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Motor protein

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Title: Motor protein  
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Subject: P-bodies, Molecular machines, Monastrol, Centromere protein E, Cell biology
Collection: Cell Movement, Molecular MacHines, Motor Proteins
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Motor protein

Motor proteins are a class of molecular motors that are able to move along the surface of a suitable substrate. They convert chemical energy into mechanical work by the hydrolysis of ATP. Flagellar rotation, however, is powered by a proton pump.

Contents

  • Cellular functions 1
  • Diseases associated with motor protein defects 2
  • Cytoskeletal motor proteins 3
    • Actin motors 3.1
      • Myosin 3.1.1
    • Microtubule motors 3.2
      • Kinesin 3.2.1
      • Dynein 3.2.2
    • Plant-specific motors 3.3
  • Other molecular motors 4
  • See also 5
  • References 6
  • External links 7

Cellular functions

The best prominent example of a motor protein is the muscle protein myosin which "motors" the contraction of muscle fibers in animals. Motor proteins are the driving force behind most active transport of proteins and vesicles in the cytoplasm. Kinesins and cytoplasmic dyneins play essential roles in intracellular transport such as axonal transport and in the formation of the spindle apparatus and the separation of the chromosomes during mitosis and meiosis. Axonemal dynein, found in cilia and flagella, is crucial to cell motility, for example in spermatozoa, and fluid transport, for example in trachea.

Diseases associated with motor protein defects

The importance of motor proteins in cells becomes evident when they fail to fulfill their function. For example, kinesin deficiencies have been identified as cause for Charcot-Marie-Tooth disease and some kidney diseases. Dynein deficiencies can lead to chronic infections of the respiratory tract as cilia fail to function without dynein. Defects in muscular myosin predictably cause myopathies, whereas defects in unconventional myosin are the cause for Usher syndrome and deafness.[1]

Cytoskeletal motor proteins

Motor proteins utilizing the cytoskeleton for movement fall into two categories based on their substrates: Actin motors such as myosin move along microfilaments through interaction with actin. Microtubule motors such as dynein and kinesin move along microtubules through interaction with tubulin. There are two basic types of microtubule motors: plus-end motors and minus-end motors, depending on the direction in which they "walk" along the microtubule cables within the cell.

Actin motors

Myosin

cytoplasm to stream in a particular direction. Eighteen different classes of myosins are known.[2]

Genomic representation of myosin motors:[3]

Microtubule motors

Kinesin

Kinesins are a group of related motor proteins that use a microtubule track in antegrade movement. They are vital to the movement of chromosomes during mitosis and are also responsible for shuttling mitochondria, Golgi bodies, and vesicles within eukaryotic cells. Kinesins typically contain two heavy chains with motor heads which move along microtubules via a pseudo-processive asymmetric walking motion, that can be towards the plus-end or the minus-end, depending on the type of kinesin. Fourteen distinct kinesin families are known, with some additional kinesin-like proteins that cannot be classified into these families.[4]

Genomic representation of kinesin motors:[3]

Dynein

Dyneins are microtubule motors capable of a retrograde sliding movement. Dynein complexes are much larger and more complex than kinesin and myosin motors. Axonemal dynein facilitates the movement of cilia and flagella, and cytoplasmic dynein facilitates transport of intracellular cargos. Compared to 15 types of axonemal dynein, only two cytoplasmic forms are known.[5]

Genomic representation of dynein motors:[3]

Plant-specific motors

In contrast to animals, fungi and non-vascular plants, the cells of flowering plants lack dynein motors. However, they contain a larger number of different kinesins. Many of these plant-specific kinesin groups are specialized for functions during plant cell mitosis.[6] Plant cells differ from animal cells in that they have a cell wall. During mitosis, the new cell wall is built by the formation of a cell plate starting in the center of the cell. This process is facilitated by a phragmoplast, a microtubule array unique to plant cell mitosis. The building of cell plate and ultimately the new cell wall requires kinesin-like motor proteins.[7]

Another motor protein essential for plant cell division is kinesin-like calmodulin-binding protein (KCBP), which is unique to plants and part kinesin and part myosin.[8]

Other molecular motors

Besides the motor proteins above, there are many more types of proteins capable of generating forces and torque in the cell. Many of these molecular motors are ubiquitous in both prokaryotic and eukaryotic cells, although some, such as those involved with cytoskeletal elements or chromatin, are unique to eukaryotes. The motor protein prestin,[9] expressed in mammalian cochlear outer hair cells, produces mechanical amplification in the cochlea. It is a direct voltage-to-force converter, which operates at the microsecond rate and possesses piezoelectric properties.

See also

References

  1. ^
  2. ^
  3. ^ a b c
  4. ^
  5. ^
  6. ^
  7. ^
  8. ^
  9. ^

External links

  • MBInfo - What are Motor Proteins?
  • Ron Vale's seminar: "Cytoskeletal Motor Proteins"
  • Biology of Motor Proteins Institute for Biophysical Chemistry, Göttingen
  • Jonathan Howard (2001), Mechanics of motor proteins and the cytoskeleton. ISBN 9780878933334
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