“They are studying an area that, to a large extent, has not been explored,” said Sol Grunerwho researches molecular biophysics at Cornell University; he was consulted for the study but was not a co-author.
Plasmagenic lipids are also found in the human brain, and their role in deep-sea membranes could help explain aspects of cell signaling. More immediately, the research reveals a new way in which life has adapted to the most extreme conditions of the deep ocean.
Crazy in the membrane
The cells of all life on Earth are surrounded by fatty molecules called lipids. If you put lipids in a test tube and add water, they automatically line up back to back: the fat, water-fighting tails of the lipids mix together to form an inner layer, and their water-loving heads water combine to form the outer layer. portions of a thin membrane. “It’s like oil and water separating in a dish,” Winnikoff said. “It’s universal for lipids, and it’s what makes them work.”
For a cell, an outer lipid membrane serves as a physical barrier that, like the exterior wall of a house, provides structure and holds together the interior of the cell. But the barrier can’t be too strong: It’s studded with proteins, which need some breathing room to accomplish their various cellular tasks, such as transporting molecules across the membrane. And sometimes a cell membrane pinches to release chemicals into the environment and then rebuilds itself.
For a membrane to be healthy and functional, it must be robust, fluid and dynamic at the same time. “The membranes are in equilibrium at the limit of stability,” Winnikoff said. “Even though the structure is very well defined, all the individual molecules that make up the sheets on each side are flowing around each other all the time. It’s actually a liquid crystal.
One of the emerging properties of this structure, he says, is that the membrane environment is very sensitive to both temperature and pressure, much more so than other biological molecules such as proteins, DNA or RNA. If you cool a lipid membrane, for example, the molecules move more slowly, “and eventually lock together,” Winnikoff said, like when you put olive oil in the refrigerator. “Biologically, that’s generally a bad thing.” Metabolic processes stop; the membrane can even crack and leak its contents.
To avoid this, many cold-adapted animals have membranes composed of a mixture of lipid molecules with slightly different structures to maintain the circulation of liquid crystals, even at low temperatures. Since high pressure also slows the flow of a membrane, many biologists assumed that deep-sea membranes were constructed in the same way.
