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Professor Publishes Study on Polymers Moving Into a Crowded Cell

April 3, 2008

MERCED - When material needs to cross the membrane into a living
cell, speed is critical. Fuel, chemical signals and genetic
information must be delivered at the right time so that cells can
perform essential functions like fighting off disease or producing
new cells.

The polymers, or long chains of proteins, that provide those
intracellular signals aren’t crossing into open space. The inside
of a cell is crowded with organelles, free-floating proteins and
the jungle-gym-like structure of the cell skeleton.

In a surprising finding published in the journal Physical Review
Letters, Professor Ajay Gopinathan of the School of Natural
Sciences at the University of California, Merced, and his colleague
Young Woon Kim at the Korea Institute for Advanced Study have
concluded that over an extremely large range of lengths, the time
it takes for a polymer to cross the cell membrane into the crowded
interior remains essentially constant.

“Understanding how cells communicate with each other and with
their microenvironment and how they respond to changes in their
environment is fundamental to understanding cell function in normal
development and in health and disease. Ajay’s innovative work on
polymers lends insight into how signals from the environment can be
delivered into cells and how polymers traverse the dense packing
inside the cell,” said Dean Maria Pallavicini of the School of
Natural Sciences. “His work is an important, interdisciplinary
piece of the big picture of biomedical research at UC Merced.”

“It’s as if you had a length of chain sitting on a table, and
you wanted to feed the chain over a large bump and through a hole
in the table to the floor,” Gopinathan explained. “For the first
little while you would have to feed it through the hole link by
link, but after a certain point there would be enough force from
the other side to pull the entire chain through very quickly.”

In that illustration, the force is gravity, but for polymers
crossing membranes, the force comes from structures within the cell
binding to the polymer. Once a sufficient number of them can grasp
the end of the chain, the entire structure can traverse a pore in
the cell membrane very quickly. That means the time it takes for a
polymer to cross is determined by how fast the initial part of the
chain pushes through the pore.

Gopinathan said this mechanism may facilitate the delivery of
multiple polymers of different lengths across pores at the same
time - but as with all scientific questions, more work remains to
be done. The researchers hope next to investigate how charges and
polymer networks may affect how materials cross cell membranes.

The study was funded by the National Science Foundation and by
Gopinathan’s start-up funding at UC Merced.

Gopinathan joined the faculty of UC Merced in 2006 following a
postdoctoral research appointment at UCSB. He is involved with
undergraduate programs in physics and applied mathematics along
with his research. He earned in Ph.D. in 2003 at the University of Chicago.

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