What is the Role of Tropomyosin in Skeletal Muscles?

Trapomyosin is a protein with multiple functions. It plays an important role in the regulation of muscle contraction and relaxation. Among its many roles, tropomyosin is responsible for maintaining skeletal muscles’ elasticity. This is achieved through its association with a thin filament called tropomyosin.

Function

Tropomyosin is a protein that modulates muscle contraction under physiological conditions. The protein has essential functions and is required by diverse species. These include worms, flies, yeast, and complex mammals. However, we don’t know exactly how tropomyosin works in the human body.

To trigger muscle contraction, tropomyosin must change its conformation. It does this by uncovering the myosin-binding site on actin. This allows the cross-bridge to form. Calcium ions bind to troponin and cause it to change conformation. Once the cross-bridge is formed, the muscles contract. This cycle continues until ATP and Ca2+ ions are depleted.

The protein is one of the many integral components of actin filaments. The tropomyosin family plays a critical role in the regulation of actin filaments. The rod-shaped dimers of tropomyosin lie along the a-helical groove of most actin filaments. This allows them to interact with each other as they wind around each actin filament.

Tropomyosin and actin are the two main components of smooth muscles. Together, they form the thin filaments that allow the muscle to contract. They are accompanied by calmodulin and caldesmon, two proteins that regulate the transition between on and off activity.

Three isoforms of tropomyosin have been studied in the skeletal muscle of mammals. The major Tpm protein in rat and human muscles is Tpm1.1. Its relative abundance is determined by muscle type and by analyzing high-resolution FT-MS data.

Structure

Tropomyosin is a protein that is a regulatory component of muscle cells. Its role is to control the interaction between actin-containing thin filaments and myosin-containing thick filaments, which is essential for muscle contraction. Tropomyosin is expressed in both muscle cells and nonmuscle cells. Currently, there are 18 different mammalian Tm isoforms.

Tropomyosin and actin are linked by a protein called nebulin. When troponin binds to actin, it forms a cross-bridge with actin filaments. The two proteins are physically linked, and the more cross-bridges a muscle fiber has, the greater the tension in the muscle fiber.

Tropomyosin is composed of three subunits: TnC, TnI, and TnT. The TnI subunit is arranged in a nearly two-fold symmetric fashion, and it has a dumbbell-like shape. The TnC subunit binds the inhibitory segment of TnI in its Ca2+-activated state.

Tm is found in the B-site of myosin, between the lower and upper 50 kD domains. This suggests that it may prevent either domain from binding to actin. However, this does not necessarily mean that Tm is a non-cooperative protein with actin.

Tropomyosin is a multifunctional protein. It is found in skeletal muscles in both human and zebrafish. It is a large actin-binding protein. It has 183 exons and produces hundreds of NEB isoforms. Its amino acid sequence contains simple repetitive regions that form the core of the molecule and actin-binding site.

A study of tropomyosin has demonstrated that it is a neurotransmitter that inhibits contraction. It works by blocking the binding sites of myosin on actin molecules. It also inhibits the action of motor neurons. In addition, it inhibits the interaction between myosin and actin molecules, which reduces the force generating capacity of the muscle.

Nebulin-binding mutants have weak affinities for tropomyosin. These mutants also show weak binding affinity to the super repeat S18. However, the b-tropomyosin-nebulin mutant contains super repeats that bind tropomyosin.

Mutations

Mutations in tropomyosin, a component of actin, have been linked to distinct inherited muscle diseases. The affected muscles display heightened cardiac muscle contraction and skeletal muscle weakness, resulting from impaired contractile function of myocytes. Both diseases are associated with mutations in tropomyosin.

Currently, there are three major tropomyosin isoforms: TPM1 (tropomyosin), TPM2 (tropomyosin), and TPM3 (troponin T). The TPM1 isoform is expressed in cardiac muscle fibres and slow-twitch skeletal muscle fibres. The TPM1 mutation has been linked to myopathy, an inherited muscle disorder that affects the skeletal muscles.

The TPM2 gene has been associated with a number of muscle disorders. In a report published in the American Journal of Human Genetics, a patient with TPM3 mutation had severe infantile nemaline myopathy. A few cases have been identified with this mutation.

Mutations in tropomyosin can result in distal arthrogryposis, myopathy, and apoptosis. Moreover, mutations in tropomyosin have a direct effect on the movement of tropomyosin towards the inner domain of actin.

The HCM and NM mutant forms of tropomyosin have different effects on the contractile function of intact myocytes. While HCM mutant Tm results in more dysfunction, NM mutant Tm shows the highest expression in adult cardiac myocytes.

Moreover, mutations in tropomyosin in NM affect the contractile function of skeletal muscle cells. Mutations of Tm in NM result in reduced force production at submaximal activating Ca2+ concentrations. In addition, the mutations in NM cause the formation of nemaline rods, a secondary consequence of this disease.

NM is a rare genetic muscle disorder that is caused by mutations in at least nine genes. The genes that cause NM are related to the nebulin protein, a giant filamentous protein found in skeletal muscle fibers. It is composed of a series of 35-residue a-helical domains that form super repeats. Each super repeat contains a tropomyosin binding site.

Some studies have linked certain tropomyosin isoforms with various cancers. In a study that studied tropomyosin expression in Lewis lung carcinoma cell lines, the authors found a significant correlation between tropomyosin 2 and Lewis lung cancer metastatic status.

Regulation

The regulation of tropomyosin in skeleton muscle is a complex process involving several mechanisms. These mechanisms enable tropomyosins to play a key role in cell transformation and change of shape. Interestingly, these mechanisms are highly dynamic and flexible.

Tropomyosins belong to a large family of integral actin filament components. They play an essential role in the regulation of actin filament function. These proteins are rod-shaped, coiled-coil homo-dimers that lie in the a-helical groove of most actin filaments. They interact with each other along the length of the actin filament. Tropomyosin dimers align head-to-tail, and interact with each other to regulate actin filament function.

There are three major isoforms of tropomyosin. One is TPM1, while the other two are TPM2 and TPM3. The TPM1 isoform is expressed predominantly in cardiac muscle, while the other two are predominantly found in skeletal muscles. TPM1 mutations are associated with inherited myopathies.

During contraction, troponin binds calcium ions. When this happens, tropomyosin moves to the surface of actin filaments, which exposes the myosin head binding site. When these two molecules bind, the muscle contracts. However, the contraction ends when the sarcoplasmic reticulum pumps calcium out of the muscle’s interior. Tropomyosin then shifts back to the off position.

Regulation of tropomyosin in the skeletal muscle is complex. The regulatory protein acts on actin filaments to relay information from a Ca2+ sensor to the actin-myosin complex. In addition, it can either be in a closed or open state.

Troponin-tropomyosin is thought to play a pivotal role in regulation of actin filament system dynamics. It influences actin filament associations with other actin-binding proteins and confers specific properties to the filament. Actin filaments are involved in a wide range of cellular processes and are capable of responding quickly to stimuli.

The regulation of tropomyosin in skeleton muscle is largely uncharacterized. However, the two muscle types share similarities in sarcomere structure and filament protein isoforms. Additionally, they are both believed to have the same actin-binding myosin motors.