Radial mass analysis of the flagellar filament of Salmonella: implications for the subunit folding

Bending stiffness characterization of Bacillus subtilis' flagellar filament

The filament of a bacterial flagellum is a tube-like organelle made of single protein - flagellin, and assembled into multiple polymorphic forms. The filament can be further discretized into four subunit domains (D0, D1, D2 and D3) along the radial direction. However, it remains unclear which subunit domain plays an important role in regulating the rigidity of the filament. In this article, we address how the absence of two outer subunit domains (D2 and D3) affects the bending stiffness of the bacterium B. subtilis' flagellar filament. We first shear off flagellar filaments from the cell body, anchor one of its ends to the wall of a microfluidic channel, and correlate the elongation of the filament with the driving background flow. A numerical model is then applied to determine the bending stiffness of the filament. We find that the bending stiffness does not change drastically when the filament transforms from normal to hyperextended forms, which is estimated to be 2-3 pN.μm². Furthermore, B. subtilis' flagellar filament has similar bending stiffness to Salmonella's, though the radius of the former is almost half of that of the latter, suggesting that the rigidity comes from the inner D0 and D1 subunit domains.

Structure and Intermolecular Interactions between L-Type Straight Flagellar Filaments

Bacterial mobility is powered by rotation of helical flagellar filaments driven by rotary motors. Flagellin isolated from the Salmonella Typhimurium SJW1660 strain, which differs by a point mutation from the wild-type strain, assembles into straight filaments in which flagellin monomers are arranged in a left-handed helix. Using small-angle x-ray scattering and osmotic stress methods, we investigated the structure of SJW1660 flagellar filaments as well as the intermolecular forces that govern their assembly into dense hexagonal bundles. The scattering data were fitted to models, which took into account the atomic structure of the flagellin subunits. The analysis revealed the exact helical arrangement and the super-helical twist of the flagellin subunits within the filaments. Under osmotic stress, the filaments formed two-dimensional hexagonal bundles. Monte Carlo simulations and continuum theories were used to analyze the scattering data from hexagonal arrays, revealing how the bundle bulk modulus and the deflection length of filaments in the bundles depend on the applied osmotic stress. Scattering data from aligned flagellar bundles confirmed the theoretically predicated structure-factor scattering peak line shape. Quantitative analysis of the measured equation of state of the bundles revealed the contributions of electrostatic, hydration, and elastic interactions to the intermolecular forces associated with bundling of straight semi-flexible flagellar filaments.

Functional analysis of a large non-conserved region of FlgK (HAP1) from Rhodobacter sphaeroides

The single subpolar flagellum of Rhodobacter sphaeroides shows an enlarged hook-filament junction. One of the two proteins that compose this section of the filament is HAP1(Rs) (FlgK(Rs)) it contains a central non-conserved region of 860 amino acids that makes this protein about three times larger than its homologue in Salmonella enterica serovar Typhimurium. We investigated the role of this central portion of the unusually large HAP1 protein of R. sphaeroides by monitoring the effects of serial deletions in flgK (Rs) , the gene encoding HAP1(Rs), on swimming and swarming. Two deletion mutants did not assemble functional flagella, two were paralyzed and five exhibited reduced free-swimming speeds. Some mutants produced unusual swarming patterns on soft agar without or with Ficoll 400. A segment of approximately 200-aa of the central region of HAP1(Rs) that aligns with the variable region of the flagellin sequence from other gamma- and beta-proteobacteria was also found. Therefore, it is possible that the origin of this large central domain of HAP1(Rs) could be associated with an event of horizontal transfer and subsequent duplications and/or insertions.

