Record-breaking crystals: Smallest ever X-rayed

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  • Published: Mar 15, 2017
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Record-breaking crystals: Smallest ever X-rayed

Intact insect virus

Atomic model of the crystalline occlusion bodies, derived from the X-ray diffraction images recorded at the LCLS. The individual proteins (right) stick together to form the building blocks (left, seen from the side; centre, seen from above) of the crystalline occlusion bodies. Credit: Dominik Oberthür, CFEL/DESY

The crystalline protein envelope of an insect virus has been examined by high-intensity X-ray pulses. The study by Henry Chapman of the Center for Free-Electron Laser Science is described in the Proceedings of the National Academy of Sciences and represents the smallest protein crystals examined so far by X-ray crystallography.

The researchers studied the "cocoon" of the Cydia pomonella granulovirus (CpGV), which infects the caterpillars of the codling moth (Cydia pomonella) and is used in agriculture as a biological pesticide. The research shows the fine details of the building blocks that make up the viral cocoon down to a scale of 0.2 nanometres. Moreover, as a proof of principle, this work opens up new opportunities in protein structure studies.

Biological pesticide

"The granulovirus attacks certain insects and kills them," explains Cornelius Gati of DESY (Deutsches Elektronen-Synchrotron). "This initially leaves it stranded inside the decaying host, so it has to protect itself, perhaps for years, against adverse environmental conditions such as heat, ultraviolet radiation and drought, until it is once again ingested by an insect. To achieve this, the virus wraps itself in a cocoon made of protein crystals, which only dissolve again once it reaches an insect's gut," he adds. Peter Metcalf of the University of Auckland in New Zealand and Johannes Jehle from the Julius Kühn Institute in Darmstadt, teamed up with the DESY scientists on this work for their own particular interests.

"One of the big challenges of [the X-ray crystallographic] procedure is growing the crystals," says Chapman. Many proteins do not readily align to form crystals, because that is not their natural state. However, usually the smaller the crystals that can be used for an analysis, the easier it is to grow them, but the harder it is to obtain good measurements from them. "We are hoping that in future we will be able to dispense altogether with growing crystals and study individual molecules directly using X-rays," he adds, "so we would like to understand the limits."

Record breaker

Gati points out that these viral particles are the smallest crystalline proteins ever examined by X-ray structure analysis. The occlusion body, the viral cocoon, has a volume of just 0.01 cubic micrometres; that's about one hundred times smaller than the smallest artificially grown protein crystals that have been analysed by crystallography previously. Of course, to be successful in their work, the team had to rely on an extremely bright X-ray beam at the SLAC National Accelerator Laboratory in the USA, generated using a free-electron laser (FEL). In this instrument, a beam of high-speed electrons is guided through a magnetic undulator causing them to emit laser-like X-ray pulses. The optics allow the team to focus each X-ray pulse on to an area very close to the size of the viral particles. "Directing the entire power of the FEL on to one tiny virus exposed it to tremendous radiation levels," Gati adds; 1.3 billion Grays, in fact.

At this radiation level, the FEL pulses completely vaporise the viral particles, but this disintegration only takes place after the femtosecond-duration pulse has passed though the object, Chapman explains. "Essentially, the structural information about the crystal is carried in the X-ray light pulse, and since nothing else moves faster than light, this information emerges from the crystal ahead of the subsequent explosion," he told us. "Simulations based on our measurements suggest that our method can probably be used to determine the structure of even smaller crystals consisting of just hundreds or thousands of molecules," says Chapman. "This takes us a huge step further towards our goal of analysing individual molecules," he says.

Related Links

Proc Natl Acad Sci (USA) 2017, 114, 2247-2252: "Atomic structure of granulin determined from native nanocrystalline granulovirus using an X-ray free-electron laser"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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