Structure of mare apolactoferrin: the N and C lobes are in the closed form

The structure of mare apolactoferrin (MALT) has been determined at 3.8 Angs trom resolution by the molecular-replacement method, using the structure of mare diferric lactoferrin (MDLT) as the search model. The MDLT structure c ontains two iron-binding sites: one in the N-terminal lobe, lying between d omains N1 and N2, and one in the C-terminal lobe between domains C1. and C2 . Both lobes have a closed structure. MALT was crystallized using the micro dialysis method with 10%(v/v) ethanol in 0.01 M Tris-HCl. The structure has been refined to a final R factor of 0.20 for all data to 3.8 Angstrom reso lution. Comparison of the structure of MALT with that of MDLT showed that t he domain arrangements in these structures are identical. However, the stru cture of MALT is very different to the structures of human apolactoferrin ( HALT) and duck apo-ovotransferrin (DAOT), in which the domain associations differ greatly. In HALT, the N lobe adopts an open conformation while the C lobe is in the closed form. On the other hand, in DAOT both the N and the C lobes adopt the open form. These results indicate the domain arrangements in these proteins to be an important structural feature related to their s pecific biological functions. Based on the structures of MALT, HALT and DAO T, it can be stated that the native apoproteins of the transferrin family a dopt three farms: (i) with both the N and the C lobes in closed forms, as o bserved in MALT, (ii) with the N lobe open and the C lobe closed, as observ ed in HALT, and (iii) with both the N and the Clobes open, as found in DAOT . All these proteins attain a convergent form when iron is bound to them, s uggesting an efficient and unique form of iron binding. The interface betwe en the N and C lobes, which is formed by N1-C1 contact in the core of the m olecule, does not change significantly.