Physical properties of a single-motif erythrocyte spectrin peptide: a highly stable independently folding unit

Mechanical role of the submembrane spectrin scaffold in red blood cells and neurons

Spectrins are large, evolutionarily well-conserved proteins that form highly organized scaffolds on the inner surface of eukaryotic cells. Their organization in different cell types or cellular compartments helps cells withstand mechanical challenges with unique strategies depending on the cell type. This Review discusses our understanding of the mechanical properties of spectrins, their very distinct organization in red blood cells and neurons as two examples, and the contribution of the scaffolds they form to the mechanical properties of these cells.

Structural Perturbations of Exon-Skipping Edits within the Dystrophin D20:24 Region

Exon skipping is a disease-modifying therapy in which oligonucleotide analogues mask specific exons, eliminating them from the mature mRNA, and also the cognate protein. That is one possible therapeutic aim, but it can also be used to restore the reading frame for diseases caused by frameshift mutations, which is the case for Duchenne muscular dystrophy (DMD). DMD most commonly arises as a result of large exonic deletions that create a frameshift and abolish protein expression. Loss of dystrophin protein leads to the pathology of the disease, which is severe, causing death generally in the second or third decade of life. Here, the primary aim of exon skipping is restoration of protein expression by reading frame correction. However, the therapeutically expressed protein is missing both the region of the underlying genetic defect and the therapeutically skipped exon. How removing some region from the middle of a protein affects its structure and function is unclear. Many different underlying deletions are known, and exon skipping can be applied in many ways, in some cases in different ways to the same defect. These vary in how severely perturbative they are, with possible clinical consequences. In this study, we examine a systematic, comprehensive panel of exon edits in a region of dystrophin and identify for the first time exon edits that are minimally perturbed and appear to keep the structural stability similar to that of wild-type protein. We also identify factors that appear to be correlated with how perturbative an edit is.

Multiple Functions of Spectrin: Convergent Effects

Spectrin is a multifunctional, multi-domain protein most well known in the membrane skeleton of mature human erythrocytes. Here we review the literature on the crosstalk of the chaperone activity of spectrin with its other functionalities. We hypothesize that the chaperone activity is derived from the surface exposed hydrophobic patches present in individual "spectrin-repeat" domains and show a competition between the membrane phospholipid binding functionality and chaperone activity of spectrin. Moreover, we show that post-translational modifications such as glycation which shield these surface exposed hydrophobic patches, reduce the chaperone function. On the other hand, oligomerization which is linked to increase of hydrophobicity is seen to increase it. We note that spectrin seems to prefer haemoglobin as its chaperone client, binding with it preferentially over other denatured proteins. Spectrin is also known to interact with unstable haemoglobin variants with a higher affinity than in the case of normal haemoglobin. We propose that chaperone activity of spectrin could be important in the cellular biochemistry of haemoglobin, particularly in the context of diseases.

Effect of pH on stability, conformation, and chaperone activity of erythroid & non-erythroid spectrin

Spectrin, a major component of the eukaryotic membrane skeleton, has been shown to have chaperone like activity. Here we investigate the pH induced changes in the structure and stability of erythroid and brain spectrin by spectroscopic methods. We also correlate these changes with modulations of chaperone potential at different pH. We have followed the pH induced structural changes by circular dichroism spectroscopy and intrinsic tryptophan fluorescence. It is seen that lowering the pH from 9 has little effect on structure of the proteins till about pH6. At pH4, there is significant change of the secondary structure of the proteins, along with a 5nm hypsochromic shift of the emission maxima. Below pH4 the proteins undergo acid denaturation. Probing exposed hydrophobic patches on the proteins using protein-bound 8-anilinonaphthalene-1-sulfonate fluorescence demonstrates that there is higher solvent accessibility of hydrophobic surfaces in both forms of spectrin at around pH4. Dynamic light scattering and 90° light scattering studies show that the both forms of spectrin forms oligomers at pH~4. Chemical unfolding data shows that these oligomers are less stable than the tetrameric form. Aggregation studies with BSA show that at pH4, both spectrins exhibit better chaperone activity. This enhancement of chaperone like activity appears to result from an increase in regions of solvent-exposed hydrophobicity and oligomeric state of the spectrins which in turn are induced by moderately acid pH. This may have in-vivo implications in cells facing stress conditions where cytoplasmic pH is lowered.

