Superbright X-Ray Free-Electron Lasers Find Missing Proteins

A protein's shape is key to understanding how it causes disease or toxicity. Traditionally, capturing quality x-ray snapshots of a protein has required as many as 1 billion copies of the same protein, stacked into a neat crystal. This is no longer the case.

A team from the Department of Energy’s Pacific Northwest National Laboratory (PNNL) and Lawrence Livermore National Laboratory (LLNL) has begun using exceptionally bright and fast x-rays to capture images that rivals those taken with conventional methods, but with a sheet of proteins that is just one molecule thick.

Called LCLS, the Linac Coherent Light Source free-electron x-ray laser (XFEL) is the world’s most powerful, producing beams that are millions of times brighter than earlier x-ray light sources.

X-ray free-electron lasers create images that accurately reflect the known structure of proteins (left), as determined by conventional modeling methods (right). Courtesy of PNNL.

The LCLS has allowed researchers to learn the structural details of almost 25 percent of known proteins, many of which are often overlooked due to their inability to stack properly.

“We're proving it's possible to use an XFEL to study individual monolayers of protein," said James Evans, PNNL microscopist and researcher. “Just being able to see any diffraction is brand-new.”

At around 8-angstrom resolution, which can make out items 1000 times smaller than the width of a hair, the proteins appear slightly blurry but match the expected view based on previous research. To get a clearer view of protein monolayers using XFEL, the researchers must now improve the resolution to 1 to 3 angstroms and capture images of the proteins at different angles.

Evans said that this level of clarity helps researchers in some cases to see how proteins change their shape as they interact with other proteins or molecules in their environment.

Over the past decade, the idea has been studied that one sheet of proteins could be visualized if the x-rays were bright enough, and flashed on and off quickly enough to limit the damage wrought by such power. The PNNL and LLNL team has discovered a way to create one-sheet-thick crystals of two different proteins: streptavidin and bacteriodopsin.

The team shined the superbright x-rays on the protein crystals for 30 fs. The resulting images look like the known structures. Soon, the researchers will try to capture proteins changing shape as they engage in a chemical reaction. For this, even shorter flashes of x-rays might be needed to see the action clearly.

The research was published in the International Union of Crystallography Journal. (doi: 10.1107/S2052252514001444)

For more information, visit: www.pnnl.gov