Physical heterogeneity of muscle glycogen phosphorylase revealed by hydrostatic pressure dissociation

The effects of osmolytes and crowding on the pressure-induced dissociation and inactivation of dimeric LADH

Compatible osmolytes are able to efficiently modulate the oligomeric state, stability and activity of enzymes at high pressures.

Equilibria of oligomeric proteins under high pressure - A theoretical description

High pressure methods have become a useful tool for studying protein structure and stability. Using them, various physico-chemical processes including protein unfolding, aggregation, oligomer dissociation or enzyme-activity decrease were studied on many different proteins. Oligomeric protein dissociation is a process that can perfectly utilize the potential of high-pressure techniques, as the high pressure shifts the equilibria to higher concentrations making them better observable by spectroscopic methods. This can be especially useful when the oligomeric form is highly stable at atmospheric pressure. These applications may be, however, hindered by less intensive experimental response as well as interference of the oligomerization equilibria with unfolding or aggregation of the subunits, but also by more complex theoretical description. In this study we develop mathematical models describing different kinds of oligomerization equilibria, both closed (equilibrium of monomer and the highest possible oligomer without any intermediates) and consecutive. Closed homooligomer equilibria are discussed for any oligomerization degree, while the more complex heterooligomer equilibria and the consecutive equilibria in both homo- and heterooligomers are taken into account only for dimers and trimers. In all the cases, fractions of all the relevant forms are evaluated as functions of pressure and concentration. Significant points (inflection points and extremes) of the resulting transition curves, that can be determined experimentally, are evaluated as functions of pressure and/or concentration. These functions can be further used in order to evaluate the thermodynamic parameters of the system, i.e. atmospheric-pressure equilibrium constants and volume changes of the individual steps of the oligomer-dissociation processes.

Without Binding ATP, Human Rad51 Does Not Form Helical Filaments on ssDNA

Construction of the presynaptic filament (PSF) of proper helical structure by Rad51 recombinases is a prerequisite of the progress of homologous recombination repair. We studied the contribution of ATP-binding to this structure of wt human Rad51 (hRad51). We exploited the protein-dissociation effect of high hydrostatic pressure to determine the free energy of dissociation of the protomer interfaces in hRad51 oligomer states and used electron microscopy to obtain topological parameters. Without cofactors ATP and Ca(2+) and template DNA, hRad51 did not exist in monomer form, but it formed rodlike long filaments without helical order. ΔG(diss) indicated a strong inherent tendency of aggregation. Binding solely ssDNA left the filament unstructured with slightly increased ΔG(diss). Adding only ATP and Ca(2+) to the buffer disintegrated the self-associated rods into rings and short helices of further increased ΔG(diss). Rad51 binding to ssDNA only with ATP and Ca bound could lead to ordered helical filament formation of proper pitch size with interface contacts of K(d) ∼ 2 × 10(-11) M, indicating a structure of outstanding stability. ATP/Ca binding increased the ΔG(diss) of protomer contacts in the filament by 16 kJ/mol. The results emphasize that ATP-binding in the PSF of hRad51 has an essential, yet purely structural, role.

Changes in rabbit skeletal myosin and its subfragments under high hydrostatic pressure

The pressure-induced denaturation of rabbit skeletal myosin and its subfragments under hydrostatic pressure were investigated. Four nanometer of red shift of the intrinsic fluorescence spectrum was observed in myosin under a pressure of 400 MPa. The ANS fluorescence of myosin increased with elevating pressure. Changes in the intrinsic fluorescence spectra of myosin and its subfragments were quantified and expressed as the center of spectral mass. The center of spectral mass of myosin and its subfragments linearly decreased with elevating pressure, and increased with lowering pressure. The fluorescence intensity of the ANS-labeled rod did not change during pressure treatment. The present results indicate that the most pressure-sensitive portion of myosin molecule is the head. Hysteresis of the center of spectral mass of S1 appeared under pressures above 300 MPa. Changes in the center of spectral mass of S1 above 350 MPa showed stronger hysteresis. The center of spectral mass did not decrease above 350 MPa during the compression process, indicating that S1 was stable in a partially denatured state at 350 MPa under pressure. The changes in the relative intensities of ANS fluorescence of S1 were measured under pressures up to 400 MPa, and the ANS fluorescence intensity increased with elevating pressure but it did not change after pressure release. The ANS fluorescence intensity increased under constant pressure suggesting that the pressure-induced denaturation of myosin was accelerated during pressurization.

