Model for the generation of toroidal and poloidal magnetic fields in a laser-produced plasma

A mechanism of simultaneous generation of toroidal and poloidal magnetic fields in an underdense region of a laser-produced plasma is discussed. The mechanism relies on the fact that at least a part of the incident transverse mode of the laser field undergoes a linear conversion into a longitudinal mode in the thermal plasma. It involves the conversion of ordered kinetic motion of the charged particles in the presence of the field into the energy of the induced magnetic fields both in poloidal and toroidal directions. The analysis is based on obtaining perturbative solutions of the two-fluid model of a hot nondissipative plasma. Our numerical results show that both the toroidal and poloidal fields increase with the laser intensity, and that the former dominates over the latter. Further, the toroidal fields decrease with increasing pulse lengths and increase rather slowly with an increase in laser wavelengths. However, the poloidal fields seem to be insensitive to the laser pulse lengths but they increase exponentially with the laser wavelengths. Finally, toroidal fields have a tendency to decrease as the critical surface is approached. The poloidal fields show a contrary behavior.

A study of energetics of cooperative interaction using a mutant lambda-repressor

A lambda-repressor mutant, S228N, which is defective in tetramer formation in the free state but retains full cooperativity, was studied in detail. Isolated single operator-bound S228N repressor shows association properties similar to those of the wild-type repressor. Fluorescence anisotropy studies with dansyl chloride-labeled repressor show a dimer-monomer dissociation constant of around 10(-5) M. The structure of the mutant repressor was studied by circular dichroism, acrylamide quenching and sulfhydryl reactivity at protein concentrations of < or =10(-6) M, where it is predominantly monomeric. The results suggest no significant perturbations in the structure of the S228N mutant repressor from that of the wild-type repressor. Urea denaturation studies also indicate no significant change in the stability of the repressor. The results were used to calculate energetics of loop formation in the cooperative binding process.

Sulfhydryls of tubulin. A probe to detect conformational changes of tubulin

The 20 cysteine residues of tubulin are heterogeneously distributed throughout its three-dimensional structure. In the present work, we have used the reactivity of these cysteine residues with 5, 5'-dithiobis(2-nitrobenzoic acid) (DTNB) as a probe to detect the global conformational changes of tubulin under different experimental conditions. The 20 sulfhydryl groups can be classified into two categories: fast and slow reacting. Colchicine binding causes a dramatic decrease in the reactivity of the cysteine residues and causes complete protection of 1.4 cysteine residues. Similarly, other colchicine analogs that bind reversibly initially decrease the rate of reaction; but unlike colchicine they do not cause complete protection of any sulfhydryl groups. Interestingly, in all cases we find that all the slow reacting sulfhydryl groups are affected to the same extent, that is, have a single rate constant. Glycerol has a major inhibitory effect on all these slow reacting sulfhydryls, suggesting that the reaction of slow reacting cysteines takes place from an open state at equilibrium with the native. Ageing of tubulin at 37 degrees C leads to loss of self-assembly and colchicine binding activity. Using DTNB kinetics, we have shown that ageing leads to complete protection of some of the sulfhydryl groups and increased reaction rate for other slow reacting sulfhydryl groups. Ageing at 37 degrees C also causes aggregation of tubulin as indicated by HPLC analysis. The protection of some sulfhydryl groups may be a consequence of aggregation, whereas the increased rate of reaction of other slow reacting sulfhydryls may be a result of changes in global dynamics. CD spectra and acrylamide quenching support such a notion. Binding of 8-anilino-1-naphthalenesulfonate (ANS) and bis-ANS by tubulin cause complete protection of some cysteine residues as indicated by the DTNB reaction, but has little effect on the other slow reacting cysteines, suggesting local effects.

