Domain closure mechanism in transferrins: new viewpoints about the hinge structure and motion as deduced from high resolution crystal structures of ovotransferrin N-lobe

The crystal structure of holo hen ovotransferrin N-lobe refined at 1.65 A resolution has been obtained. The final model gave an R-factor of 0.173 in the resolution range between 10.0 and 1.65 A. The comparison of the structure with previous high-resolution apo and Fe(3+)-loaded, domain-opened intermediate structures provides new viewpoints on the domain closure mechanism upon Fe(3+) uptake in ovotransferrin N-lobe. Overall, conformational transition follows the common mechanism that has been first demonstrated for lactoferrin N-lobe; the domains 1 and 2 rotate 49.7 degrees as rigid bodies with a translation of 2.1 A around a screw-axis that passes through the two interdomain beta-strands (89-94 and 244-249). It is generally believed that the two strands display a hinge-like motion. Here, the latter strand indeed displays an ideal hinge nature: the segments 244-246 and 248-249 behave as a part of the rigid body of domain 2 and that of domain 1, respectively, and a sharp bend upon the domain closure is largely accounted for by the changes in the torsion angles phi and psi of Val247. We find, however, that the mode of the conformational change in the first beta-strand is much more complex. Two of the five inter beta-strand hydrogen bonds undergo crucial exchanges: from Ser91-N...Val247-O and Thr89-O...Ala249-N in the open apo and intermediate structures into Tyr92-N...Val247-O and Thr90-O...Ala249-N in the closed holo structure. These exchanges, which may be triggered in the intermediate state by modulation in the topological relation between the Fe(3+)-ligated hinge residue Tyr92-OH and the anion anchor residues of helix 5, are accompanied by a large conformational change and extensive hydrogen bonding rearrangements in a long stretch of segment of Glu82 to Tyr92. Such structural transition would work as a driving force for the domain closure, which highlights a "door closer"-like role, in addition to the canonical-hinge role, for the interdomain polypeptide segment pair. As an alternative hinge that secures the correct domain motion by being placed on a significant distance from the beta-strand hinge, we point out the participation of the van der Waals contacts formed between domain 1 residue of Met331 and domain 2 residues of Trp125, Ile129 and Trp140.