The fluorescent toolbox for assessing protein location and function

Kinetic Analysis of G Protein–Coupled Receptor Signaling Using Fluorescence Resonance Energy Transfer in Living Cells

We describe and review methods for the kinetic analysis of G protein-coupled receptor (GPCR) activation and signaling that are based on optical methods. In particular, we describe the use of fluorescence resonance energy transfer (FRET) as a means of analyzing conformational changes within a single protein (for example a receptor) or between subunits of a protein complex (such as a G protein heterotrimer) and finally between distinct proteins (such as a receptor and a G protein). These methods allow the analysis of signaling kinetics in intact cells with proteins that retain their essential functional properties. They have produced a number of unexpected results: fast receptor activation kinetics in the millisecond range, similarly fast kinetics for receptor-G protein interactions, but much slower activation kinetics for G protein activation.

Site-specific protein modification: advances and applications

Although chemical methods to modify proteins in a sequence-specific manner have yet to be developed, site-specific post-translational modification of proteins has recently emerged as a major focus in biological chemistry. Post-translational modification with functionalized substrate analogues opens up several unique avenues to induce selective reactivity into proteins in a sequence-specific manner, and can be applied to protein identification and manipulation in both in vitro and in vivo contexts. Further in vivo applications of this method will enable the imaging of cellular processes, avoiding nonspecific labeling and probe scattering, major complications observed in nonenzymatic methods. Additionally, new tools for in vitro protein modification have been developed that offer more versatile ways to study protein structure and function.

Seeing is believing

Modern visualization techniques are affording a peek into complex cellular processes. A recent paper describes an automated fluorescence microscopy method to map the subcellular localization of up to 100 different proteins in the same sample.

So how do you know you have a macromolecular complex?

Structures of protein complexes offer some of the most interesting insights into biological processes. In this article, the methods required to show that the complex observed is the physiological one are investigated.

Protein in crystal form is at an extremely high concentration and yet retains the complex secondary structure that defines an active protein. The protein crystal itself is made up of a repeating lattice of protein–protein and protein–solvent interactions. The problem that confronts any crystallographer is to identify those interactions that represent physiological interactions and those that do not. This review explores the tools that are available to provide such information using the original crystal liquor as a sample. The review is aimed at postgraduate and postdoctoral researchers who may well be coming up against this problem for the first time. Techniques are discussed that will provide information on the stoichiometry of complexes as well as low-resolution information on complex structure. Together, these data will help to identify the physiological complex.

Monitoring dynamic systems with multiparameter fluorescence imaging

A new general strategy based on the use of multiparameter fluorescence detection (MFD) to register and quantitatively analyse fluorescence images is introduced. Multiparameter fluorescence imaging (MFDi) uses pulsed excitation, time-correlated single-photon counting and a special pixel clock to simultaneously monitor the changes in the eight-dimensional fluorescence information (fundamental anisotropy, fluorescence lifetime, fluorescence intensity, time, excitation spectrum, fluorescence spectrum, fluorescence quantum yield, distance between fluorophores) in real time. The three spatial coordinates are also stored. The most statistically efficient techniques known from single-molecule spectroscopy are used to estimate fluorescence parameters of interest for all pixels, not just for the regions of interest. Their statistical significance is judged from a stack of two-dimensional histograms. In this way, specific pixels can be selected for subsequent pixel-based subensemble analysis in order to improve the statistical accuracy of the parameters estimated. MFDi avoids the need for sequential measurements, because the registered data allow one to perform many analysis techniques, such as fluorescence-intensity distribution analysis (FIDA) and fluorescence correlation spectroscopy (FCS), in an off-line mode. The limitations of FCS for counting molecules and monitoring dynamics are discussed. To demonstrate the ability of our technique, we analysed two systems: (i) interactions of the fluorescent dye Rhodamine 110 inside and outside of a glutathione sepharose bead, and (ii) microtubule dynamics in live yeast cells of Schizosaccharomyces pombe using a fusion protein of Green Fluorescent Protein (GFP) with Minichromosome Altered Loss Protein 3 (Mal3), which is involved in the dynamic cycle of polymerising and depolymerising microtubules.

