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

Abstract

The structure of mare apolactoferrin (MALT) has been determined at 3.8 A Ê resolution by the molecular-replacement method, using the structure of mare diferric lactoferrin (MDLT) as the search model. The MDLT structure contains two iron-binding sites: one in the N-terminal lobe, lying between domains 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 microdialysis method with 10%(v/v) ethanol in 0.01 M Tris±HCl. The structure has been re®ned to a ®nal R factor of 0.20 for all data to 3.8 A Ê resolution. Comparison of the structure of MALT with that of MDLT showed that the domain arrangements in these structures are identical. However, the structure 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 speci®c biological functions. Based on the structures of MALT, HALT and DAOT, it can be stated that the native apoproteins of the transferrin family adopt three forms: (i) with both the N and the C lobes in closed forms, as observed in MALT, (ii) with the N lobe open and the C lobe closed, as observed in HALT, and (iii) with both the N and the C lobes open, as found in DAOT. All these proteins attain a convergent form when iron is bound to them, suggesting an ef®cient and unique form of iron binding. The interface between the N and C lobes, which is formed by N1± C1 contact in the core of the molecule, does not change signi®cantly.

Lactoferrin

“…The crystal structures of the apo forms of other Lfs and Tfs show further diversity (fi g. 4). The crystal structure of horse apoLf proved to have both lobes closed [51], whereas that of camel apoLf has both lobes wide open [18], and in the related apo-ovotransferrin, again both lobes are open [52]. We conclude from this variety of apoLf structures that in the absence of a bound metal ion to lock the two domains of each lobe together, the apo form is fl exible.…”

“…Although it probably exists mostly in the fully open form (i. e. with both lobes open), as implied by small-angle solution scattering [50], the small energy difference between open and closed forms enables it also to sample the closed state from time to time [7,13]. This energy difference may be greater in the Lfs of some species than others -for example, camel Lf seems to favour the fully open apo structure more than other Lfs [18], and horse Lf somewhat less so [51]. Nevertheless, the crystal structures also refl ect the fact that crystallization can select one out of several conformational states present in solution, so as to optimize crystal packing.…”

Single particle reconstruction of the human apo-transferrin–transferrin receptor complex

“…Spectroscopic studies (Grossmann et al, 1993) show both lobes open or closed, depending on the presence of iron, while crystal structures of human apo-lactoferrin show either the N-lobe open and the C-lobe closed (Anderson et al, 1990) or both lobes open (Baker et al, 1997). A structure for the horse apo-lactoferrin shows both lobes closed (Sharma et al, 1999). These Wndings suggest that the Tf-lobes in their iron-free state can sample both the open and closed conformations.…”

C-Lobe of Lactoferrin: The Whole Story of the Half-Molecule

“…The three-dimensional structures of lactoferrin from various species, such as human [5, 6], bovine [7, 8], buffalo [9, 10], equine [11, 12], and camel [13] have been solved. In all these cases, the overall structures of iron-saturated lactoferrin were found to have a similar folding.…”