Introduction

When it comes to the molecular mechanism of muscle contraction we are up with the explanation of Sliding Filament Theory. A sarcomere, which serves to be the basic unit of muscle contraction is made up of myofibrils which in turn are composed of actin and myosin filaments. The ratio of actin to myosin filament is about 2:1. The actin filaments are in turn composed of Actin, Troponin, and Tropomyosin. The backbone of an actin filament is a double-stranded F-actin protein molecule.

These actin filaments also contain a protein named Tropomyosin which is our main topic for discussion today. Each tropomyosin molecule has a net molecular weight of 70,000 and a length of 40 nanometers. What was very confusing for me at this point, was the statement of 70,000 in books without the unit. So let me be clear, it’s in Dalton. The tropomyosin in itself is a long coil of two polypeptide chains located in the groove between the two twisted actin strands. These molecules are wrapped spirally around the F-actin helix.

During the resting state, the tropomyosin lies on the top of the active sites of actin filaments preventing myosin and actin filaments to interact. The interaction will bring about contraction within the skeletal muscles via the phenomenon of Sliding Filament as stated above. These interactions will only take place when there’s an action potential beyond the threshold potential.

Location of tropomyosin

Tropomyosin is termed to be a cold-coiled dimer and it polymerizes all through the length of the actin filament. There is a profound head to head coiling all through the major actin grooves. This makes the counterpart to be flexible and stay strain-free.

This will allow the action potential to be sensed easily and when it crosses beyond threshold potential let myosin stick to actin. This hinge-like actin-binding and unbinding bring about the contraction. Each tropomyosin molecule is made up of a total of 284 amino acids which spans approx seven actin filaments altogether.  The molecule consists of two elongated strands that run along the length of the filament.

Structure of a tropomyosin molecule

This 60-70kdalton molecule is about 420 Angstrom long. It is associated with actin and troponin within the striated muscles. In mammals, about 40 different isoforms of tropomyosin are discovered. These isoforms serve two major functions. Firstly, allow myosin recruitment onto the actin molecules and secondly, it enhances actin’s stability in non-muscle systems of the body. According to the Structural Classification of Proteins, Tropomyosin can be classified into 2  general classes as follows:

• The coiled-coil hetero tropomyosin contributes to the alpha-helical arrangement with the dimer interface. The adjacent hydrophobic regions of 2 alpha-helical strands allow dimerization of the protein in the first space.
• The parallel coiled-coil homodimer is made along the actin length in head to tail overlap. The dimers are polymerized at certain intermolecular contacts through ionic, hydrophobic and nonpolar interactions.

Tropomyosin synthesis

Tropomyosin is a protein. The series of transcription and translation needs to be followed to synthesize this protein.

TRANSCRIPTION

It is a process by which information contained in the tropomyosin gene is replicated in the form of a newly assembled piece of messenger RNA. This action involves several RNA polymerases for the processes of initiation, elongation, and termination.

The mRNA molecule thus formed contains information about the sequence of various amino acids to make tropomyosin molecules. After post-transcriptional modifications, this mRNA travels to the ribosomes where the synthesis of tropomyosin takes place.

TRANSLATION  

It is the process by which the information present in the mRNA is translated into the amino acid sequence of a polypeptide chain to make tropomyosin molecules. The process takes place on the ribosomes and involves the steps of initiation, elongation, and termination.

If multiple copies of the tropomyosin molecule are needed, more than one ribosomes translate the same mRNA molecule making multiple polypeptide chains.

Functions of tropomyosin

The function of tropomyosin can be classified into two categories which are muscle and non-muscle systems.

Muscular Tissues

Muscle tissue is made up of several myocytes which in turn are made up of myofibrils. These myofibrils are further divided into simpler units called a sarcomere. A sarcomere extending between two successive Z discs forms the basic structural unit. In it are thick filaments named myosin and thin ones called actin, troponin, and tropomyosin. Tropomyosin in skeletal muscles inhibits myosin cross-linking to the actin filaments.

When an action potential travels along the motor nerve it reaches the muscle fibres where they secrete a neurotransmitter called acetylcholine. The Ach acts on nicotinic receptors hence causing acetylcholine gated cation channels. This allows the bulk of sodium ions to diffuse to the interior of the muscle fibre causing the muscle to undergo depolarization. This, in turn, open voltage-gated sodium channels triggering the action potential. The action potential travels through sarcolemmas depolarizing it gradually.