Closed Conformation of the Active Site Loop of Rabbit Muscle Triosephosphate Isomerase in the Absence of Substrate: Evidence of Conformational Heterogeneity

The active site loop of triosephosphate isomerase (TIM) exhibits a hinged-lid motion, alternating between the two well defined "open" and "closed" conformations. Until now the closed conformation had only been observed in protein complexes with substrate analogues. Here, we present the first rabbit muscle apo TIM structure, refined to 1.5A resolution, in which the active site loop is either in the open or in the closed conformation in different subunits of the enzyme. In the closed conformation described here, the lid loop residues participate in stabilizing hydrogen bonds characteristic of holo TIM structures, whereas chemical interactions observed in the open loop conformation are similar to those found in the apo structures of TIM. In the closed conformation, a number of water molecules are observed at the projected ligand atom positions that are hydrogen bonded to the active site residues. Additives used during crystallization (DMSO and Tris molecules and magnesium atoms) were modeled in the electron density maps. However, no specific binding of these molecules is observed at, or close to, the active site and the lid loop. To further investigate this unusual closed conformation of the apo enzyme, two more rabbit muscle TIM structures, one in the same and another in a different crystal form, were determined. These structures present the open lid conformation only, indicating that the closed conformation cannot be explained by crystal contact effects. To rationalize why the active site loop is closed in the absence of ligand in one of the subunits, extensive comparison with previously solved TIM structures was carried out, supported by the bulk of available experimental information about enzyme kinetics and reaction mechanism of TIM. The observation of both open and closed lid conformations in TIM crystals might be related to a persistent conformational heterogeneity of this protein in solution.

Fluorescence: basic concepts, practical aspects, and some anecdotes

We hope that we have conveyed information of interest and value to present and future fluorescence practitioners. Those readers with a sustaining interest in this topic may wish to consult more comprehensive sources such as Molecular Fluorescence: Principles and Applications, an excellent text by Valeur, or Principles of Fluorescence Spectroscopy by Lakowicz. Many specialized fluorescence topics are covered in the series Topics in Fluorescence Spectroscopy (Volumes 1-6), and several volumes of Methods in Enzymology (e.g., Volumes 246 and 278) have dealt with issues in fluorescence spectroscopy. Proceedings from the International Conference on Methods and Applications of Fluorescence Spectroscopy, 1997 (MAFS 97) and MAFS 98 (in press) also present fluorescence work on many different topics in biological and chemical fields. The Molecular Probes Handbook and web site (www.probes.com) are also rich sources of useful information. Finally, any reader with a question or seeking advice on some topic related to fluorescence is welcome to e-mail D.M.J. at djameson@hawaii.edu.

Effect of high hydrostatic pressure on chicken myosin subfragment-1

Hydrostatic pressure-induced structural changes in subfragment-1 (S1) of myosin molecule were studied. ATP-induced emission spectra of S1 were used to detect global structural change of S1 by pressure treatment. The fluorescence intensity of unpressurized S1 increased by addition of ATP. The increment of fluorescence of pressurized S1 up to 150 MPa was almost the same as control, whereas it became smaller above 200 MPa. ATP binding ability of S1 examined using 1, N(6)-ethenoadenosine 5'-diphosphate (epsilon -ADP) indicated that the binding of epsilon -ADP to S1 decreased in the range of 250-300 MPa. S1 pressurized below 250 MPa and unpressurized S1 similarly bound to F-actin, although binding of S1 pressurized above 250 MPa decreased. Electron microscopic observation revealed arrowhead structure in control acto-S1, while disordered arrowhead structure was observed in acto-S1 prepared from pressurized S1 at 300 MPa. S1 pressurized below 250 MPa retained the same actin activated ATPase activity as the control, whereas the activity decreased to 60% at 300 MPa. Pressure treated S1 was easily cleaved by tryptic digestion into three domains, i.e. 27 kDa (N-terminal), 50 and 20 kDa (C-terminal) fragments, which were the same as those in unpressurized one. It is concluded that pressure-induced global structural changes of S1 begin to occur about 150 MPa, and the local structural changes in ATPase and actin binding sites followed with elevating pressure to 250-300 MPa.

