Fluorescent proteins are genetically encoded, easily imaged reporters crucial in biology and biotechnology. When a protein is tagged by fusion to a fluorescent protein, interactions between fluorescent proteins can undesirably disturb targeting or function. Unfortunately, all wild-type yellow-to-red fluorescent proteins reported so far are obligately tetrameric and often toxic or disruptive. The first true monomer was mRFP1, derived from the Discosoma sp. fluorescent protein "DsRed" by directed evolution first to increase the speed of maturation, then to break each subunit interface while restoring fluorescence, which cumulatively required 33 substitutions. Although mRFP1 has already proven widely useful, several properties could bear improvement and more colors would be welcome. We report the next generation of monomers. The latest red version matures more completely, is more tolerant of N-terminal fusions and is over tenfold more photostable than mRFP1. Three monomers with distinguishable hues from yellow-orange to red-orange have higher quantum efficiencies.

All coelenterate fluorescent proteins cloned to date display some form of quaternary structure, including the weak tendency of Aequorea green fluorescent protein (GFP) to dimerize, the obligate dimerization of Renilla GFP, and the obligate tetramerization of the red fluorescent protein from Discosoma (DsRed). Although the weak dimerization of Aequorea GFP has not impeded its acceptance as an indispensable tool of cell biology, the obligate tetramerization of DsRed has greatly hindered its use as a genetically encoded fusion tag. We present here the stepwise evolution of DsRed to a dimer and then either to a genetic fusion of two copies of the protein, i.e., a tandem dimer, or to a true monomer designated mRFP1 (monomeric red fluorescent protein). Each subunit interface was disrupted by insertion of arginines, which initially crippled the resulting protein, but red fluorescence could be rescued by random and directed mutagenesis totaling 17 substitutions in the dimer and 33 in mRFP1. Fusions of the gap junction protein connexin43 to mRFP1 formed fully functional junctions, whereas analogous fusions to the tetramer and dimer failed. Although mRFP1 has somewhat lower extinction coefficient, quantum yield, and photostability than DsRed, mRFP1 matures >10 times faster, so that it shows similar brightness in living cells. In addition, the excitation and emission peaks of mRFP1, 584 and 607 nm, are Ϸ25 nm red-shifted from DsRed, which should confer greater tissue penetration and spectral separation from autofluorescence and other fluorescent proteins.T he red fluorescent protein cloned from Discosoma coral (DsRed or drFP583) (1) holds great promise for biotechnology and cell biology as a spectrally distinct companion or substitute for the green fluorescent protein (GFP) from the Aequorea jellyfish (2). GFP and its blue, cyan, and yellow variants have found widespread use as genetically encoded indicators for tracking gene expression and protein localization and as donor͞ acceptor pairs for f luorescence resonance energy transfer (FRET). Extending the spectrum of available colors to red wavelengths would provide a distinct label for multicolor tracking of fusion proteins, and together with GFP (or a suitable variant) would provide a FRET donor͞acceptor pair that should be superior to the currently preferred cyan͞yellow pair (3). However, the evolution of DsRed from a scientific curiosity to a generally applicable and robust tool has been hampered by several critical problems, including a slow and incomplete maturation and obligate tetramerization (4). Most previous attempts to address the rate and͞or extent of maturation of DsRed (5, 6), including the commercially available DsRed2 (CLONTECH), have provided only modest improvements. However, an engineered variant of DsRed, known as T1 (see Fig. 1A), has recently become available and effectively solved the problem of the slow maturation (7). Another approach to overcoming these shortcomings has been to continue the search for DsRed homologues in sea coral and anemone, an ...

Engineered fluorescent protein (FP) chimeras that modulate their fluorescence in response to changes in calcium ion (Ca(2+)) concentration are powerful tools for visualizing intracellular signaling activity. However, despite a decade of availability, the palette of single FP-based Ca(2+) indicators has remained limited to a single green hue. We have expanded this palette by developing blue, improved green, and red intensiometric indicators, as well as an emission ratiometric indicator with an 11,000% ratio change. This series enables improved single-color Ca(2+) imaging in neurons and transgenic Caenorhabditis elegans. In HeLa cells, Ca(2+) was imaged in three subcellular compartments, and, in conjunction with a cyan FP-yellow FP-based indicator, Ca(2+) and adenosine 5'-triphosphate were simultaneously imaged. This palette of indicators paints the way to a colorful new era of Ca(2+) imaging.

Fluorescent probes are one of the cornerstones of real-time imaging of live cells and a powerful tool for cell biologists. They provide high sensitivity and great versatility while minimally perturbing the cell under investigation. Genetically-encoded reporter constructs that are derived from fluorescent proteins are leading a revolution in the real-time visualization and tracking of various cellular events. Recent advances include the continued development of 'passive' markers for the measurement of biomolecule expression and localization in live cells, and 'active' indicators for monitoring more complex cellular processes such as small-molecule-messenger dynamics, enzyme activation and protein-protein interactions.

