FACS-optimized mutants of the green fluorescent protein (GFP)

Molecular studies on bromovirus capsid protein. III. Analysis of cell-to-cell movement competence of coat protein defective variants of cowpea chlorotic mottle virus

To determine whether the role of coat protein (CP) in cell-to-cell movement of dicot-adapted cowpea chlorotic mottle bromovirus (CCMV) is distinct from that of monocot-adapted brome mosaic bromovirus (BMV), two reporter genes, beta-glucuronidase (GUS) and enhanced green fluorescent protein (EGFP), were substituted for the CP in a biologically active clone of CCMV RNA3 (C3). Primary leaves of Nicotiana benthamiana, Chenopodium quinoa, and cowpea were co-inoculated with wild-type (wt) CCMV RNA 1 and -2 and either C3/delta CP-GUS or C3/delta CP-EGFP and analyzed for GUS activity or the presence of green fluorescence. The visual appearance of infections caused by GUS or EGFP variants indicated that, in CCMV, epidermal cell-to-cell movement can occur without a functional CP. By contrast, inoculation of MP defective variants of C3/delta CP-GUS or C3/delta CP-EGFP resulted in subliminal infections. Additional experiments examining the infectivity of wt BMV RNA 1 and -2 and a BMV RNA3 variant bearing the EGFP in the place of CP (B3/delta CP-EGFP) confirmed previous observations that, unlike CCMV, epidermal cell-to-cell movement of BMV is dependent on the expression of a functional CP. Taken together, the results demonstrate that BMV and CCMV use different mechanisms for initial epidermal cell-to-cell spread, and the individual role played by the respective CP genes in this active process is discussed.

An improved GFP cloning cassette designed for prokaryotic transcriptional fusions

A new gfp cloning cassette designed for prokaryotic transcriptional fusions has been constructed. This cassette consists of gfp (containing the S65T 'red-shift' [Heim et al. (1995) Nature 373, 663-664] and F64L 'protein solubility' [Cormack et al. (1996) Gene 173, 33-38] mutations) flanked by convenient restriction sites, a translational enhancer, and a consensus ribosome binding site with an optimized spacer region. gfp fusion strains containing this cassette demonstrate from 40- to 80-fold greater fluorescence intensity than wild-type gfp fusion strains. Additionally, this cassette confers sufficient fluorescence to recipient cells to be used in low copy-number plasmids, with promoters conferring low levels of transcription, and in bacterial taxa other than Escherichia, such as Pseudomonas.

Ca2+-triggered peptide secretion in single cells imaged with green fluorescent protein and evanescent-wave microscopy

Green fluorescent protein fused to human chromogranin B or neuropeptide Y was expressed in PC12 cells and caused bright, punctate fluorescence. The fluorescent points colocalized with the endogenous secretory granule marker dopamine beta-hydroxylase. Stimulation of live PC12 cells with elevated [K+], or of permeabilized PC12 cells with Ca2+, led to Ca2+-dependent loss of fluorescence from neurites. Ca2+ stimulated secretion of both fusion proteins equally well. In living cells, single fluorescent granules were imaged by evanescent-wave fluorescence microscopy. Granules were seen to migrate; to stop, as if trapped by plasmalemmal docking sites; and then to disappear abruptly, as if through exocytosis. Evidently, GFP fused to secreted peptides is a fluorescent marker for dense-core secretory granules and may be used for time-resolved microscopy of single granules.

