The N-terminal Arg2, Arg3 and Arg4 of human lactoferrin interact with sulphated molecules but not with the receptor present on Jurkat human lymphoblastic T-cells

We previously characterized a 105 kDa receptor for human lactoferrin (hLf) on Jurkat human lymphoblastic T-cells. To delineate the role of the basic cluster Arg²-Arg³-Arg⁴-Arg⁵ of hLf in the interaction with Jurkat cells, we isolated N-terminally deleted hLf species of molecular mass 80 kDa lacking two, three or four N-terminal residues (hLf^-2N, hLf^-3N and hLf^-4N) from native hLf that had been treated with trypsin. Native hLf bound to 102000 sites on Jurkat cells with a dissociation constant (K_d) of 70 nM. Consecutive removal of N-terminal arginine residues from hLf progressively increased the binding affinity but decreased the number of binding sites on the cells. A recombinant hLF mutant lacking the first five N-terminal residues (rhLf^-5N) bound to 17000 sites with a K_d of 12 nM. The binding parameters of bovine lactoferrin (Lf) and native hLf did not significantly differ, whereas the binding parameters of murine Lf (8000 sites; K_d 30 nM) resembled those of rhLf^-5N. Culture of Jurkat cells in the presence of chlorate, which inhibits sulphation, decreased the number of binding sites for both native hLf and hLf^-3N but not for rhLf^-5N, indicating that the hLf-binding sites include sulphated molecules. We propose that the interaction of hLf with a large number of binding sites (approx. 80000 per cell) on Jurkat cells is dependent on Arg²-Arg³-Arg⁴, but not on Arg⁵. Interaction with approx. 20000 binding sites per cell, presumably the hLf receptor, does not require the first N-terminal basic cluster of hLf. Moreover, the affinity of hLf for the latter binding site is enhanced approx. 6-fold after removal of the first basic cluster. Thus N-terminal proteolysis of hLf in vivo might serve to modulate the nature of its binding to cells and thereby its effects on cellular physiology.