NBD-isocolcemid-tubulin interaction: a novel one-step reaction involving no conformational adjustment of reactants

Isocolcemid, a colcemid analogue in which the positions of the C-ring methoxy and carbonyl are exchanged, is virtually inactive in binding to tubulin and inhibiting the formation of microtubule assembly. We have found that the substitution of a NBD group in the side chain of the B-ring of isocolcemid can reverse the effect of these structural alterations (at the C-ring) and the newly synthesized NBD-isocolcemid restores the lost biological activity. It inhibits microtubule assembly with an IC(50) of 12 microM and competes efficiently with [(3)H]colchicine, for binding to tubulin. NBD-isocolcemid has two binding sites on tubulin; one is characterized by fast binding, whereas the binding to the other site is slow. These two sites are independent and unrelated to each other. Colchicine and its analogues compete with NBD-isocolcemid for the slow site. Association and dissociation rate constants for the fast site, obtained from the stopped-flow measurements, are (7.37 +/- 0. 70) x 10(5) M(-1) s(-1) and 7.82 +/- 2.74 s(-1), respectively. While the interaction of colchicine and its analogues with tubulin involves two steps, NBD-isocolcemid binding to tubulin at the slow site has been found to be a one-step reaction. This is evident from the linear dependence of the observed rate constant (k(obs)) with both NBD-isocolcemid and tubulin concentrations. The interaction of NBD-isocolcemid with tubulin does not involve the conformational change of NBD-isocolcemid, as is evident from the unchanged CD spectra of the drug. The absence of enhanced GTPase activity of tubulin and the native-like protease cleavage pattern of the NBD-isocolcemid-tubulin complex suggest an unaltered conformation of tubulin upon NBD-isocolcemid binding to it as well. Implications of this on the mechanism of polymerization inhibition have been discussed.

Nucleotide-dependent bisANS binding to tubulin

Non-covalent hydrophobic probes such as 5, 5'-bis(8-anilino-1-naphthalenesulfonate) (bisANS) have become increasingly popular to gain information about protein structure and conformation. However, there are limitations as bisANS binds non-specifically at multiple sites of many proteins. Successful use of this probe depends upon the development of binding conditions where only specific dye-protein interaction will occur. In this report, we have shown that the binding of bisANS to tubulin occurs instantaneously, specifically at one high affinity site when 1 mM guanosine 5'-triphosphate (GTP) is included in the reaction medium. Substantial portions of protein secondary structure and colchicine binding activity of tubulin are lost upon bisANS binding in absence of GTP. BisANS binding increases with time and occurs at multiple sites in the absence of GTP. Like GTP, other analogs, guanosine 5'-diphosphate, guanosine 5'-monophosphate and adenosine 5'-triphosphate, also displace bisANS from the lower affinity sites of tubulin. We believe that these multiple binding sites are generated due to the bisANS-induced structural changes on tubulin and the presence of GTP and other nucleotides protect those structural changes.

Amino acid changes in the repressor of bacteriophage lambda due to temperature-sensitive mutations in its cI gene and the structure of a highly temperature-sensitive mutant repressor

The mutant cIts genes from seven different lambdacIts phages carrying tsU50, tsU9, tsU46, ts1, tsU51, tsI-22 and ts2 mutations were cloned in plasmid. The positions of these mutations and the resulting changes of amino acids in the repressor were determined by DNA sequencing. The first four mutations mapping in the N-terminal domain show the following changes: I21S, G53S, A62T and V73A, respectively. Of the three remaining mutations mapping in the C-terminal domain, cItsI-22 and cIts2 show N207T and K224E substitutions respectively, while the mutant cItsU51 gene carries F141I and P153L substitutions. Among these ts repressors, CIts2 having the charge-reversal change K224E was overexpressed from tac promoter in a plasmid and purified, and its structure and function were studied. Operator-binding studies suggest that the ts2 repressor is somewhat defective in monomer-dimer equilibrium and/or cooperativity even at permissive temperatures and loses its operator-binding ability very rapidly above 25 degrees C. Comparative studies of fluorescence and CD spectra, sulfhydryl group reactivity and elution behaviour in size-exclusion HPLC of both wild-type and ts2-mutant repressors at permissive and non-permissive temperatures suggest that the C-terminal domain of the ts2 repressor carrying a K224E substitution has a structure that does not favor tetramer formation at non-permissive temperatures.