A point mutation responsible for defective function of the aryl-hydrocarbon-receptor nuclear translocator in mutant Hepa-1c1c7 cells

A 3,4-benzopyrene-resistant mutant clone (c4) of the mouse hepatoma Hepa-1c1c7 cell line was examined for the mutation that causes the defective function of aryl-hydrocarbon receptor (AHR) nuclear translocator (Arnt). Arnt dimerizes with AHR and mediates the induction signal of aryl-hydrocarbon hydroxylase activity. The Arnt cDNAs of c4 cells were cloned by reverse-transcription/PCR to compare the sequences with that of wild-type Arnt cDNA. The Arnt cDNA of c4 cells was found to have a single point mutation, leading to replacement of Gly326 with Asp between two internal repeats in the highly conserved Per-Arnt-Sim (PAS) domain, PAS A and PAS B. The inability of [Asp326]Arnt/AHR heterodimers to enhance reporter gene transcription under the control of the CYP1A1 gene promoter and enhancer confirmed that the G326-->D substitution was a causative mutation. While fluorescence microscopy and coimmunoprecipitation experiments showed that this mutant form of Arnt was not changed from wild-type Arnt in terms of nuclear localization or heterodimer formation with AHR, the binding activity of the [Asp326]Arnt x AHR heterodimer to the xenobiotic-responsive element was reduced markedly. Determination of the turnover rate in COS-7 cells transfected with expression plasmids for mutant Arnt or normal Arnt showed that the mutant protein turned over with an accelerated rate compared with that of the normal. Moreover, the mutant protein displayed increased proteolytic digestibility in vitro with various proteases.

Human cytomegalovirus tegument protein pp71 (ppUL82) enhances the infectivity of viral DNA and accelerates the infectious cycle

Three tegument proteins of human cytomegalovirus (HCMV), ppUL82 (pp71), pUL69, and ppUL83 (pp65), were examined for the ability to stimulate the production of infectious virus from human diploid fibroblasts transfected with viral DNA. Although viral DNA alone had a low intrinsic infectivity of 3 to 8 plaques/microg of viral DNA, cotransfection of a plasmid expressing pp71 increased the infectivity of HCMV DNA 30- to 80-fold. The increase in infectivity produced by pp71 was reflected in an increased number of nuclei observed to express high levels of the major immediate-early proteins IE1 and IE2. Cotransfection of viral DNA with plasmids directing expression of IE1 and IE2 also resulted in extensive IE1 and IE2 expression in the transfected cells; however, the infectivity of viral DNA was only marginally increased. pp71 also facilitated late gene expression, virus transmission to adjacent cells, and plaque formation. In contrast, expression of pUL69 reduced the pp71- and IE1/IE2-mediated enhancement of HCMV DNA infectivity and also failed to produce any increase in the number of cells expressing IE1 and IE2 over that seen with viral DNA alone. Expression of pp65 did not alter the infectivity of HCMV DNA, nor did it modify the effects of pp71 or pUL69. These results imply that pp71 plays a critical role in the initiation of infection apart from its function as a transactivator of IE1 and IE2.

The vesicle docking protein p115 binds GM130, a cis-Golgi matrix protein, in a mitotically regulated manner

The docking of transport vesicles with their target membrane is thought to be mediated by p115. We show here that GM130, a cis-Golgi matrix protein, interacts specifically with p115 and so could provide a membrane docking site. Deletion analysis showed that the N-terminus binds to p115, whereas the C-terminus binds to Golgi membranes. Mitotic phosphorylation of GM130 or a peptide derived from the N-terminus prevented binding to p115. The peptide also inhibited the NSF- but not the p97-dependent reassembly of Golgi cisternae from mitotic fragments, unless it was mitotically phosphorylated. Together, these data provide a molecular explanation for the COPI-mediated fragmentation of the Golgi apparatus at the onset of mitosis.