Spatio-temporal localization of membrane lipid rafts in mouse oocytes and cleaving preimplantation embryos

We report for the first time the detection of membrane lipid rafts in mouse oocytes and cleaving preimplantation embryos. Cholera toxin beta (CTbeta), which binds to the raft-enriched ganglioside GM1, was selected to label rafts. In a novel application a Qdot reagent was used to detect CTbeta labeling. This is the first reported use of nanocrystals in mammalian embryo imaging. Comparative membrane labeling with CTbeta and lipophilic membrane dyes containing saturated or unsaturated aliphatic tails showed that the detection of GM1 in mouse oocytes and embryo membranes was consistent with the identification of cholesterol- and sphingolipid-enriched rafts in the cell membrane. Distribution of the GM1 was compared with the known distribution of non-raft membrane components, and disruption of membrane rafts with detergents confirmed the cholesterol dependence of GM1 on lipid raft labeling. Complementary functional studies showed that cholesterol depletion using methyl-beta-cyclodextrin inhibited preimplantation development in culture. Our results show that the membranes of the mouse oocyte and zygote are rich in lipid rafts, with heterogeneous and stage-dependent distribution. In dividing embryos, the rafts were clearly associated with the cleavage furrow. At the morula stage, rafts were also apically enriched in each blastomere. In blastocysts, rafts were detectable in the trophectoderm layer, but could not be detected in the inner cell mass without prior fixation and permeabilization of the embryo. Lipid rafts and their associated proteins are, therefore, spatio-temporally positioned to a play a critical role in preimplantation developmental events.

Recent progress in developing FRET-based intracellular sensors for the detection of small molecule nutrients and ligands

Intracellular sensors for nutrients such as sugars, metabolic precursors and signaling ligands such as amino acids will help to elucidate the complex roles of these small molecules in biology. In this update, several recently developed sensors, originating from the Frommer laboratory, which combine bacterial periplasmic-binding protein-based specificity for ligand targeting with fluorescent protein-derived resonance energy transfer for signal transduction are reviewed. The insight gained from designing these sensors, along with the preliminary results gathered from their first application, serve to illustrate the impact that they can have on improving our fundamental understanding of biology.

Mechanisms of peptide hormone secretion

According to the classical view, peptide hormones are stored in large dense-core vesicles that release all of their cargo rapidly and completely when they fuse with and flatten into the plasma membrane. However, recent imaging studies suggest that this view is too simple. Even after vesicles fuse with the plasma membrane, cells might control the rate of dispersal of vesicle cargo - either by modulating the properties of the fusion pore that connects the vesicle lumen to the extracellular solution or by storing cargo in states that disperse slowly in the extracellular space. Understanding these mechanisms is important, owing to the increasing prevalence of diseases, such as type 2 diabetes mellitus, which arise from insufficient secretion of peptide hormones.

Fluorescent proteins: maturation, photochemistry and photophysics

It has long been appreciated that green fluorescent protein (GFP) autocatalytically forms its chromophore in a host-independent process; several of the initial steps in the reaction have recently been elucidated. Nevertheless, the end points of the process are unexpectedly diverse, as six chemically distinct chromophores, including two with three rings, have been identified. All fluorescent proteins continuously produce a low level of reactive oxygen species under illumination, which, in some cases, can lead to host cell death. In one extreme but useful example, the red fluorescent protein KillerRed can be used to selectively destroy cells upon brief illumination. Finally, when photophysical processes such as excited-state proton transfer, reversible photobleaching and photoactivation are understood, useful research tools, for example, real-time biosensors and optical highlighters, can result; however, side effects of their use may lead to significant artifacts in time-dependent microscopy experiments.

