The human body consists of muscles that can be classified into 3 types; smooth, skeletal, and cardiac muscles. Among these, skeletal muscles are important for voluntary actions and movements. These muscles are composed of muscle fibers. Muscle fibers are actually made up of numerous muscle cells. The muscle fibers contain muscle cells and cytoplasm. The muscle cell further contains a large number of nuclei. The cytoplasm further contains collections of thin and thick filaments. In this manner, motor proteins such as myosin and kinesin come into existence.
Myosin vs Kinesin
The main difference between myosin and kinesin is that while myosin proteins can be found in both eukaryotic and prokaryotic cells, kinesin can only be found in eukaryotic cells. While a myosin protein weighs 223 kDa, a kinesin motor protein weighs 120 kDa. Myosin has the main function in muscle contraction. But, kinesin has the main function in making spindle apparatus.
Myosin, in simple terms, refers to a specific kind of motor muscle responsible for contractions and muscle movements. It works in association with actin and is responsible for the conversion of ATP (adenosine triphosphate) into mechanical energy. The structure of myosin motor proteins comprises a head followed by a tail. Thus, myosin can be understood as a superfamily composed of ATP-dependent members responsible for motility in all creatures.
Eukaryotic organisms have a protein in their cell, which performs the most crucial functions of the cell. It is known as kinesin. This motor protein helps to perform morphogenesis, microtubule dynamics, chromosome segregation, and transport of organelles. These cells are dependent on ATP production for generation strength to transport the essential molecules. This superfamily of cells generates the force for easy circulation through microtubules.
Comparison Table Between Myosin and Kinesin
| Parameters of Comparison | Myosin | Kinesin |
| Structure | A myosin protein has a length of about 4-5m. | The length of a kinesin motor protein is around 10 nm. |
| Count of Heavy and Light Chains | A myosin protein contains 1 pair of heavy chains and 2 pairs of light chains. | A kinesin chain contains 2 heavy chains and 2 light chains. |
| Found in | In a sarcomere, myosin proteins are present only in the A band. | The location of kinesin is yet unknown. |
| Molecular weight | The weight of a myosin protein is around 223 kDa. | The weight of a kinesin protein is around 120 kDa. |
| Motility Cycle | Myosin proteins move from one place to another with the help of actin filaments through cytokinesis. | Kinesin proteins move from one place to another through their microtubules and follow a progressive movement. |
What is Myosin?
Myosin refers to a huge multigene database consisting of molecular motor proteins which are actin-based, essential for eukaryotic homeostasis. These motor proteins play a functional role in the contraction of muscles in humans. In eukaryotes, these proteins are involved in a variety of motility processes. The term was used to refer to a family of similar ATPases present in smooth muscle tissues and striated muscle tissues. Thus, these motor proteins are ATP-dependent.
Enzymes that resemble in functioning to myosin were discovered in 1973 by Pollard and Korn. This provoked the discovery of myosin genes. The functions of myosin include tissue formation, metabolism, growth, reproduction, reshaping, movement, and communication in humans and animals. In the case of plants, myosin is responsible for the development and cytoplasmic streaming. Myosins are categorized into 35 phylogenetic classes. Humans contain about 39 myosin genes which belong to 13 classes.
The myosin superfamily comprises Myosin I, Myosin II, Myosin VI, and Myosin V, Myosin VII followed by Myosin X. Myosin I comes with a special tail domain that helps myosin in binding with multiple actin filaments and membrane lipids. Myosin II comes with coil domains that are long coiled to help it form assemblies of higher order. Myosin VI and Myson V are responsible for transporting cargo, with actin filaments being the bridge and these motor proteins being the fuel. Myosin VII plays a significant role in assembling and disassembling adhesion proteins and in events related to the filopodial extension.
What is Kinesin?
Kinesins refer to a superfamily consisting of motor proteins responsible for destabilizing microtubule filaments. Discovered in 1985, these motor proteins play significant roles in mitosis. The kinesin superfamily comprises 14 subclasses comprising 45 kinesin proteins. The initial beginning member of the family is kinesin-1. All these family members differ from one another in shape. These are microtubule bound and ATP-dependent motor proteins that have significant organelle transportation, signaling regulation, and cellular organization.
This motor protein has two dimers and a microtubule end responsible for transportation. The two common mechanisms involved in kinesin movement are hand-over-hand and inchworm. The kinesin heads surpass each other in the hand-over-hand movement mechanism, thereby taking the lead position alternatively. In the inchworm movement mechanism, a single kinesin head always takes the lead, thereby keeping the trailing head back by a step.
The existence of kinesin takes place as a heterotetramer and comprises heavy and light chains, which are two in number each. The heavy chains comprise the head of the kinesin, which is also known as the catalytic motor of the motor protein. The head reduces and eliminates the chances of complications when the molecule glides along microtubules.
Main Differences Between Myosin and Kinesin
- Myosin is responsible for muscular contractions, cell divisions, and cell motility. On the other hand, kinesin is responsible for making the spindle apparatus.
- Myosin forms cross bridges actively. On the contrary, kinesin forms cross bridges selectively.
- Myosin proteins are found in eukaryotic as well as prokaryotic cells. On the other hand, kinesin proteins are found specifically in eukaryotic cells.
- The length of a myosin protein is about 4-5m. Whereas the length of a kinesin protein is about 10nm.
- The movement of myosin takes place through actin filaments. On the contrary, the movement of kinesin takes place through microtubules.
Conclusion
Motor proteins refer to the class of molecular motors which can travel along the cytoskeletal filaments, thereby helping in the transportation of vesicles and proteins. Motor proteins have a force-dependent mechanism that is dependent on ATP hydrolysis as their energy source. For making mechanical movements, motor proteins derive emery by the combination of energy formed due to hydrolysis and NTP binding. Myosin is the first discovered protein. Both these motor proteins, i.e., myosin and kinesin, form the driving force resulting in the contraction of muscles.
References
- https://pubs.acs.org/doi/abs/10.1021/nl8010885
- https://www.nature.com/articles/35078000