Identification of the transferrin receptor of the rabbit reticulocyte

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Abstract
Reticulocytes were separated on the basis of density by isopycnic centrifugation in dextran gradients. This parameter correlated with the degree of maturity of the cells. Lactoperoxidase iodination of cells of different densities followed by sodium dodecyl sulfate (NaDodSO4) electrophoresis revaled a 190,000 MW protein which was well-labeled in early reticulocyte membranes. Efficiency of labeling decreased as the cells increased in density, and high specific activity iodination of mature erythrocytes did not result in the labeling of any species near this MW. Inclusion of rabbit transferrin prior to the iodination procedure resulted in a specific loss of labeling of this 190,000 MW species. When steps were taken to clear endogenous transferrin from the membranes, the labeling of this species was enhanced. Transferrin can apparently block the lactoperoxidase catalyzed iodination of this membrane protein by specifically associating with it. Coomassie blue and periodic acid-Schiff staining of NaDodSO4 gels of these membranes revealed that a glycoprotein present at this MW is lost during the course of reticulocyte maturation. A glycoprotein of MW 190,000 constitutes the transferrin receptor in the reticulocyte membrane.

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