U. OREGON (US) — It turns out that cell membranes behave a lot like water and cornstarch. At rest they are very fluid, but when quickly perturbed, they bounce back like rubber.

This surprising discovery overturns a long-held belief, and raises fresh new questions about the biology that regulates lipid and protein mobility.

“This changes our whole understanding of what lipid membranes are,” says Raghuveer Parthasarathy, a professor of physics at the University of Oregon. “We may need to rethink our understanding of how all sorts of the mechanical processes that occur in cell membranes work, like how proteins are pulled from one place to another, how cells respond to stretching and other forces, and how membrane-embedded proteins that serve as channels for chemical signals are able to open and close.”

For decades, researchers have been aware that biological membranes are fluid, and that this fluidity is crucial to allowing the motions and interactions of proteins and other cell surface molecules.

The new studies, however, reveal that this state is not the simple Newtonian fluidity of familiar liquids like water, but rather it is viscoelastic. The discovery—detailed Oct. 25 in the Early Edition of the Proceedings of the National Academy of Sciences—strikes down the notion that these biologically important membranes are Newtonian fluids that flow regardless of the stress they encounter.

“A lot of these mechanical tasks go awry in various diseases for reasons that remain mysterious,” Parthasarathy says. “Perhaps a deeper understanding of the mechanical environment that membranes provide will illuminate why biology functions, or fails to function, in the way it does.”

In the project, freestanding membranes of lipids—fatty molecules that form the basis of all cell membranes—were built with lipid-anchored nanoparticles as tracers that could be observed under high-powered microscopes.

Close analysis of the trajectories of these particles allowed researchers to deduce the fluid and elastic properties of the membranes under changing conditions.

The importance of membrane fluidity has been recognized for decades, but membranes’ strange character as a viscoelastic material has gone unnoticed, says Parthasarathy. “In retrospect, we shouldn’t be surprised. Nature uses viscoelasticity in lots of its other liquids, from mucus to tears. Now we’ve found that it harnesses viscoelasticity in lipid membranes as well.”

The Alfred P. Sloan Foundation, Office of Naval Research, and National Science Foundation supported the research.

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