Green fluorescent protein variants as ratiometric dual emission pH sensors. 2. Excited-state dynamics

In the preceding paper [Hanson, G. T., McAnaney, T. B., Park, E. S., Rendell, M. E. P., Yarbrough, D. K., Chu, S., Xi, L., Boxer, S. G., Montrose, M. H., and Remington, S. J. (2002) Biochemistry 41, 15477-15488], novel mutants of the green fluorescent protein (GFP) that exhibit dual steady-state emission properties were characterized structurally and discussed as potential intracellular pH probes. In this work, the excited-state dynamics of one of these new dual emission GFP variants, deGFP4 (C48S/S65T/H148C/T203C), is studied by ultrafast fluorescence upconversion spectroscopy. Following excitation of the high-energy absorption band centered at 398 nm and assigned to the neutral form of the chromophore, time-resolved emission was monitored from the excited state of both the neutral and intermediate anionic chromophores at both high and low pH and upon deuteration of exchangeable protons. The time-resolved emission dynamics and isotope effect appear to be very different from those of wild-type GFP [Chattoraj, M., King, B. A., Bublitz, G. U., and Boxer, S. G. (1996) Proc. Natl. Acad. Sci. U.S.A. 93, 8362-8367]; however, due to overlapping emission bands, the apparent difference can be analyzed quantitatively within the same framework used to describe GFP excited-state dynamics. The results indicate that the pH-sensitive steady-state emission characteristics of deGFP4 are a result of a pH-dependent modulation of the rate of excited-state proton transfer. At high pH, a rapid interconversion from the excited state of the higher energy neutral chromophore to the lower energy intermediate anionic chromophore is achieved by proton transfer. At low pH, excited-state proton transfer is slowed to the point where it is no longer rate limiting.     

Green fluorescent protein variants as ratiometric dual emission pH sensors. 3. Temperature dependence of proton transfer

In parts 1 and 2 of this series [Hanson, G. T., McAnaney, T. B., Park, E. S., Rendell, M. E. P., Yarbrough, D. K., Chu, S. Y., Xi, L. X., Boxer, S. G., Montrose, M. H., and Remington, S. J. (2002) Biochemistry 41, 15477-15488; McAnaney, T. B., Park, E. S., Hanson, G. T., Remington, S. J., and Boxer, S. G. (2002) Biochemistry 41, 15489-15494], we described the structure, excited-state dynamics, and applications of pH-sensitive, ratiometric dual emission green fluorescent protein (deGFP) variants with fluorescence emission that is modulated between blue (lambda(max) approximately equal 465 nm) and green (lambda(max) approximately equal 515 nm) depending on the pH of the bulk solvent. In this paper, we consider the energetic origin of the dual emission properties of these GFP variants by examining the temperature dependence of the steady-state absorption and fluorescence emission. In most cases, the quantum yield of the green emission decreased as the temperature was lowered, indicating that the excited-state proton transfer (ESPT) which produces the green emitting form is an activated process. The activation energies of ESPT, determined by modeling the quantum yields of both blue and green emissions between 260 and 298 K in the context of a simple photocycle, were found to be larger at low pH than at high pH. These results indicate that the ratiometric dual emission properties of deGFP mutants are due to this pH-sensitive ESPT rate, combined with a modulation of the ground-state neutral and anionic chromophore populations with pH. The time-resolved fluorescence of one of the deGFP mutants was studied in detail. The time-resolved emission spectra of this mutant are the first ultrafast spectra obtained for a GFP. These spectra demonstrate that the rising kinetics for green emission, considered a hallmark of ESPT, is the sum of the contribution from both the neutral and intermediate anionic forms of the chromophore at the probe wavelength and may not be observed in all mutants that undergo ESPT, depending on the relative contributions of the two forms.     