This will, in turn, cause the sarcoplasmic reticulum within the sarcomere to release its calcium ions reserves. The calcium ions are going to bind to the troponin complex which is bound to both tropomyosin and actin. The troponin will displace tropomyosin from the active site. These ions initiate forces between myosin and actin causing them to slide along which is a contractile process. Just after the span of 3 milliseconds calcium ions are pumped back into the sarcoplasmic reticulum as a reserve for the next action potential to arrive. As the concentration of calcium falls in the sarcoplasm troponin will no longer be able to perform its function.

Non-Muscular Tissues

In non-muscle systems, tropomyosin does a great deal of work. It recruits different myosin motors within the cell. So, the actin filaments that are covered with tropomyosin controls the motor activity of the cell spatially and has a role in cellular function. These regulate the cell cytoskeleton which somehow controls the cell’s motility.

Role of tropomyosin in smooth muscles

Smooth muscles can contract spontaneously and rhythmically. These muscles do not have regular sarcoplasmic patterns but contraction is still regulated via the interaction between myosin and actin. The thin filament is made up of actin, tropomyosin, and calmodulin. The Calcium-dependent calmodulin complex act here like troponin and controls tropomyosin transitions along with the phase.

Pathology following tropomyosin

Muscles disease

Nemaline myopathy is a disorder that affects the skeletal muscles primarily. People with such muscular disease undergo scoliosis (abnormal spine curvature) and (contractures) joint deformities. People may even have difficulty swallowing. What causes such deformity?

The cause lies in the fact of mutation. The mutation is in one of the many muscular genes which encode muscle proteins. In most cases, the mutation is experienced in one of the two genes which are NEB or ACTA1. Change in any of these genes leads to disorganized protein functioning hence, abrupt muscle contraction. Major mutations are seen in alpha-actin, tropomyosin, troponin, and nebulin. Perhaps, major beta and gamma tropomyosin mutations are seen.

Autoimmune disease

Autoimmune diseases are defined to be the ones in which the body makes an antibody against its self. A well known autoimmune disease named ulcerative colitis has found its link following antibodies production against tropomyosin. It was identified when Tropomyosin 5 and 1 were discovered to take part in the pathogenesis of ulcerative colitis. The process of inflammation was found to be aggravated by elevated productions of IgG antibodies against Tropomyosin 5. Hence, it induces a large amount of T-cell response. In addition to this, tropomyosin antibodies are also found in rheumatic fever which is again an autoimmune disease.

Cancer

Tropomyosin 1 is downregulated in many cancerous situations. Tropomyosin 1 is expressed by different types of cells. Tropomyosin with more number amino acids which are 284 units is downregulated in cancerous transformations. These conditions are mainly found in colorectal cancer, glioma, and neuroblastoma. This will increase the rate at which apoptosis takes place.

Read more about Muscle Contraction

How to study tropomyosins?

One of the major and most accountable ways to study tropomyosin’s isoforms is by studying it with the help of antibodies. The specific antibodies are useful in protein-blotting instruments and help them to be seen under a microscope. This will surely help researchers or microbiologists to identify the concentration of isoform present in a unit but also to distinguish different isoforms in accordance to its cellular structures or proteins. These antibodies are introduced enteral or parenteral into the body. This will allow the studies to be carried out via specific interactions with tropomyosins.

Are Tropomyosins allergens?

Crustaceans are known to cause food reactions in individuals who are allergic. This research article on PubMed implies the allergic reactions caused by the intake of a protein called tropomyosin. Immunologic reactions between crustaceans and cockroaches suggest tropomyosin as an important cross-sanitizing pan allergen.

How does tropomyosin influence angiogenin/actin interaction?

Tropomyosin prevents branching and depolymerization of actin filaments. Rather it acts as a stabilizer. In non-muscle cells, Tropomyosin 5 is directing the insertion and retention of transporter protein into the actin filaments. The two isoforms of tropomyosin – muscular and non-muscular at saturating concentration along with angiogenin do not affect binding to F-actin. Angiogenin belongs to the family of RNase and has some unusual biological properties. At lower concentrations of angiogenin, tropomyosin is already found to bound with F-actin but on the other hand, if angiogenesis concentration would increase somehow it will be difficult to displace it back.

How is actin’s diversity in neurons regulated by tropomyosin?

