Orientational distribution of spin-labeled actin oriented by flow

Molecular Motors as Components of Future Medical Devices and Engineered Materials

A new frontier in the development of prosthetic devices is the design of nanoscale systems which replace, augment, or support individual cells. Similar to cells, such devices will require the ability to generate mechanical movement, either for transport or actuation. Here, the development of nanoscale transport systems, which integrate biomolecular motors, is reviewed. To date, close to 100 publications have explored the design of such “molecular shuttles” based on the integration of synthetic molecules, nano- and microparticles, and micropatterned structures with kinesin and myosin motors and their associated cytoskeletal filaments, microtubules, and actin filaments. Tremendous progress has been made in addressing the key challenges of guiding, loading, and controlling the shuttles, providing a foundation for the exploration of applications in medicine and engineering.

Guidance of actin filament elongation on filament-binding tracks

Biomolecular motors, which convert chemical energy into mechanical work in intracellular processes, have high potential in bionanotechnology in vitro as molecular shuttles or nanoscale actuators. In this context, guided elongation of actin filaments in vitro could be used to lay tracks for myosin motor-based shuttles or to direct nanoscale actuators based on actin filament end-tracking motors. To guide the direction of filament polymerization on surfaces, microcontact printing was used to create tracks of chemically modified myosin, which binds to, but cannot exert force on, filaments. These filament-binding tracks captured nascent filaments from solution and guided the direction of their subsequent elongation. The effect of track width and protein surface density on filament alignment and elongation rate was quantified. These results indicate that microcontact printing is a useful method for guiding actin filament polymerization in vitro for biomolecular motor-based applications.

Protein Linear Molecular Motor-Powered Nanodevices

Myosin–actin and kinesin–microtubule linear protein motor systems and their application in hybrid nanodevices are reviewed. Research during the past several decades has provided a wealth of understanding about the fundamentals of protein motors that continues to be pursued. It has also laid the foundations for a new branch of investigation that considers the application of these motors as key functional elements in laboratory-on-a-chip and other micro/nanodevices. Current models of myosin and kinesin motors are introduced and the effects of motility assay parameters, including temperature, toxicity, and in particular, surface effects on motor protein operation, are discussed. These parameters set the boundaries for gliding and bead motility assays. The review describes recent developments in assay motility confinement and unidirectional control, using micro- and nano-fabricated structures, surface patterning, microfluidic flow, electromagnetic fields, and self-assembled actin filament/microtubule tracks. Current protein motor assays are primitive devices, and the developments in governing control can lead to promising applications such as sensing, nano-mechanical drivers, and biocomputation.

Cofilin Increases the Torsional Flexibility and Dynamics of Actin Filaments

We have measured the effects of cofilin on the conformation and dynamics of actin filaments labeled at Cys374 with erythrosin-iodoacetemide (ErIA), using time-resolved phosphorescence anisotropy (TPA). Cofilin quenches the phosphorescence intensity of actin-bound ErIA, indicating that binding changes the local environment of the probe. The cofilin concentration-dependence of the phosphorescence intensity is sigmoidal, consistent with cooperative actin filament binding. Model-independent analysis of the anisotropies indicates that cofilin increases the rates of the microsecond rotational motions of actin. In contrast to the reduction in phosphorescence intensity, the changes in the rates of rotational motions display non-nearest-neighbor cooperative interactions and saturate at substoichiometric cofilin binding densities. Detailed analysis of the TPA decays indicates that cofilin decreases the torsional rigidity (C) of actin, increasing the thermally driven root-mean-square torsional angle between adjacent filament subunits from approximately 4 degrees (C = 2.30 x 10(-27) Nm2 radian(-1)) to approximately 17 degrees (C = 0.13 x 10(-27) Nm2 radian(-1)) at 25 degrees C. We favor a mechanism in which cofilin binding shifts the equilibrium between thermal ErIA-actin filament conformers, and facilitates two distinct structural changes in actin. One is local in nature, which affects the structure of actin's C terminus and is likely to mediate nearest-neighbor cooperative binding and filament severing. The second is a change in the internal dynamics of actin, which displays non-nearest-neighbor cooperativity and increases the torsional flexibility of filaments. The long-range effects of cofilin on the torsional dynamics of actin may accelerate P(i) release from filaments and modulate interactions with other regulatory actin filament binding proteins.

