International audience; We present a proof-of-concept three-dimensional reconstruction of the giant mimivirus particle from experimentally measured diffraction patterns from an x-ray free-electron laser. Three-dimensional imaging requires the assembly of many two-dimensional patterns into an internally consistent Fourier volume. Since each particle is randomly oriented when exposed to the x-ray pulse, relative orientations have to be retrieved from the diffraction data alone. We achieve this with a modified version of the expand, maximize and compress algorithm and validate our result using new methods.

X-ray crystallography has been a defining tool for structural biology. Since the determination of the molecular structure of myoglobin in 1957, the technique has allowed researchers to determine the structure (and hence the function) of tens of thousands of proteins, nucleic acids, and other biological molecules. For the method to work, the molecules of interest have to be assembled into a crystal that can be investigated by monitoring how an x-ray beam scatters off of it. Unfortunately, many important molecules do not cooperate: they are impossible to crystallize. Addressing this limitation has been a key driver of a new generation of light sources based on x-ray free-electron lasers (XFELs). An international community of XFEL users is seeking to reconstruct molecular structure without crystals by scattering x rays off single molecules injected into powerful XFEL beams. A team lead by Janos Hajdu at Uppsala University, in Sweden, has now reported a key step towards this goal [1]. They have demonstrated that it is possible to reconstruct the three-dimensional structure of a virus from a very large number of diffraction patterns collected from a sequence of randomly oriented single viruses.