Structural basis for dual excitation and photoisomerization of the Aequorea victoria green fluorescent protein

The 2.1-A resolution crystal structure of wild-type green fluorescent protein and comparison of it with the recently determined structure of the Ser-65 --> Thr (S65T) mutant explains the dual wavelength absorption and photoisomerization properties of the wild-type protein. The two absorption maxima are caused by a change in the ionization state of the chromophore. The equilibrium between these states appears to be governed by a hydrogen bond network that permits proton transfer between the chromophore and neighboring side chains. The predominant neutral form of the fluorophore maximally absorbs at 395 nm. It is maintained by the carboxylate of Glu-222 through electrostatic repulsion and hydrogen bonding via a bound water molecule and Ser-205. The ionized form of the fluorophore, absorbing at 475 nm, is present in a minor fraction of the native protein. Glu-222 donates its charge to the fluorophore by proton abstraction through a hydrogen bond network, involving Ser-205 and bound water. Further stabilization of the ionized state of the fluorophore occurs through a rearrangement of the side chains of Thr-203 and His-148. UV irradiation shifts the ratio of the two absorption maxima by pumping a proton relay from the neutral chromophore's excited state to Glu-222. Loss of the Ser-205-Glu-222 hydrogen bond and isomerization of neutral Glu-222 explains the slow return to the equilibrium dark-adapted state of the chromophore. In the S65T structure, steric hindrance by the extra methyl group stabilizes a hydrogen bonding network, which prevents ionization of Glu-222. Therefore the fluorophore is permanently ionized, causing only a 489-nm excitation peak. This new understanding of proton redistribution in green fluorescent protein should enable engineering of environmentally sensitive fluorescent indicators and UV-triggered fluorescent markers of protein diffusion and trafficking in living cells.

Human immunodeficiency virus type 1 Vpr gene product prevents cell proliferation on mouse NIH3T3 cells without the G2 arrest of the cell cycle

Human immunodeficiency virus type 1 Vpr is a 96-amino-acid virion-associated protein that arrests cells in the G2 phase of the cell cycle in peripheral blood lymphocytes, HeLa, 293, 293T, A549, Jurkat, CEM, SupT1, CV-1 and COS1 cells. When we transfected Vpr expression vector into mouse NIH3T3 and then cultured it in the presence of G418, NIH3T3 cells were the drug resistant cells yielded. The surviving colonies, however, exhibited a degenerating morphology up to 8 approximately 20-fold smaller than the control vector colonies. In addition, the growth of NIH3T3 cells transiently transfected with Vpr expression vector declined dramatically compared with that of transfectants with control vector, suggesting that Vpr significantly interferes with cell proliferation of NIH3T3 cells. Cell cycle characterization by flow cytometry indicated that expression of Vpr did not induce G2 cessation in NIH3T3. These findings strongly suggest that Vpr has a novel pathway to retard cell growth independently and arrests the G2 phase of the cell cycle.

Following cell fate in the living mouse embryo

It has been difficult to follow many of the dramatic changes in cell fate and cell migration during mouse development. This is because there has been no enduring marker that would allow cells to be recognised in the living embryo. We believe that we have overcome this problem by developing a novel form of green fluorescent protein, named MmGFP, that proves to be easily visible and non toxic to mouse cells and does not perturb embryogenesis. We show that synthetic mRNA encoding MmGFP can be injected into blastomeres to follow the fate of their progeny during preimplantation development. We have made a stable embryonic stem cell line that expresses MmGFP and introduced these fluorescent cells into mouse embryos. For the first time, we have been able to follow the fate of embryonic stem cells in living embryos and to observe directly the contribution of these cells to distinct lineages of the postimplantation embryo. This approach should lead to a more complete description of the dynamics of cell fate in the mouse.

A novel mutation which enhances the fluorescence of green fluorescent protein at high temperatures

Green fluorescent protein (GFP) from Aequorea victoria is widely used as a marker of gene expression and protein localization in living cells from prokaryotes to eukaryotes. However, the total fluorescent signal from wild-type GFP is very weak when expressed in cells cultured at 37 degrees C compared to 30 degrees C or below. This characteristic makes GFP poorly suited to use as a marker in mammalian cells. Here we describe a new variant of GFP which carries a substitution of Ser147 to Pro (S147P GFP) and which emits a stronger fluorescent signal than the wild-type GFP at high temperature. When S147P is combined with the Ser65 to Thr mutation (S65T GFP), the resulting double mutant emits fluorescence which is several-fold stronger than GFP with a single S65T modification in both bacterial or mammalian cells. This S147P mutation should be useful for constructing new GFP variants which stably emit strong fluorescence at a wide range of culturing temperatures.

