Proteins of the transferrin family, which contains serum transferrin and lactoferrin, control iron levels in higher animals through their very tight (Kapp approximately 10(20)) but reversible binding of iron. These bilobate molecules have two binding sites, one per lobe, each housing one Fe3+ and the synergistic CO3(2-) ion. Crystallographic studies of human lactoferrin and rabbit serum transferrin in their iron-bound forms have characterized their binding sites and protein structure. Physical studies show that a substantial conformational change accompanies iron binding and release. We have addressed this phenomenon through crystal structure analysis of human apolactoferrin at 2.8 A resolution. In this structure the N-lobe binding cleft is wide open, following a domain rotation of 53 degrees, mediated by the pivoting of two helices and flexing of two interdomain polypeptide strands. Remarkably, the C-lobe cleft is closed, but unliganded. These observations have implications for transferrin function and for binding proteins in general.