Green Fluorescent Protein Mutants Research Articles

Alkalinization of Acrosome Measured by GFP as a pH Indicator and Its Relation to Sperm Capacitation

We previously targeted EGFP (a mutant of green fluorescent protein) to the lumen of the mouse sperm acrosome and reported the time course of EGFP release during the acrosome reaction. In the study reported here, we estimated the pH within the mouse sperm acrosome utilizing the pH-dependent nature of EGFP fluorescence. The average intra-acrosomal pH was estimated to be 5.3 ± 0.1 immediately after sperm preparation, gradually increasing to 6.2 ± 0.3 during 120 min of incubation in TYH media suitable for capacitation. Spontaneous acrosome reactions were noted to increase concomitantly with acrosomal alkalinization during incubation. We also demonstrated that acrosomal antigens detected by monoclonal antibodies MN7 and MC41 did not dissolve following the acrosome reaction in pH 5.3 media, but dissolved at pH 6.2. These data suggest that acrosomal alkalinization during incubation conducive for sperm capacitation may function to alter acrosomal contents and prepare them for release during the acrosome reaction.

Light-Induced Flickering of DsRed Provides Evidence for Distinct and Interconvertible Fluorescent States

Green fluorescent protein (GFP) from jellyfish Aequorea victoria, the powerful genetically encoded tag presently available in a variety of mutants featuring blue to yellow emission, has found a red-emitting counterpart. The recently cloned red fluorescent protein DsRed, isolated from Discosoma corals (Matz et al., 1999), with its emission maximum at 583 nm, appears to be the long awaited tool for multi-color applications in fluorescence-based biological research. Studying the emission dynamics of DsRed by fluorescence correlation spectroscopy (FCS), it can be verified that this protein exhibits strong light-dependent flickering similar to what is observed in several yellow-shifted mutants of GFP. FCS data recorded at different intensities and excitation wavelengths suggest that DsRed appears under equilibrated conditions in at minimum three interconvertible states, apparently fluorescent with different excitation and emission properties. Light absorption induces transitions and/or cycling between these states on time scales of several tens to several hundreds of microseconds, dependent on excitation intensity. With increasing intensity, the emission maximum of the static fluorescence continuously shifts to the red, implying that at least one state emitting at longer wavelength is preferably populated at higher light levels. In close resemblance to GFP, this light-induced dynamic behavior implies that the chromophore is subject to conformational rearrangements upon population of the excited state.

Using green fluorescent protein to study intracellular signalling.

Subcellular compartmentalisation of signalling molecules is an important phenomenon not only in defining how a signalling pathway is activated but also in influencing the desired physiological output of that pathway (e.g. cell growth or differentiation, regulation of metabolism, cytoskeletal changes etc.). Biochemical analyses of protein and lipid compartmentalisation by, for example, subcellular fractionation presents many technical difficulties. However, this aspect of cell signalling research has seen a major revolution thanks to the cloning and availability of a variety of mutant green fluorescent protein derivatives with distinct molecular properties. Mutants with increased brightness, altered excitation and emission maxima, altered stability and differential sensitivity to pH, are now in widespread use for following the trafficking and function of proteins in living cells and for monitoring the intracellular environment. In this review we focus on some of the recent developments in the use of green fluorescent proteins for studying intracellular signalling pathways often with special reference to the actions of insulin. We also discuss the future utility of these proteins to analyse protein--protein interactions in signalling pathways using fluorescence resonance energy transfer.

Design and characterization of a DNA-encoded, voltage-sensitive fluorescent protein.

Optical imaging of electrical activity has been suggested as a promising approach to investigate the multineuronal representation of information processing in brain tissue. While considerable progress has been made in the development of instrumentation suitable for high-speed imaging, intrinsic or extrinsic dye-mediated optical signals are often of limited use due to their slow response dynamics, low effective sensitivity, toxicity or undefined cellular origin. Protein-based and DNA-encoded voltage sensors could overcome these limitations. Here we report the design and generation of a voltage-sensitive fluorescent protein (VSFP) consisting of a voltage sensing domain of a potassium channel and a pair of cyan and yellow emitting mutants of green fluorescent protein (GFP). In response to a change in transmembrane voltage, the voltage sensor alters the amount of fluorescence resonance energy transfer (FRET) between the pair of GFP mutants. The optical signals respond in the millisecond time-scale of fast electrical signalling and are large enough to allow monitoring of voltage changes at the single cell level.

