Role of the First N-Terminal Basic Cluster of Human Lactoferrin (R²R³R⁴R⁵) in the Interactions with the Jurkat Human Lymphoblastic T-Cells

Summary

We previously characterized a receptor of Mr 105,000 for human lactoferrin (hLf) on Jurkat human lymphoblastic T-cells. To delineate the role of R²R³R⁴R⁵ of hLf in the interaction with cells, we studied the binding of hLf variants obtained either by tryptic proteolysis (hLf^−2N, hLf^−3N and hLf^−4N)or by mutagenesis (rhLf^−5N). Consecutive removal of N-terminal arginine residues from hLf progressively increased the binding affinity but decreased the number of binding sites on the cells. The binding parameters of bovine Lf and native hLf did not differ, whereas the binding parameters of murine Lf resembled those of rhLLf^−5N. Culture of Jurkat cells in the presence of chlorate, which inhibits sulfation, reduced the number of binding sites for both native hLf and hLf^−3N but not for rhLf^−5N indicating that the hLf binding sites include sulfated molecules. The results suggest that the interaction of hLf with about 80,000 binding sites per Jurkat cell, mainly sulfated molecules, is dependent on R²R³R⁴, but not on R⁵. Interaction with about 20,000 binding sites per cell, presumably the hLf receptor, does not require the first N-terminal basic cluster of hLf. We conclude that the deletion of R²–R⁵ from hLf may serve to modulate the nature of its binding to cells and thereby its effects on cellular physiology.