile receptors initially diffused either freely or within domains of various sizes, binding of an agonist or an antagonist increased partitioning of receptors into small domains of Ϸ190 nm, which likely are precursors of clathrin-coated pits. The binding of a ligand, therefore, resulted in modulation of the continuous, constitutive internalization. After endocytosis, receptors were directed to early endosomes for recycling. This unique mechanism of continuous internalization and recycling of OR17-40 might be instrumental in allowing rapid recovery of odor perception.cell signaling ͉ G protein-coupled receptors ͉ single-particle tracking ͉ in vivo protein labeling ͉ fluorescence imaging T he sensation of smell is mediated by a specific family of olfactory G protein-coupled receptors (GPCRs), which recognize small volatile molecules (1). Although odorant receptors (ORs) account for the largest mammalian gene family, comprising up to 1,000 members, the mechanism of signal recognition and amplification in olfactory transduction remains elusive (2). This is partly because classical methods for OR detection, based on immunocytochemistry (3, 4) or genetic fusion to GFP (5), have not permitted the simultaneous live-cell imaging of olfactory processes at the cell membrane and in cytoplasmic compartments.Comprehensive functional studies on ORs in a native cellular environment, such as isolated olfactory sensory neurons, adenovirus-infected olfactory epithelia, or genetically engineered animals, are often hampered by practical limitations: (i) olfactory sensory neurons are difficult to maintain in primary culture (6), (ii) virus-mediated gene transfer does not consistently yield functional OR expression (7), and (iii) creating transgenic animals for each OR would be quite expensive. Functional expression of ORs in heterologous cells is, therefore, an important approach for elucidating the molecular mechanism of olfaction and helps to identify specific ligands ...

CdSe quantum dots (QDs) with a high fluorescence quantum yield of 25% and a narrow size distribution were synthesized in a single step in water using glutathione as a stabilizing molecule. The exceptional optical properties enabled for the first time the detection of in-water-prepared single quantum dots at room temperature. For application as fluorescent bioanalytical probes, the QDs were coated with streptavidin. These QDs self-assemble with high contrast on micropatterned biotin while preserving their optical properties and their capability to bind in addition biotinylated molecules, a prerequisite for the development of novel supramolecular structures and bioassays.

CD8؉ cytotoxic T lymphocyte (CTL) can recognize and kill target cells that express only a few cognate major histocompatibility complex class I-peptide (pMHC) complexes. To better understand the molecular basis of this sensitive recognition process, we studied dimeric pMHC complexes containing linkers of different lengths. Although dimers containing short (10 -30-Å) linkers efficiently bound to and triggered intracellular calcium mobilization and phosphorylation in cloned CTL, dimers containing long linkers (>80 Å) did not. Based on this and on fluorescence resonance energy transfer experiments, we describe a dimeric binding mode in which two T cell receptors engage in an antiparallel fashion two pMHC complexes facing each other with their constant domains. This binding mode allows integration of diverse low affinity interactions, which increases the overall binding and, hence, the sensitivity of antigen recognition. In proof of this, we demonstrated that pMHC dimers containing one agonist and one null ligand efficiently activate CTL, corroborating the importance of endogenous pMHC complexes in antigen recognition.A hallmark of CD8ϩ CTLs 1 is their extraordinary sensitivity in recognizing and killing target cells that express only very few cognate pMHC complexes (1, 2). Although soluble TCR and CD8 have been shown to bind pMHC complexes typically with low affinities, fast dissociation kinetics, and in an independent manner (3, 4), the molecular basis of this highly sensitive recognition process is not clear.For several hormone, cytokine, and chemokine receptors, it has been shown that receptor dimerization is a means to strengthen receptor ligand binding and to promote receptor signaling (5, 6). Dimerization and aggregation, pMHC driven or not, have also been proposed for TCR (7-10); however, the evidences provided never gained general acceptance, leaving this issue open (11-13). In addition, none of these studies provided a plausible structural explanation for TCR dimerization. In view of the wealth of structural information on pMHC, TCR, CD8, pMHC-TCR, and pMHC-CD8 complexes, there should be a structural explanation for TCR dimerization, if it indeed exists. The crystal structure of TCR has uncovered three features that are unique to TCR, i.e. are not found in other immunoglobulin (Ig) super-family members. 1) In the V␣ domain, the CЈ strand forms hydrogen bonds with the D strand and not with the CЉ strand (14). 2) C␣ has only 12-18% sequence homology with other Ig-constant domains, and 12-15 residues are missing, resulting in a less ordered structure and a flatter outer surface as compared with C␤ (15). 3) C␤ has a prominent, surface-exposed FG loop (15). Because of these structural features, TCR␣ chains are more likely to dimerize than TCR␤ chains (14).Because TCR are naturally membrane-integrated and associated with CD3 units, studies on TCR dimerization/aggregation should be performed on cells (11). The CD3⑀␥␦ chains each contain an extracellular Ig domain and a cytoplasmic tail harboring one immunotyrosine...

Synthetic peptides derived from the heptad repeat (HR) of fusion (F) proteins can be used as dominant negative inhibitors to inhibit the fusion mechanism of class I viral F proteins. Here, we have performed a stapled-peptide scan across the HR2 domain of the respiratory syncytial virus (RSV) F protein with the aim to identify a minimal domain capable of disrupting the formation of the postfusion six-helix bundle required for viral cell entry. Constraining the peptides with a single staple was not sufficient to inhibit RSV infection. However, the insertion of double staples led to the identification of novel short stapled peptides that display nanomolar potency in HEp-2 cells and are exceptionally robust to proteolytic degradation. By replacing each amino acid of the peptides by an alanine, we found that the substitution of residues 506 to 509, located in a patch of polar contacts between HR2 and HR1, severely affected inhibition. Finally, we show that intranasal delivery of the most potent peptide to BALB/c mice significantly decreased RSV infection in upper and lower respiratory tracts. The discovery of this minimal HR2 sequence as a means for inhibition of RSV infection provides the basis for further medicinal chemistry efforts toward developing RSV fusion antivirals.

Cell‐membrane sheets suitable for in‐vitro functional fluorescence studies have been prepared by direct detachment from cell membranes using poly‐L‐lysine‐coated glass slides. The resulting transferred planar membranes conserve the composition as well as most properties of the original plasma membrane; in particular, both membrane leaflets remain fluid, allowing the investigation of diffusion properties of different cellular membrane components. Measurements on membrane sheets offer several advantages as compared to those on living cells. First, access to the intracellular leaflet is obtained, in particular to the intracellular part of membrane proteins and to cytoplasmic membrane‐associated proteins, opening the possibility of labeling them and modulating their properties with membrane impermeable compounds. Second, the cytosolic autofluorescence of the cells is absent, allowing ultrasensitive measurements to be performed down to the single‐molecule level. Third, the complexity of cellular processes occurring at the plasma membrane can be reduced, allowing the sequential investigation of selected events from complex biochemical networks. These advantages are illustrated by ligand‐binding studies on the α1b‐adrenergic receptor. Our results indicate that supported membrane sheets might find a broad application as an ideal in‐vitro system for the elucidation of complex signaling pathways.