Tropomyosin is a dimer where 2 parallel alpha helices are coiled around each other to form supercoils. These are having end-end connections with actin forming a continuous polymer throughout the length of actin. But there are two non-coiled terminals which are C and N terminus. The C-terminus forms a cleft into which N-terminus binds and overlaps 11  residues of C terminus. The isoforms are found to promote the axon length and even have profound effects on dendritic branching.

The major isoforms of tropomyosin

There are two isoforms in general. High molecular weight and low molecular weight isoforms. HMW made up of 284 amino acids includes isomeric forms namely Tm1, 2, and 3. These are homodimers. Whereas on the other hand, we have LMW heterodimers as well as homodimers namely Tm4, Tm5NM1, Tm5a, and Tm5b are made up of 247 amino acids.

It’s the basic arrangement of 20 basic amino acids that form diverse proteins within human nature.

In conclusion, four genes encode tropomyosins in vertebrates with bit diversification which results from alternate promoters and spliced axons.

The post-translational modifications associated with tropomyosin

These modifications take place at the N-terminus. It can be either acetylation or phosphorylation. Modification at the level of N-terminus enhances tropomyosin dimerization and affinity for actin.

Electromyogram

The primary function of muscle regardless of its kind is to convert chemical energy to mechanical work and in doing so muscle contracts or shortens. The combination of a single

motor neuron and all of the muscle fiber it controls is called a motor unit. Physiologically, the degree of skeletal muscle contraction is controlled by

Activating the desired number of motor units within the muscle

Controlling the frequency of motor neuron impulse

The detection, amplification, and recording of changes in skin voltage produced by underlying skeletal muscle contraction are called electromyography and the recording obtained is electromyogram.

Summary

• Tropomyosin is a regulatory protein that plays its role in muscle contraction as well as intracellular transport of substances.
• It is found in muscle cells as a component of actin filaments as well as in the cytoskeleton of non-muscle cells.
• Its synthesis involves the transcription of tropomyosin genes and the translation of mRNA.
• It is required for muscle contraction in cardiac as well as skeletal muscles.
• It assembles the myosin motor proteins for intracellular transport mechanisms.
• Different pathologies of tropomyosin are seen in muscular and autonomic disorders as well as cancer.
• Different isoforms of tropomyosin molecules are found in organisms, each having its own gene on a separate chromosome.
• Tropomyosin undergoes post-translational modifications at the N-terminus.

Frequently Asked Questions

What is tropomyosin?

Tropomyosin is a coil of two polypeptide chains that forms an integral component of the thin filaments, also called actin filaments, found in the skeletal muscles. 

What is the role of tropomyosin?

Tropomyosin occupies the binding sites on actin filaments. It regulates the access of actin-binding proteins to the binding sites present on the actin filaments. 

What is the role of tropomyosin in muscle contraction?

Tropomyosin blocks the myosin binding sites present on actin filaments in the absence of nerve impulse. When a muscle fibre is excited, calcium ions bind to troponin. This troponin then displaces the tropomyosin from myosin binding sites. Myosin filaments bind the actin filaments and contraction takes place. 

What would happen if there was no tropomyosin?

In the absence of tropomyosin, myosin heads would always have access to the myosin binding sites, and the muscle contraction could not have been controlled.

References

1. Pittenger, MF; Kazzaz, JA; Helfman, DM (1994). “Functional properties of non-muscle tropomyosin”. Curr Opin Cell Biol. 6 (1): 96–104. doi:10.1016/0955-0674(94)90122-8. PMID 8167032.
2. Gunning, PW; Schevzov, G; Kee, AJ; Hardeman, EC (2005). “Tropomyosin isoforms:divining rods for actin cytoskeleton function”. Trends Cell Biol. 15 (6): 333–341. doi:10.1016/j.tcb.2005.04.007. PMID 15953552.
3. Lin, JJ; Hegmann, TE; Lin, JL (1988). “Differential localization of tropomyosin isoforms in cultured nonmuscle cells”. J Cell Biol. 107(2): 563–572. doi:10.1083/jcb.107.2.563. PMC 2115218. PMID 3047141
4. Lees-Miller, JP; Helfman, DM (1991). “The molecular basis for tropomyosin isoform diversity”. BioEssays. 13 (9): 429–437. doi:10.1002/bies.950130902. PMID 1796905.