Preparing a sample for the Titan Krios
At the end of a long underground hallway on the edge of campus, a door leads to a brightly lit room. Within looms an imposing 12-foot-tall machine, whose array of wires and tubes can only be seen through a tinted window.
At a table nearby, two researchers prepare the latest sample. They huddle over a bowl from which billows a cool white cloud of liquid nitrogen. The machine behind them is a cryo-electron microscope, one of the world’s most sensitive microscopes. It is capable of capturing crisp, three-dimensional images of individual proteins, molecules essential for regulating human health.
Professor Nieng Yan is a leading expert in using cryo-electron microscopy, or cryo-EM, to obtain detailed images of proteins, the building blocks for our muscles, skin, blood, hormones and more.
Last fall Yan returned to Princeton — she’d earned her Ph.D. here in 2004 — as the first Shirley M. Tilghman Professor of Molecular Biology, to lead the University’s efforts in cryo-EM — a technology so influential that its development was awarded the 2017 Nobel Prize in Chemistry.
“The future of biology is mapping individual proteins within the cell, or even an entire cell, at atomic scale,” Yan said.
Yan was already something of a celebrity before coming to Princeton. While a professor at Tsinghua University, one of China’s top research institutions, she garnered more than 450,000 followers on Weibo, a social media platform similar to Twitter. She has published numerous groundbreaking papers on the structures of some of the body’s most important proteins, the ones that form channels through the outer membrane of the cell and dictate what comes in and out.
One of these proteins is the sodium channel, a molecule essential to the healthy functioning of the heart and brain. Defective sodium channels are linked to epilepsy, heart arrhythmias, paralysis and chronic pain. In 2017, Yan and her research team published the most detailed picture ever taken of the sodium channel. That image and others like it give biologists a 3-D map through the sodium channel — a guide to help them understand what goes wrong when it fails, and how to design drugs to fix it.
That discovery followed another 2017 publication from Yan’s team that illustrated the structure of the vital calcium channel known as Cav1.1. Calcium channels are involved in neurological, cardiovascular and muscular disorders.
The year before, Yan and her team worked out the structure of proteins that control the flux of glucose, the blood sugar that fuels our bodies. Faulty glucose transporters play a role in diabetes. Learning how to shut them off could also be key to starving cancer cells.
At its simplest level, cryo-EM shoots electrons at frozen substances to capture pictures of their atomic structures. As the “cryo” part of the name suggests, each sample needs to be flash-frozen to about minus 180 degrees Celsius to quell its normal movement and harden it against the electron beam.
A sample contains several billion copies of the protein under study, and the device collects thousands of two-dimensional images of each sample. Then a computer identifies each protein, matches like perspectives, and recombines the images together to construct a 3-D volume. By capturing images during different stages of activity, the technique can reveal how the protein functions.
Preparing a sample to go into the cryo-EM takes care, but it is much easier than its main rival technology, X-ray crystallography, which involves laboriously solidifying proteins into crystals. For decades, biologists considered cryo-EM to be crystallography’s poor cousin because its images looked more like blobs than like intricately folded chains of atoms. That changed in 2013 with the invention of a new detector technology that yields near-atomic-resolution images.
Paul Shao is Princeton’s resident expert in cryo-EM and the specialist responsible for running the machine, a Titan Krios made by Thermo Fisher Scientific. The machine is housed in the Princeton Institute for the Science and Technology of Materials’ Imaging and Analysis Center, located in the Andlinger Center for Energy and the Environment.
“High-end instrumentation like this is very sensitive to its environment. Fortunately, one of the most structurally sound places in Princeton just happens to be out that door,” said Shao, gesturing past the whirring micro-scope toward the room’s entryway. “It’s solid bedrock.”