Enhanced green fluorescent protein is a nearly ideal long-term expression tracer for hematopoietic stem cells, whereas DsRed-express fluorescent protein is not

Validated gene transfer and expression tracers are essential for elucidating functions of mammalian genes. Here, we have determined the suitability and unintended side effects of enhanced green fluorescent protein (EGFP) and DsRed-Express fluorescent protein as expression tracers in long-term hematopoietic stem cells (HSCs). Retrovirally transduced mouse bone marrow cells expressing either EGFP or DsRed-Express in single or mixed dual-color cell populations were clearly discerned by flow cytometry and fluorescence microscopy. The results from in vivo competitive repopulation assays demonstrated that EGFP-expressing HSCs were maintained nearly throughout the lifespan of the transplanted mice and retained long-term multilineage repopulating potential. All mice assessed at 15 months post-transplantation were EGFP positive, and, on average, 24% total peripheral white blood cells expressed EGFP. Most EGFP-expressing recipient mice lived at least 22 months. In contrast, Discosoma sp. red fluorescent protein (DsRed)-expressing donor cells dramatically declined in transplant-recipient mice over time, particularly in the competitive setting, in which mixed EGFP- and DsRed-expressing cells were cotransplanted. Moreover, under in vitro culture condition favoring preservation of HSCs, purified EGFP-expressing cells grew robustly, whereas DsRed-expressing cells did not. Therefore, EGFP has no detectable deteriorative effects on HSCs, and is nearly an ideal long-term expression tracer for hematopoietic cells; however, DsRed-Express fluorescent protein is not suitable for these cells.

Golgi twins in late mitosis revealed by genetically encoded tags for live cell imaging and correlated electron microscopy

Combinations of molecular tags visible in light and electron microscopes become particularly advantageous in the analysis of dynamic cellular components like the Golgi apparatus. This organelle disassembles at the onset of mitosis and, after a sequence of poorly understood events, reassembles after cytokinesis. The precise location of Golgi membranes and resident proteins during mitosis remains unclear, partly due to limitations of molecular markers and the resolution of light microscopy. We generated a fusion consisting of the first 117 residues of alpha-mannosidase II tagged with a fluorescent protein and a tetracysteine motif. The mannosidase component guarantees docking into the Golgi membrane, with the tags exposed in the lumen. The fluorescent protein is optically visible without further treatment, whereas the tetracysteine tag can be reduced acutely with a membrane-permeant phosphine, labeled with ReAsH, monitored in the light microscope, and used to trigger the photoconversion of diaminobenzidine, allowing 4D optical recording on live cells and correlated ultrastructural analysis by electron microscopy. These methods reveal that Golgi reassembly is preceded by the formation of four colinear clusters at telophase, two per daughter cell. Within each daughter, the smaller cluster near the midbody gradually migrates to rejoin the major cluster on the far side of the nucleus and asymmetrically reconstitutes a single Golgi apparatus, first in one daughter cell and then in the other. Our studies provide previously undescribed insights into Golgi disassociation and reassembly during mitosis and offer a powerful approach to follow recombinant protein distribution in 4D imaging and correlated high-resolution analysis.

G Protein-coupled Receptor Kinase and β-Arrestin-mediated Desensitization of the Angiotensin II Type 1A Receptor Elucidated by Diacylglycerol Dynamics

