Detection of single-molecule interactions using correlated thermal diffusion

Observation of discrete, single-molecule binding events allows one to bypass assumptions required to infer single-molecule properties from studies of ensembles of molecules. Optically trapped beads and glass microneedles have been applied to detect single-molecule binding events, but it remains difficult to identify signs of binding events given the large displacements induced by thermal forces. Here, we exploit thermal diffusion by using correlation between motion of optically trapped beads attached to both ends of a single actin filament to track binding events of individual myosin molecules. We use correlated diffusion to measure the stiffness of a single myosin molecule and estimate its thermal fluctuation in a poststroke state as comparable in amplitude to the measured stroke distance. The use of correlated diffusion to measure kinetics of single-molecule interactions and the stiffness of the interacting moieties should be applicable to any pair of interacting molecules, and not limited to biological motors.

Quantitative measurements of force and displacement using an optical trap

We combined a single-beam gradient optical trap with a high-resolution photodiode position detector to show that an optical trap can be used to make quantitative measurements of nanometer displacements and piconewton forces with millisecond resolution. When an external force is applied to a micron-sized bead held by an optical trap, the bead is displaced from the center of the trap by an amount proportional to the applied force. When the applied force is changed rapidly, the rise time of the displacement is on the millisecond time scale, and thus a trapped bead can be used as a force transducer. The performance can be enhanced by a feedback circuit so that the position of the trap moves by means of acousto-optic modulators to exert a force equal and opposite to the external force applied to the bead. In this case the position of the trap can be used to measure the applied force. We consider parameters of the trapped bead such as stiffness and response time as a function of bead diameter and laser beam power and compare the results with recent ray-optic calculations.

Detection of sub-8-nm movements of kinesin by high-resolution optical-trap microscopy

Kinesin is a molecular motor that transports organelles along microtubules. This enzyme has two identical 7-nm-long motor domains, which it uses to move between consecutive tubulin binding sites spaced 8 nm apart along a microtubular protofilament. The molecular mechanism of this movement, which remains to be elucidated, may be common to all families of motor proteins. In this study, a high-resolution optical-trap microscope was used to measure directly the magnitude of abrupt displacements produced by a single kinesin molecule transporting a microscopic bead. The distribution of magnitudes reveals that kinesin not only undergoes discrete 8-nm movements, in agreement with previous work [Svoboda, K., Schmidt, C. F., Schnapp, B. J. & Block, S.M. (1993) Nature (London) 365, 721-727], but also frequently exhibits smaller movements of about 5 nm. A possible explanation for these unexpected smaller movements is that kinesin's movement from one dimer to the next along a protofilament involves at least two distinct events in the mechanical cycle.

Characterization of single actin-myosin interactions

The feedback-enhanced laser trap assay (Finer et al., 1994) allows the measurement of force and displacement produced by single myosin molecules interacting with an actin filament suspended in solution by two laser traps. The average displacement of 11 nm at low load and the average force of 4 pN near isometric conditions are consistent with the conventional swinging cross-bridge model of muscle contraction (Huxley, 1969). The durations of single actin-myosin interactions at low load, 3-7 ms, suggest a relatively small duty ratio. Event durations can be increased either by reducing the ATP concentration until ATP binding is rate-limiting or by lowering the temperature. For sufficiently long interactions near isometric conditions, low frequency force fluctuations were observed within the time frame of a single event. Single myosin events can be measured at ionic strengths that disrupt weak binding actomyosin interactions, supporting the postulate of distinct weak and strong binding states. Myosin-generated force and displacement were measured simultaneously against several different loads to generate a force-displacement curve. The linear appearance of this curve suggests that the myosin powerstroke is driven by the release of a strained linear elastic element with a stiffness of approximately 0.4 pN nm-1.

Single myosin molecule mechanics: piconewton forces and nanometre steps

A new in vitro assay using a feedback enhanced laser trap system allows direct measurement of force and displacement that results from the interaction of a single myosin molecule with a single suspended actin filament. Discrete stepwise movements averaging 11 nm were seen under conditions of low load, and single force transients averaging 3-4 pN were measured under isometric conditions. The magnitudes of the single forces and displacements are consistent with predictions of the conventional swinging-crossbridge model of muscle contraction.

Force on single actin filaments in a motility assay measured with an optical trap

We have used an optical trap to measure or exert a force on single actin filaments via the attachment of polystyrene beads which were coated with NEM-modified HMM. In the simplest experiment, beads were attached to rhodamine phalloidin labelled actin filaments and observed to move on an HMM coated surface in the presence of ATP. Moving beads were steered into the vicinity of the trap using a PZT operated microscope stage. The minimum force needed to stop a moving bead was measured by lowering the trap strength until the bead resumed movement. By aligning the optical trap with the centre of a quadrant detector placed in an image plane of the microscope, it was possible to measure the force exerted on a filament by measuring the displacement of the bead position from the centre of the trap. In each of these experiments, the trap was calibrated by applying a Stokes force to a bead in free solution. The characteristics of the trap were studied, and the displacement of the bead from the centre of the trap was shown to be directly proportional to the applied force over a large part of the total range of the trap. The compliance of the trap could be substantially reduced by the use of feedback control to deflect the laser beam via an acousto-optic modulator. The advantages and limitations of this technique will be discussed.