And the goal is not to discover the recipe for a precious metal or the philosopher’s stone, but more modestly to ‘congeal’ the protein, to make it insoluble, to transform it into a crystal. ‘What is specific or particular about my work,’ explains Frédéric Kerff, ‘is that we don’t know the recipes for crystallisation. We don’t know in advance what will work. We could quite easily obtain a crystal after several days or knock ourselves out for several months without getting a result.’ The disclosure of the result under a microscope is always a moment of light intensity: has it worked or not? It’s an aesthetic moment as well. In its repetitiveness, a crystal’s structure can take on bewitching contours.

The day EXLX1 took form

A lot less poetic are the names the researchers give to their proteins. The one that Frédéric Kerff recently managed to crystallise, and which earned him the honour of a publication in the American PNAS journal (1), is called… EXLX1! It is a protein (from the bacteria Bacillus subtilis) which plays a role in the metabolism of bacteria cell walls, and is thus of interest to the CIP’s researchers. ‘But crystallisation is only the first part of the work,’ explains the researcher. ‘The other stage consists of building a three dimensional structure of the protein.’ Once he had obtained his crystal Fréderic Kerff thus set off for the Grenoble synchrotron, with his suitcase in one hand and in the other a small freezer pack holding the precious crystallized protein. Grenoble possesses a unique particle accelerator which allows researchers to carry out what is called ‘X-ray diffraction.’ The rays which come into contact with the protein crystal interfere with the atoms which make up the protein and are diffracted by them in different directions in order to form a diffraction diagram. The position of the marks and their intensity, measured by a detector, depend respectively on the regular stacking up of proteins in the crystal and on the electronic density of the different atoms which make up the protein. After a certain number of calculations the technique allows for the position of each atom to be determined and as a consequence the three dimensional structure of the protein can be ascertained: around 2000 positions for the protein in question, which contains 208 amino acids.

(1) Kerff F, Amoroso A, Herman R, Sauvage E, Petrella S, Filée P, Charlier P, Joris B, Tabuchi A, Nikolaidis N, Cosgrove DJ. (2008) Crystal structure and activity of Bacillus subtilis YoaJ (EXLX1), a bacterial expansin that promotes root colonization. Proc Natl Acad Sci U S A. 105(44):16876-81