A first S-SAD experiment carried out on the PROXIMA1 beamline by Adeline Goulet (member of the C. Cambillau group, Architecture and Function of Biological Macromolecules, Marseille, UMR6098) permitted the structure determination of one of the two main proteins forming the ATV virus (Acidianus Two-tailed Virus). This remarkable virus, that infects the archaeon* Acidianus convivator which lives under extreme conditions (85-93 °C, pH 1.5), is the first known example of a virus that can have extracellular development totally independent of its host (Ref1).
The advantage of the S-SAD (or Sulfur Single Wavelength Anomalous Diffraction) method is that it permits the structure of proteins to be determined ab initio in their natural form without having to label them with heavy atoms. Instead, the method uses the weak anomalous signal from sulfur atoms naturally present in the majority of proteins. In the present case, by measuring the diffraction of a single crystal at an energy of 6 keV (2 Å), the signal produced by only 3 sulfur and 3 chloride atoms permitted the elucidation of the position of the other 900 atoms in this protein. In principal very simple, S-SAD experiments are in practice very difficult to carry out properly. They consist of recording extremely precise measurements of the very weak anomalous signal produced by these atoms. The success of this experiment, showing the high quality of the measurements taken, marks an important milestone for the PROXIMA1 beamline.
• Archaea: unicellular micro-organisms without nuclei similar to bacteria, yet belonging to a separate group from bacteria.
• Ref1 - M. Häring et al., Independent virus development outside a host, Nature, 436:1101-1102, 2005
Figure : Cartoon representation of protein p131from the ATV virus. The anomalous electron densities originating from the sulfur and chlorine atoms are shown in yellow and red, respectively. The structure, uniquely composed of alpha helices, contains 100 amino acids.
Archeon : single-celled microorganism. Like bacteria, it has no cell nucleus, but Archae is a group different from the Bacteria group.
