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All eukaryotic cells including those of our body contain a large variety
of molecular machines that fulfill many different functions: Stepping motors, which run
along cytoskeletal filaments with about 100 steps per second; Rotary motors, which revolve
with about 100 Hz;
Membrane pumps, which transport ions against concentration gradients; Molecular
assemblers such as polymerases and ribosomes; see
motor cartoons.
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Even though cytoskeletal motors are rather small and constantly collide with many
water molecules, they perform
directed transport over many length scales
[1]
[2]
[3].
This multiscale motility involves the following interesting aspects:
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Energy conversion by single motors:
Stepping motors convert the chemical energy released from ATP hydrolysis into
mechanical work.
This process exhibits some universal features
[4],
and involves several motor cycles, which can be characterized by chemomechanical
balance conditions [5]
[2].
A systematic analysis of single molecule experiments shows that both conventional kinesin
[6]
[2]
and myosin V
[7]
switch between different motor cycles depending on nucleotide concentrations and
load force.
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Cargo transport by teams of motors:
A single team of identical motors that pulls on an intracellular cargo, leads to a
strongly increased run length
[8].
A similiar increase can be achieved by crosslinking the cargo
to the filament via additional motor molecules that diffuse along the
filament.
[9]
If two antagonistic species of motors pull on the cargo in opposite directions, they
perform a
stochastic tug-of-war ,
which induces several distinct patterns of bidirectional transport.
[10]
[11]
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Traffic of motors, cargo particles, and filaments:
In the living cell, filaments are typically crowded with molecular motors and cargo particles.
Crowding effects lead to jams in motor traffic
[12]
[13];
other types of motor patterns [13]
[14];
and
nonequilibrium phase transitions
[13]
[15]
[16]
[17].
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Cytoskeletal motors have a relatively simple dimeric architecture built up from
a few polypeptide chains. The cell also contains much more complex molecular machines such as
ribosomes,
which consist of several strands of ribosomal RNA as well as a large number of different proteins.
Ribosomes move along mRNA molecules and translate the
codon sequences of these mRNAs into proteins.
In general, several ribosomes are simultaneously loaded onto the same mRNA and then form a
polysome.
The size of these polysomes depends on the age of the mRNA
[18]. One interesting consequence
of this aging effect is
that the overall translation rate decreases with increasing length of the mRNA
[19].
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For more information, see publications on
energy conversion ,
cargo transport ,
motor traffic, and
ribosomes.
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