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Myosin

Introduction

The contraction of skeletal muscles is responsible for all kinds of movements exhibited by animals. This contraction is possible due to the contractile proteins present inside the skeletal muscle cells. The contractile proteins, along with the regulatory proteins are organized in the form of myofibrils. Myofibrils are further divided into sarcomeres that serve as contractile units. The major contractile proteins found in the skeletal muscles include actin and myosin.

Myosin forms the thick filaments of myofibrils. It plays a central role in the sliding filament model of skeletal muscle contraction. It is also responsible for the contraction of smooth and cardiac muscles as well. Myosin is also found in non-muscle cells where it serves to facilitate cell adhesion and migration.

In this article, we will discuss the structure of myosin, its synthesis, its classes, its role in the contraction of different types of muscles, as well as other aspects related to this protein. So, keep reading.

Illustration of myosin II structure

Structure

Myosin is a filamentous protein that belongs to the category of motor proteins. One myosin molecule is made up of six subunits. These include two heavy chains and four light polypeptide chains. The organization of these chains in the molecule is as follows;

  • The two heavy chains are coiled around each other to form a double helix. This helix serves as the tail of the myosin molecule. It forms the major bulk of the molecular structure of myosin.
  • The two heavy chains diverge at one end to form an angle of 120 degrees. The end of each chain undergoes folding to form a globular structure that makes the bulk of the myosin head or cross-bridge.
  • The two light chains join the globular structure to myosin heads.

In this way, a myosin molecule having two heads and one tail is formed.

Domains

In order to better understand the molecular structure of myosin, it is considered to be made up of three domains. These domains are as follows;

  • Head domain: It is a globular domain made up of one end of a heavy chain and two light chains. Two such domains are present in one molecule of myosin. This domain binds to the actin filaments and causes the power stroke during muscle contraction. It also has ATPase activity to hydrolyze ATP at the beginning of the muscle contraction.
  • Neck domain: This domain forms a link between the heads and tail of myosin molecule. It is the place where the heavy chains separate and bend to form myosin heads. This domain acts as a lever to transduce force generated by the myosin heads to the entire molecule. It also acts as a binding site for myosin light chains that are a part of myosin heads.
  • Tail domain: It is formed by the double helix of the heavy chains. Its function is to connect myosin molecules within a filament. In non-muscle cells, the tail domain is responsible for interaction with cargo molecules for intracellular transport. It also plays a role in regulating the motor activity of the cell.

Synthesis of Myosin

Myosin is a protein that can be synthesized by almost all the cells in the process of gene expression. The genes for myosin molecules are present on different chromosomes, depending on the class of molecules. Some of them are highly expressed while others are not expressed at all. The process of making myosin from its gene involves the following steps.

Read more about Skeletal Muscles

Transcription

During this process, the information in the myosin gene is copied in the form of mRNA. The nucleotide sequence of this mRNA is complementary to the nucleotide sequence in the myosin gene. Only one myosin gene is transcribed at a time and thus, only one myosin molecule is made from it.

The process of transcription takes place in the nuclei of the muscle and non-muscle cell. Recall that for protein synthesis, this mRNA has to be moved to the cytoplasm where abundant ribosomes are present. This is done by adding some post-transcriptional changes to the mRNA.

Post-transcriptional Changes

These changes are necessary for the successful movement of mRNA to the ribosomes. A cap made of GTP is added to the 5’ end of the myosin mRNA molecule. In addition, a polyadenyl tail is also added to the 3’ end of the same mRNA.

These changes serve to protect the mRNA molecule from enzymatic degradation so that it can successfully reach the ribosomes for the synthesis of myosin.

Translation

Once the mRNA carrying information for a particular myosin molecule has reached the ribosomes, protein-making machinery is assembled to begin the synthesis of myosin.

The initiation site is first recognized by the smaller ribosomal subunit that attaches to the mRNA molecule. In the next step, a tRNA molecule carrying the first amino acid to e assembled in myosin comes and attaches to the A site. The larger ribosomal subunit also binds and the translation begins.

