RECEPTOR RECOGNITION SITES RESIDE IN BOTH LOBES OF HUMAN SERUM TRANSFERRIN

The binding of iron by transferrin leads to a significant conformation al change in each lobe of the protein. Numerous studies have shown tha t the transferrin receptor discriminates between iron-saturated and ir on-free transferrin and that it modulates the release of iron. Given t hese observations, it seems likely that there is contact between each lobe of transferrin and the receptor. This is the case with chicken tr ansferrin, in which it has been demonstrated unambiguously that both l obes are required for binding and iron donation to occur [Brown-Mason and Woodworth (1984) J. Biol. Chem. 259, 1866-1873]. Further support t o this contention is added by the ability of both N- and C-domain-spec ific monoclonal antibodies to block the binding of a solution containi ng both lobes [Mason, Brown and Church (1987) J. Biol. Chem. 262, 9011 -9015]. In the present study a similar conclusion is reached for the b inding of human serum transferrin to the transferrin receptor. With th e use of recombinant N- and C-lobes of human transferrin produced in a mammalian expression system, we show that both lobes are required to achieve full binding. (Production of recombinant Globe in the baby ham ster kidney cell system is reported here for the first time.) Each lob e is able to donate iron to transferrin receptors on HeLa S-3 cells in the presence of the contralateral lobe. The results are not identical with the chicken system, because the C-lobe alone shows a limited abi lity to bind to receptors and to donate iron. Further complications ar ise from the relatively weak re-association between the two lobes of h uman transferrin compared with the re-association of the ovotransferri n lobes. However, domain-specific monoclonal antibodies to either lobe block the binding of N- and C-lobe mixtures in the human system, thus substantiating the need for both.