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1 BioMaDe Technology Foundation, 9747 AG Groningen, The Netherlands
2 NIZO food research, 6710 BA Ede, The Netherlands
3 Van’t Hoff Laboratory, Debeye Research Institute, Utrecht University, 3584 CH Utrecht, The Netherlands
4 Department of Biochemistry, University of Groningen, 9747 AG Groningen, The Netherlands
Reprint requests to: George T. Robillard, BioMaDe Technology Foundation, Nijenborgh 4, 9747 AG Groningen, The Netherlands; e-mail: Robillard{at}biomade.nl; fax: 31-50-3634429.
(RECEIVED August 13, 2003; FINAL REVISION November 26, 2003; ACCEPTED December 2, 2003)
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Abstract |
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Keywords: hydrophobin SC3; oligomerization; molecular exchange; association/dissociation; structural changes; self-assembly
Article and publication are at http://www.proteinscience.org/cgi/doi/10.1110/ps.03367304.
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Introduction |
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TOPAbstractIntroductionResultsDiscussionMaterials and methodsReferences |
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Hydrophobins are characterized by eight conserved cysteine residues that are proposed to form four disulfide bridges; otherwise their amino acid sequences are diverse and the length of the N-terminal sequence preceding the first cysteine residue is variable (de Vries et al. 1993; Wessels 1997). Two different types of hydrophobins, class I and class II, have been distinguished based on the differences in their hydropathy patterns and biophysical properties (Wessels 1994). SC3, secreted by Schizophyllum commune, is the best-characterized class I hydrophobin. It is distinguished from other hydrophobins by a long N-terminal sequence preceding the first cysteine residue. There are 16 to 22 mannose residues attached to the 12 threonine residues in this region, which are probably exposed at the hydrophilic side of assembled SC3 and, therefore, determine the surface properties of this side (de Vocht et al. 1998). Circular dichroism spectroscopy (CD) and attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) revealed that the protein formed a stable intermediate with an 
-helical signature upon the binding to a solid hydrophobic surface such as Teflon. This intermediate was able to convert to a structure with a 
-sheet signature upon adding detergent and heating the sample. In contrast, no stable -helical intermediate was seen if SC3 assembled at an air/water interface; instead, a -sheet structure was observed to form over a period of several hours. SC3 with the -helical signature appears to be monomeric when bound to a hydrophobic solid surface at low occupancy, as reported by the lack of fluorescence resonance energy transfer (FRET) between populations of fluorescence donor- and acceptor-labeled SC3. Upon heating in the presence of detergent, self-assembly occurred over a period of several minutes, followed by a slow conformational change to a stable structure characterized by a -sheet signature and the presence of rodlets (Wang et al. 2002).
Although much work has been done on the structural changes and molecular interactions of SC3 bound to a hydrophobic surface, the soluble-state of SC3, which is the starting point of engaging its surface activity, has still to be explored. SC3 had been assumed to be monomeric in solution until our previous FRET studies clearly showed that SC3 dissociates in pure trifluoroacetic acid (TFA) but associates once resuspended in buffer after pure TFA treatment (Wang et al. 2002); however, the details of the association of SC3 in solution are not known. Are there multiple association states? What are their sizes, and under which conditions can they be dissociated? Such questions have been addressed in this article in an effort to understand how subunit interactions might influence the surface activity of SC3 and its self-assembly properties at an air/water interface.
