Subpiconewton intermolecular force microscopy

Radial stiffness characteristics of the overlap regions of sarcomeres in isolated skeletal myofibrils in pre-force generating state

We have studied the stiffness of myofilament lattice in sarcomeres in the pre-force generating state, which was realized by a relaxing reagent, BDM (butane dione monoxime). First, the radial stiffness for the overlap regions of sarcomeres of isolated single myofibrils was estimated from the resulting decreases in diameter by osmotic pressure applied with the addition of Dextran. Then, the radial stiffness was also estimated from force-distance curve measurements with AFM technology. The radial stiffness for the overlap regions thus obtained was composed of a soft and a rigid component. The soft component visco-elastically changed in a characteristic fashion depending on the physiological conditions of myofibrils, suggesting that it comes from cross-bridge structures. BDM treatments significantly affected the soft radial component of contracting myofibrils depending on the approach velocity of cantilever: It was nearly equal to that in the contracting state at high approach velocity, whereas as low as that in the relaxing state at low approach velocity. However, comparable BDM treatments greatly suppressed the force production and the axial stiffness in contracting glycerinated muscle fibers and also the sliding velocity of actin filaments in the in vitro motility assay. Considering that BDM shifts the cross-bridge population from force generating to pre-force generating states in contracting muscle, the obtained results strongly suggest that cross-bridges in the pre-force generating state are visco-elastically attached to the thin filaments in such a binding manner that the axial stiffness is low but the radial stiffness significantly high similar to that in force generating state.

Mechanically unfolding protein L using a laser-feedback-controlled cantilever

Force spectroscopy using the atomic force microscope (AFM) can yield important information on the strength and lifetimes of the folded states of single proteins and their complexes when they are loaded with force. For example, by mechanically unfolding concatenated proteins at different velocities, a dynamic force spectrum can be built up that allows reconstruction of the energy landscape that the protein traverses during unfolding. To characterize fully the unfolding landscape, however, it is necessary both to explore the entire force spectrum and to characterize each species populated during unfolding. In the conventional AFM apparatus, force is applied to the protein construct through a compliant cantilever. This limits the dynamic range of the force spectrum that can be probed, and the cantilever recoil after unfolding may mask the presence of metastable intermediates. Here, we describe to our knowledge a new technique-constant-deflection AFM-in which the compliance of the AFM cantilever is removed. Using this technique, we show that protein L exhibits a more complex unfolding energy landscape than previously detected using the conventional technique. This technique is also able to detect the presence of a refolding intermediate whose formation is otherwise prevented by cantilever recoil.

Stochastic emergence of multiple intermediates detected by single-molecule quasi-static mechanical unfolding of protein

Experimental probing of a protein-folding energy landscape can be challenging, and energy landscapes comprising multiple intermediates have not yet been defined. Here, we quasi-statically unfolded single molecules of staphylococcal nuclease by constant-rate mechanical stretching with a feedback positioning system. Multiple discrete transition states were detected as force peaks, and only some of the multiple transition states emerged stochastically in each trial. This finding was confirmed by molecular dynamics simulations, and agreed with another result of the simulations which showed that individual trajectories took highly heterogeneous pathways. The presence of Ca²⁺ did not change the location of the transition states, but changed the frequency of the emergence. Transition states emerged more frequently in stabilized domains. The simulations also confirmed this feature, and showed that the stabilized domains had rugged energy surfaces. The mean energy required per residue to disrupt secondary structures was a few times the thermal energy (1-3 k_BT), which agreed with the stochastic feature. Thus, single-molecule quasi-static measurement has achieved notable success in detecting stochastic features of a huge number of possible conformations of a protein.

Direct observation of multiple and stochastic transition states by a feedback-controlled single-molecule force measurement

To overcome the ensemble-averaging barrier, single-molecule experiments have been performed, but energy landscapes comprising multiple intermediates have not yet been defined. We performed mechanical unfolding of staphylococcal nuclease using intermolecular force microscopy, modified AFM with high resolution and feedback control of the positioning. The force dropped vertically just after its peak, and multiple transition states were detected as force peaks. The multiple and stochastic intermediates found in the present study provide new important information on protein folding.

