Kinesin represents a complex of certain motor proteins in eukaryotic cells. Along with other motor proteins such as dynein or myosin and other structural proteins, it is involved in the assembly of the cytoskeleton. It serves to transport macromolecules, vesicles, and cell organelles from the cytoplasm or nucleus toward cell membranes.
What is kinesin?
Kinesins are a group of motor proteins with similar properties and functions. They consist of two heavy and two light protein chains. On the heavy protein chains are the head region, the neck, and the tail portion of the molecule. The light protein chains attach to the tail portion. Kinesin serves to transport cell organelles, vesicles and biomolecules along microtubules. Microtubules represent a system of rails made of the protein tubulin, which always grows from the nucleus towards the cell membrane. The growing microtubule end is called the plus end. Thus, kinesin transports biochemicals and cell organelles only in the direction of the plus end (anterograde transport). Transport in the direction of the minus end (retrograde transport) is prompted by a complex of other motor proteins, dynein. Kinesin exists as a dimer. Within the quaternary structure of the protein, the two heavy and two light chains form a protein complex, which has no covalent bond between the individual protein chains. Thus, kinesin has two motor domains (head domains) that are responsible for movement along microtubules.
Function, action, and roles
The main function of kinesin is to transport cell components and molecules from inside the cell toward cell membranes. This includes removing degraded cellular components from the cell, bringing enzymes for secretion, secreting hormones, bringing membrane proteins from the site of synthesis to the membrane, and more. Signaling substances for communication between cells are also transported to the extracellular area. In neurons, for example, neurotransmitters are transported within vesicles from the cell nucleus to the axons and synapses. From there, the neurotransmitters are used to transmit signals to other nerve cells. The vesicles, cell organelles or biomolecules bind to the kinesins via connecting proteins. With the help of the two motor domains (heads), the kinesin complex runs along the microtubule. In the process, the binding of one head is repeatedly released by energy transfer through cleavage of ATP to ADP, while the binding of the other kinesin head is initially retained. However, the detached head region immediately binds again to another binding site of the microtubule in the direction of the plus end, while at the same time the other head domain detaches under cleavage of ATP. The cleavage of ATP to ADP at the binding site of kinesin on the microtubules results in the conformational change of the entire kinesin complex, which triggers its locomotion. This process is repeated until the kinesin complex reaches the cell membrane. At the destination, the cell organelles or molecules to be transported are cleaved from the kinesin complex.
Formation, occurrence, properties, and optimal values
Kinesin is found in all eukaryotic cells. Among them, there are a variety of kinesin proteins. However, this protein complex has changed little in the phylogeny of eukaryotic organisms at the functionally important head region. Its function is exactly the same in unicellular eukaryotes such as the amoebae as it is in multicellular organisms of the animal and plant kingdoms. Kinesin transports cell organelles and molecules toward the cell membrane. The interaction of kinesin and microtubules also represents a universal phenomenon. Minor genetic changes occur in the tail portion of the protein complex. This region responds to the changing components that must be transported and previously naturally bound to kinesin. Kinesins are not related to dyneins, which organize the transport of molecules and molecular complexes from the cell membrane toward the nucleus. However, they are related to myosin, which, with the help of actin, is responsible for muscle movement and, within the cell, for small transport routes of cell organelles due to similar movement patterns.
Diseases and disorders
In association with mutations at the kinesin complex, disorders of intracellular transport may occur.Within these disorders, there is a complex of neurological disorders known as hereditary spastic paraplegias (HSP). There are over 50 different types of this disorder, all of which are genetic. More specifically, spastic spinal paralysis SPG 10 has been studied. In this disease, a mutation results in the defective production of a kinesin complex called KIF5A. Some active substances and cell organelles are transported incorrectly and no longer reach the site of action. These are particularly active substances that are needed in the axons of neurons. The corresponding neurons degenerate and can no longer transmit movement impulses correctly. This disorder has an impact on leg motor function. This results in increasing spastic paralysis of the legs. In the advanced stage of the disease, the affected patient is dependent on a wheelchair. However, spastic paraplegias are a group of several disorders with similar symptoms. They are based on different mutations. Thus, 48 different gene loci of HSP are known. In addition to the restriction of leg motor function, other neurological symptoms may also occur, depending on the disease. It is suspected that other neurodegenerative diseases are also caused by transport disorders within the cell. However, further research is needed to investigate the exact relationships. So far, there is growing evidence that nerve cells in particular are affected when kinesin function is disrupted. To what extent other body cells are affected is not yet sufficiently known.