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Results |
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shows a typical SEC elution pattern of purified soluble-state SC3 in 50 mM sodium phosphate (pH 7.0). Except for a small portion of large aggregates at the beginning and a broad shoulder between 10 and 18 min, three peaks, indicated as P1, P2, and P3, appeared, with retention times of 19.3, 20.5, and 24.0 min, respectively. Based on the absorption at 215 nm, >80% of the applied protein was eluted during the chromatography, and it could be estimated that P2 contained ~80% of the eluted protein. All three peaks were collected separately, lyophilized, treated with TFA, and analyzed by SDS-PAGE and Western Blot by using an anti-SC3 antibody. The same SC3 band was seen in all three fractions, indicating that these three peaks contained SC3 in different oligomeric states, which, however, could be dissociated by TFA treatment into the monomers (data not shown). To further confirm the mass of the major oligomeric state of SC3, the P2 fraction, we used SEC-MALLS, which combines SEC and multiangle laser light scattering (MALLS). With this technique, the particles can be detected in real-time by the light scattering detectors arranged at 18 different angles around the cell, and the absolute molar masses and distribution can readily be obtained. The result is shown in Figure 2A. Soluble-state SC3 again showed a major peak on SEC-MALLS as it had on normal SEC (dotted line); the molar mass was determined to be ~30 kD, which corresponds to the mass of dimeric SC3 (solid line). The initial area of this peak, the position of which corresponds to the P1 fraction in Figure 1, has a molar mass of ~70 kD, which is probably tetrameric SC3. The ratio between dimer and tetramer was estimated to be 7 : 3, and the yield of the eluted protein, including the large aggregates at the beginning, was ~70%. The remaining protein either stuck to the column material or was excluded by the column as large aggregates. This experiment therefore confirms the conclusion that SC3 mainly forms a dimer in buffer, but that certain amount of tetramer and larger oligomers are also present.
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). First, SC3 prior to chromatography was measured, as described in Materials and Methods. More than two components could be identified after data fitting, indicating that the solution was heterogeneous (Fig. 2B, left). In another experiment, SC3 was subjected to SEC, and peak P2 was collected (the concentration was >100 µg/mL, ensuring a high signal to noise ratio) and subjected to DLS measurement immediately. The resulting data showed a single component upon data fitting, indicating that the solution was relatively homogeneous (Fig. 2B, right). The apparent hydrodynamic radius of the particle in peak P2 was ~3.4 nm, which is larger than that of a dimeric SC3 if it would have a spherical shape (~2.7 nm). This may indicate that the shape of dimeric SC3 in buffer may not be spherical, but rather elongated.
Time-dependent aggregation of dimerized SC3
The stability of dimeric SC3 in buffer has been checked with SEC. Peak P2 was collected and reloaded onto the same column under the same conditions after 2-h incubation at room temperature. Only peak P2 and P3 could be seen, whereas peak P1 and its front shoulders were missing, indicating that a small amount of monomers but no tetramers or higher oligomers formed during the incubation or subsequent chromatography (data not shown). When the same peak P2 was stored overnight at 4°C and then rechromatographed, only peak P2 indicated that dimeric SC3 is very stable at low temperature.
The oligomeric state of SC3 in the P2 fractions was further characterized by fluorescence anisotropy using dansyl-labeled SC3 (dansyl-SC3). Dansyl-SC3 was used in this experiment because the long lifetime of the covalently labeled probe (
18 nsec), makes it possible to distinguish monomers from multimers on the basis of their rotational correlation times. Dansyl-SC3 also showed three peaks with the same retention times as SC3 on SEC (data not shown). The P2 fractions were collected and subjected to time-resolved fluorescence anisotropy measurement. Three lifetimes—0.8 to 0.9 nsec; 5 to 6 nsec, and 18 to 20 nsec—could be identified after fitting the time-resolved data (Fig. B; Table 1, upper panel). As a control, only one lifetime of 3.3 nsec was determined for the free dansyl group in buffer (Fig. 3A; Table 1). The three different lifetimes observed for dansyl-SC3 might reflect the structural heterogeneity of soluble-state SC3, which had also been indicated by previous fluorescence and CD measurements (Wang et al. 2002). The fluorescence anisotropy decay was then measured on collected P2 fractions of another sample of dansyl-SC3 chromatographed under the same conditions. The data were taken 1 to 2 h after chromatography due to the optimization of the experimental setup. The resulting anisotropy decay can be described by two exponentials, corresponding to rotational correlation times of 1.77 (
1) and 19.74 (2) nsec, respectively (Fig. 3C,D). Table 1, lower panel, lists the correlation times measured for the sample prior to chromatography and for the pooled P2 fractions after 1-, 8-, and 24-h incubation at room temperature. It seems unlikely that 1 reflects the tumbling of monomeric SC3 because the fraction of 1 (A1, in Table 1) is 30% to 40%, whereas, as indicated by SEC, only a very low amount of monomeric SC3 (P3) is present. Therefore, 1 probably reflects the tumbling of a partially restricted dansyl group attached to SC3, whereas 2 reflects the tumbling of the whole particle. The considerable time-dependent increase of 2 (19.7, 35.1, and 61.5 nsec for the 1-, 8-, and 24-h samples, respectively) strongly indicates that soluble-state SC3 forms larger oligomers at room temperature. 1 for dansyl-SC3 prior to chromatography resembled that determined for P2, but 2 was much larger (64 nsec), most likely due to the larger oligomers that exist in the sample.