Ultrafast excited-state dynamics in the green fluorescent protein variant S65T/H148D. 1. Mutagenesis and structural studies

Wild type green fluorescent protein (wt-GFP) and the variant S65T/H148D each exhibit two absorption bands, A and B, which are associated with the protonated and deprotonated chromophores, respectively. Excitation of either band leads to green emission. In wt-GFP, excitation of band A ( approximately 395 nm) leads to green emission with a rise time of 10-15 ps, due to excited-state proton transfer (ESPT) from the chromophore hydroxyl group to an acceptor. This process produces an anionic excited-state intermediate I* that subsequently emits a green photon. In the variant S65T/H148D, the A band absorbance maximum is red-shifted to approximately 415 nm, and as detailed in the accompanying papers, when the A band is excited, green fluorescence appears with a rise time shorter than the instrument time resolution ( approximately 170 fs). On the basis of the steady-state spectroscopy and high-resolution crystal structures of several variants described herein, it is proposed that in S65T/H148D, the red shift of absorption band A and the ultrafast appearance of green fluorescence upon excitation of band A are due to a very short (相似文献   

Use of the green fluorescent protein and its mutants in quantitative fluorescence microscopy.

We have investigated properties relevant to quantitative imaging in living cells of five green fluorescent protein (GFP) variants that have been used extensively or are potentially useful. We measured the extinction coefficients, quantum yields, pH effects, photobleaching effects, and temperature-dependent chromophore formation of wtGFP, alphaGFP (F99S/M153T/V163A), S65T, EGFP (F64L/S65T), and a blue-shifted variant, EBFP (F64L/S65T/Y66H/Y145F). Absorbance and fluorescence spectroscopy showed little difference between the extinction coefficients and quantum yields of wtGFP and alphaGFP. In contrast, S65T and EGFP extinction coefficients made them both approximately 6-fold brighter than wtGFP when excited at 488 nm, and EBFP absorbed more strongly than the wtGFP when excited in the near-UV wavelength region, although it had a much lower quantum efficiency. When excited at 488 nm, the GFPs were all more resistant to photobleaching than fluorescein. However, the wtGFP and alphaGFP photobleaching patterns showed initial increases in fluorescence emission caused by photoconversion of the protein chromophore. The wtGFP fluorescence decreased more quickly when excited at 395 nm than 488 nm, but it was still more photostable than the EBFP when excited at this wavelength. The wtGFP and alphaGFP were quite stable over a broad pH range, but fluorescence of the other variants decreased rapidly below pH 7. When expressed in bacteria, chromophore formation in wtGFP and S65T was found to be less efficient at 37 degrees C than at 28 degrees C, but the other three variants showed little differences between 37 degrees C and 28 degrees C. In conclusion, no single GFP variant is ideal for every application, but each one offers advantages and disadvantages for quantitative imaging in living cells.     

Structural and spectral response of green fluorescent protein variants to changes in pH

The green fluorescent protein (GFP) from the jellyfish Aequorea victoria has become a useful tool in molecular and cell biology. Recently, it has been found that the fluorescence spectra of most mutants of GFP respond rapidly and reversibly to pH variations, making them useful as probes of intracellular pH. To explore the structural basis for the titration behavior of the popular GFP S65T variant, we determined high-resolution crystal structures at pH 8.0 and 4.6. The structures revealed changes in the hydrogen bond pattern with the chromophore, suggesting that the pH sensitivity derives from protonation of the chromophore phenolate. Mutations were designed in yellow fluorescent protein (S65G/V68L/S72A/T203Y) to change the solvent accessibility (H148G) and to modify polar groups (H148Q, E222Q) near the chromophore. pH titrations of these variants indicate that the chromophore pKa can be modulated over a broad range from 6 to 8, allowing for pH determination from pH 5 to pH 9. Finally, mutagenesis was used to raise the pKa from 6.0 (S65T) to 7.8 (S65T/H148D). Unlike other variants, S65T/H148D exhibits two pH-dependent excitation peaks for green fluorescence with a clean isosbestic point. This raises the interesting possibility of using fluorescence at this isosbestic point as an internal reference. Practical real time in vivo applications in cell and developmental biology are proposed.     