Dielectric relaxations on erythrocyte membrane as revealed by spectrin denaturation

We studied the effect of spectrin denaturation at 49.5°C (TA) on the dielectric relaxations and related changes in the complex impedance, Z*, complex capacitance, C*, and dielectric loss curve of suspensions containing human erythrocytes, erythrocyte ghost membranes (EMs) and Triton-X-100 residues of EMs. The loss curve prior to, minus the loss curve after TA, resulted in a bell-shaped peak at 1.5MHz. The changes in the real and imaginary components of Z* and C* at TA, i.e., ΔZre, ΔZim, ΔCre and ΔCim, calculated in the same way, strongly varied with frequency. Between 1.0 and 12MHz the -ΔZim vs ΔZre, and ΔCim vs ΔCre plots depicted semicircles with critical frequencies, fcr, at 2.5MHz expressing recently reported relaxation of spectrin dipoles. Between 0.02 and 1.0MHz the -ΔZim vs ΔZre plot exhibited another relaxation whose fcr mirrored that of beta relaxation. This relaxation was absent on Triton-X-shells, while on erythrocytes and EMs it was inhibited by selective dissociation of either attachment sites between spectrin and bilayer. Considering above findings and inaccessibility of cytosole to outside field at such frequencies, the latter relaxation was assumed originating from a piezoelectric effect on the highly deformable spectrin filaments.

Probing conformational stability and dynamics of erythroid and nonerythroid spectrin: effects of urea and guanidine hydrochloride

We have studied the conformational stability of the two homologous membrane skeletal proteins, the erythroid and non-erythroid spectrins, in their dimeric and tetrameric forms respectively during unfolding in the presence of urea and guanidine hydrochloride (GuHCl). Fluorescence and circular dichroism (CD) spectroscopy have been used to study the changes of intrinsic tryptophan fluorescence, anisotropy, far UV-CD and extrinsic fluorescence of bound 1-anilinonapthalene-8-sulfonic acid (ANS). Chemical unfolding of both proteins were reversible and could be described as a two state transition. The folded erythroid spectrin and non-erythroid spectrin were directly converted to unfolded monomer without formation of any intermediate. Fluorescence quenching, anisotropy, ANS binding and dynamic light scattering data suggest that in presence of low concentrations of the denaturants (up-to 1M) hydrogen bonding network and van der Waals interaction play a role inducing changes in quaternary as well as tertiary structures without complete dissociation of the subunits. This is the first report of two large worm like, multi-domain proteins obeying twofold rule which is commonly found in small globular proteins. The free energy of stabilization (ΔGuH20) for the dimeric spectrin has been 20 kcal/mol lesser than the tetrameric from.

Stabilization of the spectrin-like domains of nesprin-1α by the evolutionarily conserved "adaptive" domain

Nesprins are located at the outer and inner membranes of the nuclear envelope and help link the cytoskeleton to the nucleoskeleton. Nesprin-1α, located at the inner nuclear membrane, binds to A-type lamins and emerin and has homology to spectrin-repeat proteins. However, the mechanical and thermodynamic properties of the spectrin-like repeats (SLRs) of nesprin-1α and the potential structural contributions of the unique central domain were untested. In other spectrin superfamily proteins, tandem spectrin-repeat domains undergo cooperatively coupled folding and unfolding. We hypothesized that the large central domain, which interrupts SLRs and is conserved in other nesprin isoforms, might confer unique structural properties. To test this model we measured the thermal unfolding of nesprin-1α fragments using circular dichroism and dynamic light scattering. The SLRs in nesprin-1α were found to have structural and thermodynamic properties typical of spectrins. The central domain had relatively little secondary structure as an isolated fragment, but significantly stabilized larger SLR-containing molecules by increasing their overall helicity, thermal stability and cooperativity of folding. We suggest this domain, now termed the 'adaptive' domain (AD), also strengthens dimerization and inhibits unfolding. Further engineering of the isolated AD, and AD-containing nesprin molecules, may yield new information about the higher-order association of cooperative protein motifs.