The Chlamydia trachomatis parasitophorous vacuolar membrane is not passively permeable to low-molecular-weight compounds

Chlamydia trachomatis is an obligately intracellular bacterial parasite of eucaryotic cells that undergoes a biphasic life cycle within a parasitophorous vacuole (PV) called an inclusion. The parasitophorous vacuolar membrane (PVM) constitutes a barrier between the replicating bacteria and the nutrient-rich environment of the host cytoplasm. To determine whether the chlamydial PVM contains pores that allow passive diffusion of metabolites between the host cytoplasm and the PV, fluorescent tracer molecules were introduced directly into the cytoplasm of infected cells by transfection or microinjection. Fluorescence microscopy and laser scanning confocal microscopy were subsequently employed to determine whether equilibration of the fluorescent tracers between the cytoplasm and the PV occurred. No movement of tracer molecules as small as 520 Da from the cytoplasm to the PV was observed. These data suggest that the chlamydial PV is not passively permeable to small molecules through open channels in the PVM.

Characterization of a notochord-specific enhancer from the Brachyury promoter region of the ascidian, Ciona intestinalis

We present evidence that the embryo of the ascidian, Ciona intestinalis, is an easily manipulated system for investigating the establishment of basic chordate tissues and organs. Ciona has a small genome, and simple, well-defined embyronic lineages. Here, we examine the regulatory mechanisms underlying the differentiation of the notochord. Particular efforts center on the regulation of a notochord-specific Ciona Brachyury gene (Ci-Bra). An electroporation method was devised for the efficient incorporation of transgenic DNA into Ciona embryos. This method permitted the identification of a minimal, 434 bp enhancer from the Ci-Bra promoter region that mediates the notochord-restricted expression of both GFP and lacZ reporter genes. This enhancer contains a negative control region that excludes Ci-Bra expression from inappropriate embryonic lineages, including the trunk mesenchyme and tail muscles. Evidence is presented that the enhancer is activated by a regulatory element which is closely related to the recognition sequence of the Suppressor of Hairless transcription factor, thereby raising the possibility that the Notch signaling pathway plays a role in notochord differentiation. We discuss the implications of this analysis with regard to the evolutionary conservation of integrative enhancers, and the subdivision of the axial and paraxial mesoderm in vertebrates.

Yeast-enhanced green fluorescent protein (yEGFP): a reporter of gene expression in Candida albicans

The green fluorescent protein (GFP) of Aequorea victoria has been developed here as a reporter for gene expression and protein localization in Candida albicans. When wild-type (wt) GFP was expressed in C. albicans, it was not possible to detect fluorescence or a translation product for the wt protein. Since this was probably due in part to the presence of the non-canonical CTG serine codon in the Aequorea sequence, this codon was changed to the leucine codon TTG. C. albicans cells expressing this construct contained GFP mRNA but were non-fluorescent and contained no detectable translation product. Hence a codon-optimized GFP gene was constructed in which all of the 239 amino acids are encoded by optimal codons for C. albicans. In this gene were also incorporated two previously identified mutations in the chromophore that increase GFP fluorescence. C. albicans cells expressing this yeast-enhanced GFP gene (yEGFP3) are fluorescent and contain GFP protein. yEGFP3 can be used as a versatile reporter of gene expression in C. albicans and Saccharomyces cerevisiae and the optimized GFP described here should have broad applications in these and other fungal species.

