Major glycoprotein of the human erythrocyte membrane: Evidence for an amphipathic molecular structure

Abstract

Treatment of the major glycoprotein of the human red cell membrane [erythrocyte glycophorin (1)] with cyanogen bromide yields five fragments. On the basis of N-terminal analyses and tryptic overlaps three of these (designated C-1, C-2, and C-5) have been aligned as unique portions of a single polypeptide chain. C-5 and C-1 represent the N-terminal fragments, in that order, and the third, C-2, is the C-terminal fragment of the original polypeptide chain. Evidence is presented for a limited polypeptide microheterogeneity of the N-terminal portion of the molecule. From the amino acid composition and carbohydrate content of the three fragments, C-5, C-1, and C-2, the molecule can be divided into three distinct regions or domains. These are (a) a receptor or carbohydrate containing N-terminal segment, (b) an internal hydrophobic domain of approximately 30 residues, and (c) a hydrophilic, proline-rich C-terminal portion. This unique molecular topography suggests an amphipathic model for the in situ orientation of this molecule in which the hydrophobic domain of the glycoprotein lies within the hydrophobic interior of the membrane. This model is supported by lactoperoxidase-catalyzed 125I-labeling experiments which suggest that the N-terminal (receptor) half of the molecule (fragment C-1) is exposed to the external environment of the cell, while the C-terminal segment (fragment C-2) extends into the interior of the cell.