Time-Resolved Small-Angle X-Ray Scattering of Protein Cage Assembly.

Recent improvements in X-ray detectors and synchrotron light sources have made it possible to measure time-resolved small-angle X-ray scattering (TR-SAXS) at millisecond time resolution. As an example, in this chapter we describe the beamline setup, experimental scheme, and the points that should be noted in stopped-flow TR-SAXS experiments for investigating the ferritin assembly reaction.

Morphological difference of Escherichia coli non-heme ferritin iron cores reconstituted in the presence and absence of inorganic phosphate.

The iron core of Escherichia coli ferritin was reconstituted in the presence and absence of phosphate. The core formed in the presence of phosphate contained phosphate in amounts comparable to the iron content. The size distribution of the core was analyzed by analytical ultracentrifugation. A continuous size distribution was observed in the presence of phosphate, whereas a multimodal distribution was found in the absence of phosphate. In the presence of phosphate, the core size observed by electron microscopy was consistent with the inner diameter of ferritin. In contrast to this, clusters of several smaller particles were observed in the absence of phosphate. The small-angle X-ray scattering was measured under contrast matching conditions to obtain information on the iron core shape. A fringe was observed in the scattering profile in the presence of phosphate, but it was not observed in the absence of phosphate. Combining all results, we conclude that a hollow spherical core was formed in the presence of phosphate, while several small particles were formed within the inner cavity in the absence of phosphate.

Mechanisms of ferritin assembly studied by time-resolved small-angle X-ray scattering.

The assembly reaction of Escherichia coli ferritin A (EcFtnA) was studied using time-resolved small-angle X-ray scattering (SAXS). EcFtnA forms a cage-like structure that consists of 24 identical subunits and dissociates into dimers at acidic pH. The dimer maintains native-like secondary and tertiary structures and can reassemble into a 24-mer when the pH is increased. The time-dependent changes in the SAXS profiles of ferritin during its assembly were roughly explained by a simple model in which only tetramers, hexamers, and dodecamers were considered intermediates. The rate of assembly increased with increasing ionic strength and decreased with increasing pH (from pH 6 to pH 8). These tendencies might originate from repulsion between assembly units (dimers) with the same net charge sign. To test this hypothesis, ferritin mutants with different net charges (net-charge mutants) were prepared. In buffers with low ionic strength, the rate of assembly increased with decreasing net charge. Thus, repulsion between the assembly unit net charges was an important factor influencing the assembly rate. Although the differences in the assembly rate among net-charge mutants were not significant in buffers with an ionic strength higher than 0.1, the assembly rates increased with increasing ionic strength, suggesting that local electrostatic interactions are also responsible for the ionic-strength dependence of the assembly rate and are, on average, repulsive.

In vitro assembly of Haemophilus influenzae adhesin transmembrane domain and studies on the electrostatic repulsion at the interface.

Haemophilus influenzae adhesin (Hia) belongs to the trimeric autotransporter family, and it mediates the adherence of these bacteria to the epithelial cells of host organisms. Hia is composed of the passenger domain, which is a virulence factor, and the translocator domain, which anchors the passenger domain into the outer membrane. The Hia transmembrane domain forms a transmembrane ß-barrel of 12 ß-strands, four of which are provided from each subunit. The ß-barrel has a pore that is traversed by three α-helices, one of which is provided from each subunit. This domain has a unique arginine arrangement inside the ß-barrel. The side chains of the arginine residues protrude from the ß-strands of three subunits toward the center of the barrel and are close to each other. Mutation of this arginine residue revealed the importance of the electrostatic repulsion between the three arginines. Electrostatic repulsion is considered to prevent misfolding and/or misassembly. The arginine clusters at the interface were found in several proteins and might generally play an important role in the assembly of the oligomer.

Structure, Function, Folding, and Aggregation of a Neuroferritinopathy-Related Ferritin Variant.

