Myosin, actin, tropomyosin, and troponin are the four major structural proteins that make up muscle. These proteins form the foundation of the basic contractile unit known as the sarcomere. The banding pattern seen in striated skeletal muscle comes from the repeating arrangement of two myofibrillar filaments, thick filaments and thin filaments.
Thick filaments are composed primarily of a large protein, myosin [~470,000 Daltons (Da)]. Myosin accounts for approximately 55% of the total percentage of structural protein in muscle. The myosin molecule is hexameric, being composed of two heavy chains and four light chains.
Trypsin can cleave the heavy chains to yield fragments of heavy meromysin (HMM), and light meromysin, (LMM) which account for the head and tail, respectively, of the myosin molecule. The HMM can be further divided into smaller fragments by papain cleavage to yield subfragments 1 and 2 (S1 and S2, respectively).
The S1 fragment of HMM is responsible for the ATPase activity of myosin, allowing actin–myosin cross-bridging during muscle contraction. The thick filament also contains C protein, M protein, and titin. The C protein is involved in cross-bridge regions of the thick filament, and the M protein contains the creatine phosphokinase enzyme. Titin is an elastic protein that is thought to provide resistance to elongation of the sarcomere and to protect against overstretch.
Thin filaments contain primarily actin, along with lesser amounts of tropomyosin and troponin proteins, which form an α-helical cylinder. The actin of thin filaments is anchored at the end of the sarcomere to the Z line. Thin filaments measure approximately 1.0 µm in length and are arranged in a hexagonal pattern around each thick filament.
Actin thin filaments also are associated with troponin and tropomyosin proteins, which serve a regulatory function during contraction and are discussed later in this section. Nebulin is a cytoskeletal protein that has been proposed to regulate the length of the thin filament during contraction and stretch.
Myofibrils demonstrate a regular, repeating arrangement of dark and light bands every 2 to 3 µm when viewed with an electron microscope. The regular arrangement of striations in skeletal muscle results from the organization of thick and thin filament proteins within the sarcomere.
The sarcomere extends from one Z band to another, with I bands and A bands in between. The Z band, or Z disk (from Zwishen-Schieben, which is German for “interim disk”), anchors thin filaments at either end of the sarcomere. The I (isotropic) band, which is made of actin, troponin, and tropomyosin, is less densely packed with proteins and, therefore, appears lighter under a polarizing microscope.
The A (anisotropic) band is composed of myosin and the actin– tropomyosin complex and corresponds with the thick filaments. It is more densely packed with proteins, giving it a darker appearance when viewed with a polarizing microscope. In the center of the A band is a protein-dense band known as the H (Heller) zone that contains the center of the sarcomere, the M (Middle) line.
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