Structural changes in actin-containing filaments of muscle

50 Years of the steric-blocking mechanism in vertebrate skeletal muscle: a retrospective

Fifty years have now passed since Parry and Squire proposed a detailed structural model that explained how tropomyosin, mediated by troponin, played a steric-blocking role in the regulation of vertebrate skeletal muscle. In this Special Issue dedicated to the memory of John Squire it is an opportune time to look back on this research and to appreciate John's key contributions. A review is also presented of a selection of the developments and insights into muscle regulation that have occurred in the years since this proposal was formulated.

Troponin-I-induced tropomyosin pivoting defines thin-filament function in relaxed and active muscle

Regulation of the crossbridge cycle that drives muscle contraction involves a reconfiguration of the troponin-tropomyosin complex on actin filaments. By comparing atomic models of troponin-tropomyosin fitted to cryo-EM structures of inhibited and Ca2+-activated thin filaments, we find that tropomyosin pivots rather than rolls or slides across actin as generally thought. We propose that pivoting can account for the Ca2+ activation that initiates muscle contraction and then relaxation influenced by troponin-I (TnI). Tropomyosin is well-known to occupy either of three meta-stable configurations on actin, regulating access of myosin motorheads to their actin-binding sites and thus the crossbridge cycle. At low Ca2+ concentrations, tropomyosin is trapped by TnI in an inhibitory B-state that sterically blocks myosin binding to actin, leading to muscle relaxation. Ca2+ binding to TnC draws TnI away from tropomyosin, while tropomyosin moves to a C-state location over actin. This partially relieves the steric inhibition and allows weak binding of myosin heads to actin, which then transition to strong actin-bound configurations, fully activating the thin filament. Nevertheless, the reconfiguration that accompanies the initial Ca2+-sensitive B-state/C-state shift in troponin-tropomyosin on actin remains uncertain and at best is described by moderate-resolution cryo-EM reconstructions. Our recent computational studies indicate that intermolecular residue-to-residue salt-bridge linkage between actin and tropomyosin is indistinguishable in B- and C-state thin filament configurations. We show here that tropomyosin can pivot about relatively fixed points on actin to accompany B-state/C-state structural transitions. We argue that at low Ca2+ concentrations C-terminal TnI domains attract tropomyosin, causing it to bend and then pivot toward the TnI, thus blocking myosin binding and contraction.

Myosin assembly of smooth muscle: from ribbons and side polarity to a row polar helical model

After decades of debate over the structure of smooth muscle myosin filaments, it is still unclear whether they are helical, as in all other muscle types, or square in shape. In both cases bipolar building units are proposed, but the deduced cross-bridge arrangements are fundamentally different. The opposite polarity of the adjusting longitudinal rows is proposed for the helical structure, while in the case of square filaments, or myosin ribbons, only their two faces are appositively polarized. Analysis of our unpublished archival data on light meromyosin (LMM) paracrystals and myosin rod assemblies as well as the filaments themselves indicated that the rods were assembled with a 6°-7° tilt angle from the rods' longitudinal axis, in contrast to the lack of tilt in LMM, both exhibiting a 14.3 nm myosin periodicity. Optical diffraction analysis of EM images of the rod assemblies and those of intact myosin confirmed their helical architecture characterized by 28 nm residue translations, 172 nm repeats and 516 nm pitch. A detailed helical model of these filaments was elucidated with bipolar tetramer building units made of two polar trimers. The filaments elongate at their two ends in a head-to-head manner, enabling targeted cross-bridge polarity of the adjacent rows, in the form of a unique Boerdijk-Coxeter type helix, similar to that of collagen or desmin fibers, with the covalent links replaced by a head-to-head clasp.

Switching Muscles On and Off in Steps: The McKillop-Geeves Three-State Model of Muscle Regulation

BJ Classic highlighting the article "Regulation of the interaction between actin and myosin subfragment 1: evidence for three states of the thin filament."

