Ultrastructure of a transmembrane glycoprotein, glycophorin A.

The major sialoglycoprotein of the human red cell membrane, glycophorin A, was isolated and examined by rotary shadowing and transmission electron microscopy. The glycophorin A molecule appeared as a cloud-like structure with a short, dense core within a large cloud. Mild acid hydrolysis in 0.05 M H2SO4, 80 degrees C for 1 hr reduced the size of the cloud significantly but left the dense core intact indicating that the original cloud represented the sialylated oligosaccharide chains of glycophorin A with the dense core being the polypeptide chain and its associated linkage proteins. Incubating glycophorin A with cationized ferritin (CF) revealed that the CF was bound only to the cloud, a finding that supports the view that the cloud is comprised of the sialylated oligosaccharide chains of the glycophorin A molecule. SDS-polyacrylamide gel electrophoresis revealed that our preparation of glycophorin A, as well as commercial preparations, consisted of monomers, dimers and oligomers of glycophorin A with trace amounts of the minor glycophorins and linkage proteins. Knowledge of the ultrastructure of this important integral protein will enable one to design studies to determine its functional role in the membrane.