Monosaccharide transport in protein-depleted vesicles from erythrocyte membranes

Treatment of human erythrocyte membranes with dilute alkali (pH 11.5) generates sealed, protein-depleted vesicles that can be isolated by density gradient centrifugation. The vesicles are 0.5 to 2.0 micrometers in diameter, and their membranes are predominantly oriented inside-out. The vesicles lack protein bands 1, 2, 5, and 6 (nomenclature of Steck, T.L. (1974) J. Cell Biol. 62, 1-19) of the erythrocyte membrane. L-Sorbose, a substrate of the monosaccharide transport system in erythrocytes, is transported by the vesicles. Based on comparisons between erythrocytes and vesicles with regard to specificity, temparture dependence, and effects of inhibitors, we conclude that sorbose uptake into the vesicles occurs by way of the monosaccharide transport system. The specific activity of the transport system in vesicles, as determined by initial rate measurements of sorbose uptake, averaged 58% of that in erythrocytes. This finding indicates that the major polypeptides of Bands 1, 2, 5, and 6 do not play an obligatory role in monosaccharide transport.