Measurement of single macromolecule orientation by total internal reflection fluorescence polarization microscopy

A new approach is presented for measuring the three-dimensional orientation of individual macromolecules using single molecule fluorescence polarization (SMFP) microscopy. The technique uses the unique polarizations of evanescent waves generated by total internal reflection to excite the dipole moment of individual fluorophores. To evaluate the new SMFP technique, single molecule orientation measurements from sparsely labeled F-actin are compared to ensemble-averaged orientation data from similarly prepared densely labeled F-actin. Standard deviations of the SMFP measurements taken at 40 ms time intervals indicate that the uncertainty for individual measurements of axial and azimuthal angles is approximately 10 degrees at 40 ms time resolution. Comparison with ensemble data shows there are no substantial systematic errors associated with the single molecule measurements. In addition to evaluating the technique, the data also provide a new measurement of the torsional rigidity of F-actin. These measurements support the smaller of two values of the torsional rigidity of F-actin previously reported.

Changes in actin and myosin structural dynamics due to their weak and strong interactions

Figure 3 summarizes the effects of actomyosin binding on the internal and global dynamics of either protein, as discussed in this chapter. These effects depend primarily on the strength of the interaction; which in turn depends on the state of the nucleotide at the myosin active site. When either no nucleotide or ADP is bound, the interaction is strong and the effect on each protein is maximal. When the nucleotide is ATP or ADP.Pi, or the equivalent nonhydrolyzable analogs, the interaction is weak and the effect on molecular dynamics of each protein is minimal. The weaker effects in weak-binding states are not simply the reflection of lower occupancy of binding sites--the molecular models in Fig. 3 illustrate the effects of the formation of the ternary complex, after correction for the free actin and myosin in the system. Thus EPR on myosin (Berger and Thomas 1991; Thomas et al. 1995) and pyrene fluorescence studies on actin (Geeves 1991) have shown that the formation of a ternary complex has a negligible effect on the internal dynamics of both [figure: see text] proteins (left side of Fig. 3, white arrows). As shown by both EPR (Baker et al. 1998; Roopnarine et al. 1998) and phosphorescence (Ramachandran and Thomas 1999), both domains of myosin are dynamically disordered in weak-binding states, and this is essentially unaffected by the formation of the ternary complex (left side of Fig. 3, indicated by disordered myosin domains). The only substantial effect of the formation of the weak interaction that has been reported is the EPR-detected (Ostap and Thomas 1991) restriction of the global dynamics of actin upon weak myosin binding (left column of Fig. 3, gray arrow). The effects of strong actomyosin formation are much more dramatic. While substantial rotational dynamics, both internal and global, exist in both myosin and actin in the presence of ADP or the absence of nucleotides, spin label EPR, pyrene fluorescence, and phosphorescence all show dramatic restrictions in these motions upon formation of the strong ternary complex (right column of Fig. 3). One implication of this is that the weak-to-strong transition is accompanied by a disorder-to-order transition in both actin and myosin, and this is itself an excellent candidate for the structural change that produces force (Thomas et al. 1995). Another clear implication is that the crystal structures obtained for isolated myosin and actin are not likely to be reliable representations of structures that exist in ternary complexes of these proteins (Rayment et al. 1993a and 1993b; Dominguez et al. 1998; Houdusse et al. 1999). This is clearly true of the strong-binding states, since the spectroscopic studies indicate consistently that substantial changes occur in both proteins upon strong complex formation. For the weak complexes, the problem is not that complex formation induces large structural changes, but that the structures themselves are dynamically disordered. This is probably why so many different structures have been obtained for myosin S1 with nucleotides bound--each crystal is selecting one of the many different substates represented by the dynamic ensemble. Finally, there is the problem that the structures of actomyosin complexes are probably influenced strongly by their mechanical coupling to muscle protein lattice (Baker at al. 2000). Thus, even if co-crystals of actin and myosin are obtained in the future, an accurate description of the structural changes involved in force generation will require further experiments using site-directed spectroscopic probes of both actin and myosin, in order to detect the structural dynamics of these ternary complexes under physiological conditions.