Fluorescence and FTIR study of pressure-induced structural modifications of horse liver alcohol dehydrogenase (HLADH)

The process of pressure-induced modification of horse liver alcohol dehydrogenase (HLADH) was followed by measuring in situ catalytic activity (up to 250 MPa), intrinsic fluorescence (0.1-600 MPa) and modifications of FTIR spectra (up to 1000 MPa). The tryptophan fluorescence measurements and the kinetic data indicated that the pressure-induced denaturation of HLADH was a process involving several transitions and that the observed transient states have characteristic properties of molten globules. Low pressure (< 100 MPa) induced no important modification in the catalytic efficiency of the enzyme and slight conformational changes, characterized by a small decrease in the centre of spectral mass of the enzyme's intrinsic fluorescence: a native-like state was assumed. Higher pressures (100-400 MPa) induced a strong decrease of HLADH catalytic efficiency and further conformational changes. At 400 MPa, a dimeric molten globule-like state was proposed. Further increase of pressure (400-600 MPa) seemed to induce the dissociation of the dimer leading to a transition from the first dimeric molten globule state to a second monomeric molten globule. The existence of two independent structural domains in HLADH was assumed to explain this transition: these domains were supposed to have different stabilities against high pressure-induced denaturation. FTIR spectroscopy was used to follow the changes in HLADH secondary structures. This technique confirmed that the intermediate states have a low degree of unfolding and that no completely denatured form seemed to be reached, even up to 1000 MPa.

Folding of a de novo designed native-like four-helix bundle protein

The folding of a model native-like dimeric four-helix bundle protein, (alpha(2))(2), was investigated using guanidine hydrochloride, hydrostatic pressure, and low temperature. Unfolding by guanidine hydrochloride followed by circular dichroism and intrinsic fluorescence spectroscopy revealed a highly cooperative transition between the native-like and unfolded states, with free energy of unfolding determined from CD data, DeltaG(unf) = 14.3 +/- 0.8 kcal/mol. However, CD and intrinsic fluorescence data were not superimposable, indicating the presence of an intermediate state during the folding transition. To stabilize the folding intermediate, we used hydrostatic pressure and low temperature. In both cases, dissociation of the dimeric native-like (alpha(2))(2) into folded monomers (alpha(2)) was observed. van't Hoff analysis of the low temperature experiments, assuming a two-state dimer 171-monomer transition, yielded a free energy of dissociation of (alpha(2))(2) of DeltaG(diss) = 11.4 +/- 0.4 kcal/mol, in good agreement with the free energy determined from pressure dissociation experiments (DeltaG(diss) = 10.5 +/- 0.1 kcal/mol). Binding of the hydrophobic fluorescent probe 4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonic acid (bis-ANS) to the pressure- and cold-dissociated states of (alpha(2))(2) indicated the existence of molten-globule monomers. In conclusion, we demonstrate that the folding pathway of (alpha(2))(2) can be described by a three-state transition including a monomeric molten globule-like state.

High pressure static fluorescence to study macromolecular structure-function

Through some typical examples, the high pressure static fluorescence method is described. The potentiality of the intrinsic and extrinsic fluorescence probes are analyzed for structural characterizations. Special attention is given to the use of fluorescence to understand the behavior of enzymatic reactions under high pressure. The application of fluorescence polarization is also presented together with some relevant spectroscopic problems inherent in data interpretation.

