Monomeric erythrocyte band 3 protein transports anions.

Abstract

The anion transport system of the human erythrocyte membrane was reconstituted in egg phosphatidylcholine membranes by using either the unmodified transport protein, band 3, or covalently crosslinked band 3 dimers. Unilamellar vesicles of a diameter of 32 ± 3 nm were then isolated from the sample by passage through a French press and subsequent gel filtration. According to sedimentation equilibrium measurements, around 85% of the vesicles were devoid of protein. The remaining 15% contained either a single band 3 monomer or, when crosslinked band 3 protein was used, a single band 3 dimer. Vesicles containing either single monomers or single dimers showed a rapid, inhibitor-sensitive sulfate efflux, and the turnover numbers of band 3 for the inhibitor-sensitive flux component were identical in both systems. This shows that monomeric band 3 protein is able to transport anions and that dimerization of the protein does not change its transport activity.involved either chemical modification of the band 3 protein (10) or treatment with a denaturant (11) and therefore may be misleading.We have tried to resolve the question of whether or not monomeric band 3 protein is able to transport anions by a conceptionally simple experiment: (i) we reconstituted the anion-transport system in lipid vesicles, starting either with unmodified band 3 protein or with covalently crosslinked band 3 dimers; (ii) the vesicles were transformed into smaller ones by French press treatment; (iii) from these samples, vesicles of 32-nm diameter were isolated that, according to an unusual application of ultracentrifugal analysis, contained at most either a single monomer or a single covalent dimer of band 3; and (iv) we compared the sulfate efflux from the two types of vesicles. It was found that both showed a stilbenedisulfonate-sensitive sulfate effiux with virtually identical turnover numbers. Band 3, the most abundant protein of the erythrocyte mem-

The NH2-terminal Half of the Tn10-specified Tetracycline Efflux Protein TetA Contains a Dimerization Domain

“…Some other membrane transport proteins of the same superfamily (24) as TetA are known to occur as multimers, including the facilitated glucose transporter GLUT1 (17,25), the erythrocyte anion exchanger Band 3 (26), and the Na ϩ /glucose cotransporter (27). The relationship between these multimerizations and function is uncertain (25,28), and at least one example exists (the lactose permease, LacY) in which the transporter almost certainly functions as a monomer (29). Our results strengthen the concept that the mechanism of action of TetA involves a multimeric state.…”

Crystal structure of the anion exchanger domain of human erythrocyte band 3

“…UraA is also made of two 7 TM repeat units forming two domains. As shown in Figure 2 AE1CTD is a physiological dimer (37) but dimerization is not necessary for transport (38,39). The dimer in the crystal consists of two AE1CTD monomers with 1092 A 2 of surface area buried at the interface (32).…”

Structural Model of the Anion Exchanger 1 (SLC4A1) and Identification of Transmembrane Segments Forming the Transport Site

“…At the molecular level, this protein is divided into two main structural domains: a cytoplasmic N-terminal domain (about 400 amino acids) and a membrane-spanning domain (about 450 amino acids) with a short C-terminal tail in the cytoplasm. These two entities seem to function independently: the large cytoplasmic domain is involved in interactions with enzymes, hemoglobin, and structural proteins, whereas the membrane-spanning domain is responsible for the transport activity of the protein (2,3). In addition to red cells, AE1 is also expressed in kidney ␣-intercalated cells and cardiac myocytes (4).…”