Spectrin organization and dynamics: new insights

Spectrin is the major constituent protein of the erythrocyte cytoskeleton which forms a filamentous network on the cytoplasmic face of the membrane by providing a scaffold for a variety of proteins. In this review, several aspects of spectrin organization are highlighted, particularly with respect to its ability to bind hydrophobic ligands and its interaction with membrane surfaces. The characteristic binding of the fluorescent hydrophobic probes Prodan and pyrene to spectrin, which allows an estimation of the polarity of the hydrophobic probe binding site, is illustrated. In addition, the contribution of uniquely localized and conserved tryptophan residues in the 'spectrin repeats' in these processes is discussed. A functional implication of the presence of hydrophobic binding sites in spectrin is its recently discovered chaperone-like activity. Interestingly, spectrin exhibits residual structural integrity even after denaturation which could be considered as a hallmark of cytoskeletal proteins. Future research could provide useful information about the possible role played by spectrin in cellular physiology in healthy and diseased states.

Spectrin domains lose cooperativity in forced unfolding

Spectrin is a multidomain cytoskeletal protein, the component three-helix bundle domains are expected to experience mechanical force in vivo. In thermodynamic and kinetic studies, neighboring domains of chicken brain alpha-spectrin R16 and R17 have been shown to behave cooperatively. Is this cooperativity maintained under force? The effect of force on these spectrin domains was investigated using atomic force microscopy. The response of the individual domains to force was compared to that of the tandem repeat R1617. Importantly, nonhelical linkers (all-beta immunoglobulin domains) were used to avoid formation of nonnative helical linkers. We show that, in contrast to previous studies on spectrin repeats, only 3% of R1617 unfolding events gave an increase in contour length consistent with cooperative two-domain unfolding events. Furthermore, the unfolding forces for R1617 were the same as those for the unfolding of R16 or R17 alone. This is a strong indication that the cooperative unfolding behavior observed in the stopped-flow studies is absent between these spectrin domains when force is acting as a denaturant. Our evidence suggests that the rare double unfolding events result from misfolding between adjacent repeats. We suggest that this switch from cooperative to independent behavior allows multidomain proteins to maintain integrity under applied force.

Structural cooperativity in spectrin type repeats motifs of dystrophin

Dystrophin is a member of the spectrin family of proteins, which are characterized as being predominantly composed the spectrin-type-repeat, a triple alpha-helical bundle motif present in multiple tandem copies, producing a rod-like shape. Whether or not this motif, which is determined by sequence homology, is correlated with biophysical domains in the intact protein is uncertain. The nature of the domain structure impacts the flexibility and shape of the rod region of this protein, which is a target for modification in several therapeutic approaches aimed at Duchenne Muscular Dystrophy, a common and fatal genetic disease caused by defective dystrophin. We examined three such motifs in dystrophin, expressing them recombinantly both singly and in tandem, and studying their thermodynamic properties by solvent and thermal denaturation. We have found that the degree to which they are independently stable and expressible varies considerably. The fourth motif appears to be largely stable and independent, whereas the third and second motifs interact strongly.

Molecular Level Investigations of the Inter- and Intramolecular Interactions of pH-Responsive Artificial Triblock Proteins

Intelligent materials that can undergo physical gelation in response to environmental stimuli have potential impacts in the bioengineering and biomedical fields where the entrapment of cellular or molecular species is desired. Here, we utilize atomic force microscopy (AFM) to perform molecular level investigations of designer artificial proteins that undergo physical gelation. These are engineered as triblock copolymers with independent interchain binding and solvent retention functions, namely, two terminal leucine zipper-like peptide sequences and a central alanylglycine rich sequence, respectively. AFM force measurements between probes and surfaces functionalized with molecules of this triblock protein revealed adhesive interactions that increased in average force and frequency as the pH was lowered from pH 11.2 to 7.4 to 4.5, reflecting an increase in the numbers of interacting molecular strands. In bulk solution, lowering the pH results in a viscous liquid to gel transition. The modular design of the triblock protein was also exploited for single molecule force spectroscopy investigations, which revealed altered intramolecular interactions in response to changes in pH. An increased understanding of the inter- and intramolecular forces involved in biomolecule driven gelation processes is not only of great fundamental interest in the study of the biomolecular systems involved but may also prove key in enabling the rational design of new generations of intelligent hydrogel systems.

