Peptide mapping of the human transferrin receptor in normal and transformed cells

Human transferrin receptor protein from various human cell lines in tissue culture (phytohemagglutinin-activated lymphocytes, normal embryonic fibroblasts (WI-38), a choriocarcinoma (Be Wo), an epidermoid carcinoma (KB), and a metastatic breast carcinoma (MCF-7)) was metabolically labeled with [35S]methionine and purified by two methods. The first method involves affinity chromatography of the transferrin receptor with a transferrin-linked Sepharose 4B resin. The second method involves direct immunoprecipitation of the receptor protein with a goat polyclonal antibody raised against placental transferrin receptor. Additionally, transferrin receptor from normal gestational placenta was similarly radiolabeled with [35S]methionine and subsequently purified with transferrin-bound resin. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the reduced receptor isolated by both purification techniques revealed an Mr = 94,000 protein to be present in all cells assayed for this study. Partial proteolytic digestion with Staphylococcus aureus protease followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed identical mapping patterns of the Mr = 94,000 protein isolated from each cell line, irrespective of the isolation technique employed to purify the receptor. These findings indicate an identical peptide structure of the human transferrin receptor in cells with a differentiated function for iron metabolism, in normal cells undergoing mitosis, and in neoplastic cells in long term culture, within the limits of detectability of the proteolytic digestion maps.