Tryptophan phosphorescence and pressure effects on protein structure

After a brief introduction of the potentialities of Trp phosphorescence spectroscopy for probing the conformation and flexibility of protein structure, this presentation summarizes the effects of hydrostatic pressure (up to 3 kbar) on the native fold of monomeric and oligomeric proteins as inferred from the variation of the intrinsic phosphorescence lifetime and the oxygen and acrylamide bimolecular quenching rate constants of buried Trp residues. The pressure/temperature response of the globular fold and modulation of its dynamical structure is analyzed both in terms of a reduction of internal cavities and of hydration of the polypeptide. The implications of these findings for the thermodynamic stability of proteins and for the determination of subunit dissociation equilibria under high pressure conditions are also discussed.

Effect of binding of Calcofluor White on the carbohydrate residues of alpha1-acid glycoprotein (orosomucoid) on the structure and dynamics of the protein moiety. A fluorescence study

Calcofluor White is a fluorescent probe that interacts with polysaccharides and is commonly used in clinical studies. Interaction between Calcofluor White and carbohydrate residues of alpha1-acid glycoprotein (orosomucoid) was previously studied at low and high concentrations of Calcofluor compared to that of the protein. alpha1-Acid glycoprotein contains 40% carbohydrate by weight and has up to 16 sialic acid residues. At equimolar concentrations of Calcofluor and alpha1-acid glycoprotein, the fluorophore displays free motions [Albani, J. R.; Sillen, A.; Coddeville, B.; Plancke, Y. D.; Engelborghs, Y. Carbohydr. Res. 1999, 322, 87-94], while at high concentration of Calcofluor, its surrounding microenvironment is rigid, inducing the rigidity of the fluorophore itself [Albani, J. R.; Sillen, A.; Plancke, Y. D.; Coddeville, B.; Engelborghs, Y. Carbohydr. Res. 2000, 327, 333-340]. In the present work, red-edge excitation spectra and steady-state anisotropy studies performed on Trp residues in the presence of Calcofluor, showed that the apparent dynamics of Trp residues are not modified. However, deconvoluting the emission spectra with two different methods into different components, reveals that the structure of the protein matrix has been disrupted in the presence of high Calcofluor concentrations.

Effects of hydrostatic pressure on the structure and biological activity of infectious bursal disease virus

The effects of high hydrostatic pressure on the structure and biological activity of infectious bursal disease virus (IBDV), a commercially important pathogen of chickens, were investigated. IBDV was completely dissociated into subunits at a pressure of 240 MPa and 0 degrees C revealed by the change in intrinsic fluorescence spectrum and light scattering. The dissociation of IBDV showed abnormal concentration dependence as observed for some other viruses. Electron microscopy study showed that morphology of IBDV had an obvious change after pressure treatment at 0 degrees C. It was found that elevating pressure destroyed the infectivity of IBDV, and a completely pressure-inactivated IBDV could be obtained under proper conditions. The pressure-inactivated IBDV retained the original immunogenic properties and could elicit high titers of virus neutralizing antibodies. These results indicate that hydrostatic pressure provides a potential physical means to prepare antiviral vaccine.

Subunit dissociation and inactivation of pyruvate kinase by hydrostatic pressure oxidation of sulfhydryl groups and ligand effects on enzyme stability

The effect of hydrostatic pressure on the stability of tetrameric rabbit muscle pyruvate kinase was investigated by enzyme activity measurements, size-exclusion chromatography, circular dichroism and fluorescence spectroscopies. Under nonreducing conditions, enzyme activity was irreversibly inhibited by increasing pressure and was completely abolished at 350 MPa. Inhibition was dependent on the concentration of pyruvate kinase, indicating that it was related to pressure-induced subunit dissociation. Size-exclusion chromatography of pressurized samples confirmed a decrease in the proportion of tetramers and an increase in monomers relative to native samples. Addition of dithiothreitol immediately following pressure release led to full recovery of both enzyme activity and of native tetramers. Furthermore, no irreversible inhibition of pyruvate kinase was observed if pressure treatment was carried out in the presence of dithiothreitol. These data suggest that pressure-dissociated monomers undergo conformational changes leading to oxidation of sulfhydryl groups, which prevents correct refolding of native tetramers on decompression. These conformational changes are relatively subtle, as indicated by the lack of significant changes in far-UV circular dichroism and intrinsic fluorescence emission spectra of previously pressurized samples. The effects of various physiological ligands on the pressure stability of pyruvate kinase were also investigated. A slight protection against inhibition was observed in the simultaneous presence of K+, Mg2+ and ADP. Both phosphoenolpyruvate and the allosteric inhibitor, phenylalanine, caused marked stabilization against pressure, suggesting significant energy coupling between binding of these ligands and stabilization of the tetramer.

