Intermembrane protein transfer. Band 3, the erythrocyte anion transporter, transfers in native orientation from human red blood cells into the bilayer of phospholipid vesicles.

Band 3, the erythrocyte anion transporter, has been shown to transfer between human erythrocytes and sonicated vesicles (Newton, A. C., Cook, S. L., and Huestis, W. H. (1983) Biochemistry 22, 6110-6117). Functional band 3 becomes associated with dimyristoylphosphatidylcholine vesicles incubated with human red blood cells. Proteolytic degradation patterns reveal that the transporter is transferred to the vesicles in native orientation. In erythrocytes, native band 3 is degraded on the exoplasmic membrane face by chymotrypsin and on the cytoplasmic surface by trypsin (Cabantchik, Z. I., and Rothstein, A. (1974) J. Membr. Biol. 15, 227-248; Jennings, M. L., Anderson, M. P., and Monaghan, R. (1986) J. Biol. Chem. 261, 9002-9010). Band 3 in intact protein-vesicle complexes is degraded by exogenous chymotrypsin but not by trypsin. In contrast, trypsin entrapped in the lumen of the vesicles proteolyses the vesicle-bound band 3 quantitatively. Band 3 remaining in the membranes of vesicle-treated cells and in cell fragments is not degraded detectably by vesicle-entrapped trypsin. These observations indicate that band 3 is unlikely to transfer between cell and vesicle membranes via a water-soluble form or to adhere nonspecifically to the vesicle surface; the aqueous contents of vesicles and cells (or membrane fragments) are not pooled during cell-vesicle incubations, hence no cell-vesicle fusion occurs; and the band 3 associated with the sonicated vesicle fraction is inserted in the vesicle bilayer in native orientation, with its cytoplasmic segment contacting the aqueous contents of the vesicle lumen.