Abstract
Muscular contraction is a fundamental phenomenon in all animals; without it life as we know it would be impossible. The basic mechanism in muscle, including heart muscle, involves the interaction of the protein filaments myosin and actin. Motility in all cells is also partly based on similar interactions of actin filaments with non-muscle myosins. Early studies of muscle contraction have informed later studies of these cellular actin-myosin systems. In muscles, projections on the myosin filaments, the so-called myosin heads or cross-bridges, interact with the nearby actin filaments and, in a mechanism powered by ATP-hydrolysis, they move the actin filaments past them in a kind of cyclic rowing action to produce the macroscopic muscular movements of which we are all aware. In this special issue the papers and reviews address different aspects of the actin-myosin interaction in muscle as studied by a plethora of complementary techniques. The present overview provides a brief and elementary introduction to muscle structure and function and the techniques used to study it. It goes on to give more detailed descriptions of what is known about muscle components and the cross-bridge cycle using structural biology techniques, particularly protein crystallography, electron microscopy and X-ray diffraction. It then has a quick look at muscle mechanics and it summarises what can be learnt about how muscle works based on the other studies covered in the different papers in the special issue. A picture emerges of the main molecular steps involved in the force-producing process; steps that are also likely to be seen in non-muscle myosin interactions with cellular actin filaments. Finally, the remarkable advances made in studying the effects of mutations in the contractile assembly in causing specific muscle diseases, particularly those in heart muscle, are outlined and discussed.
Highlights
Nature’s Linear MotorsIn human bodies and those of other animals there are beautifully designed molecular mechanisms which move our limbs, or pump our blood, or aid in peristalsis, and there are motile mechanisms in all cells that move cell organelles or other cargoes from one part of the cell to another
This kind of head interaction was first seen in 2D crystals of vertebrate smooth muscle myosin [21], but was found on all types of relaxed myosin filament [20,51,52,53,54] when studied at high enough resolution by electron microscopy and single particle analysis
Use can be made of the photon power in focused laser beams to manipulate small beads in the light microscope; so-called optical trap methods. If these beads are attached to either end of an actin filament (Figure 17c), this filament can be lowered onto an isolated myosin molecule on a pedestal or on another bead and the force generated by the interaction can be determined by the observed displacement of the actin-attached beads
Summary
In human bodies and those of other animals there are beautifully designed molecular mechanisms which move our limbs, or pump our blood, or aid in peristalsis, and there are motile mechanisms in all cells that move cell organelles or other cargoes from one part of the cell to another. The molecular tracks are actin filaments and the ATP-driven motors are myosin molecules [1]. In all non-muscle cells there are actin filament tracks with myosin-like motors moving along them [2]. Muscle fibres contain cross-striated myofibrils, often about 1 to 10 μm in diameter and very long. Vertebrate smooth muscles act on our intestines, our blood vessels and some other internal organs Like other muscles, they contain actin and myosin filaments, but the myosin filaments are of a different type specialised to allow the muscles to shorten over a very large range of muscle lengths (see for example [8]). The muscles appear smooth because the myosin and actin filaments are not regularly organised into sarcomeres, they do contain rudimentary contractile units with alternating myosin and actin filaments [8,9]
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