Fluorescence fluctuation spectroscopy

The analysis of the intensity fluctuation of a fluorescence signal from a relatively small volume and from a few molecules contains information about the distribution of different species present in the solution and about kinetic parameters of the system. The same information is generally averaged out when the fluorescence experiment is performed in a much larger volume, typically a cuvette experiment. The fundamental reason for this difference is that the fluctuations of the fluorescence signal from a few molecules directly reflect the molecular nature of the matter. Only recently, with the advent of confocal microscopy and two-photon excitation, it has become practical to achieve small excitation volumes in which only a few fluorescent molecules are present. We introduce the concept of fluctuation spectroscopy and highlight some of the technical aspects. We discuss different analysis methods used in fluctuation spectroscopy and evaluate their use for studying protein-protein interactions.

Fluorescence and FTIR study of the pressure-induced denaturation of bovine pancreas trypsin

The pressure denaturation of trypsin from bovine pancreas was investigated by fluorescence spectroscopy in the pressure range 0. 1-700 MPa and by FTIR spectroscopy up to 1000 MPa. The tryptophan fluorescence measurements indicated that at pH 3.0 and 0 degrees C the pressure denaturation of trypsin is reversible but with a large hysteresis in the renaturation profile. The standard volume changes upon denaturation and renaturation are -78 mL.mol-1 and +73 mL.mol-1, respectively. However, the free energy calculated from the data in the compression and decompression directions are quite different in absolute values with + 36.6 kJ.mol-1 for the denaturation and -5 kJ. mol-1 for the renaturation. For the pressure denaturation at pH 7.3 the tryptophan fluorescence measurement and enzymatic activity assays indicated that the pressure denaturation of trypsin is irreversible. Interestingly, the study on 8-anilinonaphthalene-1-sulfonate (ANS) binding to trypsin under pressure leads to the opposite conclusion that the denaturation is reversible. FTIR spectroscopy was used to follow the changes in secondary structure. The pressure stability data found by fluorescence measurements are confirmed but the denaturation was irreversible at low and high pH in the FTIR investigation. These findings confirm that the trypsin molecule has two domains: one is related to the enzyme active site and the tryptophan residues; the other is related to the ANS binding. This is in agreement with the study on urea unfolding of trypsin and the knowledge of the molecular structure of trypsin.

Hydrostatic pressure and calcium-induced dissociation of calpains

The dissociation of mu- and m-calpains was studied by fluorescence spectroscopy under high hydrostatic pressure (up to 650 MPa). Increasing pressure induced a red shift of the tryptophan fluorescence of the calcium-free enzyme. The concentration dependence of the spectral transition was consistent with a pressure-induced dissociation of the subunits. Rising temperature increased the stability of calpain heterodimers and confirmed the predominance of hydrophobic interactions between monomers. At saturating calcium, the spectral transition was not observed for native or iodoacetamide-inactivated calpains, indicating that they were already dissociated by calcium. The reaction volume was about -150 ml mol-1 for both isoforms, and the dissociation constants at atmospheric pressure are approximately 10-12 M and 10-15 M for mu- and m-calpains, respectively. This result indicates a tighter interaction in the isoform that requires higher calcium concentration for activity.

