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 Å resolution has been obtained. The final model gave an R-factor of 0.173 in the resolution range between 10.0 and 1.65 Å. The comparison of the structure with previous high-resolution apo and Fe3+-loaded, domain-opened intermediate structures provides new viewpoints on the domain closure mechanism upon Fe3+ 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 ° as rigid bodies with a translation of 2.1 Å around a screw-axis that passes through the two interdomain β-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 φ and ψ of Val247. We find, however, that the mode of the conformational change in the first β-strand is much more complex. Two of the five inter β-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 Fe3+-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 β-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.