Molecular rafts of different compositions are believed to be involved in cell-membrane processes, Webb says, "but the physics of raft formation is not well understood. We hope our experiments -- based on 3-D-resolved multiphoton microscopy to illuminate membrane domains and transition behavior -- will encourage others to join in this study."
Baumgart emphasizes that balloon animals are not a perfect analogy for shape-shifting vesicles. "For one thing, rubber balloons stretch and vesicle membranes don't, but seeing these shapes from a distance, we can imagine some of the same forces at work," he says.
Zooming in with the multiphoton microscope, Baumgart and his colleagues looked at a simplified model of cell membranes. In nature, vesicles are a complex mixture of several kinds of lipids, proteins and impurities; Baumgart's laboratory-grown vesicles were made of just three kinds of lipids -- sphingomyelin, dioleylphosphatidylcholine and cholesterol. Fluorescent dyes that light up under the microscope's laser beam were used to label different membrane phases either red or blue. The researchers found that, depending on the temperature, a cell membrane can have several fluid phases with different physical properties at the same time -- something like oil and water on the same surface.
Baumgart says the simplified vesicles produced a wide assortment of exotic shapes and structures as the temperature was changed, processes that appear to involve the disappearance of boundary-line tension as the two phases merge into a single one at higher temperatures.
The study was supported, in part, by a National Institutes of Health grant to the Developmental Resource for Biophysical Imaging Opto-Electronics (DRBIO) at Cornell and by a Keck Foundation grant to Baumgart. Multiphoton microscopy
Contact: Roger Segelken
Cornell University News Service