A molecular mechanics and database analysis of the structural preorganization and activation of the chromophore-containing hexapeptide fragment in green fluorescent protein

We propose that heterologous posttranslational chromophore formation in green fluorescent protein (GFP) occurs because the chromophore-forming amino acid residues 65SYG67 are preorganized and activated for imidazolinone ring formation. Based on extensive molecular mechanical conformational searching of the precursor hexapeptide fragment (64FSYGVQ69), we suggest that the presence of low energy conformations characterized by short contacts (approximately 3 A) between the carbonyl carbon of Ser65 and the amide nitrogen of Gly67 accounts for the initial step in posttranslational chromophore formation. Database searches showed that the tight turn required to establish the key short contact is a unique structural motif that is rarely found, except in other FSYG tetrapeptide sequences. Additionally, ab initio calculations demonstrated that an arginine side chain can hydrogen bond to the carbonyl oxygen of Ser65, activating this group for nucleophilic attack by the nearby lone pair of the Gly67 amide nitrogen. We propose that GFP chromophore-formation is initiated by a unique combination of conformational and electronic enhancements, identified by computational methods.

An indelible lineage marker for Xenopus using a mutated green fluorescent protein

We describe the use of a DNA construct (named GFP.RN3) encoding green fluorescent protein as a lineage marker for Xenopus embryos. This offers the following advantages over other lineage markers so far used in Xenopus. When injected as synthetic mRNA, its protein emits intense fluorescence in living embryos. It is non-toxic, and the fluorescence does not bleach when viewed under 480 nm light. It is surprisingly stable, being strongly visible up to the feeding tadpole stage (5 days), and in some tissues for several weeks after mRNA injection. We also describe a construct that encodes a blue fluorescent protein. We exemplify the use of this GFP.RN3 construct for marking the lineage of individual blastomeres at the 32- to 64-cell stage, and as a marker for single transplanted blastula cells. Both procedures have revealed that the descendants of one embryonic cell can contribute single muscle cells to nearly all segmental myotomes rather than predominantly to any one myotome. An independent aim of our work has been to follow the fate of cells in which an early regulatory gene has been temporarily overexpressed. For this purpose, we co-injected GFP.RN3 mRNA and mRNA for the early Xenopus gene Eomes, and found that a high concentration of Eomes results in ectopic muscle gene activation in only the injected cells. This marker may therefore be of general value in providing long term identification of those cells in which an early gene with ephemeral expression has been overexpressed.

Construction of GFP vectors for use in gram-negative bacteria other than Escherichia coli

A set of vectors containing a mutated gfp gene was constructed for use with Gram-negative bacteria other than Escherichia coli. These constructs were: pTn3gfp for making random promoter probe gfp insertions into cloned DNA in E. coli for subsequent introduction into host strains; pUTmini-Tn5gfp for making random promoter probe gfp insertions directly into host strains; p519gfp and p519nfp, broad host range mob+ plasmids containing gfp expressed from a lac and an npt2 promoter, respectively.

Colocalization of cell division proteins FtsZ and FtsA to cytoskeletal structures in living Escherichia coli cells by using green fluorescent protein

In the current model for bacterial cell division, FtsZ protein forms a ring that marks the division plane, creating a cytoskeletal framework for the subsequent action of other proteins such as FtsA. This putative protein complex ultimately generates the division septum. Herein we report that FtsZ and FtsA proteins tagged with green fluorescent protein (GEP) colocalize to division-site ring-like structures in living bacterial cells in a visible space between the segregated nucleoids. Cells with higher levels of FtsZ-GFP or with FtsA-GFP plus excess wild-type FtsZ were inhibited for cell division and often exhibited bright fluorescent spiral tubules that spanned the length of the filamentous cells. This suggests that FtsZ may switch from a septation-competent localized ring to an unlocalized spiral under some conditions and that FtsA can bind to FtsZ in both conformations. FtsZ-GFP also formed nonproductive but localized aggregates at a higher concentration that could represent FtsZ nucleation sites. The general domain structure of FtsZ-GFP resembles that of tubulin, since the C terminus of FtsZ is not required for polymerization but may regulate polymerization state. The N-terminal portion of Rhizobium FtsZ polymerized in Escherichia coli and appeared to copolymerize with E. coli FtsZ, suggesting a degree of interspecies functional conservation. Analysis of several deletions of FtsA-GFP suggests that multiple segments of FtsA are important for its localization to the FtsZ ring.

