The interactions between transferrin (Tf) and transferrin receptor (Tfr) as they occur during biosynthesis were studied in the human hepatoma cell line HepG2, which synthesizes both. Early during biosynthesis the Tfr monomer is converted to a disulfide-linked Tfr dimer. The Tfr monomer is not able to bind Tf, but Tf binding is observed as soon as the covalent Tfr dimer is formed and can take place in the ER. The Tf-Tfr complex is transported through the Golgi reticulum and trans-Golgi reticulum (TGR) and is ultimately delivered to an acidic compartment, where Tf releases its Fe3+. We did not observe conversion of Tf to apoTf in the TGR, showing that the part of the TGR passed by secreted Tf has a pH higher than 5.5. We conclude that when a ligand-receptor combination is synthesized by one and the same cell, ligand and receptor can interact during biosynthesis and be transported to the cell surface.

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