Position and orientation of phalloidin in F-actin determined by X-ray fiber diffraction analysis

Knowledge of the phalloidin binding position in F-actin and the relevant understanding of the mechanism of F-actin stabilization would help to define the structural characteristics of the F-actin filament. To determine the position of bound phalloidin experimentally, x-ray fiber diffraction data were obtained from well-oriented sols of F-actin and the phalloidin-F-actin complex. The differences in the layer-line intensity distributions, which were clearly observed even at low resolution (8 A), produced well-resolved peaks corresponding to interphalloidin vectors in the cylindrically averaged difference-Patterson map, from which the radial binding position was determined to be approximately 10 A from the filament axis. Then, the azimuthal and axial positions were determined by single isomorphous replacement phasing and a cross-Patterson map in radial projection to be approximately 84 degrees and 0.5 A relative to the actin mass center. The refined position was close to the position found by prior researchers. The position of rhodamine attached to phalloidin in the rhodamine-phalloidin-F-actin complex was also determined, in which the conjugated Leu(OH)(7) residue was found to face the outside of the filament. The position and orientation of the bound phalloidin so determined explain the increase in the interactions between long-pitch strands of F-actin and would also account for the inhibition of phosphate release, which might also contribute to the F-actin stabilization. The method of analysis developed in this study is applicable for the determination of binding positions of other drugs, such as jasplakinolide and dolastatin 11.

Dolastatin 11 connects two long-pitch strands in F-actin to stabilize microfilaments

Dolastatin 11, a drug isolated from the Indian Ocean sea hare Dolabella auricularia, arrests cytokinesis in vivo and increases the amount of F-actin to stabilize F-actin in vitro, like phalloidin and jasplakinolide. However, according to the previous biochemical study, the binding of dolastatin 11 to F-actin does not compete with that of phalloidin, suggesting that the binding sites are different. To understand the mechanism of F-actin stabilization by dolastatin 11, we determined the position of bound dolastatin 11 in F-actin using the X-ray fiber diffraction from oriented filament sols. Our analysis shows that the position of dolastatin 11 is clearly different from that of phalloidin. However, these bound drugs are present in the gap between the two long-pitch F-actin strands in a similar way. The result suggests that the connection between the two long-pitch F-actin strands might be a key for the control of F-actin stabilization.

Interaction of the disordered terminal regions of flagellin upon flagellar filament formation

Helical filaments of bacterial flagella are built up by a self-assembly process from thousands of flagellin subunits. To clarify how the disordered terminal regions of flagellin interact upon filament formation, polymerization ability of various terminally truncated fragments was investigated. Fragments deprived of 19 N-terminal residues were able to bind to the end of filaments, however, only a single layer was formed. Removal of C-terminal segments or truncation at both ends resulted in the complete loss of binding ability. Our observations are consistent with the coiled-coil model of filament formation, which suggests that the alpha-helical N- and C-terminal regions of axially adjacent subunits form an interlocking pattern of helical bundles upon polymerization.

Structure of the bacterial flagellar protofilament and implications for a switch for supercoiling

The bacterial flagellar filament is a helical propeller constructed from 11 protofilaments of a single protein, flagellin. The filament switches between left- and right-handed supercoiled forms when bacteria switch their swimming mode between running and tumbling. Supercoiling is produced by two different packing interactions of flagellin called L and R. In switching from L to R, the intersubunit distance ( approximately 52 A) along the protofilament decreases by 0.8 A. Changes in the number of L and R protofilaments govern supercoiling of the filament. Here we report the 2.0 A resolution crystal structure of a Salmonella flagellin fragment of relative molecular mass 41,300. The crystal contains pairs of antiparallel straight protofilaments with the R-type repeat. By simulated extension of the protofilament model, we have identified possible switch regions responsible for the bi-stable mechanical switch that generates the 0.8 A difference in repeat distance.