Chaperone-like activity of tubulin

Tubulin, a ubiquitous protein of eukaryotic cytoskeleton, is a building block unit of microtubule. Although several cellular processes are known to be mediated through the tubulin-microtubule system, the participation of tubulin or microtubule in protein folding pathway has not yet been reported. Here we show that goat brain tubulin has some functions and features similar to many known molecular chaperones. Substoichiometric amounts of tubulin can suppress the non-thermal and thermal aggregation of a number of unrelated proteins such as insulin, equine liver alcohol dehydrogenase, and soluble eye lens proteins containing beta- and gamma-crystallins. This chaperone-like activity of tubulin becomes more pronounced as temperature increases. Aging of tubulin solution at 37 degreesC also enhances its chaperone-like activity. Tubulin loses its chaperone-like activity upon removal of its flexible hydrophilic C-terminal tail. These results suggest that both electrostatic and hydrophobic interactions are important in substrate binding by tubulin and that the negatively charged C-terminal tails play a crucial role for its chaperone-like activity.

Molecular dynamics simulation of colchicinoids

Colchicine, a tricyclic alkaloid, has a remarkable range of biological activities. It binds with tubulin and prevents the formation of microtubules. This compound consists of a six membered aromatic ring (A ring), a seven membered troponoid ring (C ring) and another seven membered aliphatic ring (B ring). Using molecular mechanics and molecular dynamics simulations as tools, conformational analysis of colchicine and its several important analogs were done. Molecular mechanics studies show that conformational space of these molecules have one low energy region. Taking the low energy minima as the starting conformation, molecular dynamics simulation for 100 pico seconds is done for each of the analogs and molecular dynamics simulation in solution is done for three representative compounds colchicine,isocolchicine and A-C compound. Internal coordinate trajectories show that the value of the dihedral angle C9-C7-C1-C14 (phi), (C7-C1 bond connects the A and C ring), is within 40 degrees to 50 degrees for all the compounds with fluctuations less than 15 degrees. These calculations indicate that there is an overall similarity in the dynamically averaged structure of all the drugs. The A ring and B ring of the compounds are more or less rigid. The C ring is somewhat flexible, the average conformation and motional properties show overall similarity. The potential energy curve and dynamics behaviour of colchicine and isocolchicine suggests that the difference in binding property of colchine and isocolchicine may originate from the positional difference of carbonyl oxygen and methoxy group of C ring, which is the only difference in the structures of the two compounds and this has no effect on the motional property and average conformations of these two compounds. From our study it is proposed that the movements occuring at various positions of the drug molecules are significantly correlated. It is suggested that such correlated motion may play an important role in the biological property of these compounds.

Refolding of urea-denatured tubulin: recovery of nativelike structure and colchicine binding activity from partly unfolded states

Tubulin unfolding in urea proceeds via the formation of a partially unfolded intermediate state, stable in 2 M urea, that unfolds further in higher urea concentrations. The intermediate state had spectroscopic properties reminiscent of a molten globule and negligible colchicine binding activity. Refolding of totally unfolded tubulin in 8 M urea yielded an intermediatelike state characterized by partial burial of tryptophans and partial recovery of secondary and tertiary structures, although colchicine-binding activity of the protein was not regained. Further folding of this intermediatelike state, toward the native conformation, with respect to both structural and functional parameters did not occur. However, a significant percentage of colchicine binding activity and nativelike tertiary structure was recovered when refolding was initiated from partially denatured protein samples, viz., from <1.2 M urea. Thus, although high concentration of urea induced loss of structure and activity was irreversible, the conformational changes induced in restricted regions of tubulin by lower concentrations of urea, which are probably crucial for its various functional properties, could be reversed.