Development of a novel GFP-based ratiometric excitation and emission pH indicator for intracellular studies

We report on the development of the F64L/S65T/T203Y/L231H GFP mutant (E2GFP) as an effective ratiometric pH indicator for intracellular studies. E2GFP shows two distinct spectral forms that are convertible upon pH changes both in excitation and in emission with pK close to 7.0. The excitation of the protein at 488 and 458 nm represents the best choice in terms of signal dynamic range and ratiometric deviation from the thermodynamic pK. This makes E2GFP ideally suited for imaging setups equipped with the most widespread light sources and filter settings. We used E2GFP to determine the average intracellular pH (pH(i)) and spatial pH(i) maps in two different cell lines, CHO and U-2 OS, under physiological conditions. In CHO, we monitored the evolution of the pH(i) during mitosis. We also showed the possibility to target specific subcellular compartments such as nucleoli (by fusing E2GFP with the transactivator protein of HIV, (Tat) and nuclear promyelocytic leukemia bodies (by coexpression of promyelocytic leukemia protein).     

Crystallographic and energetic analysis of binding of selected anions to the yellow variants of green fluorescent protein

The fluorescence emission of yellow fluorescent proteins (YFPs) has been shown to respond rapidly and reversibly to changes in the concentration of some small anions such as halides; this allows for the use of YFPs as genetically encodable Cl(-) sensors that may be targeted to specific organelles in living cells. Fluorescence is suppressed due to protonation of the chromophore upon anion binding, with a stronger level of interaction at low pH values. At pH 6.0, the apparent dissociation constant (K(app)) for Cl(-) is 32 mM for YFP and 22 mM for YFP-H148Q, whereas at pH 7.5, K(app) is 777 mM and 154 mM, respectively. In the cytosol, YFP-H148Q appears most promising as a halide sensor due to its high degree of sensitivity towards I(-) (K(app)=23 mM at pH 7.5). To aid in the design of variants with improved levels of specificity and affinity for Cl(-), we solved apo and I(-)-bound crystal structures of YFP-H148Q to 2.1 A resolution. The halide-binding site is found near van der Waals contact with the chromophore imidazolinone oxygen atom, in a small buried cavity adjacent to Arg96, which provides electrostatic stabilization. The halide ion is hydrogen bonded to the phenol group of T203Y, consistent with a mutational analysis that indicates that T203Y is indispensible for tight binding. A series of conformational changes occurs in the amphiphilic site upon anion binding, which appear to be propagated to the beta-bulge region around residue 148 on the protein surface. Anion binding raises the chromophore pK(a) values, since delocalization of the phenolate negative charge over the chromophore skeleton is suppressed. Extraction of microscopic binding constants for the linked equilibrium between anion and proton binding indicates that anion selectivity by YFP is related to hydration forces. Specific suggestions to improve Cl(-) binding to YFP-H148Q based on size and hydration energy are proposed.     

Ultrafast excited-state dynamics in the green fluorescent protein variant S65T/H148D. 2. Unusual photophysical properties

In the preceding accompanying paper [Shu, X., et al. (2007) Biochemistry 46, 12005-12013], the 1.5 A resolution crystal structure of green fluorescent protein (GFP) variant S65T/H148D is presented, and the possible consequences of an unusual short hydrogen bond (相似文献   

The Single T65S Mutation Generates Brighter Cyan Fluorescent Proteins with Increased Photostability and pH Insensitivity

Cyan fluorescent proteins (CFP) derived from Aequorea victoria GFP, carrying a tryptophan-based chromophore, are widely used as FRET donors in live cell fluorescence imaging experiments. Recently, several CFP variants with near-ultimate photophysical performances were obtained through a mix of site-directed and large scale random mutagenesis. To understand the structural bases of these improvements, we have studied more specifically the consequences of the single-site T65S mutation. We find that all CFP variants carrying the T65S mutation not only display an increased fluorescence quantum yield and a simpler fluorescence emission decay, but also show an improved pH stability and strongly reduced reversible photoswitching reactions. Most prominently, the Cerulean-T65S variant reaches performances nearly equivalent to those of mTurquoise, with QY  = 0.84, an almost pure single exponential fluorescence decay and an outstanding stability in the acid pH range (pK_1/2 = 3.6). From the detailed examination of crystallographic structures of different CFPs and GFPs, we conclude that these improvements stem from a shift in the thermodynamic balance between two well defined configurations of the residue 65 hydroxyl. These two configurations differ in their relative stabilization of a rigid chromophore, as well as in relaying the effects of Glu222 protonation at acid pHs. Our results suggest a simple method to greatly improve numerous FRET reporters used in cell imaging, and bring novel insights into the general structure-photophysics relationships of fluorescent proteins.     