Fluorescent Plasmodium berghei sporozoites and pre-erythrocytic stages: a new tool to study mosquito and mammalian host interactions with malaria parasites.

To track malaria parasites for biological studies within the mosquito and mammalian hosts, we constructed a stably transformed clonal line of Plasmodium berghei, PbFluspo, in which sporogonic and pre-erythrocytic liver-stage parasites are autonomously fluorescent. A cassette containing the structural gene for the FACS-adapted green fluorescent protein mutant 2 (GFPmut2), expressed from the 5' and 3' flanking sequences of the circumsporozoite (CS) protein gene, was integrated and expressed at the endogenous CS locus. Recombinant parasites, which bear a wild-type copy of CS, generated highly fluorescent oocysts and sporozoites that invaded mosquito salivary glands and were transmitted normally to rodent hosts. The parasites infected cultured hepatocytes in vitro, where they developed into fluorescent pre-erythrocytic forms. Mammalian cells infected by these parasites can be separated from non-infected cells by fluorescence activated cell sorter (FACS) analysis. These fluorescent insect and mammalian stages of P. berghei should be useful for phenotypic studies in their respective hosts, as well as for identification of new genes expressed in these parasite stages.

Photophysics of DsRed, a Red Fluorescent Protein, from the Ensemble to the Single-Molecule Level

DsRed, a tetrameric fluorescent protein cloned from the Discosoma genus of coral, has shown promise as a longer-wavelength substitute for green fluorescent protein (GFP) mutants for in vivo protein labeling. Bulk and single-molecule studies of the recombinant protein revealed that the DsRed chromophore shows high stability against photobleaching as compared to GFP mutants. Stark modulation spectra confirm that the electronic structure of the DsRed chromophore is similar to that of GFP. However, the tetrameric nature of DsRed leads to intersubunit energy transfer, as evidenced by the molecule's unusually low fluorescence anisotropy when immobilized (0.23 ± 0.02). This value is approximately consistent with an estimate of the energy transfer rates based on preliminary crystallographic information. The fluorescence emission bleaches at a rate linear in the applied excitation intensity, implying that the cessation of emission during pumping at 532 nm is light-driven and, consistent with the tetrameric structure, several photobleaching “steps” were observed for individual complexes. Because more photons are emitted before bleaching, this study suggests that DsRed may be superior to some GFP-based labeling technologies as long as tetramerization is not an issue in physiological studies.

Excited-State Dynamics in the Enhanced Green Fluorescent Protein Mutant Probed by Picosecond Time-Resolved Single Photon Counting Spectroscopy

The complex photophysics of the wild-type green fluorescent protein (GFP), one of the most popular fluorescent probes in biology, has been extensively documented in literature. The excited-state dynamics of GFP was explained by means of a model implying excited-state proton transfer (ESPT) and three forms of the chromophore, a protonated A form absorbing at 400 nm and two deprotonated I and B forms absorbing at around 475 nm. We report here a systematic picosecond time-resolved fluorescence study of the enhanced green fluorescent protein (EGFP) variant, carrying the Ser65-Thr and Phe64-Leu mutations. By means of multiple excitation wavelength time-resolved experiments, we were able to distinguish between the fluorescence decay times of the deprotonated I* and B* states (3.4 and 2.7 ns). Spectrally, we found the I form being red shifted in comparison with the B form, both in absorption and in emission. Evidence for an excited-state reaction, namely, proton transfer, is also reported. An additional protonated species is proposed in the photophysical scheme in order to explain the excited-state dynamics of EGFP on the basis of our results as well as previous reported data. Two alternative models are presented, both of them applicable also to the data reported in relation with wild-type GFP.

Mechanism for down-regulation of CD28 by Nef.

