Actin is the collective molecules of protein from which thin bands are formed. On the other hand, Myosin is the group of protein by which thick bands are formed. Actin and Myosin are responsible for various types of cellular movements, and the most striking one is the muscle contraction, that provides the best-model to understand the role of actin and myosin.
Now, to know the function of actin-myosin, it is necessary to gather bit information about muscles contraction. The cellular and molecular movements in the body are dependent on the muscles cells. Vertebrates have three types of muscles cells – smooth muscles, cardiac muscles and skeletal muscles.
Smooth muscles are known for performing the involuntary motion in the body; cardiac muscles are known for pumping our heart regularly, while skeletal muscles play their role in all kinds of voluntary movements.
Skeletal muscles contain numbers of the muscles fibres, these are a group of numerous cells, that fused to give rise to the single large cells at the time of development. The muscles cells contain numerous nuclei, and its cytoplasm contains myofibrils, that consist of the cylindrical bundles of thick and thin filaments.
The thin filament is made up of a protein known as actin, and the thick one is made fro protein known as myosin, and the these are organized as units of repeating chain known as sarcomeres. The sarcomeres are compelled to give the striated appearance to the cardiac and the striated muscles.
Therefore, it is said that myosin and actin together work at the time of the muscles contractions, where myosin is the precursor protein that plays its critical role in converting the chemical energy (ATP) into mechanical energy. In this article, we will provide vital differences and the points on which actin and myosin vary with their similarities.
Content: Actin Vs Myosin
|Basis for Comparison
|Actin is the proteins, known to form the thin bands in the myofibrils.
|Myosin is the proteins, known to form the thick bands in the myofibrils.
|1. Actin forms a short filament of 2-2.6 um, and it is thin up to 0.005 um.
2. Actin contains troponin and tropomyosin (protein).
|1. Myosin forms a long filament of 4.5 um, which has a thickness of 0.01 um.
2. Myosin contains meromyosin (protein).
|Actin is present in A and I bands.
|Myosin is present in A bands of the sarcomere.
|Actin does not form cross-bridges.
|Myosin form cross-bridges.
|The surface of actin is smooth.
|The surface of the myosin is rough.
|Actin is numerous in numbers.
|Myosin is less in number, and they are one per six actin filaments.
|Actin slide in H zone at the time contraction.
|Myosin does not slide at the time of contraction.
Definition of Actin
As discussed above, the two primary protein filaments found in muscles are actin and myosin. It is one of the essential components of the cellular cytoskeleton, especially in eukaryotes. It is a highly conserved protein having the molecular weight of 42kDa.
Actin is present in monomeric form as G-actin or polymeric form as F actin, where ‘G’ stand for globular actin protein, while ‘F’ is for filamentous actin protein or polymeric fibrous protein. These have different cellular functions like muscles contraction, cytokinesis and cell migration.
As actin filaments play a primary role in the formation of the dynamic cytoskeleton of the cell, hence it provides the movement and shape also to the cell. The cytoskeleton also delivers communications with the neighbouring cells, also supports the interior surrounding within the cell.
Definition of Myosin
Myosin is another type of protein filaments that works in the presence of calcium ions. Myosin is known to generate the force that is required during muscle contraction. So, it is also known as motor protein.
Skeletal muscles are known for the voluntary action in the body, where actin and myosin are present as the repeating units. The thick filament of the myosin is surrounded by the thin filament of actins. Then again, the thin filament of actin is surrounded by the thick filament of the myosin. So, this continuous and repeating result in the formation of the bundle of filaments in muscles.
Myosin has three parts: head, neck and tail, and is composed of numerous light chains and two heavy chains. The globular head region has the binding site for the ATP and actin, and the neck part has alpha-helical region wheres tail has other binding sites. Head region converts ATP to ADP, by the enzyme ATPase.
As soon as the nerve sends signal muscle cell for the muscle contraction, the myosin and actin get activated. After that, the myosin starts working in releasing energy (ATP) and further the myosin along with actin filaments slide past each other.
Tropomyosin and troponin are other two muscles protein that temporary fuse with the actin and myosin for the muscle contraction. This working can be understood by the ‘sliding filament theory’.
Actin and myosin also play a critical role in nonmuscle cells. There are two actions performed consecutively by the muscles, which are contraction and relaxation. The contraction results in shortening of muscles and results in movement, while relaxation returns the muscle to its original length.
Key Differences Between Actin and Myosin
Following are the few though essential differences between actin and myosin:
- Actin and myosin are the proteins filaments found in muscle cells, and actin is known to form the thin bands in the myofibrils, while myosin is known to form the thick bands in the myofibrils.
- Actin forms a short filament of 2-2.6 um, and it is thin up to 0.005 µm, but myosin forms a long filament of 4.5 µm, which has a thickness of 0.01 um, that means actin are thinner than myosin.
- Actin contains troponin and tropomyosin (proteins), and myosin contains meromyosin (protein), it(myosin) has an ATP binding site, through which it releases energy for the muscle contraction.
- Actin is present in A and I bands, whereas myosin is present in A bands of the sarcomere.
- The surface of actin is smooth, and they are more in numbers as compared to the myosin, the ratio is one per six actin molecules. Myosin has a rough surface.
- Actin slide in H zone at the time contraction, whereas myosin does not slide at the time of contraction.
- Actin and Myosin are the protein filaments present in muscles.
- Both types of proteins are required during muscle contraction.
- Calcium ions are needed for the muscles contraction.
So we can say that apart from the muscle contraction, actin and myosin play a vital role in cell biology by participating in cell division, in functions of nonmuscle cells, etc. Myosin is thicker than actin and has darker striations. The working of muscle contraction can be understood by sliding filament theory.