From valleys to ridges: exploring the dynamic energy landscape of single membrane proteins

Membrane proteins are involved in essential biological processes such as energy conversion, signal transduction, solute transport and secretion. All biological processes, also those involving membrane proteins, are steered by molecular interactions. Molecular interactions guide the folding and stability of membrane proteins, determine their assembly, switch their functional states or mediate signal transduction. The sequential steps of molecular interactions driving these processes can be described by dynamic energy landscapes. The conceptual energy landscape allows to follow the complex reaction pathways of membrane proteins while its modifications describe why and how pathways are changed. Single-molecule force spectroscopy (SMFS) detects, quantifies and locates interactions within and between membrane proteins. SMFS helps to determine how these interactions change with temperature, point mutations, oligomerization and the functional states of membrane proteins. Applied in different modes, SMFS explores the co-existence and population of reaction pathways in the energy landscape of the protein and thus reveals detailed insights into local mechanisms, determining its structural and functional relationships. Here we review how SMFS extracts the defining parameters of an energy landscape such as the barrier position, reaction kinetics and roughness with high precision.

MEMS capacitive force sensors for cellular and flight biomechanics

Microelectromechanical systems (MEMS) are playing increasingly important roles in facilitating biological studies. They are capable of providing not only qualitative but also quantitative information on the cellular, sub-cellular and organism levels, which is instrumental to understanding the fundamental elements of biological systems. MEMS force sensors with their high bandwidth and high sensitivity combined with their small size, in particular, have found a role in this domain, because of the importance of quantifying forces and their effect on the function and morphology of many biological structures. This paper describes our research in the development of MEMS capacitive force sensors that have already demonstrated their effectiveness in the areas of cell mechanics and Drosophila flight dynamics studies.

Role of multiple bonds between the single cell adhesion molecules, nectin and cadherin, revealed by high sensitive force measurements

Nectins and cadherins, members of cell adhesion molecules (CAMs), are the primary mediators for various types of cell-cell junctions. Here, intermolecular force microscopy (IFM) with force sensitivity at sub-picoNewtons is used to characterize the extracellular trans-interactions between paired nectins and paired cadherins at the single molecule level. Three and four different bound states between paired nectins and paired cadherins are, respectively, identified and characterized based on bond strength distributions where each bound state has a unique lifetime and bond length. The results indicate that multiple domains of nectins act uncooperatively, as a zipper-like multiply bonded system whereas those of cadherins act cooperatively, as a parallel-like multiply bonded system, consistent with a "fork initiation and zipper" hypothesis for the formation of cell-cell adhesion. The observed dynamic properties among multiple bonds are expected to be advantageous such that nectins search adaptively in the cell-cell exploratory recognition process while cadherins slowly stabilize in the cell-cell zippering process.

Mechanism of muscle contraction based on stochastic properties of single actomyosin motors observed in vitro

We have previously measured the process of displacement generation by a single head of muscle myosin (S1) using scanning probe nanometry. Given that the myosin head was rigidly attached to a fairly large scanning probe, it was assumed to stably interact with an underlying actin filament without diffusing away as would be the case in muscle. The myosin head has been shown to step back and forth stochastically along an actin filament with actin monomer repeats of 5.5 nm and to produce a net movement in the forward direction. The myosin head underwent 5 forward steps to produce a maximum displacement of 30 nm per ATP at low load (<1 pN). Here, we measured the steps over a wide range of forces up to 4 pN. The size of the steps (∼5.5 nm) did not change as the load increased whereas the number of steps per displacement and the stepping rate both decreased. The rate of the 5.5-nm steps at various force levels produced a force-velocity curve of individual actomyosin motors. The force-velocity curve from the individual myosin heads was comparable to that reported in muscle, suggesting that the fundamental mechanical properties in muscle are basically due to the intrinsic stochastic nature of individual actomyosin motors. In order to explain multiple stochastic steps, we propose a model arguing that the thermally-driven step of a myosin head is biased in the forward direction by a potential slope along the actin helical pitch resulting from steric compatibility between the binding sites of actin and a myosin head. Furthermore, computer simulations show that multiple cooperating heads undergoing stochastic steps generate a long (>60 nm) sliding distance per ATP between actin and myosin filaments, i.e., the movement is loosely coupled to the ATPase cycle as observed in muscle.