Colchicine binding to tubulin monomers: a mechanistic study

The kinetic and thermodynamic parameters for colchicine-tubulin and deacetamidocolchicine-tubulin interaction, under the condition where tubulin is predominantly in its dissociated state (approximately 80% monomer), have been determined. We observe that the kinetic parameters exihibit marked change when colchicine interacts with the monomeric form of tubulin rather than with the dimeric form of tubulin. The reaction of colchicine with tubulin monomers is characterized by an enhanced association rate which is a consequence of the lowering of activation energy. Colchicine-tubulin interaction, which is only poorly reversible, becomes partially reversible under this condition. Differences were also noticed in the thermodynamic parameters: the reaction of colchicine with tubulin monomers is enthalpy driven with small positive entropy, while with tubulin dimers a large positive entropy change was reported. However, no such changes in the binding parameters were observed for the reaction involving deacetamidocolchicine (a colchicine analog devoid of a side chain at the C-7 position of B-ring) with tubulin monomers. We therefore conclude that a single subunit of tubulin is capable of binding colchicine and that the unusual properties of colchicine-tubulin interactions such as the slow association rate, high activation energy, and the poor reversibility are due to the possible contact(s) of the C-7 substituent (in the B-ring) of colchicine with the other subunit of tubulin.

Screening for depression among acutely ill geriatric inpatients with a short Geriatric Depression Scale

BACKGROUND

Depression is not uncommon among acutely ill geriatric inpatients.

METHOD

The performances of shorter versions of the Geriatric Depression Scale (GDS) in screening for depression among acutely ill geriatric inpatients were examined.

RESULTS

A cut-off of 2/3 gives the best sensitivity (88%) and specificity (75%) for the 10-item version (GDS10). A cut-off of 0/1 gives the best sensitivity (72%) and specificity (90%) for the 4-item version (GDS4). A positive response to item 6 ("Do you often feel helpless?") on the GDS10 gave a sensitivity of 76% and specificity of 75%. Patients found the GDS10 tolerable and acceptable.

CONCLUSION

Both shorter versions of the GDS may be utilized in screening for depression among acutely ill geriatric inpatients.

The colchicine-binding and pyrene-excimer-formation activities of tubulin involve a common cysteine residue in the beta subunit

Colchicine binding and pyrene excimer fluorescence of tubulin have been used to identify cysteine residue(s) essential for the colchicine binding activity of the protein. We report here that both the colchicine binding activity and the ability to form pyrene excimers of tubulin decay at an identical rate when the protein ages at 37 degrees C. Glycerol, which stabilizes the colchicine binding site also stabilizes the excimer formation equally. Thus, these two properties of tubulin are correlated and are likely to belong to the same structural domain. In an attempt to identify the excimer-forming Cys residues, we found that incubation of tubulin with N,N'ethylenebis(iodoacetamide) causes a significant inhibition of excimer fluorescence. Incubation of tubulin with colchicine prior to this treatment fully retains excimer-forming ability. It is known that Cys239 and Cys354 of beta-tubulin, which are about 0.9 nm apart in the native structure, are protected from ethylenebis(iodoacetamide) cross-linking by incubation of tubulin with colchicine [Luduena, R. F. & Roach, M. C. (1981) Pharmacol. Ther. 49, 133-152]. These residues must therefore be responsible for the excimer formation of tubulin with pyrene maleimide. Incubation of tubulin with ethylenebis(iodoacetamide) decreases the colchicine binding activity and the excimer formation at an identical rate. Since the alkylation of Cys239 of beta-tubulin (responsible for tubulin self-assembly) has no effect on colchicine binding [Bai, R., Lin, C. M., Nguyen, N. Y., Liu, T. & Hamel, E. (1989) Biochemistry 28, 5606-5612], our results suggest that excimer formation and the colchicine binding site of tubulin share Cys354 of the beta-subunit. Determination of the number of essential Cys residue(s) for colchicine binding activity, using the statistical method of Tsou [Tsou, C. L. (1962) Sci. Sin. 11, 1535-1558], also shows only one essential Cys residue.