Neuroferritinopathy is a rare, adult-onset, dominantly inherited movement disorder caused by mutations in the ferritin gene. A ferritin light-chain variant related to neuroferritinopathy, in which alanine 96 is replaced with threonine (A96T), was expressed in Escherichia coli, purified, and characterized. The circular dichroism, analytical ultracentrifugation, and small-angle X-ray scattering studies have shown that both the subunit structure and the assembly of A96T are the same as those of wild-type human ferritin light chain (HuFTL). The iron-incorporation ability was also comparable to that of HuFTL. Although the structural stability against heat, acid, and denaturant was reduced, the structure was sufficiently stable under physiological conditions. The most remarkable defects observed for A96T were a lower refolding efficiency and a stronger propensity to aggregate. The possible relationship between folding deficiency and disease is discussed.

ß-Strand twisting/bending in soluble and transmembrane ß-barrel structures.

The majority of ß-strands in globular proteins have a right-handed twist and bend. The dominant driving force for ß-strand twisting is thought to be inter-strand hydrogen bonds. We previously demonstrated that for water-soluble proteins, both the twisting and bending of ß-strand are suppressed by the polar side chains of serine, threonine, and asparagine residues. To determine whether this also holds for transmembrane ß-strands, we calculated and statistically analyzed the twist and bend angles of four-residue frames of ß-strands in both transmembrane and water-soluble ß-barrel proteins with known three-dimensional structures. In the case of transmembrane ß-strands, we found that twisting was suppressed even for frames not containing serine, threonine, or asparagine residues. The suppression of twisting in transmembrane ß-strands could be attributed to the propagation of the suppressive effect of serine, threonine, and asparagine residues within a frame to the neighboring, hydrogen-bonded strands under the restriction that all strands in the closed barrel structure must have similar twist angles. A similar tendency was also observed for water-soluble ß-barrel proteins. We previously showed that the dominant driving force for ß-strand bending is hydrophobic interactions involving aromatic residues within and outside the strand. Transmembrane ß-barrels have no hydrophobic core; however, rather hydrophilic residues predominate inside the barrel or the ß-strands of transmembrane ß-barrels have larger bend angles than those of water-soluble ß-barrels. Our results reveal that, in transmembrane ß-barrel proteins, the glycine-aromatic ring motif is important for generating the ß-strand bending necessary for barrel formation.

Optimization of Haemophilus influenzae adhesin transmembrane domain expression in Escherichia coli.

To obtain a high yield of the transmembrane domain of Haemophilus influenzae adhesin (HiaTD) in Escherichia coli, we attempted to express the HiaTD with and without a signal sequence using a T7 expression system. The expression level of HiaTD after induction was followed by quantification of the purified HiaTD, flow cytometric analysis of the outer membrane integrated HiaTD, and immunoblotting assay of fractionated cell lysate. In the expression system with a signal sequence, although the amount of cell-surface-expressed HiaTD increased over time, the number of HiaTD-expressing cells decreased, probably because of plasmid instability. As a result, the amount of purified HiaTD reached a plateau at 2 h postinduction. Although expression without the signal sequence provides a large amount of proteins as inclusion bodies in some membrane proteins, HiaTD expressed without a signal sequence was not observed as inclusion bodies and seemed to be assembled into the outer membrane during or after cell lysis.

Electrostatic Repulsion between Unique Arginine Residues Is Essential for the Efficient in Vitro Assembly of the Transmembrane Domain of a Trimeric Autotransporter.

Haemophilus influenzae adhesin (Hia) belongs to the trimeric autotransporter family and mediates the adherence of these bacteria to the epithelial cells of host organisms. Hia contains a passenger and a transmembrane domain. The transmembrane domain forms a 12-stranded ß-barrel in which four strands are provided by each subunit. The ß-barrel has a pore that is traversed by three α-helices. This domain has a unique arginine cluster, in which the side chains of the three arginine residues located at position 1077 (Arg1077) protrude into the pore of the ß-barrel. This arrangement seems to be unfavorable for assembly, because of repulsion between the positive charges. In this study, we investigated the in vitro assembly of the Hia transmembrane minimum domain (mHiaTD) and found that the dissociated mHiaTD reassembled in detergent solution. To investigate the role of Arg1077 in trimer assembly, we generated mutant proteins in which Arg1077 was replaced with methionine or lysine. The reassembly kinetics of the mutants was compared with that of the wild-type protein. The methionine mutant showed misassembly, whereas the lysine mutant showed reversible assembly, similar to that observed for the wild-type protein. These results show that electrostatic repulsion between the positive charges of Arg1077 is important for preventing the formation of misassembled oligomers by the mHiaTD in vitro.

