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Macromolecules on surface control mobility in phospholipid bilayers
James
E. Kloeppel, Physical Sciences Editor
217-244-1073; kloeppel@illinois.edu
6/20/2005
CHAMPAIGN, Ill. —
Phospholipid bilayers serve as the framework in biological membranes
in which other components are embedded. Fundamental not only in biology,
lipid bilayers are also essential in applications such as biosensors
and nanoreactors.
Forming a fluid film like the skin of a soap bubble, lipid molecules
are free to move around the membrane laterally – like couples
on a dance floor. At the same time, however, cellular proteins have
to interact in a very controlled fashion with the membrane.
Spatially resolved measurements performed by researchers at the University
of Illinois at Urbana-Champaign now show that adsorption of macromolecules
of different size can modify the mobility of underlying lipids.
“Understanding what controls the lateral mobility of individual
lipid molecules might help us better explain how cell membranes function,”
said Steve Granick, an Illinois professor of materials
science and engineering, chemistry and physics, and corresponding
author of a paper to be published the week of June 20 in the Online
Early Edition of the Proceedings of the National Academy of Sciences.
The print version will appear at a later date.
To study lipid mobility, Granick and graduate student Liangfang Zhang
first supported a bilayer made of a single type of phospholipid molecule
on a planar substrate (separated from the substrate by a thin layer
of water several nanometers thick, the lipid molecules were free to
move around). This simple bilayer mimicked the much more complex structure
of a real cell membrane comprised of hundreds of different lipids and
proteins.
Next, the researchers deposited synthetic polymer macromolecules onto
the bilayer surface to mimic the roles of membrane-associating proteins.
The polymers adsorbed onto the surface, flattening like pancakes and
covering hundreds of lipids.
Using a measurement technique called fluorescence correlation spectroscopy,
the researchers then recorded lipid movement at different spots on the
membrane.
“The measurement method is somewhat like shining a floodlight
at one spot on a dance floor, with couples waltzing in and out of the
light,” said Granick, who also is a researcher at the Frederick
Seitz Materials Research Laboratory and at the Beckman
Institute for Advanced Science and Technology. “We shine a
near-infrared laser at a very small spot on the bilayer, and watch the
motion of fluorescing molecules waltzing in and out of the illuminated
region. By analyzing how fast the fluorescence switches on and off,
we can measure the rate of mobility.”
Comparing naked regions of bilayer to areas with adsorbed polymers,
the researchers discovered that lipids moved slower when situated below
a polymer. They also found that the bigger the polymer, the slower the
lipids moved.
Individually, the lipid molecules have a very small affinity for the
adsorbed polymer, Granick said. “But collectively, if the polymer
is large enough, the many lipid binding sites add up to a strong attraction.”
Lipids on both sides of the bilayer moved at the same rate, the researchers
noted. This coordinated movement could mean that in cellular environments,
the adsorption of peripheral membrane proteins to the outside of a cell
wall may affect not just the mobility of the lipids directly beneath,
but also those on the other side of the membrane.
“This process might then enable the subtle, further changes that
proteins make on cell membranes,” Granick said. “For example,
lipid movement could affect protein distributions in the membrane and
influence docking, formation of synapses, and other membrane-mediated
functions.”
The U.S. Department of Energy funded the work.
Editor’s note: To reach Steve Granick, call 217-333-5720; e-mail: sgranick@illinois.edu.
