Identification of an internal topogenic signal sequence in human Band 3, the erythrocyte anion exchanger

The insertion of Band 3, the human erythrocyte anion exchanger, into microsomal membranes was studied in an in vitro reticulocyte lysate translation system. Band 3 consists of a 43-kDa amino-terminal cytosolic domain and a carboxyl-terminal 52-kDa membrane domain containing up to 14 transmembrane segments with a single N-glycosylation site at Asn-642. Insertion of truncated Band 3 molecules into microsomal membranes was assayed by glycosylation, resistance to alkaline extraction, and tryptic removal of the cytosolic domain. Truncations containing either the first four or the last eight putative transmembrane segments were stably integrated into microsomes showing that an intact membrane domain was not required for membrane integration. Furthermore, the extracytosolic domain following the seventh transmembrane segment was properly translocated across the microsomal membrane and glycosylated whether the seventh transmembrane segment was the first, last, or the only transmembrane segment in the construct. The ability of the entire membrane domain, the truncated domain beginning with the seventh transmembrane segment, or the seventh transmembrane segment to insert into microsomes was dependent on the presence of the signal recognition particle receptor. The seventh transmembrane segment in Band 3 therefore has the topogenic properties of an internal signal sequence.