A unique new method for protein imaging has now been tested on a lipid phasic membrane protein with excellent results. The method remains X-ray based but uses technology that could replace current X-ray-based approaches and has potential to advance the challenging field of membrane protein structural biology rapidly. It also has potential to film a protein in motion – at the molecular level.

X-ray free-electron laser (X-FEL) based serial femtosecond crystallography (SFC) is an emerging method, which has now been successfully applied to record interpretable diffraction data from micrometer-sized lipidic sponge phase crystals of a bacterial photosynthetic-reaction center membrane protein.

Two major technical challenges for imaging proteins are to create the right sized protein crystals and then to irradiate them in such a way that they do not disintegrate. The commonly used type of technology is not sufficiently light intensive and therefore requires large protein crystals, which take several years to produce. In the new study published in the journal Nature Methods on January 29, 2012, scientists have shown that it is possible to use very small crystals to determine a membrane protein structure. In addition, “we have developed a new method of creating incredibly small protein crystals,” noted Linda Johansson, lead author of the article and doctoral student at the Department of Chemistry and Molecular Biology of the University of Gothenburg (Sweden).

Earlier, Richard Neutze, senior author and professor of biochemistry at the University of Gothenburg, and his research group were among the first in the world to image proteins using very short and intensive X-ray pulses. Neutze was one of the researchers to float the idea that it might be possible to image small-crystal protein samples using free-electron lasers, which emit intensive X-ray radiation in extremely short pulses. The kind of facility that could enable such work has been available in California since 2009, and it is this unique facility that was used for the current study.

“Producing small protein crystals is easier and takes less time, so this method is much faster,” says Linda Johansson. “We hope that it’ll become the standard over the next few years. X-ray free-electron laser facilities are currently under construction in Switzerland, Japan, and Germany.”

Another key discovery was that far fewer images are needed to map the protein than previously believed. Using a free-electron laser it is possible to produce around 60 images per second, which meant that the team had over 365,000 images at its disposal. However, only 265 images were needed to create a three-dimensional model of the protein.

“We’ve managed to create a model of how this protein looks,” says Johansson; “The next step is to make films where we can look at the various functions of the protein, for example how it moves during photosynthesis.”

The study was an international collaboration carried out by researchers from Sweden, Germany, and the US.

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