Actin motion on microlithographically functionalized myosin surfaces and tracks

High-resolution e-beam patterning exposure of the surface of poly[(tert-butyl-methacrylate)-co-(methyl methacrylate)]-a common e-beam and deep-UV resist used in semiconductor microlithography-induced sharp changes in the surface hydrophobicity. These differences in hydrophobicity resulted in the selective attachment of heavy meromyosin to hydrophobic, unexposed surfaces. The movement of the actin filaments on myosin-rich and myosin-poor surfaces was statistically characterized in terms of velocity, acceleration, and angle of movement. The actin filaments have a smooth motion on myosin-rich surfaces and an uneven motion on myosin-poor surfaces. Interestingly, an excess of myosin sites has a slowing, albeit mild effect on the motion of the actin filaments. It was also found that the myosin-rich/myosin-poor boundary has an alignment-enforcement effect, especially for the filaments approaching the border from the myosin-rich side. Based on these results, we discuss the feasibility of building purposefully designed molecular motor arrays and the testing of the hypotheses regarding the functioning of the molecular motors.

Dynamics of individual flexible polymers in a shear flow

Polymer dynamics are of central importance in materials science, mechanical engineering, biology and medicine. The dynamics of macromolecular solutions and melts in shear flow are typically studied using bulk experimental methods such as light and neutron scattering and birefringence. But the effect of shear on the conformation and dynamics of individual polymers is still not well understood. Here we describe observations of the real-time dynamics of individual, flexible polymers (fluorescently labelled DNA molecules) under a shear flow. The sheared polymers exhibit many types of extended conformation with an overall orientation ranging from parallel to perpendicular with respect to the flow direction. For shear rates much smaller than the inverse of the relaxation time of the molecule, the relative populations of these two main types of conformation are controlled by the rate of the shear flow. These results question the adequacy of assumptions made in standard models of polymer dynamics.

A conformational change in F-actin when myosin binds: fluorescence resonance energy transfer detects an increase in the radial coordinate of Cys-374

Interactions of myosin with actin filaments probably induce conformational changes in actin which are crucial for its function. Fluorescence resonance energy transfer spectroscopy can determine changes in distance (range 10-100 A) between two probes and therefore can sense conformational changes in proteins. We have investigated changes in the radial coordinates of fluorescent probes bound to Cys-374 of F-actin when either of the isozymes (S1A1 and S1A2) of myosin subfragment 1 (S-1) bind. Using 5-[[2-[(iodoacetyl)amino]ethyl]amino]naphthalene-1-sulfonic acid and N-(4-dimethylamino-3,5-dinitrophenyl)maleimide as donor and acceptor probes, respectively, we calculated a radius of 13-14 A. This distance increased by approximately 4.5 A upon addition of S-1. No differences were detected between the effects of S1A1 and S1A2. This increase is reversed by MgATP. The average position of the probes on Cys-374 is closer to the filament axis than expected from the current models of F-actin. S-1 increases the radial position of Cys-374 either by direct interaction or via an allosteric conformational change associated with its binding.

Actin and the actomyosin interface: a review

This review deals with the structure of the actin monomer, its assembly into filaments and the loci on F-actin involved in binding myosin. Two distinctly different arrangements of monomers have been suggested for actin filaments. One model proposed by Holmes et al. is well developed. It places the so-called 'large' domain close to the filament axis and the so-called 'small' domain out near the surface of the filament. A second, less-well developed, model proposed by Schutt et al. locates the 'small' domain close to the filament axis and they rotate the monomer so that 'bottom' of the 'large' domain is at the highest radius. We analyze the available evidence for the models of F-actin derived from X-ray diffraction, reconstructions from electron micrographs, fluorescence resonance energy transfer spectroscopy, chemical cross-linking, antibody probes, limited proteolysis, site-directed and natural mutations, nuclear magnetic resonance spectroscopy and other techniques. The result is an actin-centered view of the loci on actin which are probably involved in its interaction with the myosin 'head'. From these multiple contacts we speculate on the sequence of steps between the initial weak-binding state of S-1 to the actin filament through to the stable strong-binding state seen in the absence of free Mg-ATP, i.e., the rigor state.

