Structural and functional consequences of an N-glycosylation mutation (HEMPAS) affecting human erythrocyte membrane glycoproteins

Band 3, the human erythrocyte anion exchanger (AE1), and the glucose transporter (GLUT1) proteins each contain a single site of N-glycosylation that is heterogeneously glycosylated. Lectin binding and enzymatic deglycosylation assays showed that the polylactosaminyl oligosaccharide structure of these glycoproteins was altered to a high mannose or hybrid glycan form in three patients with hereditary erythroblastic multinuclearity, with a positive acidified-serum lysis test (HEMPAS). Offspring from one of the HEMPAS patients had intermediate levels of polylactosaminyl oligosaccharide associated with AE1 and GLUT1, suggesting they may have been heterozygous for the genetic defect. The array of polylactosaminyl-containing glycoproteins present in EBV-transformed lymphoblasts derived from fresh blood of HEMPAS patients was similar to control lymphoblasts. HEMPAS lymphoblasts do not therefore express the defect in polylactosamine synthesis found in erythroid cells, indicating that lymphoid cells are not deficient in the processing enzymes or contain an alternative oligosaccharide processing pathway. Purified HEMPAS band 3 had an unaltered oligomeric structure but dimers aggregated more rapidly in detergent solution than normal band 3. The altered oligosaccharide structure did not affect the sensitivity of band 3 to proteolytic digestion in intact red cells but a greater amount of HEMPAS band 3 was associated with the cytoskeleton. The transport activities of AE1 and GLUT1 in HEMPAS erythrocytes were similar to those in normal controls. This shows that the HEMPAS glycosylation defect does not impair the functional accumulation of these two important erythrocyte membrane transporters even though it produces subtle structural changes in band 3 that result in its increased cytoskeletal interaction and self association in detergent solution.Key words: anion exchanger, band 3, erythrocyte, glycoprotein, HEMPAS.