Molecular Cloning of Transferrin Receptor 2

Abstract

Transferrin receptor (TfR) plays a major role in cellular iron uptake through binding and internalizing a carrier protein transferrin (Tf). We have cloned, sequenced, and mapped a human gene homologous to TfR, termed TfR2. Two transcripts were expressed from this gene: (~2.9 kilobase pairs), and β (~2.5 kilobase pairs). The predicted amino acid sequence revealed that the TfR2-α protein was a type II membrane protein and shared a 45% identity and 66% similarity in its extracellular domain with TfR. The TfR2-β protein lacked the amino-terminal portion of the TfR2-α protein including the putative transmembrane domain. Northern blot analysis showed that the α transcript was predominantly expressed in the liver. In addition, high expression occurred in K562, an erythromegakaryocytic cell line. To analyze the function of TfR2, Chinese hamster ovary TfR-deficient cells (CHO-TRVb cells) were stably transfected with FLAG-tagged TfR2-α. These cells showed an increase in biotinylated Tf binding to the cell surface, which was competed by nonlabeled Tf, but not by lactoferrin. Also, these cells had a marked increase in Tf-bound 55Fe uptake. Taken together, TfR2-α may be a second transferrin receptor that can mediate cellular iron transport.