IDENTIFICATION OF THE MEMBRANE REMNANTS OF TRANSFERRIN RECEPTOR WITH DOMAIN-SPECIFIC ANTIBODIES

Tissue culture studies with K562 and HL60 cells have demonstrated the production of a soluble form of transferrin receptor identical to that identified in human serum. The present study was undertaken to search for membrane remnants of the truncated receptor with peptide antibodi es specific for the extracellular and cytoplasmic domain of transferri n receptor. In cell membranes, a 105K remnant was identified that is c onsistent with truncation of one extracellular domain monomer of the t ransferrin receptor. In the exosomal fraction of the culture supernata nt, a smaller 20K remnant consistent with truncation of both extracell ular domains was also demonstrated. These findings provide evidence th at soluble receptor is the product of proteolytic cleavage of intact m embrane-bound transferrin receptor. Prior studies showing that the con centration of the extracellular domain in exosomes remained stable dur ing incubation in culture supernatant suggest that this cleavage possi bly occurs intracellularly.