Binding of transferrin to the core protein of fibroblast proteoheparan sulfate

Cell-surface-associated proteoheparan sulfate from confluent human skin fibroblasts appears to consist of two disulfide-bonded polypeptides of Mr approximately equal to 90,000. The transferrin receptor, a ubiquitous cell-surface component of proliferating cells, also consists of two subunits of Mr 90,000 linked by S--S bonds. Radiolabeled proteoheparan sulfate mixed with holotransferrin or apotransferrin at pH 4-5 followed by rabbit anti-human transferrin was adsorbed onto protein A-Sepharose to approximately equal to 80-90%. At pH 7.5 apotransferrin bound approximately equal to 40% of the proteoglycan, whereas approximately equal to 80% was bound to holotransferrin. Trypsin digestion of the proteoglycan markedly lowered its ability to bind transferrin. However, binding was essentially unaffected by heparan-sulfate lyase treatment and after reduction and alkylation. Over 90% of the 3H activity of an L-[3H]leucine-labeled proteoglycan was recovered by immunoprecipitation (transferrin.antitransferrin) of a heparan-sulfate lyase digest of the proteoglycan. The immunoprecipitated core protein had an apparent Mr of 150,000 before reduction and Mr of 90,000 after reduction of disulfide bonds. The core protein of the proteoglycan was recognized by the monoclonal antibody B3/25, which is known to be receptor specific. The present findings suggest that the core polypeptides of proteoheparan sulfate and the transferrin receptor may be identical or closely similar.