APR 21, 2015 09:40 AM PDT

Rafts on the Cell Membrane, Reconsidered

WRITTEN BY: Judy O'Rourke
Tiny structures made of lipid molecules and proteins have been believed to wander within the membrane of a cell, much like rafts on the water.

This "raft hypothesis" has been widely accepted, but now scientists at TU Wien, Vienna, have shown that in living cells these lipid rafts do not exist. Their findings are published in the journal "Nature Communications".
Eva Sevcsik in the bio lab.
Fluid Membranes
"We should not think of the cell membrane as a static, solid surface," says Eva Sevcsik, biophysics group. "The membrane, the outermost layer of the cell, is fluid. Its molecules-lipids and proteins-are constantly in motion."

The proteins can serve different purposes. They can act as docking stations for substances from outside, or as channels transporting molecules into the cell. As many proteins influence each other, it appeared likely that such proteins may move within the membrane together as a "nano-raft".

This hypothesis became more and more popular among cell biologists, and "rafts" have been associated with many cellular processes. But the evidence for this hypothesis has only been derived from studies with model systems or dead cells. "Rafts" have never been directly observed in a living cell.

Many researchers used to think that "rafts" are just too small and short-lived to be detected with conventional microscopic methods. In the biophysics labs at TU Wien, a combination of several cutting-edge techniques have been used to tackle this problem. "On the one hand, we use super-resolution microscopy, which allows us to study single molecules, on the other hand, we can influence the cell membrane using micro- and nanostructured surfaces," Sevcsik says. "That way we can analyze the structure of the cell membrane in completely new ways."

Molecular Lego
First, surfaces were structured on a micrometer scale, so that cells which were grown on this surface could interact with the structure. "It is like molecular Lego," Sevcsik says. "We place molecular building blocks on the microstructured surface, which bind to specific proteins in the cell membrane." The proteins attach to the structured surface and cannot travel across the cell membrane any more.

Therefore, a protein can be selected, which is considered to be an important building block of the nano-raft, it can be fixed at particular positions on the surface and then one can study how the other proteins and lipids react.

The molecules become visible using a special microscopic technique. Tiny amounts of fluorescent markers are attached to proteins or lipids, and then molecules can be filmed as they travel within the membrane. "When we study the motion of single proteins, we can see whether we are dealing with membrane rafts or not," Sevcsik says. "Such a raft, anchored at the artificial nanostructures on the surface below, would offer more resistance to the wandering proteins than the surrounding regions. Therefore, the motion of the proteins would be slower. In our measurements, however, the diffusive motion of the molecules is the same everywhere."

For Sevcsik, the fact that the raft hypothesis has remained popular for such a long time, even though there has never been any direct evidence for it, is not that surprising: "It is always tempting to interpret one's results in the context of an established hypotheses, this is a common problem in science. Our goal was to test the raft hypothesis without any bias or prejudice."

The raft hypothesis as it has been taught up until now has taken a blow. But if raft-like structures travelling across the membrane do not exist, are there other mechanisms providing order among proteins and lipids? "Perhaps the actin cytoskeleton plays a more important role than we had thought," Sevcsik says. The cytoskeleton lies directly below the cell membrane and provides stability. Sevcsik now wants to study its function using biophysical research methods.

The article is "GPI-anchored proteins do not reside in ordered domains in the live cell plasma membrane."

[Source: Vienna University of Technology]
About the Author
  • Judy O'Rourke worked as a newspaper reporter before becoming chief editor of Clinical Lab Products magazine. As a freelance writer today, she is interested in finding the story behind the latest developments in medicine and science, and in learning what lies ahead.
You May Also Like
OCT 21, 2019
Cell & Molecular Biology
OCT 21, 2019
Insight Into the Epigenetic Mechanisms Controlling Cellular Identity
Every cell type has to carefully control which genes are expressed, and new research has learned more about that process....
OCT 21, 2019
OCT 21, 2019
Bacterial Enzyme Strips Inflammatory Carbohydrate From Meat
Humans don't make a carbohydrate called Neu5Gc, but most mammals, including cows, do....
OCT 21, 2019
OCT 21, 2019
Researchers Identify Pair of "Recruiters" that Pull T Cells to the Lungs
How do CD8 T cells make it to the lungs to help in the fight against infection? Why don’t T cells remain longer in the lungs? How can science optimiz...
OCT 21, 2019
Clinical & Molecular DX
OCT 21, 2019
The Three Common Herbs Combating High Blood Pressure: Molecular Mechanism Revealed
Hypertension is also known as high blood pressure is a serious condition. According to the Centers for Disease Control and Prevention, about 1 of...
OCT 21, 2019
OCT 21, 2019
A Bacterial Pathogen Can Steal Huge Chunks of DNA From Other Microbes
Microorganisms are everywhere, and they are often engaged in a fight for resources with other microbes....
OCT 21, 2019
OCT 21, 2019
Early Ancestor of Animals Can Group Together and Coordinate Their Movements
After observing unusual microorganisms that had been collected in Curaçao, researchers saw a strange thing....
Loading Comments...