Thin Filament Structure and the Steric Blocking Model

By interacting with the troponin-tropomyosin complex on myofibrillar thin filaments, Ca2+ and myosin govern the regulatory switching processes influencing contractile activity of mammalian cardiac and skeletal muscles. A possible explanation of the roles played by Ca2+ and myosin emerged in the early 1970s when a compelling "steric model" began to gain traction as a likely mechanism accounting for muscle regulation. In its most simple form, the model holds that, under the control of Ca2+ binding to troponin and myosin binding to actin, tropomyosin strands running along thin filaments either block myosin-binding sites on actin when muscles are relaxed or move away from them when muscles are activated. Evidence for the steric model was initially based on interpretation of subtle changes observed in X-ray fiber diffraction patterns of intact skeletal muscle preparations. Over the past 25 years, electron microscopy coupled with three-dimensional reconstruction directly resolved thin filament organization under many experimental conditions and at increasingly higher resolution. At low-Ca2+, tropomyosin was shown to occupy a "blocked-state" position on the filament, and switched-on in a two-step process, involving first a movement of tropomyosin away from the majority of the myosin-binding site as Ca2+ binds to troponin and then a further movement to fully expose the site when small numbers of myosin heads bind to actin. In this contribution, basic information on Ca2+-regulation of muscle contraction is provided. A description is then given relating the voyage of discovery taken to arrive at the present understanding of the steric regulatory model.

Hugh E. Huxley: the compleat biophysicist

The sliding filament model of muscle contraction, put forward by Hugh Huxley and Jean Hanson in 1954, is 60 years old in 2014. Formulation of the model and subsequent proof was driven by the pioneering work of Hugh Huxley (1924-2013). We celebrate Huxley's integrative approach to the study of muscle contraction; how he persevered throughout his career, to the end of his life at 89 years, to understand at the molecular level how muscle contracts and develops force. Here we show how his life and work, with its focus on a single scientific problem, had impact far beyond the field of muscle contraction to the benefit of multiple fields of cellular and structural biology. Huxley introduced the use of x-ray diffraction to study the contraction in living striated muscle, taking advantage of the paracrystalline lattice that would ultimately allow understanding contraction in terms of single molecules. Progress required design of instrumentation with ever-increasing spatial and temporal resolution, providing the impetus for the development of synchrotron facilities used for most protein crystallography and muscle studies today. From the time of his early work, Huxley combined electron microscopy and biochemistry to understand and interpret the changes in x-ray patterns. He developed improved electron-microscopy techniques, thin sections and negative staining, that enabled answering major questions relating to the structure and organization of thick and thin filaments in muscle and the interaction of myosin with actin and its regulation. Huxley established that the ATPase domain of myosin forms the crossbridges of thick filaments that bind actin, and introduced the idea that myosin makes discrete steps on actin. These concepts form the underpinning of cellular motility, in particular the study of how myosin, kinesin, and dynein motors move on their actin and tubulin tracks, making Huxley a founder of the field of cellular motility.

Fifty years of fibrous protein research: a personal retrospective

As a result of X-ray fiber diffraction studies on fibrous proteins and crystallographic data on fragments derived from them, new experimental techniques across the biophysical and biochemical spectra, sophisticated computer modeling and refinement procedures, widespread use of bioinformatics and improved specimen preparative procedures the structures of many fibrous proteins have now been determined to at least low resolution. In so doing these structures have yielded insight into the relationship that exists between sequence and conformation and this, in turn, has led to improved methodologies for predicting structure from sequence data alone. In this personal retrospective a selection of progress made during the past 50years is discussed in terms of events to which the author has made some contribution.

X-ray structure analysis of thin filaments of a molluscan smooth muscle in the living relaxed state

In the small-angle x-ray diffraction pattern of the living relaxed anterior byssus retractor muscle of Mytilus edulis, the thin filaments showed the following features. The 59.8-A reflection was much stronger and a little farther from the meridian than the 51.9-A reflection, although they are both contributions of the first-order Bessel function and are comparable with each other in the height from the equator. The 381-A reflection, given by the second-order Bessel function, was weaker than the 59.8-A reflection by more than the difference between the peak values of the first- and second-order Bessel functions, and was not so distant radially from the latter as estimated from the amount of peak shift brought about by the alteration of the Bessel order. A model of the thin filament was made on the basis of inverse Fourier transformation of the scattering amplitude, and the above features were explained by the characteristic shape of actin shown in this model. The actin subunits are elongated along the genetic left-hand helix with a pitch of 59.8 A, and are bonded together along the genetic helix in the inner part of the filament.