With every new amino acid added to the myosin molecule, the ribosome crawls one nucleotide ahead on the mRNA in 5’ to 3’ direction. The process of elongating the myosin chain continues until the stop codon is reached.

When a stop codon is reached, it signals that the synthesis of myosin has been completed. The myosin molecule is released from the ribosome and is sent to the endoplasmic reticulum for post-translational modifications.

Multiple ribosomes may be attached to a single mRNA molecule to make multiple copies of myosin when necessary.

Post-translational Changes

The post-translational modifications in the myosin light chains are of major importance. They play a role in determining the final shape of the molecule. The major modifications that are normally seen are as follows;

  • Phosphorylation: It involves the addition of phosphate group to threonine, tyrosine, or serine residues in myosin light chains. This process is catalyzed by a family of enzymes called myosin light chain kinases. Phosphorylation can turn on or off the function of the protein. It is important in the contraction of smooth muscles and cardiac muscle where contraction is regulated by the activity of myosin light chain kinases.
  • Nitration: It involves the addition of the nitrate group to the tyrosine residues. It is seen in many pathologic conditions such as hypoxia, inflammation, etc.
  • Nitrosylation: It takes place on the cysteine residues. Like nitration, nitrosylation also occurs in different pathologic conditions. Both these modifications result in contractile dysfunction of myosin.

Classes

Myosin is a family of filamentous motor protein that is divided into different classes or groups. These classes differ concerning their structure, location of genes, and functions. Some of the major classes of myosin are discussed below.

  • Myosin I: It is a monomeric protein that is involved in intracellular transport of vesicles.
  • Myosin II: It is the normal muscle myosin responsible for muscle contraction. It is made up of six subunits. It is found in the cells of skeletal muscles, smooth muscles, and cardiac myocytes.
  • Myosin III: It is a poorly studied protein. It was recognized in the eyes of Drosophila where it is responsible for the transduction dependent on light.
  • Myosin V: It is a motor protein that walks along the actin filaments. It is a dimeric protein that is also involved in intracellular transport.
  • Myosin VI: It is a dimeric protein that is responsible for the transport of endocytic vesicles in animal cells.
  • Myosin VII: It is an unconventional protein that is responsible for phagocytosis and spermatogenesis in some animals. It is also found in the stereocilia of some mice species.
  • Myosin VIII:It is found only in plant cells where it is involved in the process fo cell division. It is also responsible for regulating the flow of cytoplasm in plant cell.s
  • Myosin XI: It is another dimeric unconventional myosin that is responsible for the movement of organelles within the cell such as mitochondria and plastids, etc.

Role in Muscle Contraction

The most important function of myosin is the role played by it in muscle contraction. In this section., we will discuss how myosin plays a role in the contraction of different types of muscles.

Myosin in Skeletal Muscles

In the skeletal muscle cells, myosin molecules are arranged in the form of myosin filaments or thick filaments. They are present in the center of sarcomeres with the actin filaments interdigitating at both the ends in the H-zone.

Myosin heads emerge from the thick filaments via a hinge. These heads can bind to the myosin binding sites on the actin or thin filaments.

The binding sites on the actin filaments are covered by the tropomyosin molecule in the resting state. When the muscle is excited and calcium ions are released by the sarcoplasmic reticulum, the binding sites are exposed by the action of troponin.

The myosin heads are in a charged state when bound to a molecule of ATP. When the binding sites are exposed, the charged heads bind to them and hydrolyze ATP. Hydrolysis of ATP provides the energy for the power stroke during which myosin heads bend towards the center of the sarcomere, pulling the actin filaments.

Once a power stroke is over, another ATP molecule attaches to the myosin heads and they are displaced from the binding sites. They are now ready to undergo another cycle of contraction.

The actin filaments slide over the myosin filaments. This is why it is called the sliding filament model of muscle contraction.

Myosin in Smooth Muscles

Myosin molecules have a different arrangement in smooth muscle cells. In these cells, the myosin filaments are interspersed between the actin filaments. These actin filaments are attached to the dense bodies.

