A pH-dependent reversible conformational transition of the human transferrin receptor leads to self-association

Human transferrin receptor (tfR) is a covalent homodimer of 90-kDa transmembrane subunits, which transits an endocytotic pathway involving exposure to low pH. Digestion of purified tfR at neutral pH generates a soluble noncovalent dimer of 70-kDa fragment subunits containing 95% of the extracellular tfR sequence, including the transferrin binding sites. Below pH 6, the 70-kDa fragment undergoes a conformational transition, which causes reversible association of the dimers in solution. Transferrin binding prevents both the conformational transition and the self-association. We suggest that tfR clustering in acidic compartments results from self-association due to a conformational change that is sensitive to transferrin binding. This and other observations support a concentration mechanism based on interactions between ectodomains in intracellular lumina.