Smooth and skeletal muscle single-molecule mechanical experiments

Resource Letter: LBOT-1: Laser-based optical tweezers

This Resource Letter provides a guide to the literature on optical tweezers, also known as laser-based, gradient-force optical traps. Journal articles and books are cited for the following main topics: general papers on optical tweezers, trapping instrument design, optical detection methods, optical trapping theory, mechanical measurements, single molecule studies, and sections on biological motors, cellular measurements and additional applications of optical tweezers.

Direct tests of muscle cross-bridge theories: predictions of a Brownian dumbbell model for position-dependent cross-bridge lifetimes and step sizes with an optically trapped actin filament

Force and displacement events from a single myosin molecule interacting with an actin filament suspended between optically trapped beads (Finer, J. T., R. M. Simmons, and J. A. Spudich. 1994. Nature. 368:113-119) can be interpreted in terms of a generalized cross-bridge model that includes the effects of Brownian forces on the beads. Steady-state distributions of force and displacement can be obtained directly from a generalized Smoluchowski equation for Brownian motion of the actin-bead "dumbbell," and time series from Monte Carlo simulations of the corresponding Langevin equation. When the frequency spectrum of Brownian motion extends beyond cross-bridge transition rates, the inverse mean lifetimes of force/displacement pulses are given by cross-bridge rate constants averaged over a Boltzmann distribution of Brownian noise. These averaged rate constants reflect the strain-dependence of the rate constants for the stationary filament, most faithfully at high trap stiffness. Hence, measurements of the lifetimes and displacements of single events as a function of the resting position of the dumbbell can provide a direct test of different cross-bridge theories of muscle contraction. Quantitative demonstrations are given for Huxley models with 1) faster binding or 2) slower dissociation at positive cross-bridge strain. Predictions for other models can be inferred from the averaging procedure.

Modelling and simulation of motility in actomyosin systems

We present a model mechanism for simulating the diffusive motion and fluctuations inherent in myofibrillar sarcomere and its subunits at the molecular level. The model couples Langevin dynamics with Huxley kinetics to reproduce the transient patterns of momentum transfer, force generation and resulting motility due to the interactive activities of actin and myosin crossbridges. When myosin is detached from actin, our model predicts Brownian displacements centered at 0 +/- 8 nm (mean +/- SD, n = 265,308) and it is broadly distributed due to the Brownian noise. Attachment events produced displacements with step sizes of approximately 8 +/- 6 nm (mean +/- SD, n = 34,693), which is in agreement with some recent optical-tweezers transducer experimental results. The proposed model could form the basis for a complete qualitative and quantitative description of the evolving complex interactions of the molecular proteins--actin and myosin--in the overall framework of muscular contraction studies.

F-actin-binding proteins

The study of proteins that bind filamentous actin (F-actin) is entering an exciting stage as more and more structures are determined. After more than 50 years in which the focus was on muscle proteins, emphasis has recently shifted towards understanding the complex interplay among actin-binding molecules in non-muscle cells. To date, the binding sites for eight classes of filament-binding molecules have been determined by combining low- to intermediate-resolution maps obtained by electron microscopy with atomic structures determined by X-ray crystallography and NMR. Recent results have dramatically accentuated the importance of filament geometry and actin conformation in defining these interactions.