Residual structures in the unfolded state of starch-binding domain of glucoamylase revealed by near-UV circular dichroism and protein engineering techniques.

Protein folding is a thermodynamic process driven by energy gaps between the native and unfolded states. Although a wealth of information is available on the structure of folded species, there is a paucity of data on unfolded species. Here, we analyzed the structural properties of the unfolded state of the starch-binding domain of glucoamylase from Aspergillus niger (SBD) formed in the presence of guanidinium hydrochloride (GuHCl). Although far-UV CD and intrinsic tryptophan fluorescence spectra as well as small angle X-ray scattering suggested that SBD assumes highly unfolded structures in the presence of GuHCl, near-UV circular dichroism of wild-type SBD suggested the presence of residual structures in the unfolded state. Analyses of the unfolded states of tryptophan mutants (W543L, W563A, W590A and W615L) using Similarity Parameter, a modified version of root mean square deviation as a measure of similarity between two spectra, suggested that W543 and W563 have preferences to form native-like residual structures in the GuHCl-unfolded state. In contrast, W615 was entirely unstructured, while W590 tended to form non-native ordered structures in the unfolded state. These data and the amino acid sequence of SBD suggest that local structural propensities in the unfolded state can be determined by the probability of the presence of hydrophobic or charged residues nearby tryptophan residues.

Ferritin Assembly Revisited: A Time-Resolved Small-Angle X-ray Scattering Study.

The assembly reaction of Escherichia coli ferritin A (EcFtnA) was studied using time-resolved small-angle X-ray scattering (TR-SAXS). EcFtnA forms a cagelike structure that consists of 24 identical subunits and dissociates into dimers at acidic pH. The dimer maintains nativelike secondary and tertiary structures and is able to reassemble into a 24-mer when the pH is increased. The reassembly reaction was induced by pH jump, and reassembly was followed by TR-SAXS. Time-dependent changes in the forward scattering intensity and in the gyration radius suggested the existence of a significant population of intermediate oligomers during the assembly reaction. The initial reaction was a mixture of second- and third-order reactions (formation of tetramers and hexamers) from the protein concentration dependence of the initial velocity. The time-dependent change in the SAXS profile was roughly explained by a simple model in which only tetramers, hexamers, and dodecamers were considered as intermediates.

Electrostatic Repulsion during Ferritin Assembly and Its Screening by Ions.

Escherichia coli non-heme-binding ferritin A (EcFtnA) is a spherical cagelike protein that is composed of 24 identical subunits. EcFtnA dissociates into 2-mers under acidic conditions and can reassemble into the native structure when the pH is increased. To understand how electrostatic interactions influence the assembly reaction, the dependence of the process on ionic strength and pH was investigated. The assembly reaction was initiated by stopped-flow mixing of the acid-dissociated EcFtnA solution and high-pH buffer solutions and monitored by time-resolved small-angle X-ray scattering. The rate of assembly increased with increasing ionic strength and decreased with increasing pH from 6 to 8. These dependences were thought to originate from repulsion between assembly units (2-mer in the case of EcFtnA) with the same net charge sign; therefore, to test this assumption, mutants with different net charges (net-charge mutants) were prepared. In buffers with a low ionic strength, the rate of assembly increased with a decreasing net charge. Thus, repulsion between the assembly unit net charges was demonstrated to be an important factor determining the rate of assembly. However, the difference in the assembly rate among net-charge mutants was not significant in buffers with an ionic strength of >0.1. Notably, under such high-ionic strength conditions, the assembly rate increased with an increasing ionic strength, suggesting that local electrostatic interactions are also responsible for the ionic strength dependence of the rate of assembly and are repulsive on average.

The origin of ß-strand bending in globular proteins.