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2, which is ~20 nsec. Depending on the degree of hydration, the calculated rotational correlation time for dimeric dansyl-SC3 varies from 8.8 to 13.2 nsec, assuming that the particles are spherical in shape. The obvious difference between the theoretical and measured rotational correlation time might be caused by a higher degree of hydration, an elongated shape of dimeric SC3 in buffer, or both. The later has also been indicated by the DLS data.
Factors that affect the oligomerization
When TFA-treated SC3 was resuspended in water and the chromatography carried out in pure water instead of 50 mM sodium phosphate (pH 7.0), a completely different elution pattern was observed (cf. Figs. 1 and 4A). The pH of this sample was 6.0. The presence of a broad peak in Figure 4A, with a retention time lasting from 17 to 22 min, indicates the occurrence of larger oligomers. A sharp peak, with a retention time of ~24 min, however, most likely represents the presence of monomeric SC3. This monomeric SC3, however, was not stable in water. A fraction of Pb, which contained monomeric SC3, was collected and stored for 1 h at 4°C before being loaded onto the same column again equilibrated with water. The pH of sample was checked and determined to be 5.0 to 6.0. Pb disappeared together with the emergence of a peak at the position of dimeric SC3; all monomers appear to have converted to dimers in this case (data not shown). When 50 mM sodium chloride in water (pH -6) instead of pure water was used to repeat the same experiment, the elution pattern was the same as that obtained when using 50 mM sodium phosphate (pH 7.0; Fig. 4B), indicating that increased ionic strength results in oligomerization of SC3. All efforts made to get a pure monomeric form or to keep the monomeric fraction for longer time failed, indicating that SC3 monomers have a high tendency to oligomerize.
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). Therefore, SC3 forms larger oligomers at higher pH, indicating that the sole basic amino acid in SC3, histidine, which is located at the N-terminal domain, may play an important role in keeping SC3 dimeric in solution.
Monomer–dimer–tetramer equilibrium
When P2 fractions collected from the SEC were reloaded onto the same column under the same condition, P1 shown in the first chromatography was no longer present, but the distribution of the last two peaks, P2 and P3, remained unchanged (data not shown). No apparent difference could be observed in the peak distribution whether P2 fractions from the first chromatography were reloaded 1 or 48 h after being pooled and stored at 4°C. Apparently, the equilibrium between SC3 in P1 and P2 is a relatively slow process; however, the equilibrium between SC3 in P2 and P3 is established quickly and remains stable for many hours at 4°C.
SC3 samples at different protein concentrations were studied by SEC, and the transition between the tetramer in P1, the dimer in P2, and the monomer in P3 was monitored by comparing the peak areas (Fig. 5). After dilution from the same SC3 stock solution, the sample was incubated for ~10 min at room temperature and chromatographed under the same conditions (50 mM sodium phosphate at pH 7.0). After elution, the areas of all three peaks were integrated. When the protein concentration decreased from 1 mg/mL to 20 µg/mL, the ratio between the areas of P1, P2, and P3 varied from 17.9 : 81 : 1.1 to 13.2 : 85.9 : 0.9, indicating that the distribution of the oligomeric forms did not change much in this concentration range. However, upon lowering the concentration from 20 to 3.5 µg/mL, the peak ratios changed from 13.2 : 85.9 : 0.9 to 7.9 : 85.8 : 6.3, demonstrating a substantial increase in the fraction of monomer at the low protein concentration (a few micrograms per milliliter).
