Monitoring of Actin Unfolding by Room Temperature Tryptophan Phosphorescence

Slow intramolecular mobility of native and inactivated actin from rabbit skeletal muscle during the process of protein unfolding induced by GdnHCl was studied using tryptophan room temperature phosphorescence (RTP). By this method, the conclusion was confirmed that an essentially unfolded intermediate preceded the formation of inactivated actin [Turoverov et al. Biochemistry (2002) 41, 1014-1019]. It was found that the kinetic intermediate generated at the early stage of protein denaturation has no tryptophan RTP, suggesting the high lability of its structure. Symbate changes of integral intensity and the mean lifetime of RTP during the U* --> I transition suggests a gradual increase of the number of monomers incorporated in the associate (U* --> I(1)... --> I(n)... --> I(15)), which is accompanied by an increase of structural rigidity. The rate of inactivated actin formation (I identical with I(15)) is shown to increase with the increase of protein concentration. It is shown that, no matter what the means of inactivation, actin transition to the inactivated state is accompanied by a significant increase of both integral intensity and the mean lifetime of RTP, suggesting that inactivated actin has a rigid structure.

[Kinetics of inactivation of lectin: analysis of a method of tryptophan phosphorescence at room temperature]

The internal dynamics of muscle actin during inactivation induced by guanidine hydrochloride (0.5-1.8 M) was studied by the method of room-temperature tryptophan phosphorescence (RTTP). It was shown that the essentially unfolded actin intermediate, which appears within the first minutes of incubation with guanidine hydrochloride, exhibits no RTTP, suggesting a high lability of its structure. Subsequent accumulation of associates of inactivated actin is accompanied by a significant increase in the intensity and decay time of RTTP, which is caused by the rigidity of the structure of inactivated actin. The kinetic dependencies of the intensity and lifetime of RTTP of actin during its inactivation depended on the concentration of the protein and guanidine hydrochloride.

High stability of Discosoma DsRed as compared to Aequorea EGFP

Comparative analysis of conformational stabilities was performed for two widely used genetic reporters, EGFP and DsRed, proteins exhibiting similar beta-can folds, but possessing different oligomeric organization and chromophore structures. Two factors affecting protein stability in vitro, such as elevated temperatures and a chaotropic agent guanidine hydrochloride, were studied. In vivo tolerance of the fluorescence proteins to proteasomal-based degradation was studied in insect and mammalian cells, and in Xenopus embryos. The apparent rate constants of thermal and GdmCl-induced denaturation were several orders of magnitude lower for DsRed than for EGFP. DsRed lifetimes severalfold longer than those of EGFP were observed in cultured cells and in embryos. The remarkable fluorescence stability of DsRed under the all conditions that have been studied is attributed to a significant extent to its tetrameric organization. Therefore, DsRed can be used as a genetic reporter and advanced population marker with a significantly extended intracellular lifespan.

Expression of recombinant GFP-actin fusion protein in the methylotrophic yeast Pichia pastoris

The integrative vector pPIC3 for the yeast Pichia pastoris and a cDNA fragment encoding a fusion protein consisting of green fluorescent protein (GFP) and actin 5C of the fruit fly Drosophila melanogaster were used to construct a pPIC3-GFP-actin 5C expression plasmid. The P. pastoris host strain GS115 was transformed with the pPIC3-GFP-actin 5C carrying HIS4 as a selective marker. The transformants were selected on a histidine-deficient medium, and were shown to contain the gene of GFP-actin 5C fusion protein. Expression was induced by cultivation of the transformant cells in a methanol-containing medium. Production of the fusion protein in the yeast was detected by the bright green fluorescence of the GFP tag. The pattern of yeast cytoskeleton labeling by the fusion indicated proper folding and functioning of GFP-actin 5C in a heterologous system in vivo. After cell destruction, purification of GFP-actin 5C was performed by DNase I-Sepharose. Efficient binding of the chimera to the DNase I indicated nativity of the actin 5C fusion in vitro. SDS electrophoresis and further Western blot confirmed the purified protein to exhibit the expected molecular mass of about 70 kDa. The recombinant GFP-actin 5C was used to produce polyclonal antibodies, which had not been reported so far but are extremely needed for immuno-labeling and isolation of wild-type and mutant forms of actin 5C.