SIV and HIV Nef proteins disrupt T-cell receptor machinery by down-modulating cell surface expression of CD4 and expression or signaling of CD3-TCR. Nef also down-modulates class I major histocompatibility complex (MHC) surface expression. We show that SIV and HIV-1 Nefs down-modulate CD28, a major co-stimulatory receptor that mediates effective T-cell activation, by accelerating CD28 endocytosis. The effects of Nef on CD28, CD4, CD3 and class I MHC expression are all genetically separable, indicating that all are selected independently. In cells expressing a Nef-green fluorescent protein (GFP) fusion, CD28 co-localizes with the AP-2 clathrin adaptor and Nef-GFP. Mutations that disrupt Nef interaction with AP-2 disrupt CD28 down-regulation. Furthermore, HIV and SIV Nefs use overlapping but distinct target sites in the membrane-proximal region of the CD28 cytoplasmic domain. Thus, Nef probably induces CD28 endocytosis via the AP-2 pathway, and this involves a ternary complex containing Nef, AP-2 and CD28. The likely consequence of the concerted down-regulation of CD28, CD4 and/or CD3 by Nef is disruption of antigen-specific signaling machineries in infected T cells following a productive antigen recognition event.

Measurement of mitochondrial pH in situ.

In this article, we describe the advantages and disadvantages of procedures for monitoring mitochondrial pH in situ using optical microscopic techniques. The first method employs the combination of the fluorescent pH-sensitive indicator carboxy-SNARF and laser scanning confocal microscopy. Manipulation of the loading and post-loading conditions enables relatively specific accumulation of carboxy-SNARF into mitochondria. With the use of a mitochondrial-specific marker, mitochondrial pH can be accurately monitored. More recently, mitochondrial-targeted, pH-sensitive probes have been used to monitor mitochondrial pH. In particular, mitochondrial targeting of the yellow fluorescent protein (YFP) mutant of green fluorescent protein (GFP) combines the advantages of specific mitochondrial localization, high-fluorophore quantum yield, and extinction coefficient with an appropriate pKa for measuring mitochondrial pH. The use of dual-excitation ratiometry with mitochondrially targeted YFP increases the dynamic range of mitochondrial pH measurements and corrects for differences in the amount of expression of mitochondrially targeted YFP at the level of individual mitochondria.

Optimized expression of green fluorescent protein in Toxoplasma gondii using thermostable green fluorescent protein mutants

Optimized expression of green fluorescent protein in Toxoplasma gondii using thermostable green fluorescent protein mutants

Efficient GFP mutations profoundly affect mRNA transcription and translation rates

Green fluorescent protein (GFP) variants with higher expression efficiencies have been generated by mutagenesis. Favorable mutations often improve the folding of GFP. However, an effect on protein folding fails to explain the efficiency of several other GFP mutations. In this work, we demonstrate that mutations of the GFP open reading frame and untranslated regions profoundly affect mRNA transcription and translation efficiencies. The removal of the GFP 5′ untranslated region halves the transcription rate of the GFP gene, but hugely improves its translation rate. Mutations of the GFP open reading frame or the addition of peptide sequences differentially reduce the GFP mRNA transcription rate, translation efficiency and protein stability. These previously unrecognized effects are demonstrated to be critical to the efficiency of GFP mutants. These findings indicate the feasibility of generating more efficient GFP variants, with optimized mRNA transcription and translation in eukaryotic cells.

Spatiotemporal dynamics of guanosine 3',5'-cyclic monophosphate revealed by a genetically encoded, fluorescent indicator.

To investigate the dynamics of guanosine 3',5'-cyclic monophosphate (cGMP) in single living cells, we constructed genetically encoded, fluorescent cGMP indicators by bracketing cGMP-dependent protein kinase (cGPK), minus residues 1-77, between cyan and yellow mutants of green fluorescent protein. cGMP decreased fluorescence resonance energy transfer (FRET) and increased the ratio of cyan to yellow emissions by up to 1.5-fold with apparent dissociation constants of approximately 2 microM and >100:1 selectivity for cGMP over cAMP. To eliminate constitutive kinase activity, Thr(516) of cGPK was mutated to Ala. Emission ratio imaging of the indicators transfected into rat fetal lung fibroblast (RFL)-6 showed cGMP transients resulting from activation of soluble and particulate guanylyl cyclase, respectively, by nitric oxide (NO) and C-type natriuretic peptide (CNP). Whereas all naive cells tested responded to CNP, only 68% responded to NO. Both sets of signals showed large and variable (0.5-4 min) latencies. The phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (IBMX) did not elevate cGMP on its own but consistently amplified responses to NO or CNP, suggesting that basal activity of guanylate cyclase is very low and emphasizing the importance of PDEs in cGMP recycling. A fraction of RFL cells showed slowly propagating tides of cGMP spreading across the cell in response to delocalized application of NO. Biolistically transfected Purkinje neurons showed cGMP responses to parallel fiber activity and NO donors, confirming that single-cell increases in cGMP occur under conditions appropriate to cause synaptic plasticity.