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). From 80 to 10 µg/mL, the fluorescence increased only slightly as the protein concentration was lowered, whereas at <4.5 µg/mL the fluorescence increased more dramatically. The donor fluorescence increase was obviously caused by the decrease of FRET due to the dissociation of oligomeric SC3. The fact that the protein used in this experiment was not chromatographed might be the cause of the two concentration dependencies. The shallow increase might reflect the dissociation of larger oligomers into smaller ones, which are themselves still quenched by the presence of acceptor-labeled protein within the same oligomer, whereas the steep increase might reflect mainly the monomerization from dimers, which has been indicated by the SEC experiment mentioned above.
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). Tetrameric SC3 and the sample after overnight incubation at room temperature also had the same CD spectrum (data not shown). All samples showed an ellipticity maximum at 209 nm, whereas a control, performic acid oxidized SC3 in the same buffer, showed a typical unstructured spectrum with a maximum ellipticity at ~200 nm. CD data of monomeric SC3 could not be measured due to the low protein concentration after SEC. The secondary structure elements of dimeric SC3 were then calculated from its CD spectrum based on molar ellipticity, which revealed ~12% -helix, 42% -sheet, 18% -turn, and 28% random coil. This result is consistent with a previous study that used unchromatographed SC3 (de Vocht et al. 1998). Therefore, by this criterion, oligomerization of SC3 in solution does not change the protein structure significantly; structural change only occurs when the protein self-assembles at a hydrophobic/hydrophilic interface. The fact that dimeric SC3 is structured was confirmed by a one-dimensional 1H NMR measurement (Fig. 7B). To increase the signal to noise ratio, SEC-separated, dimeric SC3 was concentrated four times at 4°C, as described in Materials and Methods. Spectral dispersion in the range of 7 to 9 ppm clearly showed that the protein is structured. The same measurement was repeated after storing the sample for 10 d at 4°C and yielded the same spectrum (data not shown). Such stability holds out hope for a successful high-resolution NMR structure determination of dimeric SC3 in the future.
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Discussion |
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SC3 oligomerization is relatively independent of pH but is dependent on ionic strength. Significant amount of monomers, along with some oligomers larger than dimer, were observed for SC3 in pure water. This monomeric form, however, is not stable. Rechromatography experiments clearly showed that monomers could convert to dimers completely within a few hours, even at low temperature, indicating that SC3 tends to form stable dimers in solution. Interestingly, the difference of using phosphate buffer or pure water to dissolve hydrophobin SC3 has been noticed previously in an experiment in which the surface activity of SC3 was compared with those of other proteins. It was found that when SC3 was dissolved in water at a concentration of 0.1 mg/mL, the water surface tension decreased from 72 to 32 mJ/m2, whereas the same protein dissolved in phosphate buffer could only decrease the surface tension to 43 mJ/m2 (Van der Vegt et al. 1996). Thus, the monomer–oligomer transition of SC3 seems to play an important role in determining the surface activity.
Despite the relative stability of the aggregates in buffer, the molecules within the aggregate are able to undergo exchange. We have demonstrated this previously by following energy transfer between fluorescence donor- and acceptor-labeled SC3 oligomer populations (Wang et al. 2002). In the current study, the dynamic exchange is visible in the redistribution of SCC3 between monomer and dimer, starting from an isolated dimer population. It is also visible from the concentration dependence of the energy transfer process in Figure 6, in which a strong decrease in quenching is reported at SC3 concentrations <5 µg/mL. A previous study of SC3 self-assembly using thioflavin T (ThT) fluorescence indicated that a critical concentration of 3.7 µg/mL existed, above which SC3 self-assembled at the air/liquid interface; below this critical concentration, the protein did not assemble (M. L. de Vocht, I. Reviakine, H. A. B. Wösten, A. Brisson, J. G. H. Wessels, and G. T. Robillard, unpubl.). The similarity between these two concentrations, as well as the stability of dimeric SC3, strongly indicates that the dimer is a fundamental building block and that dimerization is the initial event in SC3 self-assembly at an air/water interface and possibly on solid surfaces as well. The fact that, by CD criteria, the secondary structure of dimeric SC3 is identical to its higher aggregated form in solution seems to support such a conclusion, although how the dimer organizes into an assembled form and how it converts to a -sheet state as a result of assembly is still unknown. The class II hydrophobins, HFBI and HFBII from Trichoderma reesei, were recently characterized to be tetramers in solution by using small-angle X-ray scattering. The same tetramer was proposed to be the building block of the assembled form on a surface, based on the study of two-dimensional crystalline film by using atomic force microscopy (Torkkeli et al. 2002; Paananen et al. 2003). The unit cell in the film was found to have dimensions similar to that of the tetrameric aggregates in solution.
