Chemical Characterization and Surface Orientation of the Major Glycoprotein of the Human Erythrocyte Membrane

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Abstract
The major glycoprotein of the human erythrocyte membrane has been isolated by treatment with lithium di-iodosalicylate and found to be a single polypeptide chain with a molecular weight of about 50,000. This molecule, which is 60% carbohydrate and 40% protein, carries multiple blood-group antigens, the receptors for influenza viruses, and various plant agglutinins. Four unique carbohydrate-containing peptides (α-1, α-2, α-3, and β) are produced by tryptic digestion of the isolated glycoprotein; their order in the molecule has been determined by sequential tryptic digestion of intact erythrocyte membranes and partially digested glycoprotein fragments. Cleavage of the native protein with cyanogen bromide produces five fragments; two of these (C-5 and C-1) contain most of the carbohydrate in the molecule and are derived from the N-terminal half of the polypeptide chain. The nonpolar amino acids of this glycoprotein are located predominantly in the C-terminal fragment (C-2). Phytohemagglutinin conjugated to ferritin has been used to map the distribution of glycoprotein receptors over the surfaces of intact erythrocytes by freeze-etching and electron microscopy. This label localizes to sites on the membrane that overlie the intramembranous particles. These findings suggest that the glycoprotein is oriented at the cell surface with its oligosaccharide-rich N-terminal end exposed to the exterior, while its C-terminal segment interacts with other components in the interior of the membrane to form intramembranous particles.