Receptor desensitization progressively limits responsiveness of cells to chronically applied stimuli. Desensitization in the continuous presence of agonist has been difficult to study with available assay methods. Here, we used a fluorescence resonance energy transfer-based live cell assay for the second messenger diacylglycerol to measure desensitization of a model seven-transmembrane receptor, the Gq-coupled angiotensin II type 1(A) receptor, expressed in human embryonic kidney 293 cells. In response to angiotensin II, we observed a transient diacylglycerol response reflecting activation and complete desensitization of the receptor within 2-5 min. By utilizing a variety of approaches including graded tetracycline-inducible receptor expression, mutated receptors, and overexpression or short interfering RNA-mediated silencing of putative components of the cellular desensitization machinery, we conclude that the rate and extent of receptor desensitization are critically determined by the following: receptor concentration in the plasma membrane; the presence of phosphorylation sites on the carboxyl terminus of the receptor; kinase activity of G protein-coupled receptor kinase 2, but not of G protein-coupled receptor kinases 3, 5, or 6; and stoichiometric expression of beta-arrestin. The findings introduce the use of the biosensor diacylglycerol reporter as a powerful means for studying Gq-coupled receptor desensitization and document that, at the levels of receptor overexpression commonly used in such studies, the properties of the desensitization process are markedly perturbed and do not reflect normal cellular physiology.

Calcium microdomains in mitochondria and nucleus

Endomembranes modify the progression of the cytosolic Ca(2+) wave and contribute to generate Ca(2+) microdomains, both in the cytosol and inside the own organella. The concentration of Ca(2+) in the cytosol ([Ca(2+)](C)), the mitochondria ([Ca(2+)](M)) and the nucleus ([Ca(2+)](N)) are similar at rest, but may become very different during cell activation. Mitochondria avidly take up Ca(2+) from the high [Ca(2+)](C) microdomains generated during cell activation near Ca(2+) channels of the plasma membrane and/or the endomembranes and prevent propagation of the high Ca(2+) signal to the bulk cytosol. This shaping of [Ca(2+)](C) signaling is essential for independent regulation of compartmentalized cell functions. On the other hand, a high [Ca(2+)](M) signal is generated selectively in the mitochondria close to the active areas, which tunes up respiration to the increased local needs. The progression of the [Ca(2+)](C) signal to the nucleus may be dampened by mitochondria, the nuclear envelope or higher buffering power inside the nucleoplasm. On the other hand, selective [Ca(2+)](N) signals could be generated by direct release of stored Ca(2+) into the nucleoplasm. Ca(2+) release could even be restricted to subnuclear domains. Putative Ca(2+) stores include the nuclear envelope, their invaginations inside the nucleoplasm (nucleoplasmic reticulum) and nuclear microvesicles. Inositol trisphosphate, cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate have all been reported to produce release of Ca(2+) into the nucleoplasm, but contribution of these mechanisms under physiological conditions is still uncertain.

Evolving the substrate specificity of O6-alkylguanine-DNA alkyltransferase through loop insertion for applications in molecular imaging

We introduce a strategy for evolving protein substrate specificity by the insertion of random amino acid loops into the protein backbone. Application of this strategy to human O6-alkylguanine-DNA alkyltransferase (AGT) led to the isolation of mutants that react with the non-natural substrate O6-propargylguanine. Libraries generated by conventional random or targeted saturation mutagenesis, by contrast, did not yield any mutants with activity towards this new substrate. The strategy of loop insertion to alter enzyme specificity should be general and applicable to other classes of proteins. An important application of the isolated AGT mutant is in molecular imaging, where the mutant and parental AGTs are used to label two different AGT fusion proteins with different fluorophores in the same living cell or in vitro . This allowed the establishment of fluorescence-based assays to detect protein-protein interactions and measure enzymatic activities.

Three-dimensional reconstruction of cell nuclei, internalized quantum dots and sites of lipid peroxidation

BACKGROUND

The purpose of the study was to develop and illustrate three-dimensional (3-D) reconstruction of nuclei and intracellular lipid peroxidation in cells exposed to oxidative stress induced by quantum dots. Programmed cell death is characterized by multiple biochemical and morphological changes in different organelles, including nuclei, mitochondria and lysosomes. It is the dynamics of the spatio-temporal changes in the signalling and morphological adaptations which will ultimately determine the 'shape' and fate of the cell.

RESULTS

We present new approaches to the 3-D reconstruction of organelle morphology and biochemical changes in confocal live-cell images. We demonstrate the 3-D shapes of nuclei, the 3-D intracellular distributions of QDs and the accompanying lipid-membrane peroxidation, and provide methods for quantification.