Tubulin conformation and dynamics: a red edge excitation shift study

The fluorescence emission maximum of a polar fluorophore in viscous medium often shows a dependence on excitation wavelength, a phenomenon which is named red edge excitation shift (REES). We have found that the fluorescence spectra of the tubulin tryptophans exhibit a REES of about 7 nm. Also, their steady state fluorescence polarization and mean lifetimes show a dependence on both excitation and emission wavelengths. These results indicate that the average tryptophan environment in tubulin is motionally restricted. Although the tryptophan(s) responsible for the observed REES effect could not be localized, it could be concluded from energy transfer experiments with the tubulin-colchicine complex that the tryptophan(s) participating in energy transfer with bound colchicine probably does not contribute to the REES. A REES of 7 nm was also observed in the case of colchicine complexed with tubulin. However, such a REES was not seen in similar studies with the B-ring analogs of colchicine, viz. 2-methoxy-5-(2',3',4'-trimethoxyphenyl)tropone (called AC because it lacks the B ring of colchicine) and deacetamidocolchicine (which lacks the acetamido substituent at the C-7 position of the B ring). There may be two possible reasons to explain these data. (1) Structural differences between colchicine and its analogs may give rise to differences in their excited state dipole moments which will directly affect the extent of REES, and (2) The B-ring substituent, hanging outside the colchicine binding site on the beta-subunit of the tubulin dimer, probably makes contact with the alpha-subunit of tubulin and imparts a rigidity to that region of the protein, which facilitates the REES.

Fluorescence energy transfer measurement of distances between ligand binding sites of tubulin and its implication for protein-protein interaction

9-(Dicyanovinyl) julolidine (DCVJ) is a fluorescent probe, which binds to a unique site on the tubulin dimer and exhibits different properties that are dependent upon its oligomeric state (Kung & Reed, 1989). DCVJ binds to tubulin, the tubulin-colchicine complex, and the tubulin-ruthenium red complex equally well, but binds tighter to the ANS-tubulin complex than to tubulin alone. The energy transfer studies indicate a small amount of energy transfer with colchicine, but a significant energy transfer with ANS. It was shown previously that ruthenium red binds near the C-terminal tail region of the alpha-subunit. Ruthenium red causes major quenching of fluorescence of the tubulin-DCVJ complex, suggesting proximity of binding sites. The derived distances are consistent with DCVJ binding near the alpha beta interface, but on the opposite face of the colchicine binding site. Location of the binding site correlates with the observed effect of a different polymerized state of tubulin on the DCVJ spectroscopic properties. The effect of dimer-dimer association on DCVJ binding, at high protein concentrations (Kung & Reed, 1989), suggests that such an association may occur through lateral contacts of the elongated tubulin dimer, at least in a significant fraction of the cases. Transmission of ANS-induced conformational change to the DCVJ binding site, which is near important dimer-dimer contact sites, makes it possible that such conformational changes may be responsible for polymerization inhibition by anilino-naphthalene sulfonates.

Relative importance of inositol (1,4,5)trisphosphate and inositol (1,3,4,5)tetrakisphosphate in Entamoeba histolytica

[3H]Inositol tetrakisphosphate (Ins(1,3,4,5)P4) binding sites which were poorly displaced by unlabelled inositol (1,4,5)-trisphosphate (Ins(1,4,5)P3) were detected in membrane fractions of Entamoeba histolytica. Similarly, unlabelled Ins(1,3,4,5)P4 was 30-fold less efficient in displacing [3H]Ins(1,4,5)P3 binding. pH sensitivities of binding of the two isomers were markedly different. Scatchard analysis of the data revealed single binding sites and similar receptor densities for each of the two isomers. Formation of both Ins(1,4,5)P3 and Ins(1,3,4,5)P4 in E. histolytica was also demonstrated. Calcium release studies showed that after treatment with a saturating dose of either Ins(1,4,5)P3 or Ins(1,3,4,5)P4 the other inositol polyphosphate could partially revive the response to a subsequent addition of the first inducer. Our data clearly demonstrate that Ins(1,4,5)P3 and Ins(1,3,4,5)P4 are two equally important but independent second messengers in E. histolytica.

