THE TRANSFERRIN RECEPTOR CYTOPLASMIC DOMAIN DETERMINES ITS RATE OF TRANSPORT THROUGH THE BIOSYNTHETIC-PATHWAY AND ITS SUSCEPTIBILITY TO CLEAVAGE EARLY IN THE PATHWAY

The soluble human transferrin receptor (TfR) found in blood is the res ult of a proteolytic cleavage occurring in the ectodomain of the recep tor close to the transmembrane domain at Arg-100. We have discovered a nother cleavage site between Gly-91 and Val-92 even closer to the tran smembrane domain. Cleavage at Gly-91 differs markedly from the normal cleavage site. It occurs when the entire cytoplasmic portion or the pr oximal 31 amino acids of the transmembrane domain are deleted. A solub le disulfide-bonded dimer of the TfR is released into the medium in co ntrast to the cleavage at Arg-100 where a dimer lacking intersubunit d isulfide bonds is released. Whereas the cleavage at Arg-100 is generat ed by cycling through the endosomal system, pulse-chase experiments in dicate that cleavage at Gly-91 occurs predominantly during the biosynt hesis of the receptor. Pulse-chase analysis of the biosynthesis of mut ant TfRs that lack the membrane-proximal cytoplasmic domain show that they exit the endoglycosidase H-sensitive compartment at a slower rate than the wild type TfR. These results suggest that the cytoplasmic do main influences the trafficking of the TfR either by influencing the f olding of the ectodomain or by providing a positive signal for its tra nsport through the biosynthetic pathway.