CONCLUSION

This study provides an approach to 3-D organelle and nanoparticle visualization in the context of cell death; however, this approach is also applicable more generally to investigating changes in organelle morphology in response to therapeutic interventions, stressful stimuli and internalized nanoparticles. Moreover, the approach provides quantitative data for such changes, which will help us to better integrate compartmentalization of subcellular events and to link morphological and biochemical changes with physiological outcomes.

Genetically encoded Ca2+ indicators: using genetics and molecular design to understand complex physiology

This article reviews genetically encoded Ca2+ indicators (GECIs), with a focus on the use of these novel molecules in the context of understanding complex cell signalling in mammals, in vivo. The review focuses on the advantages and limitations of specific GECI design strategies and the results of experiments in which these molecules have been expressed in transgenic mice, concentrating particularly on recent experiments from our laboratory in which physiological signalling could be monitored in vivo. Finally, newer strategies for effective genetic specification of GECIs are briefly reviewed.

Different subcellular locations of secretome components of Gram-positive bacteria

Gram-positive bacteria contain different types of secretion systems for the transport of proteins into or across the cytoplasmic membrane. Recent studies on subcellular localization of specific components of these secretion systems and their substrates have shown that they can be present at various locations in the cell. The translocons of the general Sec secretion system in the rod-shaped bacteriumBacillus subtilishave been shown to localize in spirals along the cytoplasmic membrane, whereas the translocons in the coccoidStreptococcus pyogenesare located in a microdomain near the septum. In both bacteria the Sec translocons appear to be located near the sites of cell wall synthesis. The Tat secretion system, which is used for the transport of folded proteins, probably localizes in the cytoplasmic membrane and at the cell poles ofB. subtilis. InLactococcus lactisthe ABC transporter dedicated to the transport of a small antimicrobial peptide is distributed throughout the membrane. Possible mechanisms for maintaining the localization of these secretion machineries involve their interaction with proteins of the cytoskeleton or components of the cell wall synthesis machinery, or the presence of lipid subdomains surrounding the transport systems.

Pharmacophotonics: utilizing multi-photon microscopy to quantify drug delivery and intracellular trafficking in the kidney

The recent introduction of multi-photon microscopy coupled with advances in optics, computer sciences and the available fluorophores used to label molecules of interest have empowered investigators to study the dynamic events within the functioning kidney at cellular and subcellular levels. This emerging technique, with improved spatial and temporal resolution and sensitivity, enables investigators to follow the cell specific uptake of large and small molecules, determine the mode of cellular uptake, intracellular trafficking and drug metabolism in complex heterogeneous organs such as the kidney over time. Repeat determinations over seconds to hours to days allow for multiple observations within the same animal, thereby minimizing animal use and inter-animal variability. This can be particularly useful for preclinical studies. Furthermore, the ability to obtain volumetric data (3-D) makes quantitative 4-D (time) analysis possible. Finally, up to three fluorophores can be visualized simultaneously allowing for three different or interactive processes to be observed and resolved.

Directing neuron-specific transgene expression in the mouse CNS

Recent advances in molecular genetics have produced many novel strategies for directing the expression of both functional and regulatory elements in transgenic mice. With the application of such approaches, the specific populations that comprise CNS networks can be both visualized and manipulated. Transgenic methods now range from the use of specific enhancer elements and large genomic regions assembled using BACs and PACs, to the use of gene targeting to a specific locus. In addition, the advent of transactivators and site-specific recombinases has provided unprecedented spatial and temporal control for directing expression in the CNS using a combination of appropriate alleles. As a result, the promise of being able to use transgenics to target specific neuronal populations is now being realized.