Ultrafast excited-state dynamics in the green fluorescent protein variant S65T/H148D. 3. Short- and long-time dynamics of the excited-state proton transfer

Steady-state emission, femtosecond pump-probe spectroscopy, and time-correlated single-photon counting (TCSPC) measurements were used to study the photophysics and the excited-state proton transfer (ESPT) reactions in the green fluorescent protein (GFP) variant S65T/H148D at three pH values: 6.0, 7.9, and 9.5. Selective mutation of GFP caused a dramatic change in the steady-state and excited-state behavior as compared to the wild-type GFP (wt-GFP) studied earlier. An excitation wavelength dependence of the quantum yield of the strong emission band at 510 nm (I* band) indicates the competition between adiabatic and non-adiabatic excited-state proton-transfer reactions. Pump-probe measurements show that the signal buildup probed at 510 nm is biphasic, where 0.8 of the signal amplitude is ultrashort, <150 fs, and 0.2 of the signal decreases with a approximately 10 ps time constant. This effect is a summary result of adiabatic ESPT to the carboxylate group of Asp148 and nonradiative processes. When compared with the luminescence of wt-GFP, the steady-state intensity at 450 nm is lower by a factor of about 10. This very weak emission at 450 nm has a nonexponential decay. It is relatively pH insensitive and, at about 25 ps, is almost twice as long as in wt-GFP. The former exhibits a surprisingly small kinetic deuterium isotope effect (KDIE), compared with the KDIE of about 5 for wt-GFP. Such weak proton dependence may indicate that this emission comes from the species not directly involved in the ESPT. In contrast, pH and H/D isotope dependence of the intense nonexponential luminescence decay of the S65T/H148D deprotonated form measured at 510 nm may result from an isomerization-induced deactivation that is accompanied by the proton recombination quenching. The data are complementary to the femtosecond time-resolved emission data obtained by ultrafast fluorescence up-conversion spectroscopy, found in the preceding paper (Shi et al.). The spectroscopic results are discussed on the basis of the detailed X-ray structure of the mutant published in the preceding paper (Shu et al.).     

Transducer-binding and transducer-mutations modulate photoactive-site-deprotonation in sensory rhodopsin I.

Sensory rhodopsin I (SRI) is a seven-transmembrane helix retinylidene protein that mediates color-sensitive phototaxis responses through its bound transducer HtrI in the archaeon Halobacterium salinarum. Deprotonation of the Schiff base attachment site of the chromophore accompanies formation of the SRI signaling state, S(373). We measured the rate of laser flash-induced S(373) formation in the presence and absence of HtrI, and the effects of mutations in SRI or HtrI on the kinetics of this process. In the absence of HtrI, deprotonation occurs rapidly (halftime 10 micros) if the proton acceptor Asp76 is ionized (pK(a) = approximately 7), and only very slowly (halftime > 10 ms) when Asp76 is protonated. Transducer-binding, although it increases the pK(a) of Asp76 so that it is protonated throughout the range of pH studied, results in a first order, pH-independent rate of S(373) formation of approximately 300 micros. Therefore, the complexation of HtrI facilitates the proton-transfer reaction, increasing the rate approximately 50-fold at pH6. Arrhenius analysis shows that HtrI-binding accelerates the reaction primarily by an entropic effect, suggesting HtrI constrains the SRI molecule in the complex. Function-perturbing mutations in SRI and HtrI also alter the rate of S(373) formation and the lambda(max) of the parent state as assessed by laser flash-induced kinetic difference spectroscopy, and shifts to longer wavelength are correlated with slower deprotonation. The data indicate that HtrI affects electrostatic interactions of the protonated Schiff base and not only receives the signal from SRI but also optimizes the photochemical reaction process for SRI signaling.     

Molecular basis for pH sensitivity and proton transfer in green fluorescent protein: protonation and conformational substates from electrostatic calculations.