So far there is only one class I hydrophobin, EAS, whose soluble-state structure in solution has been studied systematically (Mackay et al. 2001). Three different isoforms of EAS are distinguishable on reverse-phase HPLC. Two of the three isoforms were present in low amounts and were determined to be unstructured monomer, whereas the dominant isoform could not be studied further due to its propensity to aggregate. The present study shows that SC3 mainly forms structured dimers in solution. Because of its low concentration and high propensity to dimerize, the monomeric form of SC3 could not be characterized, and therefore, we cannot exclude the possibility that the monomeric form of SC3 is also unstructured. According to the modern protein structure–function paradigm, native proteins (or their functional regions) can exist in any of the four thermodynamic states: ordered, molten globule, premolten globule, and random coil. The function of the protein can arise from any of the four conformations and transitions between them (Dunker et al. 2001; Uversky 2002). Accordingly, structured (ordered) oligomeric and possibly unstructured monomeric (premolten globule or random coil) hydrophobin can coexist, and transitions between them can take place. Actually some studies have already shown that self-association of an intrinsically unstructured protein induced folding into a molten globule or ordered structure (Ferre-D’Amare et al. 1993; Uversky et al. 2002). Obviously, such a disorder–order transition benefits proteins in different ways. For hydrophobins, the most important advantage might be preventing aggregation in the cell in the monomeric state and gaining the ability of binding to surfaces in the associated state. Furthermore, the formation of a structured oligomer might make hydrophobins thermodynamically stable and protease resistant in solution.
Stroud et al. (2003) recently proposed a model describing the association process of SC3, which involved three states: a monomer or multimer in solution (U-SC3), solution-assembled SC3 with less-ordered structure (S-SC3), and the interfacially assembled SC3 with a highly ordered and stacked -sheet structure (I-SC3). S-SC3 formation is a time-dependent self-assembly from a quiescent solution in the absence of an air/water or oil/water interface, whereas I-SC3 formation needs both an air/water interface and sufficient energy, such as vortexing. We have checked the time-dependent aggregation, or assembly, of SC3 by using both SEC and fluorescence anisotropy (Table 1). The aggregates resulting from long incubations without disturbance showed CD spectra similar to the original sample and chromatographed dimeric SC3. This is consistent with the conclusion of Stroud et al. (2003), derived from SDS-PAGE and ThT fluorescence. Furthermore, our study shows that soluble-state SC3 in solution mainly exists as the dimer not the monomer, and thus, the dimer is the principle building block for both solution aggregation and interfacial assembly. The estimation of the fraction of soluble-state SC3 in the various oligomeric forms is also consistent in both studies. In our SEC experiment, dimeric SC3 was estimated to be >70% of the total mass by comparing the peak areas, whereas Stroud et al. found that >48% of the soluble SC3 was monomeric or multimeric (U-SC3) based on density gradient centrifugation and SDS-PAGE. They also reported an inverse soluble-state SC3 (U-SC3) concentration dependence for the solution aggregation (S-SC3), which they were unable to explain but which can be understood in light of the present study. At low protein concentration (<6 µg/mL), monomerization takes place and the resulting unstable monomers rapidly associate into large SDS-insoluble aggregates. In contrast, a stable dimeric form dominates when the protein concentration is high, which prevents or retards the aggregation process. This assumption of course needs to be further investigated.