Orientation of actin filaments during motion in in vitro motility assay

Rhodamine-phalloidin was added to F-actin, and the orientation of transition dipoles of the dye was measured in single actin filaments by polarization of fluorescence. Rhodamine-phalloidin was well immobilized on the surface of actin, indicating that changes in orientation of the dye reported changes in orientation of actin monomers. In stationary filaments the dipoles were inclined at 49.3 degrees with respect to the filament axis. The disorganization of dipoles in stationary filaments was insignificant. When the filaments were made to translate, the average orientation of the dye did not change, but disorganization slightly increased. Disorganization increased significantly when filaments were free in solution. We concluded that, within the accuracy of our measurements (approximately 18%), actin monomers did not undergo major reorientations during motion, but that binding of myosin heads deformed the structure of filaments.

Orientation and rotational dynamics of spin-labeled phalloidin bound to actin in muscle fibers

We have used electron paramagnetic resonance spectroscopy (EPR) to investigate the orientational distribution of actin in thin filaments of glycerinated muscle fibers in rigor, relaxation, and contraction. A spin-labeled derivative of a mushroom toxin, phalloidin (PHSL), was bound to actin in the muscle fibers (PHSL-fibers). The EPR spectrum of unoriented PHSL-labeled myofibrils consisted of three sharp lines with a splitting between the outer extrema (2T parallel') of 42.8 +/- 0.1 G, indicating that the spin labels undergo restricted nanosecond rotational motion within an estimated half-cone angle of 76 degrees. When the PHSL-fiber bundle was oriented parallel to the magnetic field, the splitting between the zero-crossing points (2T') was 42.7 +/- 0.1 G. When the fiber bundle was perpendicular to the magnetic field, 2T' decreased to 34.5 +/- 0.2 G. This anisotropy shows that the motion of the probe is restricted in orientation by its binding site on actin, so that the EPR spectrum of PHSL-fiber bundles would be sensitive to small changes in the mean axial orientation of the PHSL-actin interface. No differences in the EPR spectra were observed in fibers during rigor, relaxation, or contraction, indicating that the mean axial orientation of the PHSL binding site changes by less than 5 degrees, and that the amplitude of nanosecond probe rotational motion, which should be quite sensitive to the local environment of the phalloidin, changes by no more than 1 degree.(ABSTRACT TRUNCATED AT 250 WORDS)

Polarization of fluorescently labeled myosin subfragment-1 fully or partially decorating muscle fibers and myofibrils

Fluorescently labeled myosin heads (S1) were added to muscle fibers and myofibrils at various concentrations. The orientation of the absorption dipole of the dye with respect to the axis of F-actin was calculated from polarization of fluorescence which was measured by a novel method from video images of muscle. In this method light emitted from muscle was split by a birefringent crystal into two nonoverlapping images: the first image was created with light polarized in the direction parallel to muscle axis, and the second image was created with light polarized in the direction perpendicular to muscle axis. Images were recorded by high-sensitivity video camera and polarization was calculated from the relative intensity of both images. The method allows measurement of the fluorescence polarization from single myofibril irrigated with low concentrations of S1 labeled with dye. Orientation was also measured by fluorescence-detected linear dichroism. The orientation was different when muscle was irrigated with high concentration of S1 (molar ratio S1:actin in the I bands equal to 1) then when it was irrigated with low concentration of S1 (molar ratio S1:actin in the I bands equal to 0.32). The results support our earlier proposal that S1 could form two different rigor complexes with F-actin depending on the molar ratio of S1:actin.