The molecular structure of green fluorescent protein

The crystal structure of recombinant wild-type green fluorescent protein (GFP) has been solved to a resolution of 1.9 A by multiwavelength anomalous dispersion phasing methods. The protein is in the shape of a cylinder, comprising 11 strands of beta-sheet with an alpha-helix inside and short helical segments on the ends of the cylinder. This motif, with beta-structure on the outside and alpha-helix on the inside, represents a new protein fold, which we have named the beta-can. Two protomers pack closely together to form a dimer in the crystal. The fluorophores are protected inside the cylinders, and their structures are consistent with the formation of aromatic systems made up of Tyr66 with reduction of its C alpha-C beta bond coupled with cyclization of the neighboring glycine and serine residues. The environment inside the cylinder explains the effects of many existing mutants of GFP and suggests specific side chains that could be modified to change the spectral properties of GFP. Furthermore, the identification of the dimer contacts may allow mutagenic control of the state of assembly of the protein.

Crystal structure of the Aequorea victoria green fluorescent protein

The green fluorescent protein (GFP) from the Pacific Northwest jellyfish Aequorea victoria has generated intense interest as a marker for gene expression and localization of gene products. The chromophore, resulting from the spontaneous cyclization and oxidation of the sequence -Ser65 (or Thr65)-Tyr66-Gly67-, requires the native protein fold for both formation and fluorescence emission. The structure of Thr65 GFP has been determined at 1.9 angstrom resolution. The protein fold consists of an 11-stranded beta barrel with a coaxial helix, with the chromophore forming from the central helix. Directed mutagenesis of one residue adjacent to the chromophore, Thr203, to Tyr or His results in significantly red-shifted excitation and emission maxima.

Use of green fluorescent protein variants to monitor gene transfer and expression in mammalian cells

Two mutants of the green fluorescent protein (GFP), RSGFP4 and GFPS65T, have been recently created which differ from the wildtype GFP of A. victoria in their excitation maxima. Here we show that human fibroblasts transfected with either of the two mutant GFP genes emit a green fluorescence that is 18-fold brighter than the cells transfected with the wildtype GFP gene. Retroviral vectors expressing the improved GFP gene were also constructed to determine their suitability for stable gene transduction into mammalian cells. The inclusion of the RSGFP4 gene in a retroviral vector did not reduce the viral titer and resulted in a fluorescent signal in viable transduced cells detectable by both fluorescence microscopy and fluorescence-activated cell sorter (FACS) analysis. Therefore, the improved mutant GFP provides a vital marker for monitoring gene transfer and expression in mammalian cells.

Applications for green fluorescent protein (GFP) in the study of host-pathogen interactions

The green fluorescent protein (GFP) from Aequorea victoria is a novel fluorescent marker that has potential use in the study of bacterial pathogenicity. To explore some of the potential applications of GFP to the study of host-parasite interactions, we constructed two GFP expression vectors suitable for different facultative intracellular bacterial pathogens. The first expression vector was tested in the enteric pathogens, Salmonella typhimurium and Yersinia pseudotuberculosis, and the second vector tested in Mycobacterium marinum (Mm). Both expression vectors were found to be stable and to direct high levels of GFP synthesis. Standard epifluorescence microscopy was used to detect all three bacterial pathogenic species during the early and late stages of infection of live mammalian cells. Mm expressing gfp was also visualized in infected animal tissues. gfp expression did not adversely affect bacterial survival, nor did it compromise entry into mammalian cells or their survival within macrophages. In addition, all three gfp-expressing bacterial pathogens could be detected and sorted in a flow cytometer, either alone or in association with epithelial cells or macrophages. Therefore, GFP not only provides a convenient tool to image pathogenic bacteria, but allows the quantitative measurement of bacterial association with mammalian cells.