Two glycoprotein populations of band 3 dimers are present in human erythrocytes

The human erythrocyte Band 3 anion exchanger contains a single site of N-glycosylation that contains either a short complex oligosaccharide or an extended polylactosaminyl oligosaccharide. Approximately equal amounts of the different glycosylated forms of Band 3 are found in human red cells. As Band 3 exists predominantly as dimers, they may be uniform and consist of a subunit containing an extended oligosaccharide paired with a subunit containing a short oligosaccharide chain. Alternatively, Band 3 dimers may be comprised of subunits that either contain polylactosaminyl or short oligosaccharide chains. To distinguish between these two extremes, the ability of Band 3 membrane domain dimers to bind to immobilized tomato lectin, which specifically binds polylactosaminyl oligosaccharide, was tested. The dimeric membrane domain of Band 3 could be resolved into two fractions by tomato lectin chromatography. This shows that Band 3 dimers are not homogeneous and that two pools exist in red cells, some with a long polylactosaminyl oligosaccharide and the other with a short complex type. The amount of short chain form recovered in the unbound fraction was higher than expected for a random distribution of oligosaccharide chains on Band 3 dimers. Detergent extraction experiments showed that Band 3 glycoforms did not display a differential interaction with the cytoskeleton. The ability to separate Band 3 dimers into two glycoform populations suggests that subunit exchange between dimers does not occur in the membrane or in detergent solution. Furthermore, the results show that while one population of Band 3 dimers is processed to contain polylactosaminyl oligosaccharide, the other largely escapes this processing step.