Based on our experimental data, we propose to extend the three-state model of Stroud et al. (2003) to a four-state model, including a distinct monomer–dimer transition (Fig. 8). In nature, monomeric SC3 might dimerize immediately after being excreted from fungal hyphae into the aqueous medium. Therefore, apart from the known stabilizing effect of four intramolecular disulfide bonds (de Vocht et al. 2000), dimerization may also play an important role in preventing SC3 being degraded or prematurely assembled. Dimeric SC3 can probably aggregate in solution without largely changing its structure, provided that the solution is not disturbed externally. However, given some external energy (e.g., heating or vortexing) or the presence of some hydrophobic/hydrophilic interfaces (e.g, air bubbles or a Teflon surface), dimeric SC3 assembles into a two-dimensional film with dramatic structural changes, which ends with the so called "-sheet state." Therefore, dimeric SC3 is the basis for establishing both solution aggregates (S-SC3) and interfacial self-assembly (I-SC3). Whether unstable and possibly unstructured monomeric SC3 can convert directly to S-SC3 and I-SC3 remains to be determined.
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Materials and methods |
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Size-exclusion chromatography
A Bio-Sil TSK-250 gel filtration column (600 x 7.5 mm, Bio-Rad) installed on a ChemStation HPLC system (Hewlett-Packard) was equilibrated with various buffers or water at a flow rate of 1 mL/min at room temperature. Lyophilized SC3 was dissolved in a small amount of TFA, dried with a flow of nitrogen gas, and then suspended in either buffer or pure water to a final concentration of 1 to 3 mg/mL. A 200 µL volume of soluble-state SC3 was injected into the column that had been equilibrated by the same buffer or water, followed by elution at a flow rate of 1 mL/min. The elution of SC3 was monitored at 215 nm. The eluate was collected and stored at 4°C, if necessary. The elution pattern was then analyzed by using the instrument software.
SEC-MALLS
SEC-MALLS was used to determine the molecular mass of soluble-state SC3. A TSK 2000SW column (7.5 x 300 mm, Agilent Technologies) was connected to an HPLC system, which was coupled to an Optilab DSP refractive index (RI) detector and a DAWN 18-angle lighter scattering detector (Wyatt Technology). The eluate from the SEC column could then be detected in terms of both refractive index and light scattering, and the absolute masses and molar mass distributions of particles in the eluate could be obtained in real-time. The whole system was equilibrated at a flow rate of 0.5 mL/min with filtered 50 mM sodium phosphate (pH 7.0), for >4 d before use, in order to remove all the large particles and dust from the system. Soluble-state SC3 at 23 mg/mL in the same buffer was centrifuged at 14,000 rpm for 10 min at 4°C, and the supernatant was injected into the column. The experiment was run for 45 min at the same flow rate as used for equilibration.
The data analysis was done by using Wyatt software, which follows the theory of classical light scattering:
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where R(,c) is the excess Rayleigh ratio of the solution as a function of scattering angle and concentration c. It is directly proportional to the intensity of the scattered light in excess of the light scattered by the pure solvent. c is the solute (SC3) concentration, Mw is the weight-averaged solute molar mass, and A2 is the second virial coefficient in the virial expansion of the osmotic pressure. K* is the constant 4p2(dn/dc)2n02/Na
04. dn/dc is the refractive index increment and was set to be 0.15, which is an average value for proteins, Na is Avogadro’s number, n0 is the index of refraction of the solvent, and 0 is the vacuum wavelength of the laser. P() describes the angular dependence of the scattered light and can be related to the root mean square radius.
Dynamic light scattering
Measurements were performed at 4°C by using a DynaPro instrument (Protein Solutions Inc.) equipped with a thermostated cell. This instrument uses light from an Nd:YAG laser lasing at 883 nm to measure the fluctuation in intensity of light scattered by molecules, which is then analyzed by using single-photon-counting electronics hardware. Under the assumption of Brownian motion and a hard-sphere model for the particle, the diffusion coefficient can be converted to the hydrodynamic radius (Rh) by using the Stokes-Einstein equation:
Rh = kBT/6
DT, where kB is Boltzmann’s constant, T is the absolute temperature, and is the viscosity.