Effect of ionic strength on the organization and dynamics of tryptophan residues in erythroid spectrin: A fluorescence approach

The ionic strength of the medium plays an important role in the structure and conformation of erythroid spectrin. The spectrin dimer is a flexible rod at physiological ionic strength. However, lower ionic strength results in elongation and rigidification (stiffening) of spectrin as shown earlier by electron microscopy and hydrodynamic studies. The ionic strength induced structural transition does not involve any specific secondary structural changes. In this article, we have used a combination of fluorescence spectroscopic approaches that include red edge excitation shift (REES), fluorescence quenching, time-resolved fluorescence measurements, and chemical modification of the spectrin tryptophans to assess the environment and dynamics of tryptophan residues of spectrin under different ionic strength conditions. Our results show that while REES, fluorescence anisotropy, lifetime, and chemical modification of spectrin tryptophans remain unaltered in low and high ionic strength conditions, quenching of tryptophan fluorescence by the aqueous quencher acrylamide (but not the hydrophobic quencher trichloroethanol) and resonance energy transfer to a dansyl-labeled fatty acid show differences in tryptophan environment. These results, which report tertiary structural changes in spectrin upon change in ionic strength, are relevant in understanding the molecular details underlying the conformational flexibility of spectrin.

Pathway shifts and thermal softening in temperature-coupled forced unfolding of spectrin domains

Pathways of unfolding a protein depend in principle on the perturbation-whether it is temperature, denaturant, or even forced extension. Widely-shared, helical-bundle spectrin repeats are known to melt at temperatures as low as 40-45 degrees C and are also known to unfold via multiple pathways as single molecules in atomic force microscopy. Given the varied roles of spectrin family proteins in cell deformability, we sought to determine the coupled effects of temperature on forced unfolding. Bimodal distributions of unfolding intervals are seen at all temperatures for the four-repeat beta(1-4) spectrin-an alpha-actinin homolog. The major unfolding length corresponds to unfolding of a single repeat, and a minor peak at twice the length corresponds to tandem repeats. Increasing temperature shows fewer tandem events but has no effect on unfolding intervals. As T approaches T(m), however, mean unfolding forces in atomic force microscopy also decrease; and circular dichroism studies demonstrate a nearly proportional decrease of helical content in solution. The results imply a thermal softening of a helical linker between repeats which otherwise propagates a helix-to-coil transition to adjacent repeats. In sum, structural changes with temperature correlate with both single-molecule unfolding forces and shifts in unfolding pathways.

Mutation of a highly conserved isoleucine disrupts hydrophobic interactions in the alpha beta spectrin self-association binding site

We studied an infant with severe neonatal hemolytic anemia and hyperbilirubinemia that evolved into a partially compensated ellipto-poikilocytic anemia. His father had typical elliptocytosis. Their erythrocyte membranes demonstrated structural and functional defects in spectrin. Genetic studies revealed that the proband and his father were heterozygous for an alpha-spectrin mutation, Ile24Thr, in the alpha beta spectrin self-association binding site. The proband also carried the low expression allele alpha(LELY) in trans, influencing the clinical phenotype. The importance of isoleucine in this position of the proposed triple helical model of spectrin repeats is highlighted by its evolutionary conservation in all alpha spectrins from Drosophila to humans. Molecular modeling demonstrated that replacement of a hydrophobic isoleucine with a hydrophilic threonine disrupts highly conserved hydrophobic interactions in the interior of the spectrin triple helix critical for spectrin function.

Pathways and intermediates in forced unfolding of spectrin repeats

Spectrin repeats are triple-helical coiled-coil domains found in many proteins that are regularly subjected to mechanical stress. We used atomic force microscopy technique and steered molecular dynamics simulations to study the behavior of a wild-type spectrin repeat and two mutants. The experiments indicate that spectrin repeats can form stable unfolding intermediates when subjected to external forces. In the simulations the unfolding proceeded via a variety of pathways. Stable intermediates were associated to kinking of the central helix close to a proline residue. A mutant stabilizing the central helix showed no intermediates in experiments, in agreement with simulation. Spectrin repeats may thus function as elastic elements, extendable to intermediate states at various lengths.