5'-AMP-activated protein kinase controls insulin-containing secretory vesicle dynamics

Changes in 5'-AMP-activated protein kinase (AMPK) activity have recently been implicated in the control of insulin secretion by glucose (da Silva Xavier, G., Leclerc, I., Varadi, A., Tsuboi, T., Moule, S. K., and Rutter, G. A. (2003) Biochem. J. 371, 761-774). Here, we examine the possibility that activation of AMPK may regulate distal steps in insulin secretion, including vesicle movement and fusion with the plasma membrane. Vesicle dynamics were imaged in single pancreatic MIN6 beta-cells expressing lumen-targeted pH-insensitive yellow fluorescent protein, neuropeptide Y.Venus, or monomeric red fluorescent protein by total internal reflection fluorescence and Nipkow disc confocal microscopy. Overexpression of a truncated, constitutively active form of AMPK (AMPKalpha1, 1-312, T172D; AMPK CA), inhibited glucose-stimulated (30 versus 3.0 mM) vesicle movements, and decreased the number of vesicles docked or fusing at the plasma membrane, while having no effect on the kinetics of individual secretory events. Expression of the activated form of AMPK also prevented dispersal of the cortical actin network at high glucose concentrations. Monitored in permeabilized cells, where the effects of AMPK CA on glucose metabolism and ATP synthesis were bypassed, AMPK CA inhibited Ca2+ and ATP-induced insulin secretion, and decreased ATP-dependent vesicle movements. These findings suggest that components of the vesicle transport network, including vesicle-associated motor proteins, may be targets of AMPK in beta-cells, dephosphorylation of which is required for vesicle mobilization at elevated glucose concentrations.

Single-molecule measurement of elasticity of serine-, glutamate- and lysine-rich repeats of invertebrate connectin reveals that its elasticity is caused entropically by random coil structure

Invertebrate connectin (I-connectin) is a 1960 kDa elastic protein linking the Z line to the tip of the myosin filament in the giant sarcomere of crayfish claw closer muscle (Fukuzawa et al., 2001 EMBO J 20: 4826-4835). I-Connectin can be extended up to 3.5 microns upon stretch of giant sarcomeres. There are several extensible regions in I-connectin: two long PEVK regions, one unique sequence region and Ser-, Glu- and Lys-rich 68 residue-repeats called SEK repeats. In the present study, the force measurement of the single recombinant SEK polypeptide containing biotinylated BDTC and GST tags at the N and C termini, respectively, were performed by intermolecular force microscopy (IFM), a refined AFM system. The force vs. extension curves were well fit to the wormlike chain (WLC) model and the obtained persistence length of 0.37 +/- 0.01 nm (n = 11) indicates that the SEK region is a random coil along its full length. This is the first observation of an entropic elasticity of a fully random coil region that contributes to the physiological function of I-connectin.

Actomyosin motility on nanostructured surfaces

We have here, for the first time, used nanofabrication techniques to reproduce aspects of the ordered actomyosin arrangement in a muscle cell. The adsorption of functional heavy meromyosin (HMM) to five different resist polymers was first assessed. One group of resists (MRL-6000.1XP and ZEP-520) consistently exhibited high quality motility of actin filaments after incubation with HMM. A second group (PMMA-200, PMMA-950, and MRI-9030) generally gave low quality of motility with only few smoothly moving filaments. Based on these findings electron beam lithography was applied to a bi-layer resist system with PMMA-950 on top of MRL-6000.1XP. Grooves (100-200nm wide) in the PMMA layer were created to expose the MRL-6000.1XP surface for adsorption of HMM and guidance of actin filament motility. This guidance was quite efficient allowing no U-turns of the filaments and approximately 20 times higher density of moving filaments in the grooves than on the surrounding PMMA.