Scanning two-photon fluctuation correlation spectroscopy: particle counting measurements for detection of molecular aggregation

Scanning fluctuation correlation spectroscopy (FCS) is an experimental technique capable of measuring particle number concentrations by monitoring spontaneous equilibrium fluctuations in the local concentration of a fluorescent species in a small (femtoliter) subvolume of a sample. The method can be used to detect molecular aggregation for dilute, submicromolar samples by directly "counting particles". We introduce the application of two-photon excitation to scanning FCS and discuss its important advantages for this technique. We demonstrate the capability of measuring particle number concentrations in solution, first with dilute samples of monodisperse 7-nm and 15-nm radius latex spheres, and then with B phycoerythrin. The detection of multiple species in a single sample is shown, using mixtures containing both sphere sizes. The method is then applied to study protein aggregation in solution. We monitor the concentration-dependent association/ dissociation equilibrium for glycogen phosphorylase A and malate dehydrogenase. The measured dissociation constants, 430 nM and 144 nM respectively, are in good agreement with previously published values. In addition, oligomer dissociation induced by pH titration from pH 8 to pH 5.0 is detectable for the enyme phosphofructokinase. The possibility of measuring dissociation kinetics by scanning two-photon FCS is also demonstrated using phosphofructokinase.

Concentration dependence of the subunit association of oligomers and viruses and the modification of the latter by urea binding

A theoretical model is presented that accounts for the facilitation of the pressure dissociation of R17 phage, and for the partial restoration of the concentration dependence of the dissociation, by the presence of subdenaturing concentrations of urea. As an indifferent osmolyte urea should promote the stability of the protein aggregates under pressure, and the decrease in pressure stability with urea concentration demonstrates that such indirect solvent effects are not significant for this case, and that the progressive destabilization is the result of direct protein-urea interactions. By acting as a "homogenizer" of the properties of the phage particles, urea addition converts the pressure-induced deterministic dissociation of the phage into a limited stochastic equilibrium. The model establishes the origin of the uniform progression from the stochastic equilibrium of dimers, to the temperature-dependent and partially concentration-dependent association of tetramers, to the fully deterministic equilibrium observed in many multimers and in the virus capsids.

Pressure-induced dissociation and denaturation of allophycocyanin at subzero temperatures

The thermodynamics of assembly of the allophycocyanin hexamer was examined employing hydrostatic pressures in the range of 1 bar to 2.4 kbar and temperatures of 20 to -12 degrees C, the latter made possible by the decrease of the freezing point of water under pressure. The existence of two processes, dissociation of the hexamer into dimers, (alpha beta)3-->3 (alpha beta), and dissociation of the alpha beta dimers into monomers, (alpha beta)-->alpha + beta have been recognized previously by changes in the absorbance and fluorescence of the tetrapyrrolic chromophores owing to added ligands. The same changes are observed in the absence of ligands at pressures of under 2.4 kbar and temperatures down to -12 degrees C. On decompression from 2.4 kbar at 0 degrees C, appreciable hysteresis and a persistent loss of 50% in the absorbance at 653 nm is observed. It results from the conformational drift of the isolated subunits and is reduced to 10% when the highest pressure is limited to 1.6 kbar. The thermodynamic parameters of the reaction alpha + beta-->alpha beta can be determined from pressure effects on perchlorate solutions of allophycocyanin, which consist of dimers alone. Their previous knowledge permits estimation, under suitable hypotheses, of the thermodynamic parameters of the reaction 3(alpha beta)-->(alpha beta)3 from the overall pressure effects on the hexamers. Both association reactions have positive enthalpy changes, and the whole hexamer assembly is made possible by the excess entropy.

Structural and functional roles of cysteine 90 and cysteine 240 in S-adenosylmethionine synthetase

Site-specific mutagenesis was performed on the structural gene for Escherichia coli S-adenosylmethionine (AdoMet) synthetase to introduce mutations at cysteines 90 and 240, residues previously implicated by chemical modification studies to be catalytically and/or structurally important. The AdoMet synthetase mutants (i.e. MetK/C90A, MetK/C90S, and MetK/C240A) retained up to approximately 10% of wild type activity, demonstrating that neither sulfhydryl is required for catalytic activity. Mutations at Cys-90 produced a mixture of noninterconverting dimeric and tetrameric proteins, suggesting a structural significance for Cys-90. Dimeric Cys-90 mutants retained approximately 1% of wild type activity, indicating a structural influence on enzyme activity. Both dimeric and tetrameric MetK/C90A had up to a approximately 70-fold increase in Km for ATP, while both dimeric and tetrameric MetK/C90S had Km values for ATP similar to the wild type enzyme, suggesting a linkage between Cys-90 and the ATP binding site. MetK/C240A was isolated solely as a tetramer and differed from wild type enzyme only in its 10-fold reduction in specific activity, suggesting that the mutation affects the rate-limiting step of the reaction, which for the wild type enzyme is the joining of ATP and L-methionine to yield AdoMet and tripolyphosphate. Remarkably all of the mutants are much more thermally stable than the wild type enzyme.