Structural restoration of inactive recombinant fish growth hormones by chemical chaperonin and solvent restraint approaches

Recombinant proteins may undergo conformational distortion, leading to aggregation and loss of function, when they are expressed in heterologous systems. The structural and functional restoration of such inactive proteins is highly desirable. We have over-expressed recombinant growth hormones from the fish ayu (Plecoglossus altivelis) and yellow grouper (Epinephelus awoara) by a pET expression system. Both recombinant proteins accumulate as insoluble form in Escherichia coli. We refolded these inactive proteins into the active form using a stepwise refolding process with a dilute denaturing agent as a steric blocker and chemical chaperonin. Optical characterization showed that stable folding intermediates with a helical conformation can be detected in the molten globule state. Moreover, the function of restored recombinant growth hormones was demonstrated by its ability to stimulate proliferation in zebrafish liver cells.

alpha beta Spectrin coiled coil association at the tetramerization site

On the basis of sequence homology studies, it has been suggested that the association of human erythrocytes alpha and beta spectrin at the tetramerization site involves interactions between helices. However, no empirical details are available, presumably due to the experimental difficulties in studying spectrin molecules because of its size and/or its structural flexibility. It has been speculated that erythrocyte tetramerization involves helical bundling rather than coiled coil association. We have used recombinant spectrin peptides to model alpha and beta spectrin to study their association at the tetramerization site. Two alpha peptides, Sp alpha 1-156 and Sp alpha 1-368, and one beta peptide, Sp beta 1898-2083, were used as model peptides to demonstrate the formation of the alpha beta complex. We also found that the replacement of R28 in Sp alpha 1-368 to give Sp alpha 1-368R28C abolished complex formation with the beta peptide. Circular dichroism techniques were used to monitor the secondary structures of the individual peptides and of the complex, and the results showed that both Sp alpha 1-156 and Sp beta 1898-2083 peptides in solution, separately, included helices that were not paired with other helices in the absence of their binding partners. However, in a mixture of Sp alpha 1-156 and Sp beta 1898-2083 and formation of the alpha beta complex, the unpaired helices associated to form coiled coils. Since the sequences of these two peptides that are involved in the coiled coil association are derived from a native protein, the information obtained from this study also provides insight toward a better understanding of naturally occurring coiled coil subunit-subunit association.

Free energies of urea and of thermal unfolding show that two tandem repeats of spectrin are thermodynamically more stable than a single repeat

Free energies of both urea and thermal denaturation have been measured for three pairs of one- and two-repeat fragments, cloned in tandem from the cytoskeletal protein, alpha-spectrin, from chicken brain to ascertain whether one- and two-repeat fragments are equally stable. One- and two-repeat fragments of each pair were designed with the same N-terminus, whereas the C-terminus of the two-repeat fragment was 106 residues or the length of one repeat downstream from that of the one-repeat fragment. The averaged free energies of urea and thermal denaturation of the paired fragments, (R16)(00) and (R16R17)(00), (R16)(0+3) and (R16R17)(0+3), and (R16)(+8-4) and (R16R17)(+8-4) [subscripts represent the N- and C-terminal positions with "00" referring to the N- and C-termini defining a repeat according to X-ray crystal structures of two repeat fragments [Grum, V. L., Li, D., MacDonald, R. I., and Mondragón, A. (1999) Cell 98, 523-535] and "+" and "-" referring to positions upstream and downstream therefrom, respectively], increased from 3.7 +/- 0.4 kcal/mol for (R16)(00), 3.7 +/- 0.5 kcal/mol for (R16)(0+3), 4.4 +/- 0.4 kcal/mol for (R16)(+8-4), 6.2 +/- 0.6 kcal/mol for (R16R17)(+8-4), 8.3 +/- 0.4 kcal/mol for (R16R17)(00) to 9.9 +/- 1.0 kcal/mol for (R16R17)(0+3). Thus, the two-repeat fragment of each pair was significantly more thermodynamically stable than the single repeat by both urea and thermal denaturation. Differences in phasing among single repeats did not have the same effect as the same differences in phasing among two-repeat fragments. Addition of nine residues to the C-terminus of (R16R17)(00) yielded a free energy of unfolding of 7.9 +/- 0.8 kcal/mol, whereas addition of seven residues to the C-terminus of (R16)(+8-4) yielded a free energy of unfolding of 5.9 +/- 0.3 kcal/mol.