Thermodynamics of colchicinoid-tubulin interactions. Rrol of B-ring and C-7 substituent

The quenching of tryptophan fluorescence has been used to determine the kinetic and thermodynamic parameters of binding of B-ring analogs of colchicine to tubulin. The on rate, activation energy, off-rate, and thermodynamics of binding reaction have been found to be controlled at different points of analog structure. The on-rate and off-rate of deacetamidocolchicine (DAAC) binding with tubulin is 17 times slower than that of 2-methoxy-5-(2',3',4'-trimethoxyphenyl)tropone-tubulin (AC-tubulin) interaction, although both reactions have very similar activation energies. The presence of B-ring alone does not significantly affect the thermodynamics of the binding reactions either, since both AC-tubulin and DAAC-tubulin interactions are enthalpy driven. Introduction of a NH2 group at C-7 position of the B-ring, as in deacetylcolchicine (NH2-DAAC) lowers the on-rate further with a significant rise in the value of the activation energy. However, bulkier substitutions at the same position, as in demecolcine (NHMe-DAAC) and N-methyldemecolcine (NMe2-DAAC) have no significant additional effect either on the on-rate or on the value of activation energy. Introduction of NH2 group in the C-7 position of B-ring also increases the positive entropy of the binding reaction to a significant extent, and it is maximum when NMe2 is substituted instead of NH2 group. Thus, interaction of NH2-DAAC, NHMe-DAAC, and NMe2-DAAC with tubulin are entropy driven. Our results suggest that the B-ring side chain of aminocolchicinoids makes contact(s) with dimeric tubulin molecules.

A study of aggregation of 9-(dicyano-vinyl)julolidine

Di(cyano vinyl)julolidine (DCVJ) is a fluorescent probe which has been used to monitor the local mobility of its binding sites on proteins. It shows a concentration dependence of its emission spectrum in water. At higher DCVJ concentrations, a longer wavelength band appears. The latter increases relative to the shorter wavelength band as a function of increased DCVJ concentration. Absorption and excitation spectra indicate that the concentration dependent emission in the longer wavelength is a consequence of association in the ground state and subsequent excimer formation. DCVJ forms two types of complexes with gamma-cyclodextrin, one of which shows the longer wavelength emission band. Analysis of stoichiometry of association also suggests that longer wavelength emission band may be a consequence of association of two molecules of DCVJ in the gamma-cyclodextrin cavity. Possible uses of such excimer formation in biological systems have also been discussed.

Aging of tubulin monomers using 5,5'-bis(8-anilino-1-naphthalenesulfonate) as a probe

The fluorescent probe bis(8-anilino-1-naphthalenesulfonate) (bis-ANS) has been used to monitor the time- and temperature-dependent aging of tubulin, whereby new hydrophobic binding sites of lower affinity are generated on the protein [Prasad, A. R. S., et al. (1986a) Biochemistry 25, 739-742]. We carried out a detailed analysis of this phenomenon and found that, in addition to antimitotic drugs like colchicine or vinblastine, other parameters, viz., low temperature and protein stabilizers (e.g., glycerol and sucrose), inhibit the extent of enhanced binding of bis-ANS. Moreover, the generation of additional bis-ANS binding sites are also suppressed at high concentrations of tubulin. Cleavage of the carboxy-termini of tubulin (bound to bis-ANS) by subtilisin causes a significant reduction in the enhanced fluorescence, but has no effect on the high-affinity binding site of bis-ANS. All of these observations can be explained by the correlation of the presence of additional binding with the dissociation of heterodimeric tubulin into monomers. Enhanced binding of bis-ANS is due to tubulin dimers that have undergone dissociation, resulting in a loosening of its tertiary structure with the generation of a plethora of hydrophobic sites.

