Investigators from the Lawrence Livermore National Laboratory (LLNL), Stanford and UC Davis, using high-resolution secondary ion mass spectrometer called NanoSIMS, are taking a close look at cellular membranes. How close? Read on:
That machine is a highly specialized mass spectrometer, which analyzes the mass of small molecular ions formed when a focused ion beam runs across the surface of a sample. “You take everything in the beam’s focal area, which is about 100 nanometers in diameter and about 10 nanometers deep for our experiment, and you obliterate it,” Boxer said, explaining how the machine works. [Steven Boxer, the Camille and Henry Dreyfus Professor in Chemistry @ Stanford –ed.] “Then you sample the fragments by mass spectrometry. Then you move over and you go another 100 nanometers and you obliterate everything. And now you see if what’s in each 100 nanometer region is the same or different from the next region. And so you just raster this beam across the surface, and by rastering over and over and over again, you build an image…”
“Imagine how a cell could divide if it weren’t for the fact that the membranes were flexible,” Boxer said. “They must be flexible. You have endocytosis, exocytosis, all these processes which involve dynamic reorganizations of membranes…”
What scientists really want to know is what membrane components are near what other components, how that organization changes over time and how that organization leads to the emergence of function, Boxer said…
For the Science study, Boxer and colleagues made a spherical lipid vesicle to model a cell membrane, placed it on a small silicon wafer to make the lipid bilayer flatten into two dimensions and organized the flat membrane with a pattern of chrome grids to provide “landmarks” on the surface. That system let the scientists track how lipids moved and measure how many of each type of lipid resided in an area…
Scientists have long known that certain proteins get pieces of grease stuck on them–become “prenylated,” in chemical parlance. The addition of a hydrophobic group to a protein forms a lipid anchor that attaches the protein to the lipid membrane. Associations of different proteins in a section of compositionally distinct fatty membrane have been called “lipid rafts.”
Just as a raft is a bunch of logs associated because a rope binds them together, allowing it to perform the function of floating, proteins associated with other proteins on a lipid raft may perform functions, Boxer said, though evidence for this is limited. These rafts may serve many functions such as reacting to stress, conferring immunity through antibody response, adhering to other cells and countering bacteria and toxins.
“Many proteins live in three dimensions for part of their life and in two dimensions for part of their life,” Boxer said. “They go back and forth. When you’re in two dimensions, your chance of bumping into something else is a lot higher than when you’re in three dimensions. This is the idea of the rafts. You get one of these pieces of grease stuck on you, and now you become associated with the membrane and you find yourself in one of these rafts, whatever they are, and now you meet your friends, and the result is specialized function. It’s an organizing principle, if you like, in an otherwise fluid environment.”
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