Distinct structural changes detected by X-ray fiber diffraction in stabilization of F-actin by lowering pH and increasing ionic strength

Lowering pH or raising salt concentration stabilizes the F-actin structure by increasing the free energy change associated with its polymerization. To understand the F-actin stabilization mechanism, we studied the effect of pH, salt concentration, and cation species on the F-actin structure. X-ray fiber diffraction patterns recorded from highly ordered F-actin sols at high density enabled us to detect minute changes of diffraction intensities and to precisely determine the helical parameters. F-actin in a solution containing 30 mM NaCl at pH 8 was taken as the control. F-actin at pH 8, 30 to 90 mM NaCl or 30 mM KCl showed a helical symmetry of 2.161 subunits per turn of the 1-start helix (12.968 subunits/6 turns). Lowering pH from 8 to 6 or replacing NaCl by LiCl altered the helical symmetry to 2.159 subunits per turn (12.952/6). The diffraction intensity associated with the 27-A meridional layer-line increased as the pH decreased but decreased as the NaCl concentration increased. None of the solvent conditions tested gave rise to significant changes in the pitch of the left-handed 1-start helix (approximately 59.8 A). The present results indicate that the two factors that stabilize F-actin, relatively low pH and high salt concentration, have distinct effects on the F-actin structure. Possible mechanisms will be discussed to understand how F-actin is stabilized under these conditions.

Roles of partly unfolded conformations in macromolecular self-assembly

From genes to cells there are many steps of hierarchical increments in building up complex frameworks that provide intricate networks of macromolecular interactions, through which cellular activities such as gene expression, signal processing, energy transduction and material conversion are dynamically organized and regulated. The self-assembly of macromolecules into large complexes is one such important step, but this process is by no means a simple aggregation of macromolecules with predefined, rigid complementary structures. In many cases the component molecules undergo either domain rearrangements or folding of disordered portions, which occurs only following binding to their correct partners. The partial disorder is used in some cases to prevent spontaneous assembly at inappropriate times or locations. It is also often used for finely tuning the equilibrium and activation energy of reversible binding. In other cases, such as protein translocation across membranes, an unfolded terminus appears to be the prerequisite for the process as an initiation signal, as well as the physical necessity to be taken into narrow channels. Self-assembly processes of viruses and bacterial flagella are typical examples where the induced folding of disordered chains plays a key role in regulating the addition of new components to a growing assembly. Various aspects of mechanistic roles of natively unfolded conformations of proteins are overviewed and discussed in this short review.

The bacterial flagellar cap as the rotary promoter of flagellin self-assembly

The growth of the bacterial flagellar filament occurs at its distal end by self-assembly of flagellin transported from the cytoplasm through the narrow central channel. The cap at the growing end is essential for its growth, remaining stably attached while permitting the flagellin insertion. In order to understand the assembly mechanism, we used electron microscopy to study the structures of the cap-filament complex and isolated cap dimer. Five leg-like anchor domains of the pentameric cap flexibly adjusted their conformations to keep just one flagellin binding site open, indicating a cap rotation mechanism to promote the flagellin self-assembly. This represents one of the most dynamic movements in protein structures.

Crystallization of the F41 fragment of flagellin and data collection from extremely thin crystals

Flagellin, which constructs supercoiled filaments of the bacterial flagellum, is very difficult to crystallize because of its strong tendency to polymerize. We therefore crystallized the F41 fragment of flagellin, which does not polymerize because terminal regions that play important roles in polymerization are cleaved off. F41 was crystallized by the hanging drop vapor diffusion method in a mixture of polyethylene glycol, glycerol, and isopropanol, with a reservoir solution covered with silicon oil. The two key factors for success in growing sufficiently large crystals were isopropanol and silicon oil, which worked well to reduce the otherwise very high nucleation rate that resulted in hundreds of tiny crystals. The crystals were grown to very thin plates with thickness less than 10 microm, which made the collection of diffraction data very difficult. Freezing and annealing of the crystals and irradiation at synchrotron beamlines had to be carried out by specific methods and under specific conditions for its structure analysis at 2.0-A resolution.

