Texas -- Microtubules, essential structural elements
in living cells, grow stiffer as they grow longer,
an unexpected property that could lead to advances
in nano-materials development, an international team
of biophysicists has found.
The team, from The University of Texas at Austin,
the European Molecular Biology Laboratory (EMBL)
in Heidelberg, Germany and Ludwig Maximilians University
of Munich, reported their findings in Proceedings
of the National Academy of Science on July 5.
"We found that the microtubules grow stiffer as
they grow longer, a very unusual and surprising result," said
Ernst-Ludwig Florin, assistant professor with the
Center of Nonlinear Dynamics at The University of
Texas at Austin. "This will have a big impact on
our understanding of how microtubules function in
the cell and on advancing materials research.
"To my knowledge, no manmade material has this property--to
become stiffer as it elongates," said Florin. "This
research could lead to the design of novel materials
based on this biological structure."
Microtubules, which are about 25 nanometers in diameter,
play an essential role in many cellular processes,
acting as girders of support for the cell and tracks
along which organelles--structures in cells that
perform specialized functions--can move. They are
also essential components of flagella and cilia,
the extensions of some cells that give them movement.
and his colleagues measured the stiffness and length
of cellular microtubules using a "single-particle
tracking" technique. They attached yellow-green fluorescent
beads to the tips of microtubules of various lengths
and measured the position of the bead by analyzing
frame-by-frame videos of the beads moving in solution.
(The beads were 250 or 500 nanometers in diameter.)
The changes in the beads' position were used to
calculate the stiffness of the filaments they were
attached to, through a method recently developed
by the theoretical physicists on the research team.
To the surprise of the scientists, they found that
the longer the filament, the more rigid it became.
Florin and his coauthors attribute the microtubules'
unique properties to their molecular architecture.
The nanometer-sized filaments are hollow tubes made
of tubulin proteins that bind to each other in ways
that give them the ability to be both flexible and
stiff. Flexibility is important for microtubules
as they grow and change in cells, while rigidity
is important when cells need support.
"Microtubules are optimally designed to give the
maximum of mechanical performance at a minimum cost
for the cell," said Francesco Pampaloni, a physical
chemist at EMBL.
The new finding about the microtubules' properties
could provide insights into using the filaments as
models for the development of nano-materials.