Amyloid fibrils are self-associating filamentous structures, the deposition of which is considered to be one of the most important factors in the pathogenesis of Alzheimer's disease and various other disorders. Here we used single molecule manipulation methods to explore the mechanics and structural dynamics of amyloid fibrils. In mechanically manipulated amyloid fibrils, formed from either amyloid beta (Abeta) peptides 1-40 or 25-35, beta-sheets behave as elastic structures that can be "unzipped" from the fibril with constant forces. The unzipping forces were different for Abeta1-40 and Abeta25-35. Unzipping was fully reversible across a wide range of stretch rates provided that coupling, via the beta-sheet, between bound and dissociated states was maintained. The rapid, cooperative zipping together of beta-sheets could be an important mechanism behind the self-assembly of amyloid fibrils. The repetitive force patterns contribute to a mechanical fingerprint that could be utilized in the characterization of different amyloid fibrils.

Amyloid fibrils are self-associating filamentous structures formed from the 39- to 42-residue-long amyloid beta peptide (Abeta peptide). The deposition of Abeta fibrils is one of the most important factors in the pathogenesis of Alzheimer's disease. Abeta25-35 is a fibril-forming peptide that is thought to represent the biologically active, toxic form of the full-length Abeta peptide. We have recently shown that beta sheets can be mechanically unzipped from the fibril surface with constant forces in a reversible transition, and the unzipping forces differ in fibrils composed of different peptides. In the present work, we explored the effect of epsilon-amino acetylation of the Lys28 residue on the magnitude of the unzipping force of Abeta25-35 fibrils. Although the gross structure of the Lys28-acetylated (Abeta25-35_K28Ac) and wild-type Abeta25-35 (Abeta25-35wt) fibrils were similar, as revealed by atomic force microscopy, the fundamental unzipping forces were significantly lower for Abeta25-35_K28Ac (20 +/- 4 pN SD) than for Abeta25-35wt (42 +/- 9 pN SD). Simulations based on a simple two-state model suggest that the decreased unzipping forces, caused most likely by steric constraints, are likely due to a destabilized zippered state of the fibril.

The functionally elastic, I-band part of the myofibrillar protein titin (connectin) contains differentially expressed arrays of serially linked immunoglobulin (Ig)-like domains, the length and composition of which vary among the titin isoforms. The biological rationale of the differential expression as well as the contribution of the Ig domain mechanical characteristics to the overall mechanical behavior of titin are not exactly known. The paper briefly reviews the relevant works that have addressed the Ig-domain mechanics problems and presents the authors' experimental approach to studying the mechanical behavior of Ig domains. The mechanics of an eight domain segment from the differentially expressed tandem Ig region of titin (155-62) was studied with an atomic force microscope specially used for stretching single molecules, and the results were compared to known mechanical properties of other domains and segments.

Titin is the main determinant of passive muscle force. Physiological extension of titin derives largely from its PEVK (Pro-Glu-Val-Lys) domain, which has a different length in different muscle types. Here we characterized the elasticity of the full-length, human soleus PEVK domain by mechanically manipulating its contiguous, recombinant subdomain segments: an N-terminal (PEVKI), a middle (PEVKII), and a C-terminal (PEVKIII) one third. Measurement of the apparent persistence lengths revealed a hierarchical arrangement according to local flexibility: the N-terminal PEVKI is the most rigid and the C-terminal PEVKIII is the most flexible segment within the domain. Immunoelectron microscopy supported the hierarchical extensibility within the PEVK domain. The effective persistence lengths decreased as a function of ionic strength, as predicted by the Odijk-Skolnick-Fixman model of polyelectrolyte chains. The ionic strength dependence of persistence length was similar in all segments, indicating that the residual differences in the elasticity of the segments derive from nonelectrostatic mechanisms.

Titin (also known as connectin) is the main determinant of physiological levels of passive muscle force. This force is generated by the extensible I-band region of the molecule, which is constructed of the PEVK domain and tandem-immunoglobulin segments comprising serially linked immunoglobulin (Ig)-like domains. It is unresolved whether under physiological conditions Ig domains remain folded and act as "spacers" that set the sarcomere length at which the PEVK extends or whether they contribute to titin's extensibility by unfolding. Here we focused on whether Ig unfolding plays a prominent role in stress relaxation (decay of force at constant length after stretch) using mechanical and immunolabeling studies on relaxed human soleus muscle fibers and Monte Carlo simulations. Simulation experiments using Ig-domain unfolding parameters obtained in earlier single-molecule atomic force microscopy experiments recover the phenomenology of stress relaxation and predict large-scale unfolding in titin during an extended period (> approximately 20 min) of relaxation. By contrast, immunolabeling experiments failed to demonstrate large-scale unfolding. Thus, under physiological conditions in relaxed human soleus fibers, Ig domains are more stable than predicted by atomic force microscopy experiments. Ig-domain unfolding did not become more pronounced after gelsolin treatment, suggesting that the thin filament is unlikely to significantly contribute to the mechanical stability of the domains. We conclude that in human soleus fibers, Ig unfolding cannot solely explain stress relaxation.