Transient detection of spin-labeled myosin subfragment 1 conformational states during ATP hydrolysis

We have used time-resolved electron paramagnetic resonance spectroscopy and caged ATP to detect nucleotide-induced changes in the conformational state of spin-labeled myosin heads (IASL-S1). Changes in the internal rotational dynamics of IASL-S1 were monitored with millisecond time resolution during the pre-steady-state phase of ATP hydrolysis. The changes in the internal protein dynamics were rigorously correlated with specific biochemical kinetic transitions, allowing us to observe directly the dynamic sequence of structural changes in IASL-S1 during the binding and hydrolysis of ATP. When caged ATP was photolyzed (producing 500 microM ATP) in the presence of 100 microM IASL-S1, the EPR signal intensity rose transiently to the steady-state ATPase level, indicating increased rotational motion about the SH1 region of the myosin head. Kinetic and spectral analyses have resolved two phases of this transient, one representing the population of the M*.ATP state and the other representing the population of the M**.ADP.Pi state. We conclude that two motionally distinct states of the myosin head are present during ATP hydrolysis and that these states represent distinct conformational states that can be correlated with specific biochemical intermediates. Since specific labeling of myosin heads with IASL has been achieved in skinned muscle fibers, this study establishes the feasibility for the first direct detection and resolution of myosin's conformational transients during muscle contraction.

Measuring orientation of actin filaments within a cell: orientation of actin in intestinal microvilli

Orientational distribution of actin filaments within a cell is an important determinant of cellular shape and motility. To map this distribution we developed a method of measuring local orientation of actin filaments. In this method actin filaments within cells are labeled with fluorescent phalloidin and are viewed at high magnification in a fluorescent microscope. Emitted fluorescence is split by a birefringent crystal giving rise to two images created by light rays polarized orthogonally with respect to each other. The two images are recorded by a high-sensitivity video camera, and polarization of fluorescence at any point is calculated from the relative intensity of both images at this point. From the value of polarization, the orientation of the absorption dipole of the dye, and thus orientation of F-actin, can be calculated. To illustrate the utility of the method, we measured orientation of actin cores in microvilli of chicken intestinal epithelial cells. F-actin in microvillar cores was labeled with rhodamine-phalloidin; measurements showed that the orientation was the same when microvillus formed a part of a brush border and when it was separated from it suggesting that "shaving" of brush borders did not distort microvillar structure. In the absence of nucleotide, polarization of fluorescence of actin cores in isolated microvilli was best fitted by assuming that a majority of fluorophores were arranged with a perfect helical symmetry along the axis of microvillus and that the absorption dipoles of fluorophores were inclined at 52 degrees with respect to the axis. When ATP was added, the shape of isolated microvilli did not change but polarization of fluorescence decreased, indicating statistically significant increase in disorder and a change of average angle to 54 degrees. We argue that these changes were due to mechanochemical interactions between actin and myosin-I.

Simulation of saturation transfer electron paramagnetic resonance spectra for rotational motion with restricted angular amplitude

We have simulated both conventional (V1) and saturation transfer (V'2) electron paramagnetic resonance spectra for the case of Brownian rotational diffusion restricted in angular amplitude. Numerical solutions of the diffusion-coupled Bloch equations were obtained for an axially symmetric 14N nitroxide spin label with its principal axis rotating within a Gaussian angular distribution of full width delta theta at half maximum. Spectra were first calculated for a macroscopically oriented system with cylindrical symmetry (e.g., a bundle of muscle fibers or a stack of membrane bilayers), with the Gaussian angular distribution centered at theta 0 with respect to the magnetic field. These spectra were then summed over theta 0 to obtain the spectrum of a randomly oriented sample (e.g., a dispersion of myofibrils or membrane vesicles). The angular amplitude delta theta was varied from 0 degrees, corresponding to isotropic motion (order parameter = 0). For each value of delta theta, the rotational correlation time, tau r, was varied from 10(-7) to 10(-2) s, spanning the range from maximal to minimal saturation transfer. We provide plots that illustrate the dependence of spectral parameters on delta theta and tau r. For an oriented system, the effects of changing delta theta and tau r are easily distinguishable, and both parameters can be determined unambiguously by comparing simulated and experimental spectra. For a macroscopically disordered system, the simulated spectra are still quite sensitive to delta theta, but a decrease in tau r produces changes similar to those from an increase in delta theta. If delta theta can be determined independently, then the results of the present study can be used to determine tau r from experimental spectra. Similarly, if tau r is known, then delta theta can be determined.