Mechanical design of proteins studied by single-molecule force spectroscopy and protein engineering

Mechanical unfolding and refolding may regulate the molecular elasticity of modular proteins with mechanical functions. The development of the atomic force microscopy (AFM) has recently enabled the dynamic measurement of these processes at the single-molecule level. Protein engineering techniques allow the construction of homomeric polyproteins for the precise analysis of the mechanical unfolding of single domains. alpha-Helical domains are mechanically compliant, whereas beta-sandwich domains, particularly those that resist unfolding with backbone hydrogen bonds between strands perpendicular to the applied force, are more stable and appear frequently in proteins subject to mechanical forces. The mechanical stability of a domain seems to be determined by its hydrogen bonding pattern and is correlated with its kinetic stability rather than its thermodynamic stability. Force spectroscopy using AFM promises to elucidate the dynamic mechanical properties of a wide variety of proteins at the single molecule level and provide an important complement to other structural and dynamic techniques (e.g., X-ray crystallography, NMR spectroscopy, patch-clamp).

States and transitions during forced unfolding of a single spectrin repeat

Spectrin is a vital and abundant protein of the cytoskeleton. It has an elongated structure that is made by a chain of so-called spectrin repeats. Each repeat contains three antiparallel alpha-helices that form a coiled-coil structure. Spectrin forms an oligomeric structure that is able to cross-link actin filaments. In red cells, the spectrin/actin meshwork underlying cell membrane is thought to be responsible for special elastic properties of the cell. In order to determine mechanical unfolding properties of the spectrin repeat, we have used single molecule force spectroscopy to study the states of unfolding of an engineered polymeric protein consisting of identical spectrin domains. We demonstrate that the unfolding of spectrin domains can occur in a stepwise fashion during stretching. The force-extension patterns exhibit features that are compatible with the existence of at least one intermediate between the folded and the completely unfolded conformation. Only those polypeptides that still contain multiple intact repeats display intermediates, indicating a stabilisation effect. Precise force spectroscopy measurements on single molecules using engineered protein constructs reveal states and transitions during the mechanical unfolding of spectrin. Single molecule force spectroscopy appears to open a new window for the analysis of transition probabilities between different conformational states.

Initiation of spectrin dimerization involves complementary electrostatic interactions between paired triple-helical bundles

The spectrin heterodimer is formed by the antiparallel lateral association of an alpha and a beta subunit, each of which comprises largely a series of homologous triple-helical motifs. Initiation of dimer assembly involves strong binding between complementary motifs near the actin-binding end of the dimer. In this study, the mechanism of lateral spectrin association at this dimer nucleation site was investigated using the analytical ultracentrifuge to analyze heterodimers formed from recombinant peptides containing two or four homologous motifs from each subunit (alpha20-21/beta1-2; alpha18-21/beta1-4). Both the two-motif and four-motif dimer associations were weakened substantially with increasing salt concentration, indicating that electrostatic interactions are important for the dimer initiation process. Modeling of the electrostatic potential on the surface of the alpha20 and beta2 motifs showed that the side of the motifs comprising the A and B helices is the most favorable for association, with an area of positive electrostatic potential on the AB face of the beta2 motif opposite negative potential on the AB face of the alpha20 motif and vise versa. Protease protection analysis of the alpha20-21/beta1-2 dimer showed that multiple trypsin and proteinase K sites in the A helices of the beta2 and alpha21 motifs become buried upon dimer formation. Together, these data support a model where complementary long range electrostatic interactions on the AB faces of the triple-helical motifs in the dimer nucleation site initiate the correct pairing of motifs, i.e. alpha21-beta1 and alpha20-beta2. After initial docking of these complementary triple-helical motifs, this association is probably stabilized by subsequent formation of stronger hydrophobic interactions in a complex involving the A helices of both subunits and possibly most of the AB faces. The beta subunit A helix in particular appears to be buried in the dimer interface.