These muscles lack troponin and tropomyosin to regulate the contraction. Instead, muscle contraction is regulated by the amount of ATP in the cells. calcium ions are also needed for the contraction.

Myosin heads can only bind to actin filaments when their light chains are phosphorylated. This phosphorylation is carried out by myosin light chain kinase. In the resting state, this enzyme is non-functional. The influx of calcium ions after the excitation of smooth muscle cells activates the kinase.

Once the kinase is activated, myosin light chains are phosphorylated. The myosin heads bind to actin filaments and cause contraction of the smooth muscle.

Myosin in Cardiac Muscles

In the cardiac myocytes, myosin filaments are arranged in the form of sarcomeres just like the skeletal muscles. They undergo contraction by the same sliding-filament model.

Summary

Myosin is a contractile protein found in the muscles of animals as well as non-muscle cells. It is responsible for muscle contraction as well as intracellular transport.

Myosin is made of six subunits including two heavy chains and four light chains. The molecule is considered to be made up of three domains.

  • Tail domain is made of heavy chains
  • Neck domain is where the heavy chains separate to form a hinge
  • Head domain is made of two light chains and end of a heavy chain

Myosin is made by the central dogma of gene expression. The genes for myosin molecules are located on different chromosomes depending on the class of molecules. The steps in its synthesis are;

  • Transcription, where the nucleotide sequence in a myosin gene is copied to form mRNA
  • mRNA undergoes modification to safely reach ribosomes in the cytoplasm
  • Translation, during which amino acids are arranged in a sequence according to mRNA to make myosin molecule
  • Different post-translational modification for various purposes such as phosphorylation, nitration, and nitrosylation

Myosin proteins are divided into different classes based on their structure, location, and functions.

In skeletal muscle, myosin filaments are present in the center of the sarcomeres. They interact with actin filaments once the binding sites are exposed and cause contraction according to the sliding filament model.

In smooth muscles, myosin filaments are present in between the actin filaments that are attached to the dense bodies. The contraction occurs when the myosin light chains are phosphorylated so that they can bind to the actin filaments and pull them.

In cardiac muscles, myosin filaments are arranged in the same pattern as in the skeletal muscles and the contraction also occurs by the same process.

Frequently Asked Questions

What is myosin?

Myosin is a contractile protein found in eukaryotes. It is present in the cytoskeletal components as well as muscle fibres.

What is the function of myosin?

Myosin moves along the cytoskeleton of the cell and helps in the intracellular transport of various molecules. It is also present in muscles and helps in muscle contraction. 

What is the structure of myosin?

Myosin is made of two heavy chains and four light chains. These peptide chains twist around each other to form myosin molecules. 

What is the role of myosin in skeletal muscles?

Myosin is present in the thick filaments of skeletal muscles. It helps in the contraction of skeletal muscle by interacting with the thin filaments. The myosin head binds to the actin filament and pulls it towards the centre of the sarcomere during muscle contraction.

References

  1. McMahon, T. A. 1984. Muscles, Reflexes and Locomotion. 1st Edition. Princeton University Press. ISBN 978-0-691-02376-2
  2. Tyska MJ, Warshaw DM (January 2002). “The myosin power stroke”. Cell Motility and the Cytoskeleton. 51 (1): 1–15. doi:10.1002/cm.10014PMID 11810692.
  3. von der Ecken J, Heissler SM, Pathan-Chhatbar S, Manstein DJ, Raunser S (June 2016). “Cryo-EM structure of a human cytoplasmic actomyosin complex at near-atomic resolution”. Nature. 534 (7609): 724–8. doi:10.1038/nature18295PMID 27324845.
  1. Sellers JR (March 2000). “Myosins: a diverse superfamily”. Biochimica et Biophysica Acta (BBA) – Molecular Cell Research. 1496 (1): 3–22. doi:10.1016/S0167-4889(00)00005-7PMID 10722873.
  2. Mehta AD, Rock RS, Rief M, Spudich JA, Mooseker MS, Cheney RE (August 1999). “Myosin-V is a processive actin-based motor”. Nature. 400 (6744): 590–3. doi:10.1038/23072PMID 10448864.