The place of inactivated actin and its kinetic predecessor in actin folding-unfolding

The kinetics of actin unfolding induced by guanidine hydrochloride of different concentrations was studied. The parametric representation of the kinetic dependencies of tryptophan fluorescence intensity changes recorded at two wavelengths allowed us to detect and characterize a new essentially unfolded kinetic intermediate. Its characteristics suggested that this intermediate state is a premolten globule. It was shown that the equilibrium transition between inactivated and completely unfolded states is also a two-step process and proceeds via an essentially unfolded kinetic intermediate. The new kinetic pathway of actin unfolding--refolding was proposed. According to it, the founded essentially unfolded kinetic state is the on-pathway intermediate, while inactivated actin is the off-pathway misfolded state stabilized by aggregation of partially folded macromolecules of protein.

Unraveling multistate unfolding of rabbit muscle creatine kinase

GdmCl-induced unfolding of rabbit muscle creatine kinase, CK, has been studied by a variety of physico-chemical methods including near and far UV CD, SEC, intrinsic fluorescence (intensity, anisotropy and lifetime) as well as intensity and lifetime of bound ANS fluorescence. The formation of several stable unfolding intermediates, some of which were not observed previously, has been established. This was further confirmed by representation of fluorescence data in terms of "phase diagram", i.e. I(lambda1) versus I(lambda2) dependence, where I(lambda1) and I(lambda2) are fluorescence intensity values measured on wavelengths lambda(1) and lambda(2) under the different experimental conditions for a protein undergoing structural transformations. The unfolding behavior of CK was shown to be strongly affected by association of partially folded intermediates. A model of CK unfolding, which takes into account both structural perturbations and association of partially folded intermediates has been elaborated.

Kinetics of actin unfolding induced by guanidine hydrochloride

The kinetics of actin unfolding induced by guanidine hydrochloride has been studied. On the basis of obtained experimental data a new kinetic pathway of actin unfolding was proposed. We have shown that the transition from native to inactivated actin induced by guanidine hydrochloride (GdnHCl) passes through essential unfolding of the protein. This means that inactivated actin should be considered as the off-pathway species rather than an intermediate conformation between native and completely unfolded states of actin, as has been assumed earlier. The rate constants of the transitions that give rise to the inactivated actin were determined. At 1.0-2.0 M GdnHCl the value of the rate constant of the transition from native to essentially unfolded actin exceeds that of the following step of inactivated actin formation. It leads to the accumulation of essentially unfolded macromolecules early in the unfolding process, which in turn causes the minimum in the time dependencies of tryptophan fluorescence intensity, parameter A, characterizing the intrinsic fluorescence spectrum position, and tryptophan fluorescence anisotropy.

Partially folded conformations in the folding pathway of bovine carbonic anhydrase II: a fluorescence spectroscopic analysis

GdmCl-, urea-, and pH-induced unfolding pathways of bovine carbonic anhydrase II have been analyzed by using changes induced by different denaturing agents in intensity, anisotropy, life time, and parameter A value of intrinsic fluorescence as well as intensity and life time of ANS (ammonium salt of 8-anilinonaphthalene-1-sulfonic acid) fluorescence. The formation of several stable unfolding intermediates, some of which were not observed previously, has been established. This was further confirmed by representation of fluorescence data in terms of a "phase diagram", that is, I(lambda1) versus I(lambda2) dependence, where I(lambda1) and I(lambda2) are the fluorescence intensity values measured at wavelengths lambda(1) and lambda(2), respectively.

