CUBIC, TETRAGONAL AND ORTHORHOMBIC CRYSTAL FORMS OF HORSE SPLEEN APOFERRITIN. Geoffre, S., Gallois, B., Dautant, A., Granier, T., Langlois d'Estaintot, B., Michaux, M.A. & Précigoux, G., Unité de Biophysique Structurale, CNRS, Université de Bordeaux, 33405 Talence, France.

Ferritin is the universal iron storage protein utilized by most living cells to uptake and store iron, in a bio-available form , via redox mechanisms. X-ray structural studies show that haem is able to bind horse spleen apoferritin in a site similar to that observed in bacterioferritins with a stoichiometry 1:2.

In the crystal structures, and whatever the studied metalloporphyrin, the protoporphyrin IX is always observed free of metal. Beside that property, horse spleen apoferritin can be crystallized in three different space groups : cubic, F432 ; tetragonal, P42₁2 and orthorhombic P2₁2₁2. The two later crystal forms were described some years ago (Harrison 1963, Hoy & al. 1974) but their crystal quality (6 Å resolution) could not allow a full structure investigation.

Thanks to a careful protein purification procedure, both horse spleen apoferritin crystals of tetragonal and orthorhombic forms, which diffract beyond 2.4 Å have been obtained. X-ray diffraction data were collected, on the three different crystal forms, with the LURE synchrotron radiation facilities.

We present a comparison of the structures of the three crystal forms : molecular packing and conformational differences will be discussed in relation with crystal symmetry differences.