The estimated MW is based on the assumption that the protein particles are approximate spheres. The MW was calculated by using an algorithm MW = (1.549Rh)2.426, which is based on linear calibration curves (log Rh versus log MW) constructed by using 18 different proteins. In practice, Rh and MW were calculated by using the software Dynapro V.4, which uses the above equations and autocorrelation coefficients. Soluble-state SC3 in 50 mM sodium phosphate (pH 7.0) with a concentration of 1 mg/mL was centrifuged at 14,000 rpm for 10 min at 4°C. Supernatant with a volume of >20 µL was added to the cuvette, and the measurement was started immediately to collect 10 to 20 data points. The theoretical two-exponential autocorrelation function (bimodal analysis) was used to fit the data. In the case of quantitatively determining the MW of SC3 in solution, only a highly homogeneous protein solution after purification with gel filtration was used. For a typical Rh of SC3 of ~3.4 nm, only those measurements with standard deviations in calculated hydrodynamic radius (Rh) of <0.2 nm were considered.
Specific labeling of hydrophobin SC3
The specific labeling of SC3 with amine reactive fluorescence dyes was performed as described previously (Wang et al. 2002). Briefly, to obtain soluble-state SC3, an amount of lyophilized SC3 was weighed, treated with chilled pure TFA, and then dried with a stream of nitrogen gas. Buffer containing 0.1M NaHCO3 (pH 7.2, for dansyl labeling) or 0.1M NaHCO3 (pH 8.3) plus 0.1% Empigen (for dabcyl labeling) was added to the dried material to a concentration of ~5 mg/mL. Concentrated fluorescence dye in dimethylformamide, either succinimidyl ester of 6-((5-dimethylaminonaphthalene-1-sulfonyl)amino)hexanoic acid (dansyl) or succinimidyl ester of 4-(4-(dimethylamino)azo) benzoic acid (dabcyl; Molecular Probes), was added slowly to the stirred solution of soluble-state SC3 until the molar ratio of fluorescence dye to SC3 reached 20 : 1; the volume added was <10% of the total volume. The reaction mixture was stirred slowly overnight in the dark and then centrifuged at 10,000g for 15 min. Subsequently, 0.5 mL supernatant was applied immediately to water-equilibrated desalting PD-10 column, followed by elution with water. The stoichiometry of labeling with dansyl was determined by MALDI-TOF mass spectrometry; 50% to 60% of SC3 was labeled. The CD spectrum showed that labeling occurred without gross conformational changes. In the case of dabcyl SC3, the stoichiometry of the reaction was based on the absorbance of dabcyl at 453 nm using an extinction coefficient of 32,000M-1cm-1. The labeling yield was 90% to 100%, assuming that one dabcyl molecule coupled to one SC3 molecule. The CD spectra were again similar to those of the unlabeled protein.
Steady-state fluorescence and time-resolved fluorescence measurements
A SPF-500C spectrofluorometer (SLM Aminco) with a 300-W Xenon lamp type 300 UV was used to measure the steady-state fluorescence of dansyl-labeled protein. Soluble-state dansyl-SC3 was prepared in 10 mM sodium phosphate (pH 7.0). The fluorescence spectra were then recorded with an excitation wavelength of 340 nm and an emission wavelength ranging from 400 to 650 nm. The resulting spectra were corrected for buffer and instrumental distortions.