[Intrinsic UV-fluorescence of lysozyme and microenvironment of its tryptophan residues]

The localization of tryptophan residues in hen egg-white lysozyme macromolecule was studied on the basis of the known 3D structure. The polarity and packing density of their microenvironments were evaluated. All residues that can affect the tryptophan fluorescence were revealed. It was shown that the orientation of these active groups relative to the indole ring of tryptophan plays a dramatic role in the efficiency of their influence. Tryptophan--tryptophan nonradiative energy transfer was evaluated from distances between tryptophan residues and their mutual orientation. The conformation of the side chains of tryptophan residues was determined. Special attention was paid to microenvironment of Trp108 responsible for the minor absorption band at 305 nm.

[Tryptophan phosphorescence of nascent and inactivated actin at the room temperature]

Millisecond internal dynamics of native and inactivated actin from rabbit skeletal muscle was examined using room temperature phosphorescence. Inactivated actin was prepared by incubation of G-actin at 70 degrees C, by treatment with 4 M urea or 1.5 M guanidinium hydrochloride, renaturation from fully unfolded state or by Ca2+ ion removal. It was shown that inactivation of actin, irrespective of the denaturation procedure applied, leads to a sharp decrease of millisecond fluctuations of the protein structure. Restriction of the slow intramolecular mobility in inactivated actin can result from changes of the protein conformation and/or specific association of macromolecules.

Understanding the contribution of individual tryptophan residues to intrinsic lysozyme fluorescence

Abstract:

Packing density and other properties of the microenvironment of tryptophan residues in hen egg-white lysozyme macromolecule have been studied on the basis of the known 3D structure of this protein. Results presented here suggest that the efficiency of action of the group which, in principal, can affect tryptophan fluor scence depends not only on the distance between this group and the indole ring, but to a great extent upon the location of this group relative to the indole ring.

Effect of self-association on the structural organization of partially folded proteins: inactivated actin

The propensity to associate or aggregate is one of the characteristic properties of many nonnative proteins. The aggregation of proteins is responsible for a number of human diseases and is a significant problem in biotechnology. Despite this, little is currently known about the effect of self-association on the structural properties and conformational stability of partially folded protein molecules. G-actin is shown to form equilibrium unfolding intermediate in the vicinity of 1.5 M guanidinium chloride (GdmCl). Refolding from the GdmCl unfolded state is terminated at the stage of formation of the same intermediate state. An analogous form, known as inactivated actin, can be obtained by heat treatment, or at moderate urea concentration, or by the release of Ca(2+). In all cases actin forms specific associates comprising partially folded protein molecules. The structural properties and conformational stability of inactivated actin were studied over a wide range of protein concentrations, and it was established that the process of self-association is rather specific. We have also shown that inactivated actin, being denatured, is characterized by a relatively rigid microenvironment of aromatic residues and exhibits a considerable limitation in the internal mobility of tryptophans. This means that specific self-association can play an important structure-forming role for the partially folded protein molecules.

Contribution of separate tryptophan residues to intrinsic fluorescence of actin. Analysis of 3D structure

The location of tryptophan residues in the actin macromolecule was studied on the basis of the known 3D structure. For every tryptophan residue the polarity and packing density of their microenvironments were evaluated. To estimate the accessibility of the tryptophan residues to the solvent molecules it was proposed to analyze the radial dependence of the packing density of atoms in the macromolecule about the geometric center of the indole rings of the tryptophan residues. The proposed analysis revealed that the microenvironment of tryptophan residues Trp-340 and Trp-356 has a very high density. So these residues can be regarded as internal and inaccessible to solvent molecules. Their microenvironment is mainly formed by non-polar groups of protein. Though the packing density of the Trp-86 microenvironment is lower, this tryptophan residue is apparently also inaccessible to solvent molecules, as it is located in the inner region of macromolecule. Tryptophan residue Trp-79 is external and accessible to the solvent. All residues that can affect tryptophan fluorescence were revealed. It was found that in the close vicinity of tryptophan residues Trp-79 and Trp-86 there are a number of sulfur atoms of cysteine and methionine residues that are known to be effective quenchers of tryptophan fluorescence. The most essential is the location of SG atom of Cys-10 near the NE1 atom of the indole ring of tryptophan residue Trp-86. On the basis of microenvironment analysis of these tryptophan residues and the evaluation of energy transfer between them it was concluded that the contribution of tryptophan residues Trp-79 and Trp-86 must be low. Intrinsic fluorescence of actin must be mainly determined by two other tryptophan residues--Trp-340 and Trp-356. It is possible that the unstrained conformation of tryptophan residue Trp-340 and the existence of aromatic rings of tyrosine and phenylalanine and proline residues in the microenvironments of tryptophan residues Trp-340 and Trp-356 are also essential to their blue fluorescence spectrum.

