3D Mapping of Individual Ion Channels on Living Cells

Detection of protein-protein interactions in the live cell plasma membrane by quantifying prey redistribution upon bait micropatterning

Our understanding of complex biological systems is based on high-quality proteomics tools for the parallelized detection and quantification of protein interactions. Current screening platforms, however, rely on measuring protein interactions in rather artificial systems, rendering the results difficult to confer on the in vivo situation. We describe here a detailed protocol for the design and the construction of a system to detect and quantify interactions between a fluorophore-labeled protein ("prey") and a membrane protein ("bait") in living cells. Cells are plated on micropatterned surfaces functionalized with antibodies to the bait exoplasmic domain. Bait-prey interactions are assayed via the redistribution of the fluorescent prey. The method is characterized by high sensitivity down to the level of single molecules, the capability to detect weak interactions, and high throughput, making it applicable as a screening tool. The proof-of-concept is demonstrated for the interaction between CD4, a major coreceptor in T-cell signaling, and Lck, a protein tyrosine kinase essential for early T-cell signaling.

Improved single particle localization accuracy with dual objective multifocal plane microscopy

In single particle imaging applications, the number of photons detected from the fluorescent label plays a crucial role in the quantitative analysis of the acquired data. For example, in tracking experiments the localization accuracy of the labeled entity can be improved by collecting more photons from the labeled entity. Here, we report the development of dual objective multifocal plane microscopy (dMUM) for single particle studies. The new microscope configuration uses two opposing objective lenses, where one of the objectives is in an inverted position and the other objective is in an upright position. We show that dMUM has a higher photon collection efficiency when compared to standard microscopes. We demonstrate that fluorescent labels can be localized with better accuracy in 2D and 3D when imaged through dMUM than when imaged through a standard microscope. Analytical tools are introduced to estimate the nanoprobe location from dMUM images and to characterize the accuracy with which they can be determined.

Single hepatitis-B virus core capsid binding to individual nuclear pore complexes in Hela cells

We investigate the interaction of hepatitis B virus capsids lacking a nuclear localization signal with nuclear pore complexes (NPCs) in permeabilized HeLa cells. Confocal and wide-field optical images of the nuclear envelope show well-spaced individual NPCs. Specific interactions of capsids with single NPCs are characterized by extended residence times of capsids in the focal volume which are characterized by fluorescence correlation spectroscopy. In addition, single-capsid-tracking experiments using fast wide-field fluorescence microscopy at 50 frames/s allow us to directly observe specific binding via a dual-color colocalization of capsids and NPCs. We find that binding occurs with high probability on the nuclear-pore ring moiety, at 44 +/- 9 nm radial distance from the central axis.

Kinetics of the Initial Steps of G Protein-Coupled Receptor-Mediated Cellular Signaling Revealed by Single-Molecule Imaging

We report on an in vivo single-molecule study of the signaling kinetics of G protein-coupled receptors (GPCR) performed using the neurokinin 1 receptor (NK1R) as a representative member. The NK1R signaling cascade is triggered by the specific binding of a fluorescently labeled agonist, substance P (SP). The diffusion of single receptor-ligand complexes in plasma membrane of living HEK 293 cells is imaged using fast single-molecule wide-field fluorescence microscopy at 100 ms time resolution. Diffusion trajectories are obtained which show intra- and intertrace heterogeneity in the diffusion mode. To investigate universal patterns in the diffusion trajectories we take the ligand-binding event as the common starting point. This synchronization allows us to observe changes in the character of the ligand-receptor-complex diffusion. Specifically, we find that the diffusion of ligand-receptor complexes is slowed down significantly and becomes more constrained as a function of time during the first 1000 ms. The decelerated and more constrained diffusion is attributed to an increasing interaction of the GPCR with cellular structures after the ligand-receptor complex is formed.

Fluorescence spectroscopy of single molecules under ambient conditions: methodology and technology

This review presents an overview of the fluorescence detection and spectroscopy of single molecules (SMS) in liquids and on surfaces under ambient conditions. The various techniques of SMS, such as confocal epifluorescence detection and wide-field imaging are presented and discussed, together with the different methods of data analysis such as fluorescence correlation spectroscopy and burst-by-burst analysis. Selected applications of the various techniques in physics, chemistry, and biology are described.

Progress in focus: recent advances in histochemistry and cell biology

Advances in histochemical and cell biological techniques enable increasingly refined investigations into the cellular and subcellular distribution of specific molecules and into their role in dynamic processes; thus progress in these fields complements the growing knowledge in genomics and proteomics. The present review summarizes recent technical progress and novel applications.

Controlling pore assembly of staphylococcal gamma-haemolysin by low temperature and by disulphide bond formation in double-cysteine LukF mutants

Staphylococcal LukF and Hlg2 are water-soluble monomers of gamma-haemolysin that assemble into oligomeric pores on the erythrocyte membranes. Here, we have created double-cysteine LukF mutants, in which single disulphide bonds connect either the prestem domain and the cap domain (V12C-T136C, Cap-Stem), or two beta-strands within the prestem domain (T117C-T136C, Stem-Stem) to control pore assembly of gamma-haemolysin at intermediate stages. The disulphide-trapped mutants were inactive in erythrocyte lysis, but gained full haemolytic activity if the disulphide bonds were reduced. The disulphide bonds blocked neither the membrane binding ability nor the intermediate prepore oligomerization, but efficiently inhibited the transition from prepores to pores. The prepores of Cap-Stem were dissociated into monomers in 1% SDS. In contrast, the prepores of Stem-Stem were stable in SDS and had ring-shaped structures similar to those of wild-type LukF, as observed by transmission electron microscopy. The transition of both mutants from prepores to pores could even be achieved by reducing disulphide bonds at low temperature (2 degrees C), whereas prepore oligomerization was effectively inhibited by low temperature. Finally, real-time transition of Stem-Stem from prepores to pores on ghost cells, visualized using a Ca2+-sensitive fluorescent indicator (Rhod2), was shown by the sequential appearance of fluorescence spots, indicating pore-opening events. Taken together, these data indicate that the prepores are legitimate intermediates during gamma-haemolysin pore assembly, and that conformational changes around residues 117 and 136 of the prestem domain are essential for pore formation, but not for membrane binding or prepore oligomerization. We propose a mechanism for gamma-haemolysin pore assembly based on the demonstrated intermediates.