Mechanical design of proteins studied by single-molecule force spectroscopy and protein engineering

Mechanical unfolding and refolding may regulate the molecular elasticity of modular proteins with mechanical functions. The development of the atomic force microscopy (AFM) has recently enabled the dynamic measurement of these processes at the single-molecule level. Protein engineering techniques allow the construction of homomeric polyproteins for the precise analysis of the mechanical unfolding of single domains. alpha-Helical domains are mechanically compliant, whereas beta-sandwich domains, particularly those that resist unfolding with backbone hydrogen bonds between strands perpendicular to the applied force, are more stable and appear frequently in proteins subject to mechanical forces. The mechanical stability of a domain seems to be determined by its hydrogen bonding pattern and is correlated with its kinetic stability rather than its thermodynamic stability. Force spectroscopy using AFM promises to elucidate the dynamic mechanical properties of a wide variety of proteins at the single molecule level and provide an important complement to other structural and dynamic techniques (e.g., X-ray crystallography, NMR spectroscopy, patch-clamp).

Structures of two repeats of spectrin suggest models of flexibility

Spectrin is a vital component of the cytoskeleton, conferring flexibility on cells and providing a scaffold for a variety of proteins. It is composed of tandem, antiparallel coiled-coil repeats. We report four related crystal structures at 1.45 A, 2.0 A, 3.1 A, and 4.0 A resolution of two connected repeats of chicken brain alpha-spectrin. In all of the structures, the linker region between adjacent units is alpha-helical without breaks, kinks, or obvious boundaries. Two features observed in the structures are (1) conformational rearrangement in one repeat, resulting in movement of the position of a loop, and (2) varying degrees of bending at the linker region. These features form the basis of two different models of flexibility: a conformational rearrangement and a bending model. These models provide novel atomic details of spectrin flexibility.

Single-molecule biomechanics with optical methods

Single-molecule observation and manipulation have come of age. With the advent of optical tweezers and other methods for probing and imaging single molecules, investigators have circumvented the model-dependent extrapolation from ensemble assays that has been the hallmark of classical biochemistry and biophysics. In recent years, there have been important advances in the understanding of how motor proteins work. The range of these technologies has also started to expand into areas such as DNA transcription and protein folding. Here, recent experiments with rotary motors, linear motors, RNA polymerase, and titin are described.

Single molecule force spectroscopy of spectrin repeats: low unfolding forces in helix bundles

Spectrin repeats fold into triple helical coiled-coils comprising approximately 106 amino acid residues. Using an AFM-related technique we measured the force required to mechanically unfold these repeats to be 25 to 35 pN. Under tension, individual spectrin repeats unfold independently and in an all-or-none process. The dependence of the unfolding forces on the pulling speed reveals that the corresponding unfolding potential is shallow with an estimated width of 1.5 nm. When the unfolded polypeptide strand is relaxed, several domains refold within less than a second. The unfolding forces of the alpha-helical spectrin domains are five to ten times lower than those found in domains with beta-fold, like immunoglobulin or fibronectin Ill domains, where the tertiary structure is stabilized by hydrogen bonds between adjacent strands. This shows that the forces stabilizing the coiled-coil lead to a mechanically much weaker structure than multiple hydrogen-bonded beta-sheets.