Two-color fluorescence correlation spectroscopy of one chromophore: Application to the E222Q mutant of the green fluorescent protein

Fluorescence correlation spectroscopy (FCS) is an important method for investigations of diffusion processes as well as of photophysical properties of fluorescing molecules. It has lately been applied in studies of the photodynamics of the green fluorescent protein (GFP). In this case FCS yields valuable information about the population of dark, non-fluorescing states of the molecule. For three-level systems rate constants into and out of the dark state can easily be determined with FCS. This task however becomes significantly more complex for molecules that possess several dark states. Here we present two-color FCS with simultaneous two-color excitation as a method that also yields spectroscopic information about the dark states. This makes the complete analysis of a molecular four-level system possible. The analysis of the GFP mutant E222Q is given as an example of two-color FCS that is readily applicable to other molecules with photoconvertible dark states. For E222Q we determine all the rate constants within the four-level system. With these data we calculate the population of the different molecular states in bulk experiments as encountered, e.g., in microscopic studies.

Luminescent proteins from Aequorea victoria: applications in drug discovery and in high throughput analysis.

Recent progress in generating a vast number of drug targets through genomics and large compound libraries through combinatorial chemistry have stimulated advancements in drug discovery through the development of new high throughput screening (HTS) methods. Automation and HTS techniques are also highly desired in fields such as clinical diagnostics. Luminescence-based assays have emerged as an alternative to radiolabel-based assays in HTS as they approach the sensitivity of radioactive detection along with ease of operation, which makes them amenable to miniaturization. Luminescent proteins provide the advantage of reduced reagent and operating costs because they can be produced in unlimited amounts through the use of genetic engineering tools. In that regard, the use of two naturally occurring and recombinantly produced luminescent proteins from the jellyfish Aequorea victoria, namely, aequorin and the green fluorescent protein (GFP), has attracted attention in a number of analytical applications in diverse research areas. Aequorin is naturally bioluminescent and has therefore, virtually no associated background signal, which allows its detection down to attomole levels. GFP has become the reporter of choice in a variety of applications given that it is an autofluorescent protein that does not require addition of any co-factors for fluorescence emission. Furthermore, the generation of various mutants of GFP with differing luminescent and spectral properties has spurred additional interest in this protein. In this review, we focus on the use of aequorin and GFP in the development of highly sensitive assays that find applications in drug discovery and in high throughput analysis.

Green Fluorescent Protein Mutant as Label in Homogeneous Assays for Biomolecules

The green fluorescent protein (GFP) and its mutants have been extensively used to study various cellular processes and, more recently, as labels in binding assays. We have employed a mutant of GFP, an enhanced GFP (EGFP), in the development of homogeneous assays for biotin and cortisol. To demonstrate the feasibility of using EGFP as a label with different kinds of binders in the development of homogeneous assays, we employed the biotin–avidin and an antigen–antibody as the binding pairs. Biotin and cortisol were chemically conjugated to EGFP. A quenching of fluorescence intensity of EGFP was observed upon binding of avidin to the EGFP–biotin conjugate. The percentage fluorescence quenching observed decreased as the concentration of free biotin in the sample increased due to the fewer binding sites on avidin available for binding to the EGFP–biotin conjugate. A dose–response curve for biotin was generated by relating percentage fluorescence quenched with free biotin in the sample. To determine whether EGFP can undergo a similar type of homogeneous change when used as a label for antigen–antibody type of binding, cortisol was selected as a model analyte. In the presence of an anti-cortisol antibody the fluorescence signal of the EGFP–cortisol conjugate was quenched. A dose–response curve for cortisol was generated by relating the quenching in the fluorescence signal with varying amounts of free cortisol in the sample. This is the first time that GFP or one of its mutants has been employed as a label in homogeneous assays, thus enhancing the versatility of employing GFP or its mutants in a number of bioanalytical applications, such as clinical analysis and high-throughput screening systems.