Concerted evolution of duplicate fla genes in Campylobacter

Campylobacters have two similar copies (flaA and flaB) of their flagellin gene. It has been hypothesized that the two copies can serve for antigenic phase variation. Analysis of polymorphisms within aligned multiple DNA sequences of the Campylobacter flagellin genes revealed high pairwise homoplasy indexes between flaB/flaB pairs that were not observed between any flaA/flaA pairings or flaA/flaB pairings. Thus it seems there are constraints on the sequence of flaB that distinguish it from flaA. Nevertheless, segments of the two genes that are highly variable between strains are conserved between the flaA and flaB copies of the genes within a strain. The patterns of synonymous and non-synonymous differences suggest that one segment of the flagellin sequence is under selective pressure at the amino acid sequence level. Another segment of the protein is maintained within a strain by conversion or recombination. Comparisons of strict consensus amino acid sequences did not reveal any motifs that are uniquely FlaA or FlaB, but there are differences between FlaA and FlaB in those amino acids available for post-translational modification. The observed pattern of concerted evolution of portions of a structural gene is an unusual finding in bacteria and should be searched for with other duplicated genes. Concerted evolution was unexpected for genes involved in phase variation since it minimizes the antigenic repertoire that can be expressed by a single clone in the face of the host immune response.

The flagellar filament of Rhodobacter sphaeroides: pH-induced polymorphic transitions and analysis of the fliC gene

Flagellar motility in Rhodobacter sphaeroides is notably different from that in other bacteria. R. sphaeroides moves in a series of runs and stops produced by the intermittent rotation of the flagellar motor. R. sphaeroides has a single, plain filament whose conformation changes according to flagellar motor activity. Conformations adopted during swimming include coiled, helical, and apparently straight forms. This range of morphological transitions is larger than that in other bacteria, where filaments alternate between left- and right-handed helical forms. The polymorphic ability of isolated R. sphaeroides filaments was tested in vitro by varying pH and ionic strength. The isolated filaments could form open-coiled, straight, normal, or curly conformations. The range of transitions made by the R. sphaeroides filament differs from that reported for Salmonella enterica serovar Typhimurium. The sequence of the R. sphaeroides fliC gene, which encodes the flagellin protein, was determined. The gene appears to be controlled by a sigma(28)-dependent promoter. It encodes a predicted peptide of 493 amino acids. Serovar Typhimurium mutants with altered polymorphic ability usually have amino acid changes at the terminal portions of flagellin or a deletion in the central region. There are no obvious major differences in the central regions to explain the difference in polymorphic ability. In serovar Typhimurium filaments, the termini of flagellin monomers have a coiled-coil conformation. The termini of R. sphaeroides flagellin are predicted to have a lower probability of coiled coils than are those of serovar Typhimurium flagellin. This may be one reason for the differences in polymorphic ability between the two filaments.

The bacterial flagella motor

The bacterial flagellum is probably the most complex organelle found in bacteria. Although the ribosome may be made of slightly more subunits, the bacterial flagellum is a more organized and complex structure. The limited number of flagella must be targeted to the correct place on the cell membrane and a structure with cytoplasmic, cytoplasmic membrane, outer membrane and extracellular components must be assembled. The process of controlled transcription and assembly is still not fully understood. Once assembled, the motor complex in the cytoplasmic membrane rotates, driven by the transmembrane ion gradient, at speeds that can reach many 100 Hz, driving the bacterial cell at several body lengths a second. This coupling of an electrochemical gradient to mechanical rotational work is another fascinating feature of the bacterial motor. A significant percentage of a bacterium's energy may be used in synthesizing the complex structure of the flagellum and driving its rotation. Although patterns of swimming may be random in uniform environments, in the natural environment, where cells are confronted with gradients of metabolites and toxins, motility is used to move bacteria towards their optimum environment for growth and survival. A sensory system therefore controls the switching frequency of the rotating flagellum. This review deals primarily with the structure and operation of the bacterial flagellum. There has been a great deal of research in this area over the past 20 years and only some of this has been included. We apologize in advance if certain areas are covered rather thinly, but hope that interested readers will look at the excellent detailed reviews on those areas cited at those points.

