Exploring the Potential Roles of Band 3 and Aquaporin-1 in Blood CO2 Transport-Inspired by Comparative Studies of Glycophorin B-A-B Hybrid Protein GP.Mur

Abstract

The Cl^-/HCO₃^- exchanger band 3 is functionally relevant to blood CO₂ transport. Band 3 is the most abundant membrane protein in human red blood cells (RBCs). Our understanding of its physiological functions mainly came from clinical cases associated with band 3 mutations. Severe reduction in band 3 expression affects blood HCO₃^-/CO₂ metabolism. What could happen physiologically if band 3 expression is elevated instead? In some areas of Southeast Asia, about 1-10% of the populations express GP.Mur, a glycophorin B-A-B hybrid membrane protein important in the field of transfusion medicine. GP.Mur functions to promote band 3 expression, and GP.Mur red cells can be deemed as a naturally occurred model for higher band 3 expression. This review first compares the functional consequences of band 3 at different levels, and suggests a critical role of band 3 in postnatal CO₂ respiration. The second part of the review explores the transport of water, which is the other substrate for intra-erythrocytic CO₂/HCO₃^- conversion (an essential step in blood CO₂ transport). Despite that water is considered unlimited physiologically, it is unclear whether water channel aquaporin-1 (AQP1) abundantly expressed in RBCs is functionally involved in CO₂ transport. Research in this area is complicated by the fact that the H₂O/CO₂-transporting function of AQP1 is replaceable by other erythrocyte channels/transporters (e.g., UT-B/GLUT1 for H₂O; RhAG for CO₂). Recently, using carbonic anhydrase II (CAII)-filled erythrocyte vesicles, AQP1 has been demonstrated to transport water for the CAII-mediated reaction, CO_2(g) + H₂O ⇌ HCO₃^-_(aq) + H⁺_(aq). AQP1 is structurally associated with some population of band 3 complexes on the erythrocyte membrane in an osmotically responsive fashion. The current findings reveal transient interaction among components within the band 3-central, CO₂-transport metabolon (AQP1, band 3, CAII and deoxygenated hemoglobin). Their dynamic interaction is envisioned to facilitate blood CO₂ respiration, in the presence of constantly changing osmotic and hemodynamic stresses during circulation.

Keywords: CO2 transport; GP.Mur; aquaporin-1; band 3; erythrocytes; glycophorin; metabolon.

FIGURE 1

Glycophorin B-A-B hybrid protein GP.Mur interacts with band 3. (Top) Protein sequence alignment of GPA, GPB, and GP.Mur, which was first published in Blood (Hsu et al., 2009), is modified here with color coding. The sequences of GPA and GPB are color-coded with blue and pink, respectively. The antigenic Mur peptide at the cross-over region in GP.Mur is color-coded purple. The configuration of glycophorin B-A-B is revealed by the combination of pink-(purple)-blue-pink colors. (Bottom) GPA, GPB, and GP.Mur are homologous membrane proteins, each with a single transmembrane span. Their N-terminal sequences are located extracellular, with heavy glycosylation (not shown); their C-terminal sequences are intracellular. Both GPA and GP.Mur interact with band 3 (gray cylinder), and GPB does not.

FIGURE 2

The expression levels of band 3 are directly correlated with the efficiencies of blood CO₂/HCO₃^— exchange. (Top) Intraerythrocytic CO₂/HCO₃^— exchange in tissues or in lung alveoli is summarized in the reversible chemical reaction catalyzed by CAII [CO_2(g) + H₂O ⇌ HCO₃^—_(aq) + H⁺_(aq)]. When RBCs circulate to systemic capillaries, tissue metabolite CO₂ enters RBCs to be converted into HCO₃^— by intraerythrocytic CAII. HCO₃^—, one of the two products from this forward reaction, exits from RBCs through band 3 (shown by the blue arrow which indicates the direction of the reaction in tissues). The direction of the counterion Cl^— flux through band 3 is indicated by a black arrow. Proton, the other product generated by the forward reaction, is absorbed by deoxy Hb^— that is transiently associated with band 3 (shown by a dotted line connecting Hb^— and H⁺). When RBCs circulate to the lungs, HCO₃^— rushes into RBCs to be converted into CO₂ for expiration (shown by the pink arrow which indicates the reverse direction of the reaction). Band 3 is symbolized as the green dimeric gate. (Bottom) The spectrum of band 3 expression levels in RBCs is outlined in the shaded green bar; their differential impacts on blood CO₂ transport are illustrated in the four cartoon diagrams below. The four bottom diagrams from left to right: (1) Absence of band 3 reduces much HCO₃^— transport across the RBC membrane, rendering CO₂/HCO₃^— exchange to be very inefficient. (2) Low band 3 expression can be found in some band 3/GPA mutations that result in semi-dysfunctional band 3 transporters, as described in the section of “Low or no band 3 expression” (shown here by the brown/green gate symbols for “structurally abnormal band 3”). (3) Normal band 3 expression. (4) Higher band 3 expression in GP.Mur RBCs (shown by more green gates on the cell membrane) increases the efficiency as well as the capacity of intraerythrocytic CO₂/HCO₃^— conversion and HCO₃^— transport across the RBC membrane. The different numbers of CO₂/HCO₃^— in the bottom four diagrams represent schematically the different magnitudes of CO₂/HCO₃^— fluxes (metabolic flows) that correspond to the different levels of band 3 expression on the red cell membrane; they do not reflect extracellular concentrations of CO₂ and HCO₃^—.

FIGURE 3

A revised model for the “CO₂-transport metabolon.” As illustrated in the cartoon diagram, the CO₂-transport metabolon can be deemed as a transiently extended CAII machinery, which is built upon transient interactions among the four proteins: AQP1, erythroid AE1, CAII, and deoxy Hb^— (shown by red dotted lines). The C-terminal and N-terminal regions of AE1 are indicated by (C) and (N). Illustrated here in gray arrow lines is the direction of the conversion reaction in systemic capillaries: CO₂ is converted into HCO₃^—, followed by export of bicarbonate and absorption of proton by Hb^—.