Engineering Single-Molecule Fluorescence Dynamics for Advanced Biomolecular Applications

The green fluorescent protein (GFP) of the Aequorea victoria jellyfish is a uniquely fluorescent label that is opening the way to advanced biomolecular studies. Fundamental processes on a nanoscopic scale can, in fact, be directly addressed by imaging and spectroscopy of GFP-tagged molecules. To this end, the design of labels with novel spectral and dynamic properties plays an important role to enhance GFP applicability. We investigated and engineered the fluorescence dynamics of two GFP mutants, the F64L/S65T and the F64L/S65T/T203Y, down to the single-molecule level in order to evaluate their suitability for a broad range of experimental systems. We shall argue that the first mutant is suitable for quantitative fluorescence microscopy and displays fluorescence almost unaffected by the environment, while the second offers the important additional advantage of undergoing photobleaching reversal after ultraviolet illumination. As a consequence, the latter mutant is a viable candidate for applications when prolonged imaging is required. As an example of novel possibilities provided by GFP technology, we shall show studies of HIV-1 Tat activity in cells and, in particular, we shall demonstrate Tat interaction with cyclin T1 by fluorescence-resonance energy transfer. Manuscript received: 8 November 2000.

Synthesis of Functional Protein in Liposome

The liposome consisting of eggPC, cholesterol, and DSPE-PEG5000 with a molar ratio of 1.5:1:0.08 was used to entrap cell-free protein synthesis reaction mixture. The synthesis of a mutant green fluorescent protein in the liposome was confirmed by the fluorescence emitted from the liposome on flow cytometry analysis and fluorescence microscopy. The protein synthesized in the liposome is hence functional.

Synthesis of functional protein in liposome

The liposome consisting of eggPC, cholesterol, and DSPE-PEG5000 with a molar ratio of 1.5:1:0.08 was used to entrap cell-free protein synthesis reaction mixture. The synthesis of a mutant green fluorescent protein in the liposome was confirmed by the fluorescence emitted from the liposome on flow cytometry analysis and fluorescence microscopy. The protein synthesized in the liposome is hence functional.

A Mutant of Green Fluorescent Protein with Enhanced Sensitivity at 488nm Excitation and Its Application

A Mutant of Green Fluorescent Protein with Enhanced Sensitivity at 488nm Excitation and Its Application

Trafficking of non-regulated secretory proteins in insulin secreting (INS-1) cells.

Sorting of proinsulin to the regulated secretory pathway of pancreatic beta cells and retention of insulin in dense-core granules of this pathway is remarkably efficient. To monitor the specificity of these events, the secretion of two exogenous secretory proteins not known to carry information for sorting or retention in the regulated pathway was investigated in INS-1 cells. SEGFP, a fusion protein consisting of a signal peptide N-terminal to EGFP (mutant green fluorescent protein with enhanced fluorescence) and secreted alkaline phosphatase (SEAP) were expressed in INS-1 cells by transfection and by infection with recombinant adenovirus, respectively. Secretion of SEGFP was monitored by quantitative western blotting and that of SEAP by its activity. Secreted alkaline phosphatase showed high basal secretion (6.6% total) but only modest (3.6-fold) stimulation of secretion by secretagogues, in keeping with secretion largely through the constitutive pathway. By contrast SEGFP had a secretory pattern similar to insulin, with low basal secretion (0.8% total) and 16-fold stimulation by secretagogues. Granular localization of SEGFP was confirmed by high resolution electron microscopy immunocytochemistry. Pulse-chase experiments indicated retention of SEGFP in granules at least 24 h after synthesis. The secretory SEGFP, but not cytosolic EGFP, formed disulphide-linked oligomers. This could be implicated in its regulated secretion. These data indicate that in INS-1 cells SEGFP, but not SEAP, is unexpectedly handled as a regulated secretory protein and stored along with insulin in granules. This raises questions about the specificity and mechanism of the sorting of proteins to granules in INS-1 cells or their retention therein or both.