December 16, 1999
Learning How a Cell's Tiny Motor Powers its Mobility
Researchers have for the first time shown how the world's smallest
moving machines generate the motion needed to transport their chemical
cargo throughout cells. The discovery of how one tiny component of the
motor protein kinesin powers its movement represents an important
insight into one of the most fundamental aspects of biology.
"All cells are churning with internal motion, which involves
transport of materials from one place in the cell to another," said
Howard Hughes Medical Institute investigator Ronald
Vale of the University of California, San Francisco. "It's like
trafficking goods within a city. These goods include chromosomes during
cell division, and transport of membranes or proteins within cells.
“An electron micrograph image of a microtubule track (cylindrical tube) with bound kinesin motors. The colored balls are gold particles that are chemically attached to specific locations on the kinesin motors. By following the positions of these gold particles, researchers can determine if a particular region of the kinesin protein changes its shape or location as the motor moves along its track.”
"The kinesin motors responsible for this transport are the world's
smallest moving machines, even the smallest in the protein world," he
said. "So, besides their biological significance, it's exciting to
understand how these very compact machinesmany orders of magnitude
smaller than anything humans have producedhave evolved that ability to
generate motion."
Basically, the kinesin protein links with another kinesin to form a
two-molecule ferry that moves cellular cargo along tram tracks composed
of infinitesimal filaments called microtubules that criss-cross the
cell's interior.
In the December 16, 1999, issue of the journal Nature, Vale
and his colleagues describe how they analyzed the motion of individual
kinesin molecules, ultimately pinpointing the portion of the kinesin
protein responsible for generating movement.
The researchers' analyses showed that a tiny piece of the kinesin
protein dubbed the "neck linker" abruptly stiffens like
Velcro; zipping up when the energy molecule ATP attaches to
kinesin. This stiffening throws the neck linker forward and provides
the mechanical force that puts the kinesin molecule in motion along the
microtubule tracks. The discovery that motion is generated by the neck
linker, which is composed of only 15 amino acids, also helped the
scientists understand how two linked kinesin molecules coordinate their
movement along the microtubule, said Vale.
"The kinesin motor walks along the microtubule much like a person
walks along steppingstones across a pond," said Vale. "Just as a person
has to step from stone to stone, there are only certain points where
kinesin molecules can attach to a microtubule. Basically, the neck
linker zippers up and throws its rearward partner forward to the next
attachment site, like swinging the rear leg forward to the next
steppingstone."
According to Vale, the linked kinesins take step after step along
the microtubule by coordinating the cycling of ATP molecules, first
onto one kinesin, then onto its partnerwith the ATPs alternately
attaching, releasing their energy, and detaching as spent products.
Vale and his colleagues used several analytical techniques, each of
which uncovered a different aspect of kinesin's motion mechanism. To
begin their experiments, the scientists created kinesin molecules that
included specific attachment points for various marker molecules that
would help reveal how the neck linker moves. To obtain "snapshots" of
the marker-carrying molecules at specific stages, the scientists
treated the kinesins with altered versions of ATP, called analogues,
that "froze" the kinesins at various stages of activity.
For example, in one experiment, the scientists attached a gold
particle to the neck linker and used electron microscopy (performed by
Ron Milligan at Scripps Research Institute) to obtain images of the
kinesin at different stages. Those images revealed that in the absence
of ATP analogues, the linker neck could pivot either forward or
backward, but the binding of an ATP analogue locked the piece of
protein in the forward position. After the kinesin released the ATP
analogue, however, the neck linker again became mobile.
Another critical experiment using mutant kinesin molecules showed
that neck linker motion was necessary for kinesin movement along the
microtubule. "We studied two mutants that are both stuck at the
ATP-binding step," said Vale. "However, one of these mutants can take a
single step along the microtubule, and the other one can't. We
predicted that if the neck linker motion was actually necessary for
kinesin to take a step, then we should see such motion in the mutant
that can take a step, but not in the one that can't. And that's what we
saw very clearly."
According to Vale, basic understanding of how kinesin motors work
could lead to medical therapies that either inhibit or stimulate
kinesin activity.
"Humans have perhaps 50 different kinds of these kinesin motors, and
if we understand how they work we might be able to selectively inhibit
those involved in chromosome segregation in mitosis," said Vale. "Since
cancer cells are constantly dividing, such inhibitors might have
application as cancer chemotherapeutic agents.
"Also, there is some indication that certain neurodegenerative
diseases might result from kinesin-related deficiencies in transport.
In such cases, a therapy that stimulates the transport system might be
effective in treatment."
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