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-gl ycosylation that contains either it short complex oligosaccharide or an ext ended polylactosaminyl oligosaccharide. Approximately equal amounts of the different glycosylated forms of Band 3 are found in human red cells, As Ban d 3 exists predominantly as dimers, they may be uniform and consist of a su bunit containing an extended oligosaccharide paired with a subunit containi ng a short oligosaccharide chain. Alternatively, Band 3 dimers may be compr ised of subunits that either contain polylactosaminyl or short oligosacchar ide chains. To distinguish between these two extremes, the ability of Band 3 membrane domain dimers to bind to immobilized tomato lectin, which specif ically binds polylactosaminyl oligosaccharide, was tested. The dimeric memb rane domain of Band 3 could be reserved 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 oligosacch aride and the other with a short complex type. The amount of short chain fo rm recovered in the unbound fraction was higher than expected for a random distribution of oligosaccharide chains on Band 3 dimers. Detergent extracti on experiments showed that Band 3 glycoforms did not display a differential interaction with the cytoskeleton. The ability to separate Band 3 dimers i nto 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 con tain polylactosaminyl oligosaccharide, the other largely escapes this proce ssing step.