Structure and function of the transferrin receptor--a possible role in the recognition of natural killer cells.

Abstract

The monoclonal antibody OKT9 reacts specifically with the receptors for transferrin in human cells and has been used to isolate and characterise this receptor [1]. The receptor is a dimeric glycoprotein (Mr = 180,000) composed of two apparently identical subunits (Mr = 90,000) which are disulphide linked. The transferrin receptor appears to be a transmembrane molecule and is phosphorylated, the phosphate group being predominantly on serine residues. The cell surface form of the molecule possesses both complex and high mannose oligosaccharide chains, which do not appear to have a direct role in antibody (OKT9) binding. The molecule can be cleaved from the cell surface into a 70,000 molecular weight fragment, suggesting that the major part of the receptor is exposed to the extracellular environment. The released 70,000 molecular weight fragments are not disulphide linked and possess antibody (OKT9) binding sites. Cross-linking studies using radiolabelled transferrin suggest that two molecules of transferrin are bound to each 180,000 molecular weight receptor dimer. In addition, each 70,000 molecular weight fragment can independently bind one molecule of transferrin (Fig. 1).