The structure and dynamics of partially folded actin

Steady-state and time-resolved intrinsic fluorescence, fluorescence quenching by acrylamide, and surface testing by hydrophobic label ANS were used to study the structure of inactivated alpha-actin. The results are discussed together with that of earlier experiments on sedimentation, anisotropy of fluorescence, and CD spectrum in the near- and far-UV regions. A dramatic increase in ANS binding to inactivated actin in comparison with native and unfolded protein indicates that the inactivated actin has solvent-exposed hydrophobic clusters on the surface. It results in specific association of actin macromolecules (sedimentation constants for native and inactivated actin are 3 and 20 S, respectively) and, consequently, in irreversibility of native-inactivated actin transition. It was found that, though the fluorescence spectrum of inactivated actin is red-shifted, the efficiency of the acrylamide collision quenching is even lower than that of the intact protein. It suggests that tryptophan residues of inactivated actin are located in the inner region of protein formed by polar groups, which are highly packed. It correlates with the pronounced near-UV CD spectrum of inactivated actin. The experimentally found tryptophan fluorescence lifetimes allowed evaluation rotational correlation times on the basis of Perrin plots. It is found that oscillations of tryptophan residues in inactivated actin are restricted in comparison with native one. The inactivated actin properties were invariant with experimental conditions (ionic strength, the presence of reducing agents), the way of inactivation (Ca2+ and/or ATP removal, heating, 3-5 M urea or 1.5 M GdmCl treatment), and protein concentration (within the limits 0.005-1.0 mg/mL). The same state of actin appears on the refolding from the completely unfolded state. Thermodynamic stability, pronounced secondary structure, and the existing hydrophobic clusters, tested by ANS fluorescence and reversibility of transition inactivated-unfolded forms, allowed us to suggest that inactivated actin can be intermediate in the folding-unfolding pathway.

Unusual Combination Of The Distorted Structure And Frozen Internal Mobility In Inactivated Actin Molecule

Abstract:

The structural properties of inactivated actin were studied by means of circular dichroism and fluorescence techniques. We have shown that inactivated actin, being denatured, is characterized by a relatively rigid microenvironment of aromatic amino acid residues and exhibits a considerable limitation in the internal mobility oftryptophans. This can be due to the specific self-association of partially folded protein molecules.

[Features of the structural organization of inactivated actin--an intermediate form of the protein during the folding-unfolding process]

The structure of inactivated actin was studied by the methods of intrinsic fluorescence upon stationary and pulse excitation, selective fluorescence quenching with acrylamide, and testing the protein surface with a hydrophobic probe, 8-anilino-1-naphthalenesulfonic acid (ANS). The results are discussed along with earlier data on actin sedimentation, near- and far-UV CD spectra, and fluorescence anisotropy. The thermodynamic stability of inactivated actin, the presence of a secondary structure characteristic of the native protein, and the reversibility of the inactivated actin-completely unfolded actin transition allow inactivated actin to be considered an intermediate form in the process of protein folding into the native globular structure. In vitro actin inactivation is accompanied by specific association of actin macromolecules resulting in the formation of homogeneous stable complexes. The tendency toward aggregation (or specific association, in the case of actin), which is determined by the presence of extended hydrophobic clusters on the molecule surface, appears to be one of the intrinsic properties of any protein in the intermediate state. The mobility of the amino acid side chains in the inactivated actin differs considerably from that in the completely unfolded actin. The relaxation properties of the microenvironment of tryptophan residues determine relatively long-wave fluorescence spectra of the inactivated actin. However, the mobility observed is insufficient to compensate the asymmetry of the microenvironment of aromatic residues, which is confirmed by a characteristic and intense CD spectrum in the near-UV region. The mobility of the indole rings of tryptophans located in the internal regions of the inactivated actin that are solvent-inaccessible although polar is even considerably lower than that in the native actin.