For time-resolved fluorescence measurements, polarized fluorescence decay curves were measured by the time-correlated single photon counting technique. The laser system consisted of a Verdi-5W laser that pumped the mode-locked Mira Ti:Sapphire laser, followed by a pulse picker and a harmonics generator delivering subpicosecond pulses. The vertically polarized excitation provided an excitation wavelength of 350 nm with a pulse frequency of 1.9 MHz. Fluorescence emission was measured from 400 to 700 nm, using a WG360 cutoff filter (Schott). For fluorescence anisotropy decay measurements, the emitted light passed through a polarizer and was recorded with a Hamamatsu streak camera model C5680 equipped with a Cromex 250IS imaging spectrograph. Because the polarizer could be oriented both horizontally and vertically, the intensity of parallel Ih(t) and perpendicular Iv(t) components could be measured separately. For fluorescence lifetime measurements, the emitted light was recorded directly as I(t) without the polarizer. Fluorescence intensities, either polarized or not polarized, were then recorded in a time range of 50 nsec at 20°. To determine the time-dependent fluorescence anisotropy, r(t), the equation r = (Ih - GIv)/(Ih + 2G Iv) was used, where G is the grating factor that corrects for wavelength-dependent distortion of the polarizing system. It was determined by comparing the fluorescence intensities using horizontally polarized excitation light, with the polarizer in both the horizontal and vertical direction. The data, I(t) or r(t), were then fit to a sum of exponential decays by using a nonlinear least-squares algorithm, Y(t) = 
iexp(-t/xi), in which Y(t) is the fluorescence intensity or anisotropy at time t, and i is the amplitude of the ith component xi such that ii = 1. xi can be lifetime 
i or rotational correlation time i. The adequacy of the fit was judged by inspection of the plots of weighted residuals, which should be randomly distributed for a satisfactory fit. The average fluorescence lifetime () was calculated according to = ii/i, which is similar to that used to calculate the average rotational correlation time . The correlation time of the Brownian rotation of a protein () depends on the hydrated volume (V) and the shape of the protein, as well as on the temperature (T) and viscosity () of the solution as given by the Stokes-Einstein equation (spherical particle): = V/KT = M(
+ h) /RT, where K is the Boltzmann constant, M is the molecular mass of the protein, is the partial specific volume (milliliters per gram), and h the amount of hydration (milliliters per gram) of the rotating particle. Therefore, the rotational correlation time, , of monomeric SC3, with MW of 14 kD, can be calculated to be 4.37 (hydration is 0), 5.5 (hydration is 0.2), or 6.63 (hydration is 0.4).
Sample preparation for FRET
Based on the assumption that SC3 molecules associate in solution but dissociate in TFA, two different ways to prepare the samples for FRET measurements were used, as described previously (Wang et al. 2002). In the present study, only the second method was used. Briefly, the same amount of dansyl (energy donor)- and dabcyl (energy acceptor)-labeled protein was used. After mixing of the two species, the solution was freeze-dried, treated with TFA, dried with nitrogen gas, and finally dissolved in buffer to a total protein concentration of 10 µg/mL. Because dabcyl does not fluoresce, FRET can only be determined by the fluorescence quenching of the donor. The fluorescence intensity of donor was measured as described in Steady-State Fluorescence and Time-Resolved Fluorescence Measurements.
Circular dichroism spectroscopy
CD spectra of SC3 were recorded over the wavelength range from 190 to 250 nm on an Aviv 62A DS CD spectrometer, using a 1-mm quartz cuvette. The temperature was kept at 4°C, and the sample compartment was continuously flushed with nitrogen gas. The final spectra were obtained by averaging six scans, using a bandwidth of 1 nm, a stepwidth of 1 nm, and a 5-sec averaging per point. The spectra were then corrected for the background signal by using a reference solution without the protein.
Soluble-state SC3 and the chromatographed samples were prepared as mentioned. To prepare performic acid oxidized SC3, lyophilized SC3 was treated with TFA, dried with a flow of nitrogen gas, and then suspended in formic acid. The sample was incubated on ice for 5 min, followed by adding freshly made performic acid (a 1 : 10 mixture of hydrogen peroxide and formic acid incubated for 2 h at room temperature) to a final concentration of 10%. After 15-min incubation on ice, SC3 was separated from performic acid by elution through a PD-10 column, which was equilibrated with water. Fractions containing SC3 were pooled and lyophilized.
One-dimensional 1H NMR
Soluble-state SC3 in 50 mM sodium phosphate (pH 7.0) was chromatographed, and the peak P2 was collected and concentrated by using a Centricon centrifugal filter YM-10 (Millipore; MW cut-off, 10,000). The centrifugation was performed for 1 h at 4°C at 5000g. Subsequently, D2O was added to the concentrated sample to a final volume of 8%; the final protein concentration was ~1 mg/mL. One-dimensional NMR data, with water suppression, were recorded for 15 h at 4°C on a Varian Unity INOVA 600 NMR spectrometer. In total, 23,856 scans were acquired and averaged.
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