Difference Between Actin and Myosin

Difference Between Actin and Myosin

Actin are collective protein molecules in which thin bands are formed. On the other hand, myosin is a group of proteins that form thick bands. Actin and myosin are responsible for various types of cellular movement, and most notably muscle contraction, which provides the best model for understanding the roles of actin and myosin.

Now, in order to know the function of actin-myosin, it is necessary to gather a little information about muscle contraction. Cellular and molecular movements in the body depend on muscle cells. Vertebrates have three types of muscle cells – smooth muscle, cardiac muscle, and skeletal muscle.

Smooth muscles are known for carrying out involuntary movements in the body; Cardiac muscle is known to pump our heart regularly, while skeletal muscle plays its role in all kinds of voluntary movements.

Skeletal muscle contains a large number of muscle fibers, these are numerous clusters of cells, which fuse to give rise to a single large cell during development. Muscle cells contain many nuclei and their cytoplasm contains myofibrils which consist of bundles of thick and thin filaments.

The thin filaments are made up of a protein known as actin, and the thick one is made up of a protein known as myosin, and these are arranged as repeating chain units known as sarcomas. Sarcomas are forced to give a striated appearance to the heart and striated muscles.

Therefore, it is said that Myosin and actin work together during muscle contraction, where Myosin is a precursor protein that plays an important role in converting chemical energy (ATP) into mechanical energy. In this article, we will provide the vital differences and the points where actin and myosin differ with their similarities.

Definition of Actin

As discussed above, the two main protein filaments found in muscles are actin and myosin. It is one of the important components of the cellular cytoskeleton, especially in eukaryotes. It is a highly conserved protein having a molecular weight of 42kDa.

Actin is present in monomeric form as G-actin or polymeric form as F actin, where ‘G’ stands for globular actin protein, while ‘F’ is for filamentous actin protein or polymeric fibrous protein. It has different cellular functions like muscle contraction, cytokinesis and cell migration.

Since actin filaments play a major role in the formation of the dynamic cytoskeleton of the cell, it gives movement and shape to the cell as well. The cytoskeleton also provides communication with neighboring cells, also supporting the interior within the cell.

Difference Between Actin and Myosin-1

Definition of Myosin

Myosin is another type of protein filament that acts in the presence of calcium ions. Myosin is known to produce the required force during muscle contraction. So, it is also known as motor protein.

Skeletal muscles are known for their voluntary actions in the body, where actin and myosin are present as repeating units. Myosin thick filaments are surrounded by actin thin filaments. Then again, the thin filaments of actin are surrounded by thick filaments of myosin. So, this continuous and repeated results in the formation of filamentous bundles in the muscles.

Myosin has three parts: head, neck and tail, and consists of many light chains and two heavy chains. The globular head region has binding sites for ATP and actin, and the neck region has an alpha-helical region where the tail has other binding sites. The head region converts ATP to ADP, by the enzyme ATPase.

As soon as the nerve sends the muscle cell a signal for muscle contraction, myosin and actin are activated. After that, Myosin begins to work in releasing energy (ATP) and then Myosin along with the actin filaments slide past each other.

Tropomyosin and troponin are other two muscle proteins that temporarily fuse with actin and myosin for muscle contraction. This work can be understood by the ‘filament shear theory’.

Actin and myosin also play important roles in nonmuscle cells. There are two actions performed sequentially by muscles, namely contraction and relaxation. Contraction results in shortening of the muscle and produces movement, while relaxation returns the muscle to its original length.

Difference Between Actin and Myosin

  • Actin and myosin are protein filaments found in muscle cells, and actin is known to form thin bands on myofibrils, whereas myosin is known to form thick bands on myofibrils.
  • Actin forms short filaments 2-2.6 m, and is thin to 0.005 m, but myosin forms 4.5 m long filaments, which are 0.01 m thick, which means actin is thinner than myosin.
  • Actin contains troponin and tropomyosin (protein), and Myosin contains meromyosin (protein), it (Myosin) has ATP binding sites, where it releases energy for muscle contraction.
  • Actin is present in the A and I bands, whereas Myosin is present in the A bands of the sarcomere.
  • The surface of actin is smooth, and there are more in it compared to myosin, the ratio is one-sixth actin molecule. Myosin has a rough surface.
  • Actin slides in the H zone during contraction, whereas myosin does not slide during contraction.

Similarity

  • Actin and Myosin are protein filaments present in muscles.
  • Both types of protein are required during muscle contraction.
  • Calcium ions are required for muscle contraction.

Red Thread | So we can say that apart from muscle contraction, actin and myosin play important roles in cell biology by participating in cell division, in nonmuscle cell function, etc. Myosin is thicker than actin and has darker striations. How muscle contraction works can be understood by shifting the filament theory.