Interaction of a fluorescent analog of N-deacetyl-N-methyl-colchicine (colcemid) with liver alcohol dehydrogenase

The evidence for specific binding of N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)--colcemid (NBD-colcemid), a fluorescent analog of colcemid (N-deacetyl-N-methyl-colchicine), to liver alcohol dehydrogenase is presented. Alcohol dehydrogenase bound NBD-colcemid in a time-dependent manner, enhanced the fluorescence intensity, and caused a large blue shift of the emission maximum of the free drug. The specificity of binding was determined for both the colchicine nucleus and the NBD moiety. The binding was not affected by the presence of alcohol or NAD in the reaction mixture. Preincubation of horse liver alcohol dehydrogenase with colcemid inhibited the binding to a considerable extent. NBD-colcemid inhibited the enzymic activity of alcohol dehydrogenase in a mixed-type noncompetitive mode with a Ki value of 32 microM, whereas colcemid showed noncompetitive inhibition with a Ki of 100 microM. The association rate constant of NBD-colcemid binding with liver alcohol dehydrogenase was 587 M-1 s-1 at 25 degrees C. The stoichiometry and dissociation constant of the binding reaction were 0.62/dimer and 12 microM, respectively. Donor quenching experiments showed that both tryptophans of alcohol dehydrogenase transferred energy to the bound NBD-colcemid. Thus, this study reports the binding of a colchicine analog to a protein other than tubulin with high affinity. It is concluded that NBD-colcemid binding to dehydrogenases is a general phenomenon, but the common structural element(s) that is responsible for the binding activity, and which exists among tubulin and dehydrogenases, has yet to be determined.

A partially folded intermediate during tubulin unfolding: its detection and spectroscopic characterization

The unfolding reaction of the dimeric protein tubulin, isolated from goat brain, was studied using fluorescence and circular dichroism techniques. The unfolding of the tubulin dimer was found to be a two-step process at pH 7. The first step leads to the formation of an intermediate conformation, stable at around 1-2 M urea, followed by a second step that was due to unfolding of the intermediate state. At pH 3, the urea-induced biphasic unfolding profiles obtained at pH 7 became a one-step process indicating that a stable intermediate was also formed at this pH. The intermediate at pH 3 was more stable toward urea denaturation than that at pH 7. The intermediate state has about 60% secondary structure, partially exposed aromatic residues, and less tertiary structure as compared to the native states. Also, hydrophobic surfaces were more exposed in the intermediate than in the native or unfolded states. These results indicate that the intermediate state observed during tubulin unfolding is not only distinct from both the native and unfolded forms but also possesses some properties characteristic of a molten globule.

Role of the C-terminal tail region in the self-assembly of lambda-repressor

Acrylamide quenching of the tryptophan fluorescence of the lambda-repressor at different protein concentrations indicates that one of the three tryptophan residues, W129, W142, and W230, undergoes a change in environment upon self-assembly, from dimer to associated species. Quenching data suggest that this tryptophan residue is inaccessible to low concentrations of acrylamide and is blue-shifted in the associated form. In the dimer, this tryptophan residue is highly accessible to acrylamide and is red-shifted. NBS oxidation, at protein concentrations which favor the associated form, showed that this tryptophan is also significantly protected from NBS oxidation. HPLC peptide mapping of NBS-oxidized lambda-repressor, amino acid analysis, and sequencing indicate that the protected, blue-shifted tryptophan is tryptophan 230. A mutant repressor (F235C) was specifically labeled at Cys 235 with an environment-sensitive probe, acrylodan. The acrylodan fluorescence of the labeled F235C lambda-repressor undergoes a significant blue-shift, accompanied by fluorescence enhancement, upon protein association. Along with other genetic evidence, these results suggest involvement of the C-terminal tail region in the self-assembly of the lambda-repressor.