Crystallization and X-ray diffraction studies of a soluble form of the human transferrin receptor

A soluble tryptic fragment of the human transferrin receptor (residues 121 to 760) has been crystallized from 2.8 M-KCl (pH 6.2) and polyethylene glycol 8000. This fragment retains the transferrin-binding activity of intact transferrin receptor. Although the trypsin treatment removes the intermolecular disulfide bonds, the receptor fragment is dimeric both under physiological conditions and at the high salt concentrations used for crystallization. The receptor fragment crystallizes in the orthorhombic space group P2(1)2(1)2(1), a = 105.5 A, b = 224.5 A, c = 363.5 A. The crystals are extremely radiation sensitive. Their diffraction extends to 3.8 A, and there is some diffuse scatter with helical characteristics. Analysis of these diffraction patterns indicates that the transferrin receptor fragments are arranged in continuous 8-fold symmetric helical columns parallel to the c axis, with a total of 32 receptor fragment monomers in the unit cell. A structure determination is in progress.