BACKGROUND: Many ß-strands are not flat but bend and/or twist. However, although almost all ß-strands have a twist, not all have a bend, suggesting that the underlying force(s) driving ß-strand bending is distinct from that for the twist. We, therefore, investigated the physical origin(s) of ß-strand bends. METHODS: We calculated rotation, twist and bend angles for a four-residue short frame. Fixed-length fragments consisting of six residues found in three consecutive short frames were used to evaluate the twist and bend angles of full-length ß-strands. RESULTS: We calculated and statistically analyzed the twist and bend angles of ß-strands found in globular proteins with known three-dimensional structures. The results show that full-length ß-strand bend angles are related to the nearby aromatic residue content, whereas local bend angles are related to the nearby aliphatic residue content. Furthermore, it appears that ß-strands bend to maximize their hydrophobic contacts with an abutting hydrophobic surface or to form a hydrophobic side-chain cluster when an abutting hydrophobic surface is absent. CONCLUSIONS: We conclude that the dominant driving force for full-length ß-strand bends is the hydrophobic interaction involving aromatic residues, whereas that for local ß-strand bends is the hydrophobic interaction involving aliphatic residues.

A Physicochemical and Mutational Analysis of Intersubunit Interactions of Escherichia coli Ferritin A.

Ferritin A from Escherichia coli (EcFtnA) is 24-meric protein, which forms spherical cagelike structures called nanocages. The nanocage structure is stabilized by the interface around 4-, 3-, and 2-fold symmetric axes. The subunit structure of EcFtnA comprises a four-helix bundle (helices A-D) and an additional helix E, which forms a 4-fold axis. In this study, we examined the contribution of the interface around three symmetric axes. pH-induced dissociation experiments monitored by analytical ultracentrifugation and small-angle X-ray scattering showed that the dimer related by 2-fold symmetry is the most stable unit. Mutations located near the 3-fold axis revealed that the contribution of each interaction was small. A mutant lacking helix E at the 4-fold axis formed a nanocage, suggesting that helix E is not essential for nanocage formation. Further truncation of the C-terminus of helix D abrogated the formation of the nanocage, suggesting that a few residues located at the C-terminus of helix D are critical for this process. These properties are similar to those known for mammalian ferritins and seem to be common principles for nanocage formation. The difference between EcFtnA and mammalian ferritins was that helix E-truncated EcFtnA maintained an iron-incorporating ability, whereas mammalian mutants lost it.

Relationship between chain collapse and secondary structure formation in a partially folded protein.

Chain collapse and secondary structure formation are frequently observed during the early stages of protein folding. Is the chain collapse brought about by interactions between secondary structure units or is it due to polymer behavior in a poor solvent (coil-globule transition)? To answer this question, we measured small-angle X-ray scattering for a series of ß-lactoglobulin mutants under conditions in which they assume a partially folded state analogous to the folding intermediates. Mutants that were designed to disrupt the secondary structure units showed the gyration radii similar to that of the wild type protein, indicating that chain collapse is due to coil-globule transitions.

Effect of non-native helix destabilization on the folding of equine ß-lactoglobulin.

ß-lactoglobulin forms a non-native α-helix during an early stage of folding. To address the role of the non-native structure in the folding process, we designed several mutants of equine ß-lactoglobulin with reduced helical propensity in the non-native helix region. One of them, A123T, showed a similar structure to that of the wild-type protein; its folding kinetics was investigated by stopped-flow circular dichroism (CD) and fluorescence. Although A123T showed a reduced burst-phase CD intensity, its folding rate was similar to that of the wild-type protein, which indicated that the formation of the non-native helix does not accelerate or decelerate the folding reaction.

Transient non-native helix formation during the folding of ß-lactoglobulin.

In ideal proteins, only native interactions are stabilized step-by-step in a smooth funnel-like energy landscape. In real proteins, however, the transient formation of non-native structures is frequently observed. In this review, the transient formation of non-native structures is described using the non-native helix formation during the folding of ß-lactoglobulin as a prominent example. Although ß-lactoglobulin is a predominantly ß-sheet protein, it has been shown to form non-native helices during the early stage of folding. The location of non-native helices, their stabilization mechanism, and their role in the folding reaction are discussed.

Delineation of solution burst-phase protein folding events by encapsulating the proteins in silica gels.