We performed a theoretical study to elucidate the coupling between protonation states and orientation of protein dipoles and buried water molecules in green fluorescent protein, a versatile biosensor for protein targeting. It is shown that the ionization equilibria of the wild-type green fluorescent protein-fluorophore and the internal proton-binding site E222 are mutually interdependent. Two acid-base transitions of the fluorophore occur in the presence of neutral (physiologic pH) and ionized (pH > 12) E222, respectively. In the pH-range from approximately 8 to approximately 11 ionized and neutral sites are present in constant ratio, linked by internal proton transfer. The results indicate that modulation of the internal proton sharing by structural fluctuations or chemical variations of aligning residues T203 and S65 cause drastic changes of the neutral/anionic ratio-despite similar physiologic fluorophore pK(a) s. Moreover, we find that dipolar heterogeneities in the internal hydrogen-bond network lead to distributed driving forces for excited-state proton transfer. A molecular model for the unrelaxed surrounding after deprotonation is discussed in relation to pathways providing fast ground-state recovery or slow stabilization of the anion. The calculated total free energy for excited-state deprotonation ( approximately 19 k(B)T) and ground-state reprotonation ( approximately 2 k(B)T) is in accordance with absorption and emission data (相似文献   

An alternative excited-state proton transfer pathway in green fluorescent protein variant S205V

Wild-type green fluorescent protein (wt-GFP) has a prominent absorbance band centered at approximately 395 nm, attributed to the neutral chromophore form. The green emission arising upon excitation of this band results from excited-state proton transfer (ESPT) from the chromophore hydroxyl, through a hydrogen-bond network proposed to consist of a water molecule and Ser205, to Glu222. Although evidence for Glu222 as a terminal proton acceptor has already been obtained, no evidence for the participation of Ser205 in the proton transfer process exists. To examine the role of Ser205 in the proton transfer, we mutated Ser205 to valine. However, the derived GFP variant S205V, upon excitation at 400 nm, still produces green fluorescence. Time-resolved emission spectroscopy suggests that ESPT contributes to the green fluorescence, and that the proton transfer takes place approximately 30 times more slowly than in wt-GFP. The crystal structure of S205V reveals rearrangement of Glu222 and Thr203, forming a new hydrogen-bonding network. We propose this network to be an alternative ESPT pathway with distinctive features that explain the significantly slowed rate of proton transfer. In support of this proposal, the double mutant S205V/T203V is shown to be a novel blue fluorescent protein containing a tyrosine-based chromophore, yet is incapable of ESPT. The results have implications for the detailed mechanism of ESPT and the photocycle of wt-GFP, in particular for the structures of spectroscopically identified intermediates in the cycle.     

Base catalysis of chromophore formation in Arg96 and Glu222 variants of green fluorescent protein

In green fluorescent protein (GFP), chromophore biosynthesis is initiated by a spontaneous main-chain condensation reaction. Nucleophilic addition of the Gly67 amide nitrogen to the Ser65 carbonyl carbon is catalyzed by the protein fold and leads to a heterocyclic intermediate. To investigate this mechanism, we substituted the highly conserved residues Arg96 and Glu222 in enhanced GFP (EGFP). In the R96M variant, the rate of chromophore formation is greatly reduced (time constant = 7.5 x 10(3) h, pH 7) and exhibits pH dependence. In the E222Q variant, the rate is also attenuated at physiological pH (32 h, pH 7) but is accelerated severalfold beyond that of EGFP at pH 9-10. In contrast, EGFP maturation is pH-independent and proceeds with a time constant of 1 h (pH 7-10). Mass spectrometric results for R96M and E222Q indicate accumulation of the pre-cyclization state, consistent with rate-limiting backbone condensation. The pH-rate profile implies that the Glu222 carboxylate titrates with an apparent pK(a) of 6.5 in R96M and that the Gly67 amide nitrogen titrates with an apparent pK(a) of 9.2 in E222Q. These data suggest a model for GFP chromophore synthesis in which the carboxylate of Glu222 plays the role of a general base, facilitating proton abstraction from the Gly67 amide nitrogen or the Tyr66 alpha-carbon. Arg96 fulfills the role of an electrophile by lowering the respective pK values and stabilizing the alpha-enolate. Modulating the base strength of the proton-abstracting group may aid in the design of fast-maturing GFPs with improved characteristics for real-time monitoring of cellular events.     

