Transferrin receptor of the rabbit reticulocyte

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Abstract
A Triton X-100 solubilized macromolecular complex of transferrin and a membrane constituent can be isolated by gel chromatography from rabbit reticulocytes previously incubated with 125I-labeled transferrin. The apparent MW of this complex is close to that of ferritin, or about 445,000. On sodium dodecyl sulfate gel electrophoresis the complex displays 2 glycoprotein subunits, of MW 176,000 and 95,000, in addition to transferrin. A transferrin-binding fraction with a MW near 400,000, containing these subunits, can also be identified in membranes of nonincubated reticulocytes. The corresponding membrane fraction from mature erythrocytes, which have lost transferrin-binding activity, displays both protein subunits, but the 176,000 MW component fails to give a PAS [periodic acid Schiff] stain for carbohydrate. Treatment of reticulocytes with pronase, which destroys the ability of the cells to form specific complexes with transferrin, degrades both components. These results are consistent with the hypothesis that the primary transferrin receptor of the rabbit reticulocyte is a glycoprotein of MW in the range 350,000-400,000, comprised of a combination of 2 subunits with MW 176,000 and 95,000, respectively. Transferrin-binding activity appears to depend on the carbohydrate moiety of the 176,000 subunit.