Heterogeneity in molecular recognition by restriction endonucleases: osmotic and hydrostatic pressure effects on BamHI, Pvu II, and EcoRV specificity

The cleavage specificity of the Pvu II and BamHI restriction endonucleases is found to be dramatically reduced at elevated osmotic pressure. Relaxation in specificity of these otherwise highly accurate and specific enzymes, previously termed "star activity," is uniquely correlated with osmotic pressure between 0 and 100 atmospheres. No other colligative solvent property exhibits a uniform correlation with star activity for all of the compounds tested. Application of hydrostatic pressure counteracts the effects of osmotic pressure and restores the natural selectivity of the enzymes for their canonical recognition sequences. These results indicate that water solvation plays an important role in the site-specific recognition of DNA by many restriction enzymes. Osmotic pressure did not induce an analogous effect on the specificity of the EcoRV endonuclease, implying that selective hydration effects do not participate in DNA recognition in this system. Hydrostatic pressure was found to have little effect on the star activity induced by changes in ionic strength, pH, or divalent cation, suggesting that distinct mechanisms may exist for these observed alterations in specificity. Recent evidence has indicated that BamHI and EcoRI share similar structural motifs, while Pvu II and EcoRV belong to a different structural family. Evidently, the use of hydration water to assist in site-specific recognition is a motif neither limited to nor defined by structural families.

Hydrostatic and osmotic pressure as tools to study macromolecular recognition

Clearly, hydrostatic and osmotic pressure techniques offer unique potential in the study of fundamental problems of molecular recognition in biological systems. With the recent advances in technology such investigations are rapidly becoming commonplace. We look forward to further advances and their report in succeeding compendiums such as this volume.

Pressure effects and thermal stability of myosin rods and rod minifilaments: fluorescence and circular dichroism studies

In the present study hydrostatic pressure was applied upon both skeletal myosin rod molecules and rod minifilaments to learn more of the intra- and intermolecular interaction behavior of myosin. Applied pressure disassembled the rod minifilaments into individual rod molecules and dissociated each myosin rod molecule into two chains of alpha-helix. The dissociation and disassembly profiles of these systems were obtained by measuring their fluorescent anisotropy under pressure. The mid-disassembly pressure of rod minifilaments at 0.4 mg/mL concentration was 430-490 bar. However, dissociation of two helical strands of rod molecules occurred at a much higher pressure, with a mid-disassembly pressure of 1300 bar at this concentration. These results indicate that the intramolecular interactions occurring between two alpha-helical chains of a rod molecule are much more stable under pressure than the intermolecular interactions that occur among rod molecules in a minifilament. The regions in the rod molecules involved in filament assembly were investigated through usage of both the intrinsic fluorescence of tryptophan residues and the extrinsic fluorescence of 6-acryloyl-2-(dimethylamino)naphthalene (acrylodan) labeled cysteine residues. The blue spectral shifts upon minifilament formation suggest the participation of both light meromyosin (LMM) and subfragment-2 (S-2) regions of myosin rods in the filament formation. Profiles of thermal unfolding of myosin rod molecules and rod minifilaments were obtained by circular dichroism measurement. The multiple transitions exhibited upon unfolding profiles indicated the presence of more than one structural domain, each correlating with a cooperative transition. The domain transitional temperatures were found to be 1-4 degrees C higher for rods in minifilaments than those for rod molecules in a solution of similar ionic composition, indicating that all structural domains are involved in filament assembly. Furthermore, the domain transitional temperatures for rod molecules in a buffer containing 0.6 M NaCl were 6-8 degrees C higher than those for rod molecules in 5 mM sodium pyrophosphate buffer, suggesting that each structural domain of a rod molecule becomes stabilized at 0.6 M NaCl solution.