The utility of green fluorescent protein in transgenic plants

The green fluorescent protein (GFP) from the jellyfish Aequorea victoria has proven to be a powerful tool in plant genetic transformation studies. This paper reviews the history and the progression of the expression of GFP variants in transgenic plants. The distinguishing features of the most useful GFPs, such as those including the S65T chromophore mutation and those with dual excitation peaks, are discussed. The review also focuses on the utility of GFP as a visual selectable marker in aiding the plant transformation process; GFP has been more important in monocot transformation compared with dicot transformation. Finally, the potential utility of new fluorescent proteins is speculated upon.     

Green fluorescent protein ground states: the influence of a second protonation site near the chromophore

The photophysical properties of most green fluorescent protein mutants (GFPs) are strongly affected by pH. This effect must be carefully taken into account when using GFPs as fluorescent probes or indicators. Usually, the pH-dependence of GFPs is rationalized on the basis of the ionization equilibrium of the chromophore phenol group. Yet many different mutants show spectral behavior that cannot be explained by ionization of this group alone. In this study, we propose a general model of protonation comprising two ionization sites (2S model). Steady-state optical measurements at different pH and temperature and pH-jump relaxation experiments were combined to highlight the thermodynamic and kinetic properties of paradigmatically different GFP variants. Our experiments support the 2S model. For the case of mutants in which E222 is the second protonation site, thermodynamic coupling between this residue's and the chromophore's ionization reactions was demonstrated. In agreement with the 2S model predictions, X-ray analysis of one of these mutants showed the presence of two chromophore populations at high pH.     

Molecular dynamics simulations of enhanced green fluorescent proteins: effects of F64L,S65T and T203Y mutations on the ground-state proton equilibria

Molecular dynamics simulations with the Amber force field are carried out to study two mutants of the green fluorescent protein (GFP), namely EGFP (F64L/S65T) and T203Y-EGFP (E(2)GFP). Those variants display an opposite equilibrium between the structural A and B states, associated with neutral and anionic protonation forms of the chromophore. Configurations of those two states are simulated for each variant and the energetics of their equilibrium in the two mutants is studied by evaluating the change in the relative free energy of A and B states (DeltaG(AB)) upon T203Y mutation. The resulting DeltaDeltaG(AB) agrees with the value inferred from absorption measurements. A comparison of the hydrogen bond network around the chromophore rationalizes the different population of state A and B in EGFP and E(2)GFP. On the basis of structural and energetic considerations, a mechanism for destabilization of the neutral chromophore in S65T mutants is proposed. Simulations of the B state of the S65T variant and of WT GFP are also performed for comparison and to test the force field parameters of the chromophore derived for the present calculations. Possible paths of proton transfer leading to nonfluorescent states of the chromophore are discussed in light of the photodynamical behavior of GFP, as revealed by fluorescence correlation spectroscopy and single-molecule experiments.     

Spectroscopic and structural study of proton and halide ion cooperative binding to gfp

This study reports the influence of halogens on fluorescence properties of the Aequorea victoria Green Fluorescent Protein variant S65T/T203Y (E(2)GFP). Halide binding forms a specific nonfluorescent complex generating a substantial drop of the fluorescence via static quenching. Spectroscopic analysis under different solution conditions reveals high halogen affinity, which is strongly dependent on the pH. This evidences the presence in E(2)GFP of interacting binding sites for halide ions and for protons. Thermodynamic link and cooperative interaction are assessed demonstrating that binding of one halide ion is associated with the binding of one proton in a cooperative fashion with the formation, in the pH range 4.5-10, of a single fully protonated E(2)GFP.halogen complex. To resolve the structural determinants of E(2)GFP sensitivity to halogens, high-resolution crystallographic structures were obtained for the halide-free and I(-), Br(-), and Cl(-) bound E(2)GFP. Remarkably the first high-resolution (1.4 A) crystallographic structure of a chloride-bound GFP is reported. The chloride ion occupies a specific and unique binding pocket in direct contact (3.4 A) with the chromophore imidazolidinone aromatic ring. Unanticipated flexibility, strongly modulated by halide ion interactions, is observed in the region surrounding the chromophore. Furthermore molecular dynamics simulations identified E222 residue (along with the chromophore Y66 residue) being in the protonated state when E(2)GFP.halogen complex is formed. The impact of these results on high-sensitivity biosensor design will be discussed.     