2-color photobleaching experiments reveal distinct intracellular dynamics of two components of the Hsp90 complex

The abundant molecular chaperone Hsp90 functions in association with co-chaperones including p23 to promote the folding and maturation of a subset of cytosolic proteins. "Fluorescence recovery after photobleaching" (FRAP) experiments showed that the dynamics of p23 in live cells is dictated by Hsp90. Since Hsp90 is present in large excess over p23, the mobility of Hsp90 could conceivably be quite different. To facilitate the analysis and to allow a direct comparison with p23, we developed a 2-color FRAP technique. Two test proteins are expressed as fusion proteins with the two spectrally separable fluorescent proteins mCherry and enhanced green fluorescent protein (EGFP). The 2-color FRAP technique is powerful for the concomitant recording of two proteins located in the same area of a cell, two components of the same protein complex, or mutant and wild-type versions of the same protein under identical experimental conditions. 2-color FRAP of Hsp90 and p23 is virtually indistinguishable, consistent with the notion that they are both engaged in a multitude of large protein complexes. However, when Hsp90-p23 complexes are disrupted by the Hsp90 inhibitor geldanamycin, p23 moves by free diffusion while Hsp90 maintains its low mobility because it remains bound in remodeled multicomponent complexes.

Phospholemman phosphorylation alters its fluorescence resonance energy transfer with the Na/K-ATPase pump

Phospholemman (PLM) or FXYD1 is a major cardiac myocyte phosphorylation target upon adrenergic stimulation. Prior immunoprecipitation and functional studies suggest that phospholemman associates with the Na/K-pump (NKA) and mediates adrenergic Na/K-pump regulation. Here, we tested whether the NKA-PLM interaction is close enough to allow fluorescence resonance energy transfer (FRET) between cyan and yellow fluorescent (CFP/YFP) fusion proteins of Na/K pump and phospholemman and whether phospholemman phosphorylation alters such FRET. Co-expressed NKA-CFP and PLM-YFP in HEK293 cells co-localized in the plasma membrane and exhibited robust FRET. Selective acceptor photobleach increased donor fluorescence (F(CFP)) by 21.5 +/- 4.1% (n = 13), an effect nearly abolished when co-expressing excess phospholemman lacking YFP. Activation of protein kinase C or A progressively and reversibly decreased FRET assessed by either the fluorescence ratio (F(YFP)/F(CFP)) or the enhancement of donor fluorescence after acceptor bleach. After protein kinase C activation, forskolin did not further reduce FRET, but after forskolin pretreatment, protein kinase C could still reduce FRET. This agreed with phospholemman phosphorylation measurements: by protein kinase C at both Ser-63 and Ser-68, but by protein kinase A only at Ser-68. Expression of PLM-YFP and PLM-CFP resulted in even stronger FRET than for NKA-PLM (F(CFP) increased by 37 +/- 1% upon YFP photobleach), and this FRET was enhanced by phospholemman phosphorylation, consistent with phospholemman multimerization. Co-expressed PLM-CFP and Na/Ca exchange-YFP were highly membrane co-localized, but FRET was undetectable. We conclude that phospholemman and Na/K-pump are in very close proximity (FRET occurs) and that phospholemman phosphorylation alters the interaction of Na/K-pump and phospholemman.

Imaging of cell migration

Cell migration is an essential process during many phases of development and adult life. Cells can either migrate as individuals or move in the context of tissues. Movement is controlled by internal and external signals, which activate complex signal transduction cascades resulting in highly dynamic and localised remodelling of the cytoskeleton, cell-cell and cell-substrate interactions. To understand these processes, it will be necessary to identify the critical structural cytoskeletal components, their spatio-temporal dynamics as well as those of the signalling pathways that control them. Imaging plays an increasingly important and powerful role in the analysis of these spatio-temporal dynamics. We will highlight a variety of imaging techniques and their use in the investigation of various aspects of cell motility, and illustrate their role in the characterisation of chemotaxis in Dictyostelium and cell movement during gastrulation in chick embryos in more detail.

