The use of x-rays to reveal the atomic-scale 3-D structures of proteins has led to many advances in understanding how these molecules work in bacteria, viruses, plants, and humans. It has also guided the development of precision drugs to combat diseases including cancer and AIDS.
However, many proteins can't be grown into crystals large enough to be deciphered. To deal with this challenge, scientists at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory and colleagues at Columbia University have developed a new approach for solving protein structures from tiny crystals.
The technique relies on unique sample-handling, signal-extraction, and data-assembly approaches, and a beamline capable of focusing intense x-rays at Brookhaven's National Synchrotron Light Source II (NSLS-II), a DOE Office of Science user facility, to a millionth-of-a-meter spot, about one-fifth the width of a human hair.
The corresponding author on the study, Qun Liu, Brookhaven Lab scientist, said that their technique indeed opens the door to dealing with microcrystals that have been previously inaccessible such as difficult-to-crystallize cell-surface receptors and other membrane proteins, flexible proteins, and many complex human proteins. The study was published in IUCrJ, a journal of the International Union of Crystallography.
To handle the tiny crystals, the researchers developed sample grids patterned with micro-sized wells. After pouring solvent containing the microcrystals over these well-mount grids, the scientists removed the solvent and froze the crystals that were trapped on the grids.
Liu explained that they still have a challenge since they can't see where the tiny crystals are on their grid. To solve this problem, they used microdiffraction at NSLA-II's Frontier Microfocusing Macromolecular Crystallography (FMX) beamline to survey the whole grid. They scanned line by line to find where those crystals are hidden.
The leading beamline scientist at FMX, Martin Fuch, said that the FMX beamline could focus the full intensity of the x-ray beam down to a size of one micron, or a millionth of a meter. They can control the beam size to match it to the size of the crystals, five microns in the case of the current experiment. These capabilities are critical to obtaining the best signal.
Liu explained further that to adapt to the environment through evolution, these proteins are malleable and have lots of non-uniform modifications. He added that it is hard to get a lot of repeat copies in a crystal because they don't pack well. Receptors in human are common targets for drugs and knowing their various structures could help guide the development of new, more targeted pharmaceuticals. The method may not be restricted to small crystals. Lui explained that the technique they developed could handle small protein crystals, but it can also be used for any size protein crystals any time there is a need to combine data from more than one sample.