Domain organization of flagellar hook protein from Salmonella typhimurium

Hook forms a universal joint, which mediates the torque of the flagellar motor to the outer helical filaments. Domain organization of hook protein from Salmonella typhimurium was investigated by exploring thermal denaturation properties of its proteolytic fragments. The most stable part of hook protein involves residues 148 to 355 and consists of two domains, as revealed by deconvolution analysis of the calorimetric melting profiles. Residues 72-147 and 356-370 form another domain, while the terminal regions of the molecule, residues 1-71 and 371-403, avoid a compact tertiary structure in the monomeric state. These folding domains were assigned to the morphological domains of hook subunits known from EM image reconstructions, revealing the overall folding of hook protein in its filamentous state.

Mechanism of self-association and filament capping by flagellar HAP2

HAP2 forms a capping structure, which binds very tightly to the distal end of flagellar filaments and still allows insertion of flagellin subunits below the cap by an unknown mechanism. Terminal regions of HAP2 from Salmonella typhimurium were found to be quickly degraded by various proteases, indicating that HAP2 also possesses disordered terminal regions like other axial proteins of bacterial flagellum. Removal of these portions by trypsin results in a fragment of 40 kDa (HP40), which lacks 42 NH2-terminal and 51 COOH-terminal residues. HAP2 in solution readily associates into a decameric structure without any significant population of intermediate oligomeric forms. The HP40 fragments, however, do not form decamers, while they can assemble into pentamers, as revealed by chemical cross-linking and analytical ultracentrifugation. Decameric HAP2 also dissociates into pentamers and smaller oligomers upon a heat induced conformational transition around 36 degreesC. While the highly mobile terminal regions are immobilized in decameric HAP2 complexes, they are still largely disordered in the pentameric state. These results demonstrate that the intersubunit interactions within the pentamers are mainly through the HP40 portions, whereas the terminal regions are responsible for association of pentamers into decameric complexes. Several observations indicate that HAP2 performs its capping function as a pentamer. We suggest that binding of the pentameric HAP2 cap to the filament is mediated by the highly flexible terminal regions. Indeed, HP40 fragments are unable to cap the end of filaments, while removal of about 30 residues from both terminal regions of HAP2 results in a highly reduced capping ability. A model is presented to explain the molecular mechanism of capping, in which conformational entropy in the disordered terminal regions moderates the otherwise too tight HAP2-filament interactions to allow insertion of flagellin subunits below the cap.

Effect of the length and effective diameter of F-actin on the filament orientation in liquid crystalline sols measured by x-ray fiber diffraction

We examined factors that affect the filament orientation in F-actin sols to prepare highly well-oriented liquid crystalline sols suitable for x-ray fiber diffraction structure analysis. Filamentous particles such as F-actin spontaneously align with one another when concentrated above a certain threshold concentration. This alignment is attributed to the excluded volume effect of the particles. In trying to improve the orientation of F-actin sols, we focused on the excluded volume to see how it affects the alignment. The achievable orientation was sensitive to the ionic strength of the solvent; the filaments were better oriented at lower ionic strengths, where the effective diameter of the filament is relatively large. Sols of longer filaments were better oriented than those of shorter filaments at the same concentration, but the best achievable orientation was limited, probably because of the filament flexibility. The best strategy for making well-oriented F-actin sols is therefore to concentrate F-actin filaments of relatively short length (<1 micrometer) by slow centrifugation in a low-ionic-strength solvent (<30 mM).

Role of the outermost subdomain of Salmonella flagellin in the filament structure revealed by electron cryomicroscopy

A mutant strain of Salmonella typhimurium, SJW46, has flagellar filaments supercoiled in the same form as the wild-type strain, SJW1103, and swims normally. However, its flagellar filaments are mechanically unstable and show anomalous behaviors of polymorphism. Flagellin from SJW46 has a large central deletion from Ala204 to Lys292 of SJW1103 flagellin, which has been thought to be located in the outer surface of the filament. Since the filament structure is determined by intersubunit interactions of the terminal regions in the densely packed core of the filament, no serious involvement of the deleted portion was expected in the filament stability and polymorphism. In order to locate the deleted portion and to understand the underlying mechanism of these anomalous characteristics, we carried out structure analysis of the L-type straight filament reconstituted from a mutant flagellin of SJW46 (SJW46S) and compared the structure with that of the SJW1660 filament, which is also the L-type but composed of flagellin with no deletion. The deleted portion was identified as the outermost subdomain, and the structure in the core region showed no appreciable differences. The structure revealed the previously identified folding of flagellin in further detail, and the significance of intersubunit interactions between outer domains, which are present in the SJW1660 filament but absent in the SJW46 filament. This suggests that these contacts have a significant contribution to the filament stability and polymorphic behavior, despite the fact that the contacting surface area occupies only a minor portion of the whole intersubunit interactions.