Imaging today's infectious animalcules

The study of pathogens and their interactions with host cells has advanced hand-in-hand with developments in optical microscopy. Whereas microbiology benefits enormously from modern imaging technologies, for example, digital imaging and confocal microscopy, it also presents unique challenges. To overcome these, microbiologists are adept at customising imaging methods, and recently there have been studies using state-of-the-art quantitative imaging methods to probe host-pathogen interactions at the single-cell level. Of particular interest are the studies using combined light and electron microscopy methods, bi-arsenical tetra-cysteine tag labelling and automated image-acquisition and analysis for high-throughput/high-content experimentation. These applications demonstrate how imaging methodologies, adapted for microbiology, continue to open avenues for studies that previously have proven inaccessible.

Broad-spectrum multi-modality image registration: from PET, CT, and MRI to autoradiography, microscopy, and beyond

Image registration and fusion are increasingly important components of both clinical and small-animal imaging and have lead to the development of a variety of pertinent hardware and software tools, including multi-modality, e.g. PET-CT, devices. At the same time, advances in microscopic imaging, including phosphor-plate digital autoradiography and immunohistochemistry, now allow ultra-high (sub-100 microm)-resolution molecular characterization of tissue sections. To date, however, in vivo imaging of intact subjects and ex vivo imaging of harvested tissues sections have remained separate and distinct, making it difficult to reliably inter-compare the former and the latter. The Department of Medical Physics and the Radiation Biophysics Laboratory at Memorial Sloan-Kettering Cancer Center, under the direction of Dr. Clifton Ling, has now designed, fabricated, and tested a stereotactic imaging system for so-called "broad-spectrum" image registration, from coarser-resolution in vivo imaging modalities such as PET, CT, and MRI to ultra-high-resolution ex vivo imaging techniques such as histology, autoradiography, and immunohistochemistry.

Ratiometric fluorescence detection of a tag fused protein using the dual-emission artificial molecular probe

We have successfully developed a ratiometric detection system for protein of interest using the complementary recognition pair of the tetra-aspartate peptide tag and the SNARF-appended Zn(ii)-DpaTyr probe.

Rational design of homogenous protein kinase assay platforms that allow both fluorometric and colorimetric signal readouts

Protein kinases play important roles in signaling pathways that regulate many cellular biological processes, including apoptosis, cell growth, and differentiation in response to extracellular stimuli. Design of homogenous protein kinase assay platforms including design of potent protein kinase substrates is essential for exploration of the phosphoproteome. Here, we describe a unique chromism-based assay (CHROBA) technique for the direct measurement of protein kinase activities. The CHROBA is a novel chemosensor system that produces signals based on the photochromic and thermodynamic properties of a spiropyran derivative incorporated into peptide substrates. The CHROBA technique for detecting protein kinase activities involves the following five steps: (i) phosphorylation, (ii) photobleaching of the reaction mixture, (iii) addition of ionic polymer(s), (iv) incubation in the dark, and (v) signal readout. This simple 'end-point' assay method allows quantitative measurements of protein kinase A, Src protein tyrosine kinase, c-Abl protein tyrosine kinase, and protein kinase Calpha activities even with excess ATP. Our results showed that spiropyran-containing peptide substrates with net charges between +2 and 0 are suitable for the present CHROBA method. This information should aid in the rational design of diverse protein kinase assay platforms. The present CHROBA technique can be adapted to a microplate format with both fluorometric and colorimetric readouts and would be useful for high-throughput drug discovery and analysis of the phosphoproteome.

Photoinduced charge and energy transfer involving fullerene derivatives

In this feature article, a brief overview over the photoinduced energy and charge transfer mechanisms involving fullerenes will be presented. The photoinduced charge separation between organic donor and acceptor molecules is the basic photophysical mechanism for natural photosynthesis and nearly all organic solar cell concepts. We will give a short introduction to the mechanisms of excited state charge transfer and resonant energy transfer and present examples of relevant applications in organic optoelectronics and photodynamic tumor therapy.