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How Actin and Myosin Interact With Each Other to Shorten a Sarcomere

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Imagine you are sitting in a rowboat on a still lake. To move across the lake, you must place your oars in the water and pull backwards. At the end of your stroke, you lift the oars out of the water, move them forward and dip them back into the lake for the next stroke. Each movement of the oar propels the boat across the water. Your muscles work in a similar fashion

Muscles are composed of two major protein filaments: a thick filament composed of the protein myosin and a thin filament composed of the protein actin. Muscle contraction occurs when these filaments slide over one another in a series of repetitive events. Let's see how myosin molecules play a role similar to the oars of a rower.

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What does it mean to say that muscles contract? The short answer is - no pun intended - muscles shorten when they contract. This begs the question of how muscles shorten and, thus, contract. To answer this question, we must first examine the components of the sarcomere, which is the fundamental functional unit of striated muscle - that is, skeletal and cardiac muscle. To say that the sarcomere is the functional unit means that all the components needed for contraction are contained within each sarcomere. In fact, muscle is composed of millions of tiny sarcomeres, and each sarcomere shortens, thus resulting in muscular contraction. It's important to note that smooth muscles do not contain sarcomeres. Rather, each smooth muscle is like one giant sarcomere

In this lesson, we will describe the basic components of a sarcomere and how they interact to contract our striated muscles.Now, let's take a look at an individual myofibril within the muscle cell. At this level, we can see the sarcomeres are butted up end to end, running the length of each myofibril. sarcomere is end to end A given myofibril contains approximately 10,000 sarcomeres, each of which is about 3 micrometers in length. While each sarcomere is small, several sarcomeres added together span the length of the muscle fiber. Each sarcomere consists of thick and thin bundles of proteins referred to as myofilaments. If we magnify a portion of the myofilaments, we can identify the molecules that compose them. Thick filaments contain myosin, while thin filaments contain actin. Actin and myosin collectively are referred to as the contractile proteins, which cause muscle shortening when they interact with each other. Additionally, thin filaments contain the regulatory proteins troponin and tropomyosin, which regulate interaction between the contractile proteins. The I band is that part of the sarcomere that contains thin filaments, while the A band contains an area of overlap between the thin and the thick filaments. As you can see, a single I band spans two neighboring sarcomeres. A Z line attaches those neighboring sarcomeres. The thin filaments are attached to the Z lines on each end of the sarcomere, while the thick filaments reside in the middle of the sarcomere.

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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 (Frank D., F, 2011). 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.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 (Gautel M., 2016)

In all cases, molecules which are enzymes that can utilise the energy stored in adenosine triphosphate (ATP) move along molecular tracks.

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Finally, the third stage of cell crawling, retraction of the trailing edge, is the least understood. The attachments of the trailing edge to the substratum are broken, and the rear of the cell recoils into the cell body

The process appears to require the development of tension between the front and rear of the cell, generating contractile force that eventually pulls the rear of the cell forward. This aspect of cell locomotion is impaired in mutants of Dictyostelium lacking myosin II, consistent with a role for myosin II in contracting the actin cortex and generating the force required for retraction of the trailing edge.

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Gautel M., Djinović-Carugo K. The sarcomeric cytoskeleton: From molecules to motion. J. Exp. Biol. 2016;219:135–145. doi: 10.1242/jeb.124941.

Frank D., Frey N. Cardiac Z-disc Signaling Network. J. Biol. Chem. 2011;286:9897–9904. doi: 10.1074/jbc.R110.174268.

Bienert S., Waterhouse A., de Beer T.A.P., Tauriello G., Studer G., Bordoli L., Schwede T. The SWISS-MODEL Repository—New features and functionality. Nucleic Acids Res. 2017;45:D313–D319. doi: 10.1093/nar/gkw1132.

McNamara J.W., Li A., Dos Remedios C.G., Cooke R. The role of super-relaxed myosin in skeletal and cardiac muscle. Biophys. Rev. 2015;7:5–14. doi: 10.1007/s12551-014-0151-5.

Lee K.H., Sulbarán G., Yang S., Mun J.Y., Alamo L., Pinto A., Sato O., Ikebe M., Liu X., Korn E.D., et al. Interacting-heads motif has been conserved as a mechanism of myosin II inhibition since before the origin of animals. Proc. Natl. Acad. Sci. USA. 2018

Woodhead J.L., Zhao F.-Q., Craig R., Egelman E.H., Alamo L., Padron R. Atomic model of a myosin filament in the relaxed state. Nature. 2005

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