[Intrinsic fluorescence of globular actin. Features of localization of tryptophan residues]

The contribution of individual Trp residues to alpha-actin fluorescence was evaluated by means of an analysis of their microenvironment, which was done on the basis of PIR-International protein sequence database information. The contribution of Trp79 and Trp86 was shown to be low due to an effective nonradiating energy transfer according to the inductive resonance mechanism between the Trp residues and the fluorescence quenching of Trp86 by S gamma of Cys10, an efficient fluorescence quencher. The intrinsic fluorescence of actin was found to be determined mainly by Trp340 and Trp356, which are internal, inaccessible to solvent, and have a high density microenvironment formed mainly by nonpolar groups of protein. It is possible that the side chain conformation of Trp340 (t-isomer; chi 1 190 degrees, chi 2 89 degrees), aromatic rings of Tyr and Phe residues, and Pro residues in the microenvironment of Trp340 and Trp356 substantially contribute to the short-wavelength fluorescence spectrum of actin.

[A complex of apparatus and programs for the measurement of spectral, polarization and kinetic characteristics of fluorescence in solution]

A description of instruments for investigation of spectral, polarizational and kinetic characteristics of fluorescence that have been designed and constructed, and are in operation at the Institute of Cytology is presented. The spectrofluorimeter with the steady state excitation allows recording fluorescence spectra, spectral parameter A, the value of fluorescence polarization, and fluorescence polarization excitation and emission spectra in the wavelength range of 250-500 and 300-600 nm, respectively. The pulse spectrofluorimeter is designed to record fluorescence decay curves and fluorescence anisotropy decay curves in the same spectral region. A procedure of decay curve deconvolution in multiexponential approximation within the frame of the least square method, using the Marquardt algorithm, is described in detail. The lower limit of the determined fluorescence life-time is 0.5 ns.

[What determines the characteristics of the intrinsic UV-fluorescence of proteins? Analysis of the properties of the microenvironment and features of the localization of their tryptophan residues]

To elucidate the dependence of protein intrinsic fluorescence characteristics on the microenvironment of their tryptophan residue localization, more than a hundred tryptophan residues of a number of proteins were analysed and compared with experimental data on their intrinsic fluorescence. Some factors were revealed, which determine the fluorescence spectrum position of certain tryptophan residues and their contribution to the total protein fluorescence. Specifically, the role of aromatic residues and proline, as well as of tryptophan residue side chain conformation, in the formation of the unique blue fluorescence spectrum of a number of proteins was demonstrated. It was shown that the quenching effect of sulphur atoms of cysteine and methionine, imidazole rings of histidine, guanyl groups of arginine, etc. depends not only on their distance from the indole ring of the tryptophan residue but, to a great extent, on their orientation to indole ring.

[Spectrofluorometric analysis of protein fractions of blood serum of health donors and patients with kidney diseases]

A comparison was made of ultra-violet fluorescence characteristics of albumin enriched protein fractions of serum blood of 35 glomerulonephritis (GN) patients, 30 uremic haemodialysis patients, and 40 donors. It has been found that fluorescence spectra of the albumin enriched fractions of serum from GN patients are blue shifted as compared with those of donors' serum preparations. At the same time fluorescence spectra of the albumin enriched fractions of donors' serum are similar, while those of GN patients vary significantly. The ratio of fluorescence intensities at 320 and 365 nm (A = I320/I365), characterizing the spectrum position is 1.27 +/- 0.05, for protein preparations of donors, while that for GN patients varies within the limits of 1.3-2.1. Fluorescence spectra of protein fractions from blood serum of uremic patients are red shifted (A = 0.77-1.29) in comparison with those of donors' blood. Chromatographic investigations show that protein preparations of GN patients' blood contain monomers and dimers of albumin, that can be divided according to their molecular masses or hydrophobic properties of the surface. Joint chromatographic and spectral analysis allowed to distinguish up to six albumin enriched fractions. A protein fraction with blue fluorescence spectrum was obtained by gel-filtration and ion exchange chromatography. A higher concentration of this fraction determines a high value of parameter A, that is typical for protein preparations of GN patients' blood. Amino acid sequence shows that one component of this fraction is beta-haptoglobin.

