Lipid domains chemically identified by SIMS (top) match images from AFM (bottom).


A chance meeting with a cosmochemist has led Steven Boxer to a new way to precisely image lipid locations. With Mary Kraft (Stanford University, Stanford, CA) and colleagues, he hopes to test ideas generated by the raft hypothesis. “A lot of this is cartoons,” says Boxer. “We want to translate these cartoons of membrane molecules into reality.”

The technique fills in a gap between FRET (operating over a maximum of a few nanometers) and optical microscopy (several hundreds of nanometers or more). Now, the NanoSIMS (secondary ion mass spectrometry) machine identifies lipid distributions with a lateral resolution of ∼100 nm.

The NanoSIMS sweeps a focused beam of cesium ions over the sample in around 10 minutes. The high-energy beam almost completely fragments proteins and lipids. Thus, molecules can be easily identified only if they are labeled with a particular isotope. The advantage, however, is that “we are reducing this thing to dust and we get a lot of dust per molecule,” says Boxer. “So sensitivity is very high.” In the future, different types of beams may allow identification without the need for labeling.

The next trick is to get access to the instrument. The 5 instruments in the US cost $2–3 million each, and are tricky to run. They are not yet biofriendly, as their original use was the analysis of specks of dust from comets. “These instruments were the private domain of that [cosmochemist] community,” says Boxer.

But he thinks they complement the alternative: atomic force microscopy (AFM), which feels the shape of the protein and lipid landscape. “AFM has no chemical information,” he says, “and is notorious for confusing real signals and debris. Those are the most expensive measurements of dirt you are ever going to see.”

He remains skeptical of much of the raft concept, but asserts that “there must be organizing principles of some sort.” Those principles should emerge once experiments are applied not only to the current lipid mixtures made in vitro but also to membrane samples isolated from cells.


Kraft, M.L., et al.