Combination of AFM with an objective-type total internal reflection fluorescence microscope (TIRFM) for nanomanipulation of single cells

A new instrument was constructed by combining an objective-type total internal reflection fluorescence microscope with an atomic force microscope (AFM). Our purpose of constructing such an instrument is to detect and confirm the result of cellular level manipulations made with the AFM part through the detection system of the highly sensitive fluorescence microscope part. In this combination, manipulations are now possible from the nanometer to the micrometer scales and the fluorescence detection system is sensitive enough even for localizing single molecules. In this paper, we applied the system as a precise intracellular injector (nanoplanter). Fluorescent beads were first chemically immobilized onto a ZnO whisker that was glued to an AFM tip and were injected into a living BALB/3T3 cell together with the whisker. It was demonstrated that the system could clearly show the result of injection, that is, the presence of a small number of fluorescent beads in the cell.

Thermal noise reduction of mechanical oscillators by actively controlled external dissipative forces

We show that the thermal fluctuations of very soft mechanical oscillators, such as the cantilever in an atomic force microscope (AFM), can be reduced without changing the stiffness of the spring or having to lower the environment temperature. We derive a theoretical relationship between the thermal fluctuations of an oscillator and an actively controlled external dissipative force. This relationship is verified by experiments with an AFM cantilever where the external active force is coupled through a magnetic field. With simple instrumentation, we have reduced the thermal noise amplitude of the cantilever by a factor of 3.4, achieving an apparent temperature of 25 K with the environment at 295 K. This active noise reduction approach can significantly improve the accuracy of static position or static force measurements in a number of practical applications.

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.

A single myosin head moves along an actin filament with regular steps of 5.3 nanometres

Actomyosin, a complex of actin filaments and myosin motor proteins, is responsible for force generation during muscle contraction. To resolve the individual mechanical events of force generation by actomyosin, we have developed a new instrument with which we can capture and directly manipulate individual myosin subfragment-1 molecules using a scanning probe. Single subfragment-1 molecules can be visualized by using a fluorescent label. The data that we obtain using this technique are consistent with myosin moving along an actin filament with single mechanical steps of approximately 5.3 nanometres; groups of two to five rapid steps in succession often produce displacements of 11 to 30 nanometres. This multiple stepping is produced by a single myosin head during just one biochemical cycle of ATP hydrolysis.

Single molecule imaging of fluorophores and enzymatic reactions achieved by objective-type total internal reflection fluorescence microscopy

Imaging of single fluorescent molecules has been achieved in a relatively simple manner using objective-type total internal reflection fluorescence microscopy (TIRFM). Switching from epi-fluorescence microscopy to objective-type TIRFM was achieved by translation of a single mirror in the system. Clear images of single molecules of an orange fluorescent dye, Cy3, were obtained with a fluorescence-to-background ratio of 12, using a conventional high aperture objective (PlanApo, 100 x, NA 1.4) with ordinary coverslips and immersion oil. This method allowed visualization of single molecules under scanning probe microscopes. Taking advantage of the technique of single molecule imaging, individual ATP turnovers have been visualized with a fluorescent ATP analogue, Cy3-ATP, using a simple experimental strategy. Clear on/off signals were obtained that correspond to the association and dissociation of single Cy3-ATP/ADP molecules with a single myosin head molecule. This method will allow a variety of single-molecular assays of biomolecular functions to be performed using fluorescently labeled substrates, ligands, messengers, and biologically active molecules. Thus, the present technique provides a simple yet powerful and universal tool for researchers to probe the events of single molecules.