The dependence of the latency relaxation on temperature in single muscle fibres of the frog

The time course of the latency relaxation was studied at various temperatures in the range 0-26 degrees. Over the entire range the time of onset of the drop in tension, t1, was independent of sarcomere length. At temperatures above 12-15 degrees C the falling phase had a point of inflexion, while at lower temperatures there was an interval during which the tension fell at a constant rate. The time when the rate of drop in tension had passed its maximum value tL2, the time to the maximum drop in tension t2, and the time when the tension crossed the resting level t3, all showed linear dependence on sarcomere length in the range from 2.1 to 2.7-3.4 microgram. In this range the durations of the intervals tL2-t1, t2-t1, and t3-t1 were nearly proportional to the distance from the Z-line to the end of the zone of overlap between the thick and the thin filaments. This could be explained as the activation being a longitudinal process starting from the Z-line. The slopes (dt/dS) of the linear portions of the time variables t1.2, t2, and t3 in a time-sarcomere length (S) diagram all had the same dependence on temperature giving a Q10 of 1.75. Under the assumption that the activation process followed a diffusion of calcium from the Z-line region to the zone of overlap a diffusion coefficient was estimated. At room temperature it had a magnitude of about 1/20 of that for calcium chloride in water. It had a dependence on temperature corresponding to an Arrhenius activation energy of about 37 kJ/mol which is about twice the activation energy for a simple diffusion of calcium in water. The results can be interpreted in terms of the time course of the latency relaxation mainly reflecting a longitudinal diffusion of calcium ions in the sarcoplasm.

The effect of a small stretch on the latency relaxation and the short-range elastic stiffness in isolated frog muscle fibres

The amplitude of the latency relaxation and the stiffness of the short-range elastic component (SREC) were determined in single muscle fibres before and immediately after subjecting the fibre to a sudden small stretch at constant velocity. After the stretch the amplitude of the latency relaxation was reduced compared to the value obtained before the stretch provided that the velocity of stretch exceeded about 1 mm . ms-1 per half-sarcomere and that the amplitude of the stretch was sufficient to reach the stationary response of the SREC. The effect of the stretch on the amplitude of the latency relaxation subsided after about 4 ms. A reduction of the amplitude of the latency relaxation was accompanied by a reduction of the short-range elastic stiffness. This result is discussed in terms of the various hypotheses put forward to account for the generation of the latency relaxation.

Sarcomere lengthening and tension drop in the latent period of isolated frog skeletal muscle fibers

A laser diffraction technique has been developed for registering small changes in sarcomere length. The technique is capable of resolving changes as small as 0.2 A in isolated frog skeletal muscle fibers. The small sarcomere lengthening that accompanies the drop in tension in the latent period of contraction was investigated. We suggest this lengthening be named latency elongation (LE). The LE is present in a completely slack fiber and must, therefore, be caused by a forcible lengthening process. Furthermore, the LE is dependent on the existence of an overlap between thin and tick filaments. The rate of elongation and the time interval between stimulation and maximum elongation may vary along the fiber. The maximum elongation was 3-5 A per sarcomere. At any instant the drop in tension is a product of the sum of sarcomere lengthenings along the fiber and the slope stiffness of the series elasticity. The latency relaxation (LR) could be registered in the sarcomere length range from 2.2 mum to 3.6-3.7 mum. The amplitude went through a sharp maximum at 3.0-3.1 mum. In the sarcomere length range from 2.2 to 2.8 mum the delay from onset to maximum LR was nearly proportional to the distance from the Z-line to the overlap zone. A working hypothesis is presented. It is suggested that the LE is caused by a lengthening of the thin filaments.