title: Biosynthesis of the major human red cell sialoglycoprotein, glycophorin A. A review.
abstract: The human leukemia cell line K562 is erythroid and expresses the major red cell sialoglycoprotein, glycophorin A. With this cell line we have studied the biosynthesis of glycophorin A after pulse-chase labeling with [35S] methionine. Using lectin-Sepharose affinity chromatography and immune precipitation with specific anti-glycophorin A antiserum followed by polyacrylamide slab gel electrophoresis a precursor of glycophorin A was visualized. This had an apparent molecular weight of 37000 and contained an incompleted N-glycosidic oligosaccharide and unfinished O-glycosidic oligosaccharides. After chase for 10 min, the completed glycophorin A with an apparent molecular weight of 39000 was seen and it appeared at the cell surface in about 30 min. Using tunicamycin N-glycosylation was inhibited but not O-glycosylation. The absence of the N-glycosidic oligosaccharide did not affect the migration of the protein to the cell surface but the yield of glycophorin A was diminished. Translation of glycophorin A messenger-RNA was achieved in a rabbit reticulocyte cell-free system. This yielded a non-glycosylated protein with an apparent molecular weight of 19500, which exceeded that of the glycophorin A apoprotein with about 5000. This indicates the presence of a "signal sequence" in the preprotein. When the translation was performed in the presence of microsomal membranes from dog pancreas the glycophorin A apoprotein aws both N-and O-glycosylated and the apparent molecular weight (37000) of the synthesized protein was identical to that of the precursor obtained from cells.
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D004285:Dogs; D006021:Glycophorins / Q000096:biosynthesis*; D006801:Humans; D007425:Intracellular Membranes / Q000378:metabolism; D008861:Microsomes / Q000032:analysis / Q000648:ultrastructure; D010179:Pancreas / Q000648:ultrastructure; D011498:Protein Precursors / Q000032:analysis / Q000302:isolation & purification / Q000378:metabolism; D012795:Sialoglycoproteins / Q000096:biosynthesis*; D014415:Tunicamycin / Q000494:pharmacology
chemical_list: D006021:Glycophorins; D011498:Protein Precursors; D012795:Sialoglycoproteins; D014415:Tunicamycin