Erythrocyte membrane protein band 3: its biosynthesis and incorporation into membranes.

Abstract

Band 3, a transmembrane protein that provides the anion channel of the erythrocyte plasma membrane, crosses the membrane more than once and has a large amino terminal segment exposed on the cytoplasmic side of the membrane. The biosynthesis of band 3 and the process of its incorporation into membranes were studied in vivo in erythroid spleen cells of anemic mice and in vitro in protein synthesizing cell-free systems programmed with polysomes and mRNA. In intact cells newly synthesized band 3 is rapidly incorporated into intracellular membranes where it is glycosylated and it is subsequently transferred to the plasma membrane where it becomes sensitive to digestion by exogenous chymotrypsin. The appearance of band 3 in the cell surface is not contingent upon its glycosylation because it proceeds efficiently in cells treated with tunicamycin. The site of synthesis of band 3 in bound polysomes was established directly by in vitro translation experiments with purified polysomes or with mRNA extracted from them. The band-3 polypeptide synthesized in an mRNA-dependent system had the same electrophoretic mobility as that synthesized in cells treated with tunicamycin. When microsomal membranes were present during translation, the in vitro synthesized band-3 polypeptide was cotranslationally glycosylated and inserted into the membranes. This was inferred from the facts that when synthesis was carried out in the presence of membranes, the product had a lower electrophoretic mobility and showed partial resistance to protease digestion. The primary translation product of band-3 mRNA apparently is not proteolytically processed either co- or posttranslationally. The incorporation of band 3 into the endoplasmic reticulum (ER) membrane may be initiated by a permanent insertion signal. To account for the cytoplasmic exposure of the amino terminus of the polypeptide, this signal may be located within the interior of the polypeptide. A mechanism that explains the final transmembrane disposition of band 3 in the plasma membrane as resulting from the mode of its incorporation into the ER is presented.