The thick filament is made up of bipolar, elongated proteins called myosin. There are about myosin molecules in a sarcomere and they are located at the center of the sarcomere. Thick filaments will self assemble in vitro under the right ionic conditions. In Get Body Smart. Muscle cells are specialized to generate force and movement.
Surprisingly, skeletal muscle cells not only synthesize a series of myofibrillar proteins but also regulate the assembly of those proteins into the sarcomere structures. However, authentic sarcomere structures cannot be reconstituted by combining purified myofibrillar proteins in vitro, therefore there must be an elaborate mechanism ensuring the correct formation of myofibril structure in skeletal muscle cells.
This review discusses the role of myosin, a main component of the thick filament, in thick filament formation and the dynamics of myosin in skeletal muscle cells. Changes in the number of myofibrils in myofibers can cause muscle hypertrophy or atrophy.
Therefore, it is important to understand the fundamental mechanisms by which myofibers control myofibril formation at the molecular level to develop approaches that effectively enhance muscle growth in animals. One of the most distinctive features of skeletal muscle is its plasticity, as it can increase or decrease its mass hypertrophy or atrophy, respectively in response to environmental factors such as growth, aging, and exercise.
However, little attention has been paid to the mechanisms underlying the assembly and structural maintenance of myofibrils. In this review, to better understand these fundamental issues regarding hypertrophy and atrophy in skeletal muscle, we focus on the thick filament and its main component, myosin. First, we describe the structure of myofibrils in the skeletal muscle, then consider myosin and thick filament structure, and finally discuss myosin replacement and maintenance of thick filament structure in myofibrils.
Skeletal muscle tissue is composed of bundles of myofibers packed by intramuscular connective tissues consisting of extracellular matrix molecules. Each myofiber is a single, long, multinucleated cell approximately 0. Thus, skeletal muscle fibers are entirely dedicated to generating force. Structure of myofibrils in the myofiber. Skeletal muscle tissue consists of bundles of myofibers.
Each myofiber contains millions of myofibrils that are comprised of longitudinally aligned sarcomeres. A transmission electron micrograph of the sarcomere structure is shown at the bottom of panel. The figure was created using BioRender. Although the length of the thin filament is known to vary depending on developmental stage Ohtsuki, , Gokhin et al.
It has a strictly uniform dimension of 1. The sarcomeric myosins specifically expressed in skeletal and cardiac muscle cells to generate contractile force are categorized as either conventional myosins or class II myosins.
Schematic model of sarcomeric myosin. Sarcomeric myosin is formed by a pair of myosin heavy chains, essential light chains, and regulatory light chains. Sarcomeric myosin has the remarkable ability to form highly organized bipolar thick filaments in myofibrils. Myosin forms filaments in an antiparallel fashion at the center of the thick filament, while myosin forms filaments in a parallel way in the rest of the thick filament. Consequently, a bipolar thick filament is formed, leaving a central bare zone in the middle.
Which myosin domain is critical for forming the thick filament in skeletal muscle cells? S1 is dispensable for thick filament formation Ojima, Oe, et al. Furthermore, LMM is incorporated into the restricted region of the thick filament, in the vicinity of the central bare zone. These results demonstrate that the LMM domain is not sufficient for thick filament formation in cultured muscle cells although it is essential for forming myosin filaments or paracrystals in vitro Sohn et al.
However, the molecular structure of the thick filament has not yet been completely determined in vertebrates Irving, How often are myosin molecules exchanged in the thick filaments? The exchange rate of myofibrillar proteins in other sarcomere components has also been determined by other research groups. Besides molecular size, the structural complexity of the thick filament may account for the slow myosin exchange rate in skeletal muscle cells. In nonmuscle cells, approximately 30 nonmuscle myosin molecules form a bipolar filament.
The molecular masses of these two distinct myosins nonmuscle and sarcomeric myosins are almost identical, however, the number of myosin molecules is 10 times higher in a single thick filament than in a single nonmuscle filament.
The intricate structure of thick filaments may reduce the exchange rate of myosin molecules in skeletal muscle cells. As myosin molecules are continuously replaced in the thick filament of myofibrils, what are the sources of these replacement myosin molecules?
In fact, blocking myosin biosynthesis using a translation inhibitor significantly slows the substitution rate of myosin in myofibrils Ojima, Ichimura, et al. The secondary source is cytosolic myosin that is not incorporated into the thick filament. These results also demonstrate that myosin allocated for replacement is present in the myocytosol, although myosin forms aggregates or filaments in physiological ionic conditions in vitro Perry, In other words, the myosin exchange rate in thick filaments declines when myosin supply sources, including de novo synthesized myosin and pooled cytosolic myosin, are depleted.
Heat shock proteins HSP are a family of molecular chaperones which assist in protein folding and remodeling, and are rapidly upregulated in response to cellular stressors. We investigated the effects of HSP90 on myosin substitution in thick filaments. HSP90 overexpression increased the rate of myosin replacement, while inhibiting its chaperone activity reduced myosin substitution in myofibrils, indicating that HSP90 chaperone activity is closely associated with myosin replacement Ojima et al.
Upregulated MYH gene expression is also observed in myotubes overexpressing HSP90, alongside an increase in myosin content, however, the molecular mechanisms underlying the upregulation of MYH expression by HSP90 remain unclear.
What are the conditions in muscle cells overexpressing HSP90? Calcium is the trigger for contraction of all muscle types. They are ATP-dependent and responsible for actin-based motility.
Each skeletal muscle fiber is a single cylindrical muscle cell. Within the fasciculus, each individual muscle cell , called a muscle fiber, is surrounded by connective tissue called the endomysium. The arrangement of the thick myosin filaments across the myofibrils and the cell causes them to refract light and produce a dark band known as the A Band.
In between the A bands is a light area where there are no thick myofilaments, only thin actin filaments. Dysfunctional actin -ATP binding may result in abnormal thin filament formation and impair muscle contraction, leading to muscle weakness and the other signs and symptoms of actin -accumulation myopathy.
In some people with actin -accumulation myopathy, no ACTA1 gene mutations have been identified. Thick filaments consist primarily of the protein myosin. Each thick filament is approximately 15 nm in diameter, and each is made of several hundred molecules of myosin. Thin filaments , 7 nm in diameter, consist primarily of the protein actin, specifically fibrous F actin.
Myofibrils consist of two types of protein filaments , called thin filaments and thick filaments. Troponin molecules, together with tropomyosin and actin molecules, form the thin filaments which have a helix structure - as shown in the diagram below. Is myosin a thick or thin filament? What is the largest known protein? What is a group of Myofibrils called? What is an example of a Myofilament? What is the Sarcolemma?
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