Observation of the protonated semiquinone intermediate in isolated reaction centers from Rhodobacter sphaeroides: implications for the mechanism of electron and proton transfer in proteins.

A proton-activated electron transfer (PAET) mechanism, involving a protonated semiquinone intermediate state, had been proposed for the electron-transfer reaction k(2)AB [Q(A)(-)(*)Q(B)(-)(*) + H(+) <--> Q(A)(-)(*)(Q(B)H)(*) --> Q(A)(Q(B)H)(-)] in reaction centers (RCs) from Rhodobacter sphaeroides [Graige, M. S., Paddock, M. L., Bruce, M. L., Feher, G., and Okamura, M. Y. (1996) J. Am. Chem. Soc. 118, 9005-9016]. Confirmation of this mechanism by observing the protonated semiquinone (Q(B)H)(*) had not been possible, presumably because of its low pK(a). By replacing the native Q(10) in the Q(B) site with rhodoquinone (RQ), which has a higher pK(a), we were able to observe the (Q(B)H)(*) state. The pH dependence of the semiquinone optical spectrum gave a pK(a) = 7.3 +/- 0.2. At pH < pK(a), the observed rate for the reaction was constant and attributed to the intrinsic electron-transfer rate from Q(A)(-)(*) to the protonated semiquinone (i.e., k(2)AB = k(ET)(RQ) = 2 x 10(4) s(-)(1)). The rate decreased at pH > pK(a) as predicted by the PAET mechanism in which fast reversible proton transfer precedes rate-limiting electron transfer. Consequently, near pH 7, the proton-transfer rate k(H) > 10(4) s(-)(1). Applying the two step mechanism to RCs containing native Q(10) and taking into account the change in redox potential, we find reasonable values for the fraction of (Q(B)H)(*) congruent with 0.1% (consistent with a pK(a)(Q(10)) of approximately 4.5) and k(ET)(Q(10)) congruent with 10(6) s(-)(1). These results confirm the PAET mechanism in RCs with RQ and give strong support that this mechanism is active in RCs with Q(10) as well.     

Green fluorescent protein as a noninvasive intracellular pH indicator.

It was found that the absorbance and fluorescence of green fluorescent protein (GFP) mutants are strongly pH dependent in aqueous solutions and intracellular compartments in living cells. pH titrations of purified recombinant GFP mutants indicated >10-fold reversible changes in absorbance and fluorescence with pKa values of 6.0 (GFP-F64L/S65T), 5.9 (S65T), 6.1 (Y66H), and 4.8 (T203I) with apparent Hill coefficients of 0.7 for Y66H and approximately 1 for the other proteins. For GFP-S65T in aqueous solution in the pH range 5-8, the fluorescence spectral shape, lifetime (2.8 ns), and circular dichroic spectra were pH independent, and fluorescence responded reversibly to a pH change in <1 ms. At lower pH, the fluorescence response was slowed and not completely reversed. These findings suggest that GFP pH sensitivity involves simple protonation events at a pH of >5, but both protonation and conformational changes at lower pH. To evaluate GFP as an intracellular pH indicator, CHO and LLC-PK1 cells were transfected with cDNAs that targeted GFP-F64L/S65T to cytoplasm, mitochondria, Golgi, and endoplasmic reticulum. Calibration procedures were developed to determine the pH dependence of intracellular GFP fluorescence utilizing ionophore combinations (nigericin and CCCP) or digitonin. The pH sensitivity of GFP-F64L/S65T in cytoplasm and organelles was similar to that of purified GFP-F64L/S65T in saline. NH4Cl pulse experiments indicated that intracellular GFP fluorescence responds very rapidly to a pH change. Applications of intracellular GFP were demonstrated, including cytoplasmic and organellar pH measurement, pH regulation, and response of mitochondrial pH to protonophores. The results establish the application of GFP as a targetable, noninvasive indicator of intracellular pH.