THE MONOCLINIC CRYSTAL STRUCTURE FORM OF BACTERIOFERRITIN FROM E. COLI. Hospital, M.1, Dautant, A.1, Yariv, J.1, Précigoux, G.1, Kalb (Gilboa), A.J.2, Frolow, F.2 & Sweet, R.M.3 1Unité de Biophysique Structurale, CNRS, Université de Bordeaux, 33405 Talence, France. 2Departments of Structural Chemistry and Chemical Services, The Weizmann Institute of Science, Rehovot, Israel. 3Biology Department, Brookhaven National Laboratory, Upton, New York 11973-50000, USA.
The structure of a monoclinic, P21, crystal form of bacterioferritin from E. coli (cytochrome b1) was solved by molecular replacement and refined using as model the fundamental unit of this protein that consists of two protein subunits and a single haem. The haem is positioned in a special position on the two-fold axis of the dimer. The asymmetric unit of the monoclinic crystal consists of twelve such dimers and corresponds to the molecule of bacterioferritin (MW = 450 kD).
First, the orientation of the molecule has been successfully determined with a standard self-rotation followed by a locked self-rotation, then, the position in the unit cell, has been localized with the program AMoRe (Navaza, 1992). The model used was built from the coordinates of the tetragonal structure of cytochrome b1 (Frolow & al., 1994). Even at the 2.9 Å resolution, the following facts about this structure emerged. Thus it is confirmed that the haem is located at the interface of two subunits with as axial ligands the sulfur atoms of two symmetrically related Met52. Furthermore the presence of a di-metal center is observed in the inside of a four-helix bundle. The residues involved in the metal coordination spheres are four glutamates and two histidines. Both histidines ligands bind to the metals through their d-nitrogen atoms.
The crystal packing corresponds to a situation halfway between the well known hexagonal closest packing and the orthorhombic all-face centred one.