Many studies have shown that during the early stages of the folding of a protein, chain collapse and secondary structure formation lead to a partially folded intermediate. Thus, direct observation of these early folding events is crucial if we are to understand protein-folding mechanisms. Notably, these events usually manifest as the initial unresolvable signals, denoted the burst phase, when monitored during conventional mixing experiments. However, folding events can be substantially slowed by first trapping a protein within a silica gel with a large water content, in which the trapped native state retains its solution conformation. In this study, we monitored the early folding events involving secondary structure formation of five globular proteins, horse heart cytochrome c, equine ß-lactoglobulin, human tear lipocalin, bovine α-lactalbumin, and hen egg lysozyme, in silica gels containing 80% (w/w) water by CD spectroscopy. The folding rates decreased for each of the proteins, which allowed for direct observation of the initial folding transitions, equivalent to the solution burst phase. The formation of each initial intermediate state exhibited single exponential kinetics and Arrhenius activation energies of 14-31 kJ/mol.

Local sequence of protein ß-strands influences twist and bend angles.

ß-Sheet twisting is thought to be mainly determined by interstrand hydrogen bonds with little contribution from side chains, but some proteins have large, flat ß-sheets, suggesting that side chains influence ß-structures. We therefore investigated the relationship between amino acid composition and twists or bends of ß-strands. We calculated and statistically analyzed the twist and bend angles of short frames of ß-strands in known protein structures. The most frequent twist angles were strongly negatively correlated with the proportion of hydrophilic amino acid residues. The majority of hydrophilic residues (except serine and threonine) were found in the edge regions of ß-strands, suggesting that the side chains of these residues likely do not affect ß-strand structure. In contrast, the majority of serine, threonine, and asparagine side-chains in ß-strands made contacts with a nitrogen atom of the main chain, suggesting that these residues suppress ß-strand twisting.

α-helix formation rate of oligopeptides at subzero temperatures.

In 1999, Clarke et al. ((1999) Proc. Natl. Acad. Sci. USA 96, 7232-7237) reported that the nucleation rate of α-helix of oligopeptide AK16 is as slow as 60 ms. In the present study, we measured the nucleation rate of oligopeptide, C17 (DLTDDIMCVKKILDKVG, corresponding to α-helical region of 84th to 100th amino acids of bovine α-lactalbumin) using the same method as Clarke et al. We found only initial bursts of the increase of α-helices at temperatures higher than -50°C in the presence of 70% methanol. The result with AK16 was the same as Clarke et al. reported. We also found that the folding rate of polyglutamic acid is too fast to be detected by the stopped-flow apparatus at 4°C. These results demonstrate that the α-helix formation rates in C17, AK16 and polyglutamic acid are shorter than the dead time of the stopped-flow apparatus (6 ms).

Dependence of α-helical and ß-sheet amino acid propensities on the overall protein fold type.

BACKGROUND: A large number of studies have been carried out to obtain amino acid propensities for α-helices and ß-sheets. The obtained propensities for α-helices are consistent with each other, and the pair-wise correlation coefficient is frequently high. On the other hand, the ß-sheet propensities obtained by several studies differed significantly, indicating that the context significantly affects ß-sheet propensity. RESULTS: We calculated amino acid propensities for α-helices and ß-sheets for 39 and 24 protein folds, respectively, and addressed whether they correlate with the fold. The propensities were also calculated for exposed and buried sites, respectively. Results showed that α-helix propensities do not differ significantly by fold, but ß-sheet propensities are diverse and depend on the fold. The propensities calculated for exposed sites and buried sites are similar for α-helix, but such is not the case for the ß-sheet propensities. We also found some fold dependence on amino acid frequency in ß-strands. Folds with a high Ser, Thr and Asn content at exposed sites in ß-strands tend to have a low Leu, Ile, Glu, Lys and Arg content (correlation coefficient = -0.90) and to have flat ß-sheets. At buried sites in ß-strands, the content of Tyr, Trp, Gln and Ser correlates negatively with the content of Val, Ile and Leu (correlation coefficient = -0.93). "All-ß" proteins tend to have a higher content of Tyr, Trp, Gln and Ser, whereas "α/ß" proteins tend to have a higher content of Val, Ile and Leu. CONCLUSIONS: The α-helix propensities are similar for all folds and for exposed and buried residues. However, ß-sheet propensities calculated for exposed residues differ from those for buried residues, indicating that the exposed-residue fraction is one of the major factors governing amino acid composition in ß-strands. Furthermore, the correlations we detected suggest that amino acid composition is related to folding properties such as the twist of a ß-strand or association between two ß sheets.