Multiple-step method for making exceptionally well-oriented liquid-crystalline sols of macromolecular assemblies

X-ray fiber diffraction is potentially powerful in solving the atomic structure of filamentous assemblies of macromolecules, as demonstrated for tobacco mosaic virus. However, it requires extremely well-oriented sols to allow for extraction of intensities on closely located layer-lines. A high degree of orientation requires a high filament concentration to restrain the orientational freedom, but orienting concentrated sols is hampered by their high viscosity. Here, we report a systematic method that reproducibly produces extremely good orientation, which involves three steps; liquid crystallization, centrifugation and magnetic orientation. We found that a slow centrifugation can trigger a dynamic self-orientation process to form perfectly homogeneous liquid-crystalline sols, and further centrifugation to concentrate sols followed by magnetic orientation produces exceptionally well-oriented sols. The best-oriented flagellar sol showed a disorientation angle of 0.6 degrees as 1sigma of its Gaussian distribution. The new method has been successfully applied to many other systems, such as tobacco mosaic virus and F-actin.

Plugging interactions of HAP2 pentamer into the distal end of flagellar filament revealed by electron microscopy

Bacterial flagellum has a cap structure tightly attached to its distal end. The cap is an oligomeric assembly of HAP2 protein (also called FliD) and plays an essential role in the filament growth in vivo by preventing flagellin monomers from leaking out without polymerization. Electron micrographs of the HAP2 complex formed in solution showed exclusively a pentagonal shape, called "star-cap", which was thought to be the end-on view of the cap. The molecular mass roughly corresponded to a dodecamer of HAP2, and therefore a double-layered star-cap was modeled to be the cap. Here, we have observed the side view of the complex in electron micrographs. The images clearly show a rectangular shape, about 80 A wide and 180 A long, with a bipolar feature in its long axis, indicating that the complex is a bipolar pair of pentamers. A thin plate feature is identified at each end of the particle, which looks exactly like the one observed as the structure of the native filament cap. Together with the structure of the filament previously analyzed by electron cryomicroscopy, the results suggest that the cap is a pentamer with its thin plate exposed to the solvent and the other half plugged into the hole at the distal end of the filament, which is almost twice wider than its central channel. This also allows us to model the axial domain arrangement of flagellin subunit in the filament.

Structure and switching of bacterial flagellar filaments studied by X-ray fiber diffraction

Bacterial motility involves switching between the left and right supercoiled states of the flagellar filament. The polymorphism of this assembly of identical flagellin molecules has presented a structural puzzle. Supercoiling has been attributed to coexistence of two conformational states of the 11 nearly axially aligned protofilament strands of subunits. The helical parameters of straight filaments in the left (L) and right (R) lattice states have now been accurately determined by X-ray fiber diffraction. The 9 A resolution electron density map of the R-type filament, refined from the X-ray data, reveals the interlocked alpha-helical segments of the core portion, which constitute the inner and outer tubes. While the inner-tube domain interactions remain invariant, the strand joints in the outer tube can switch between the L- and R-state by 2-3 A axial shifts, which change the strand periodicity of approximately 50 A by 0.8 A. This bi-stable quaternary switching results in supercoiling. Based on the measured helical parameters of the L and R lattices and the switching model, the twist and curvature calculated for the ten possible supercoils are in quantitative accord with observed supercoiled forms of flagellar filaments.