[Effect of sterols on melittin incorporation into liposomal membranes]

Effects of various sterols on the values of intensity, spectrum position and polarization of tryptophan fluorescence (P) of melittin incorporated in lecithin liposomes at different lipid/protein molar ratios (Ri) were studied. A difference in sterol effects has been revealed at the values of Ri > 5. At Ri < 5, fluorescence parameters were determined mainly by melittin in the aqueous solution. Assuming that melittin was bound to lecithin, the lecithin/melittin binding ratio was found to be in the range of 25-50. Unlike cholesterol and stigmasterol, the incorporation into membranes of ergosterol and 7-dehydrocholesterol produced a decrease in the intensity of tryptophan fluorescence and an increase in the P value. This difference might result from the presence of an additional double bond in one of sterol rings of ergosterol and 7-dehydrocholesterol molecules. Analysis of the results obtained enabled us to suggest that the structure of the steroid B ring is responsible for the effect exerted by sterols on melittin-lipid interactions.

[Intrinsic UV-fluorescence of proteins as a tool for studying their dynamics]

A review of literature and our own data on intramolecular mobility of aromatic amino acids in proteins are presented. The necessity to take into account a possible existence of the nanosecond intramolecular mobility with the rotational relaxation time independent of solvent viscosity is pointed out. A comparison of spectral and polarizational characteristics of a number of proteins allowed to solve the problem of the mobility of tryptophan residues localized in the inner regions of macromolecules. According to experimental data, such tryptophan residues can be even more mobile than those located closer to the surface but in the surrounding of polar groups. It is most evident when comparing the intramolecular mobility of tryptophan residues of native proteins and some proteins in the intermediate state of the "molten globule" type. The comparison of experimental data with molecular dynamic simulations contributes substantially to the concepts of tryptophan residue mobility in proteins.

[The effect of biologically active compounds on lysosome fusion with phagosomes and the F-actin content in mouse peritoneal macrophages and on the status of the lysosomal membranes in mouse hepatocytes]

Effects of biologically active compounds bilirubin (BR, 0.1 and 0.2 mM), chelerythrine (CR, 0.1 and 0.5 mM) and farmorubicin (FR, 0.6 and 6.0 mM) on the phagosome-lysosome fusion (P-LF) were studied using fluorescent dye acridine orange for lysosomal labelling and yeast cells as a target. To investigate mechanisms of these effects, changes in fluidity of lysosomal membranes from murine liver were studied by measuring of fluorescence intensity, lifetime and polarization of the fluorescent membrane probes: DPH (1,6-diphenyl-1,3,5-hexatriene) and TMA-DPH [1-(4-triphenylamino)-6-phenyl-1,3,5-hexatriene] incorporated in isolated murine liver lysosomes. In order to characterize the induced cytoskeleton changes, the F-actin content in murine peritoneal macrophages was determined. It was found that all three compounds tested enhanced P-LF. Our results demonstrate that BR induces a decrease in DPH and TMA-DPH fluorescence polarization, FR increases DPH and TMA-DPH fluorescence polarization, and CR causes an increase in TMA-DPH fluorescence polarization in lysosomal membranes. All the three compounds increase F-actin content in peritoneal macrophages. Thus, the action of BR extended on P-LF is associated with increasing lysosomal membranes fluidity and cytoskeleton changes. The enhancement of P-LF under the action of FR and CR can be most likely explained by changes of cytoskeleton.