2020年3月21日 星期六

Motor Proteins: Cytoskeleton Filament Motor Proteins


Motor proteins are proteins that transform chemical energy into mechanical work. They are divided into three categories: cytoskeleton filaments motor proteins, nucleic acid motor proteins, and rotary motor proteins. In this article, we will talk in-depth about cytoskeleton filament motor proteins.

Cytoskeleton filament motor protein


Cytoskeleton filament motor proteins are motor proteins that associate and move along cytoskeleton filaments. This includes myosins, kinesins, and dyneins.

Myosin


Myosins are motor proteins that move along microfilaments; thus, they are also called actin motor proteins (because they interact with the actin of microfilaments). Myosins hydrolyze ATP as the source of energy and use it to propel their movements toward the plus end of an actin filament.

There are as many as 18 types of myosins that are known. Myosin II, for instance, is responsible for generating muscle contraction and dividing a cell during cytokinesis. Myosin V is involved in vesicle and organelle transport. Myosin XI is involved in cytoplasmic streaming, wherein movement along  microfilament networks in the cell allows organelles and cytoplasm to stream in a particular direction.

The movement of myosin is characterized by a release of actin during every cycle. What does this mean? Take muscle-contracting myosin II as an example. Myosin attaches to an actin component, moves once, and then dissociates from the actin. It then attaches again onto actin. This is the cycle of myosin movement.

Kinesin


Kinesins associate and move along microtubules, involving in the anterograde movement [1], which directs the transport of cargoes toward the plus end of microtubules. However, kinesins can also travel toward the minus-end, depending on whether the kinesin has an N-terminal or a C-terminal cargo-binding region. Anyway, just keep in mind the relationship between kinesin and anterograde transport.

Kinesins are primarily involved in the separation of chromosomes during cell division and also the shuttling of mitochondria, Golgi bodies, and vesicles within eukaryotic cells.

Unlike the movements of myosins, kinesins are rather highly processive. This means that kinesins move a great deal of “steps” before dissociating their carriages. Recall the shape of a kinesin [click]. The two heavy chains that attach to the microtubule function like legs, “walking” on the microtubule for a cycle of as much as hundreds of steps, while the two light chains attach to vesicles or organelles like hands.

Dynein


Dyneins move along microtubules through the retrograde movement. Dyneins are larger and more complex than kinesin and myosin motors, composing of two or three heavy chains and a large and variable number of associated light chains. Dyneins move toward the minus end of microtubules, where the nucleus locates.

There are mainly two types of dyneins. Axonemal dyneins facilitate the beating of cilia and flagella by sliding microtubules. Cytoplasmic dyneins facilitate the transport of intracellular cargos. 15 types of axonemal dynein are presently discovered, but only two cytoplasmic forms are known.

Summary


Myosin – microfilament - muscle contraction - cytokinesis (microfilament)
Kinesin – microtubule - anterograde - separation of chromosome - transport
Dynein – microtubule – retrograde - beating of flagella and cilia - transport








Reference:

Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology, 4th edition. W. H. Freeman, 2000.
Berg JM, Tymoczko JL, Stryer L. Biochemistry, 5th edition. W. H. Freeman, 2002.
Anatoly B. Kolomeisky. Motor Proteins and Molecular Motors: How to Operate Machines at Nanoscale. NCBI, 2013.

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