Locations of terminal segments of flagellin in the filament structure and their roles in polymerization and polymorphism

Terminal regions of flagellin, about 180 NH2 and 100 COOH-terminal residues, are well conserved and play important roles in polymerization and polymorphism of bacterial flagellar filaments. About 65 NH2 and 45 COOH-terminal residues are disordered in the monomeric form, but become folded upon filament formation. Taking advantage of the facts that relatively small segments can be cleaved off these disordered termini by limited proteolysis, and isolated fragments still form straight filaments, locations of those terminal segments have been mapped out in the filament structure by electron cryomicroscopy and helical image reconstruction. The fragments studied are F(1-486), F(20-494), F(1-461), F(30-461) and F(30-452). Regardless of the size and terminal side of truncation, the structures of the filaments reconstituted from the truncated fragments all have identical subunit packing arrangements of the Lt-type symmetry. Structural differences compared to the filament reconstituted from intact flagellin are found only around the filament axis, namely in the inner-tube region, and no obvious changes are observed in the outer-tube or the outer part of the filament. Truncation of only a few terminal residues results in misfolding of the inner-tube domains and their aggregation around the filament axis; further truncation reduces the densities of different parts of the aggregate. The filament reconstituted from F(30-461) fragment shows complete disappearance of the density corresponding to the inner-tube. When a further nine residues are removed, the spoke-like features left on the inner wall of the outer-tube become significantly smaller. Based on the structures and radial mass distributions of the filaments obtained, the previous amino acid sequence assignment to the morphological domains has been confirmed and further refined. The roles of terminal segments in the assembly regulation, and those of the double-tubular structure in the polymorphic mechanism are discussed.

Adiabatic compressibility of flagellin and flagellar filament of Salmonella typhimurium

The partial specific volume and adiabatic compressibility of flagellin, its F40 fragment deprived of the disordered terminal regions, from Ala-1 to Arg-65 and from Ser-451 to Arg-494, and the flagellar filament of Salmonella typhimurium were determined from the density and the sound velocity measurements at 15 degrees C. The partial specific volumes were 0.728 cm3/g, 0.745 cm3/g, and 0.734 cm3/g, and the partial specific adiabatic compressibilities were 4.0 x 10(-12) cm2/dyn, 6.7 x 10(-12) cm2/dyn, and 4.7 x 10(-12) cm2/dyn, for flagellin, F40, and the filament, respectively. The smaller values of flagellin than those of F40 are reasonably explained by the presence of disordered terminal regions, which are supposed to be highly hydrated by water molecules. The volume increase upon polymerization of flagellin into the filament is also confirmed by depolymerization under a high pressure. The smaller volume and compressibility of the filament compared with those of F40 suggest an extensive hydration of the filament on its complex surface structure, which surpasses the effect on the volume and compressibility by a possible increase in the cavity volume at intersubunit interfaces upon polymerization.

Direct interaction of flagellin termini essential for polymorphic ability of flagellar filament

We report the structures of flagellar filaments reconstituted from various flagellins with small terminal truncations. Flagellins from Salmonella typhimurium strains SJW1103 (wild type), SJW1660, and SJW1655 were used, which form a left-handed supercoil, the L- and R-type straight forms, respectively. Structure analyses were done by electron cryomicroscopy and helical image reconstruction with a help of x-ray fiber diffraction for determining precise helical symmetries. Truncation of either terminal region, irrespective of the original flagellin species, results in a straight filament having a helical symmetry distinct either from the L- or R-type. This filament structure is named Lt-type. Although the local subunit packing is similar in all three types, a close comparison shows that the Lt-type packing is almost identical to the R-type but distinct from the L-type, which demonstrates the strong two-state preference of the subunit interactions. The structure clearly suggests that both termini are located in the inner tube of the concentric double-tubular structure of the filament core, and their proper interaction is responsible for the correct folding of fairly large terminal regions that form the inner tube. The double tubular structure appears to be essential for the polymorphic ability of flagellar filaments, which is required for the swimming-tumbling of bacterial taxis.