We recently introduced a method (Griffin, B. A.; Adams, S. R.; Tsien, R. Y. Science 1998, 281, 269-272 and Griffin, B. A.; Adams, S. R.; Jones, J.; Tsien, R. Y. Methods Enzymol. 2000, 327, 565-578) for site-specific fluorescent labeling of recombinant proteins in living cells. The sequence Cys-Cys-Xaa-Xaa-Cys-Cys, where Xaa is an noncysteine amino acid, is genetically fused to or inserted within the protein, where it can be specifically recognized by a membrane-permeant fluorescein derivative with two As(III) substituents, FlAsH, which fluoresces only after the arsenics bind to the cysteine thiols. We now report kinetics and dissociation constants ( approximately 10(-11) M) for FlAsH binding to model tetracysteine peptides. Affinities in vitro and detection limits in living cells are optimized with Xaa-Xaa = Pro-Gly, suggesting that the preferred peptide conformation is a hairpin rather than the previously proposed alpha-helix. Many analogues of FlAsH have been synthesized, including ReAsH, a resorufin derivative excitable at 590 nm and fluorescing in the red. Analogous biarsenicals enable affinity chromatography, fluorescence anisotropy measurements, and electron-microscopic localization of tetracysteine-tagged proteins.

All-optical electrophysiology—spatially resolved simultaneous optical perturbation and measurement of membrane voltage—would open new vistas in neuroscience research. We evolved two archaerhodopsin-based voltage indicators, QuasAr1 and 2, which show improved brightness and voltage sensitivity, microsecond response times, and produce no photocurrent. We engineered a novel channelrhodopsin actuator, CheRiff, which shows improved light sensitivity and kinetics, and spectral orthogonality to the QuasArs. A co-expression vector, Optopatch, enabled crosstalk-free genetically targeted all-optical electrophysiology. In cultured neurons, we combined Optopatch with patterned optical excitation to probe back-propagating action potentials in dendritic spines, synaptic transmission, sub-cellular microsecond-timescale details of action potential propagation, and simultaneous firing of many neurons in a network. Optopatch measurements revealed homeostatic tuning of intrinsic excitability in human stem cell-derived neurons. In brain slice, Optopatch induced and reported action potentials and subthreshold events, with high signal-to-noise ratios. The Optopatch platform enables high-throughput, spatially resolved electrophysiology without use of conventional electrodes.

(2) to aromatic amino acids, typically Tyr. The resulting -stacking and increased local polarizability immediately adjacent to the chromophore are believed to be responsible for the ϳ20-nm shift to longer excitation and emission wavelengths (3). However, the changes in internal hydrogen bonding and steric packing also made the fluorescence more vulnerable to photobleaching (4, 5), decolorization by protonation (6 -10), and quenching by many anions (10 -12), of which chloride is the physiologically most relevant. These sensitivities can be exploited for specialized applications such as measuring fluorescence recovery after photobleaching and sensing pH and halide concentrations, but are deleterious for using YFPs either as simple fusion tags or as acceptors for fluorescence resonance energy transfer (FRET). YFPs are becoming very popular in such roles, particularly as partners for cyan fluorescent protein (CFP) mutants of GFP (2, 5, 13-15). CFPs and YFPs are spectroscopically well enough separated to be easily distinguishable in either excitation or emission spectra, yet the emission wavelengths of CFPs and excitation wavelengths of YFPs overlap well enough to make them good partners for FRET. They have largely superseded the initial pairing of blue mutants and improved green forms of GFP (16), because the blue mutants were too dim and photobleachable, and because shorter wavelengths generically excite more autofluorescence and raise more concerns of phototoxicity. Measurements of FRET between CFP and YFP are becoming increasingly common to monitor protein-protein interactions nondestructively in live cells (5,13,17). The potential partners are fused to CFP and YFP, respectively, and coexpressed in cells. Because FRET requires that the CFP and YFP be within a few nanameters of each other, it can detect proximity at molecular dimensions, with 2 orders of magnitude higher spatial resolution than simple co-localization of the two colors.

Over the past 20 years, protein engineering has been extensively used to improve and modify the fundamental properties of fluorescent proteins (FPs) with the goal of adapting them for a fantastic range of applications. FPs have been modified by a combination of rational design, structure-based mutagenesis, and countless cycles of directed evolution (gene diversification followed by selection of clones with desired properties) that have collectively pushed the properties to photophysical and biochemical extremes. In this review, we attempt to provide both a summary of the progress that has been made during the past two decades, and a broad overview of the current state of FP development and applications in mammalian systems.