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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