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.