Trypanosoma brucei transferrin receptor can bind C-lobe and N-lobe fragments of transferrin

Abstract

Transferrin (Tf) is a dumbbell-shaped iron transport protein composed of two homologous lobes (C-lobe and N-lobe) and is an essential growth factor for the protozoan parasite Trypanosoma brucei. The trypanosomal receptor for Tf uptake (TbTfR) is a heterodimeric complex that bears no structural similarity with the human Tf receptor. As a first step in identifying the region of Tf involved in binding to the TbTfR, C-lobe and N-lobe fragments were assessed for their capability to interact with the receptor. Preparations of C-lobe and N-lobe fragments were obtained by digestion of iron-loaded bovine Tf with proteinase K-agarose. The individual fragments were then purified by concanavalin A affinity chromatography. Uptake experiments with bloodstream forms of T. brucei demonstrated that both C-lobe and N-lobe fragments were ingested by the parasites. The uptake of the isolated lobes could be inhibited by an excess of Tf and vice versa. Dot blot binding assays showed that both C-lobe and N-lobe fragments were capable of binding to the TbTfR. Both isolated lobes were also able to support the growth of bloodstream forms of T. brucei when cultured in Tf-depleted medium. However, the C-lobe fragment was more efficiently taken up and more potent in supporting parasite growth. The results indicate that the interaction of Tf with the TbTfR is different from that with the human Tf receptor. This difference may be exploited for the development of agents specifically interfering with the binding of Tf to the TbTfR.