Major O-glycosylated sialoglycoproteins of human hematopoietic cells: Differentiation antigens with poorly understood functions

All human hematopoietic cells seem to contain a major, heavily O‐glycosylated sialoglycoprotein. Glycophorin A is specific for the erythroid lineage of cells, and leukocytes have a major sialoglycoprotein, also called leukosialin or sialophorin. Cell differentiation results in patterns of O‐glycosylation in these proteins, which reflect the stage of differentiation within a cell lineage as well as lineage specificity. The altered carbohydrate compositions may influence the interactions of the cells with external ligands. Healthy individuals lacking glycophorin A in their red cells are known, whereas a deficiency of the leukocyte sialoglycoprotein may result in immunological disease. Although little is known about the physiological functions of these proteins, they form interesting models for studies on regulation of glyco‐sylation, biosynthesis of O‐glycosylated glyoproteins, and function of cell surface receptors.