PRIMARY RECEPTOR-RECOGNITION SITE OF HUMAN TRANSFERRIN IS IN THE C-TERMINAL LOBE

The role of the transferrin receptor in capturing and conveying transf errin through the cell during the iron-donating cycle of receptor-medi ated endocytosis has been studied extensively. Nevertheless, almost no thing is known of how human transferrin binds to its receptor. In an i nitial approach toward delineating the receptor-recognition site(s) of human transferrin, we have studied the interactions of proteolyticall y-cleaved, single-sited fragments of transferrin, representing the N- and C-lobes of the molecule respectively, with cells expressing the tr ansferrin receptor on their plasma membranes. Only the C-fragment was found capable of donating iron to hepatoma-derived HuH-7 cells or of b inding to surface receptors of HuH-7 and leukemic K562 cells. Although no association of N- and C-fragments could be demonstrated by gel chr omatography, the presence of excess N-fragment strengthened the bindin g of C-fragment by an order of magnitude. An explanation of these obse rvations is that the primary receptor recognition site of human transf errin is on the C-lobe of the protein, but that prior binding of this lobe to receptor enables the N-lobe to respond to receptor as well, ei ther directly or by interaction with the bound C-lobe.