Purification and characterization of transporter proteins from human erythrocyte membrane.

1. Introduction Functions and biochemical properties of several membrane transporter proteins from human erythrocyte, in particular, the glucose transporter (Glut1) and anion exchanger (AE1, also called Band 3) have been extensively characterized. Glut1 is a member of the mammalian facilitative glucose transporter family Glut1-13 (1,2). The 50-kDa integral membrane protein is expressed in all human cells, and is particularly abundant in human erythrocytes, fibroblasts and endothelial cells. In erythrocytes it is responsible for the uptake of glucose—the cell's major energy source. Glut1 consists of 492 amino acids and is N-glycosy-lated on residue Asn45 in a highly heterogeneous manner. The protein, which functions as a monomer, is predicted to span the membrane 12 times in the form of ␣-helices (1). A member of the anion exchanger family, AE1 is a 95-kDa integral membrane protein with multiple functions (3). It is responsible for the electroneutral exchange of HCO 3 − and Cl − across the erythrocyte membrane, thereby facilitating CO 2 transport by the blood. The AE1 protein consists of two structurally distinct domains, an amino-terminal cytosolic domain (residues 1–360) and a carboxyl-terminal membrane domain (residues 361–911) (4). The cytosolic domain is approx 43 kDa, and provides binding sites for hemoglobin and cytoskeletal proteins. On the other hand, the 52-kDa membrane domain (AE1MD), which is glycosylated, is solely responsible for the protein's ion transport activity, and retains functionality following enzymatic removal of the cytosolic domain. Easy access to human blood and their abundance in the erythrocyte mem