Titin is a giant polypeptide that spans half of the striated muscle sarcomere and generates passive force upon stretch. To explore the elastic response and structure of single molecules and oligomers of titin, we carried out molecular force spectroscopy and atomic force microscopy (AFM) on purified full-length skeletal-muscle titin. From the force data, apparent persistence lengths as long as ∼1.5 nm were obtained for the single, unfolded titin molecule. Furthermore, data suggest that titin molecules may globally associate into oligomers which mechanically behave as independent wormlike chains (WLCs). Consistent with this, AFM of surface-adsorbed titin molecules revealed the presence of oligomers. Although oligomers may form globally via head-to-head association of titin, the constituent molecules otherwise appear independent from each other along their contour. Based on the global association but local independence of titin molecules, we discuss a mechanical model of the sarcomere in which titin molecules with different contour lengths, corresponding to different isoforms, are held in a lattice. The net force response of aligned titin molecules is determined by the persistence length of the tandemly arranged, different WLC components of the individual molecules, the ratio of their overall contour lengths, and by domain unfolding events. Biased domain unfolding in mechanically selected constituent molecules may serve as a compensatory mechanism for contour- and persistence-length differences. Variation in the ratio and contour length of the component chains may provide mechanisms for the fine-tuning of the sarcomeric passive force response.

A representative cross-section of elastic biomolecules is covered in this volume, which combines seventeen contributions from leading research groups. State-of-the-art molecular mechanics experiments

Titinis a giant elastic protein responsible for passive force generated by the stretched striated-muscle sarcomere. Passive force develops in titin's extensible region which consists of the PEVK segment in series with tandemly arranged immunoglobulin (Ig)-like domains. Here we studied the mechanics of tandem Ig segments from the differentially spliced (I65–70) and constitutive (I91–98) regions by using an atomic force microscope specialized for stretching single molecules. The mechanical stability of I65–70 domains was found to be different from that of I91–98 domains. In the range of stretch rates studied (0.05–1.00 μm/s) lower average domain unfolding forces for I65–70 were associated with a weaker stretch-rate dependence of the unfolding force, suggesting that the differences in the mechanical stabilities of the segments derive from differences in the zero force unfolding rate (K0u) and the characteristic distance (location of the barrier) along the unfolding reaction coordinate (ΔXu). No effect of calcium was found on unfolding forces and persistence length of unfolded domains. To explore the structural basis of the differences in mechanical stabilities of the two fragment types, we compared the amino acid sequence of I65–70 domains with that of I91–98 domains and by using homology modeling analyzed how sequence variations may affect folding free energies. Simulations suggest that differences in domain stability are unlikely to be caused by variation in the number of hydrogen bonds between the force-bearing β-strands at the domain's N- and C-termini. Rather, they may be due to differences in hydrophobic contacts and strand orientations.

A polymer strand can be influenced by an array of mechanical effects as it translates through a space with dimensions similar to its own. We summarize our experiments in which the mechanical behavior of single-molecules of the giant protein titin was characterized. Titin is a filamentous polypeptide embedded in the lattice space of the vertebrate muscle sarcomere. A single titin molecule stretches from the Z- to the M-line of the sarcomere, thereby spanning a distance of approximately I pin which is huge on the molecular scale. Physiologically, titin is important in generating passive muscle force and in maintaining the structural integrity of the sarcomere through its elastic properties. We analyzed the elastic properties of titin by stretching single molecules with laser tweezers. The force-response of the titin molecule revealed an entropic polymer whose behaviour can be well fitted with the wormlike chain model. Unfolding occurs in titin upon reaching high forces during stretch, and refolding takes place at low forces during the relaxation of the molecule. The force hysteresis implies folding non-equilibrium due to the slow unfolding and refolding rates relative to the stretch and release rates, respectively. Folding non-equilibrium as the source of force hysteresis can be demonstrated by the disappearance of hysteresis upon the addition of chemical denaturant. Repeated stretch and release cycles lead to the mechanical wear-out or fatigue of titin, which may have far-reaching implications for its physiological function.

The properties of titin, a giant, structurally and mechanically complex filamentous protein were explored here by using single-molecule techniques. Fluorescently labeled titin molecules deposited on a glass substrate surface were directly visualized by using confocal microscopy. The molecules appeared as bright particles with a spatially distributed Gaussian fluorescence intensity profile. The physical dimensions of the space occupied by the 1-mum-long molecule on the surface are below the 0.25 mum resolution limit of the instrument used here, thereby directly demonstrating the flexible nature of the titin filament. The kinetics of titin's surface adsorption revealed that the process is governed by diffusion and the molecules are irreversibly bound to the substrate surface. From the kinetics data the molecule's translational diffusion constant was calculated.

The global configuration of individual, surface-adsorbed molecules of the giant muscle protein titin, labeled with rhodamine conjugates, was followed with confocal microscopy. Fluorescence-emission intensity was reduced because of self-quenching caused by the close spacing between rhodamine dye molecules that formed dimers. In the presence of chemical denaturants, fluorescence intensity increased, reversibly, up to 5-fold in a fast reaction; the kinetics were followed at the single-molecule level. We show that dimers formed in a concentrated rhodamine solution dissociate when exposed to chemical denaturants. Furthermore, titin denaturation, followed by means of tryptophan fluorescence, is dominated by a slow reaction. Therefore, the rapid fluorescence change of the single molecules reflects the direct action of the denaturants on rhodamine dimers rather than the unfolding/refolding of the protein. Upon acidic denaturation, which we have shown not to dissociate rhodamine dimers, fluorescence intensity change was minimal, suggesting that dimers persist because the unfolded molecule has contracted into a small volume. The highly contractile nature of the acid-unfolded protein molecule derives from a significant increase in chain flexibility. We discuss the potential implications this finding could have for the passive mechanical behavior of striated muscle.