Ionic strength effect on the thermal unfolding of alpha-spectrin peptides

In previous work, we have shown that the ionic strength-mediated differences found for the hydrodynamic dimensions of the human erythrocyte spectrin are not caused by secondary structural changes, but are caused more probably by subtle changes in tertiary interactions (LaBrake, C. C., Wang, L., Keiderling, T. A., and Fung, L. W.-M. (1993) Biochemistry 32, 10296-10302.). The substructure of spectrin has been suggested to be composed largely of triple alpha-helical bundle structural domains in tandem. In the present study, we used fluorescence and circular dichroism methods to study ionic strength effects on intact spectrin dimers and on recombinant peptides of spectrin domains of different lengths. We observed little ionic strength effect on the thermal unfolding temperature, Tm, values in these systems. However, we found that ionic strength-induced cooperativity in the unfolding processes was similar for the spectrin dimer and for peptides with two or three domains, as measured by entropy changes (DeltaSm). Although single-domain peptides exhibited rather variable DeltaSm values, depending on the specific domain, they showed little salt effects on the DeltaSm values themselves. This suggests that spectrin undergoes subtle ionic strength-induced conformational changes, probably near the interdomain regions of the molecule. These conformational changes may be responsible for the observed hydrodynamic and unfolding properties in intact spectrin under different ionic strength conditions. We suggest that recombinant peptides of various lengths may serve as models for studying the structural flexibility in spectrin.

Solution structure of the spectrin repeat: a left-handed antiparallel triple-helical coiled-coil

Cytoskeletal proteins belonging to the spectrin family have an elongated structure composed of repetitive units. The three-dimensional solution structure of the 16th repeat from chicken brain alpha-spectrin (R16) has been determined by NMR spectroscopy and distance geometry-simulated annealing calculations. We used a total of 1035 distance restraints, which included 719 NOE-based values obtained by applying the ambiguous restraints for iterative assignment (ARIA) method. In addition, we performed a direct refinement against 1H-chemical shifts. The final ensemble of 20 structures shows an average RMSD of 1.52 A from the mean for the backbone atoms, excluding loops and N and C termini. R16 is made up of three antiparallel alpha-helices separated by two loops, and folds into a left-handed coiled-coil. The basic unit of spectrin is an antiparallel heterodimer composed of two homologous chains, beta and alpha. These assemble a tetramer via a mechanism that relies on the completion of a single repeat by association of the partial repeats located at the C terminus of the beta-chain (two helices) and at the N terminus of the alpha-chain (one helix). This tetramer is the assemblage able to cross-link actin filaments. Model building by homology of the "tetramerization" repeat from human erythrocyte spectrin illuminates the possible role of point mutations which cause hemolytic anemias.

Site-directed mutagenesis of either the highly conserved Trp-22 or the moderately conserved Trp-95 to a large, hydrophobic residue reduces the thermodynamic stability of a spectrin repeating unit

As reported previously (MacDonald, R. I., Musacchio, A., Holmgren, R. A., and Saraste, M. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 1299-1303), an unfolded peptide was obtained by site-directed mutagenesis of Trp-22 to Ala in the cloned, wild type 17th repeating unit (alpha17) of chicken brain alpha-spectrin. Trp occurs in position 22 of nearly all repeating units of spectrin. In the present study, Trp-22 was mutated to Phe or to Tyr to compare thermodynamic stabilities of urea-induced unfolding of alpha16 and mutants thereof. alpha16 was chosen for this study instead of alpha17, because alpha16 has two tryptophans, allowing urea-induced unfolding to be tracked by the fluorescence of the Trp remaining in each mutant peptide and by circular dichroism in the far UV. The free energies of unfolding of W22Y and W22F were 50% that of alpha16, showing that Trp-22 is crucial in stabilizing the triple helical bundle motif of the spectrin repeating unit. Mutation of the moderately conserved Trp-95 of alpha16 to Val, which occupies position 95 in alpha17, also yielded a peptide with 50% of the free energy of unfolding of alpha16. Thus, the thermodynamic stability of a given spectrin repeating unit may depend on both moderately and highly conserved tryptophans. Different structural roles of Trp-22 and Trp-95 in alpha16 are suggested by the slightly higher wavelength of maximum emission of Trp-22, the greater acrylamide quenching of Trp-95 than Trp-22, and the longer lifetime of Trp-95. For comparison with alpha16, urea-induced unfolding of spectrin dimer isolated from human red cells was monitored by far UV-CD and by tryptophan fluorescence. Thermodynamic parameters could not be rigorously derived for the stability of spectrin dimer because unfolding of spectrin dimer involved more than two states, unlike unfolding of cloned repeating units. However, the similar midpoints of CD-monitored denaturation curves of alpha16 and spectrin dimer, i. e. 2.7 and 3.2 M urea, respectively, indicate that investigation of cloned repeating units of spectrin can provide physiologically relevant information on these structures.