Southeast Asian ovalocytosis in White persons

Cell Physiology and Molecular Mechanism of Anion Transport by Erythrocyte Band 3/AE1

The major transmembrane protein of the red blood cell, known as band 3, AE1, and SLC4A1, has two main functions: 1) catalysis of Cl^-/HCO₃^- exchange, one of the steps in CO₂ excretion; 2) anchoring the membrane skeleton. This review summarizes the 150 year history of research on red cell anion transport and band 3 as an experimental system for studying membrane protein structure and ion transport mechanisms. Important early findings were that red cell Cl^- transport is a tightly coupled 1:1 exchange and band 3 is labeled by stilbenesulfonate derivatives that inhibit anion transport. Biochemical studies showed that the protein is dimeric or tetrameric (paired dimers) and that there is one stilbenedisulfonate binding site per subunit of the dimer. Transport kinetics and inhibitor characteristics supported the idea that the transporter acts by an alternating access mechanism with intrinsic asymmetry. The sequence of band 3 cDNA provided a framework for detailed study of protein topology and amino acid residues important for transport. The identification of genetic variants produced insights into the roles of band 3 in red cell abnormalities and distal renal tubular acidosis. The publication of the membrane domain crystal structure made it possible to propose concrete molecular models of transport. Future research directions include improving our understanding of the transport mechanism at the molecular level and of the integrative relationships among band 3, hemoglobin, carbonic anhydrase, and gradients (both transmembrane and subcellular) of HCO₃^-, Cl^-, O₂, CO₂, pH, and NO metabolites during pulmonary and systemic capillary gas exchange.

Southeast Asian ovalocytosis and a sickle cell trait in a young patient with sudden retinal stroke: a fortuitous association?

We report a case of retinal stroke in a patient from the Comoros Islands with both sickle cell trait and Southeast Asian ovalocytosis (SAO). Southeast Asian ovalocytosis is a dominantly inherited trait, frequent in Southeast Asia, caused by a 27 nucleotide deletion in the SLC4A1 gene that encodes band 3, leading to a decreased anion exchange but an increased cation leak across the erythrocyte membrane. We hypothesized that the red cell dehydration that can be induced by this cation leak can facilitate polymerization of Hb S [beta6(A3)Glu -->Val, GAG>GTG]. Southeast Asian ovalocytosis could then be a risk factor for rare microvascular complications in sickle cell trait.

Physiological roles and regulation of mammalian sulfate transporters

All cells require inorganic sulfate for normal function. Sulfate is among the most important macronutrients in cells and is the fourth most abundant anion in human plasma (300 microM). Sulfate is the major sulfur source in many organisms, and because it is a hydrophilic anion that cannot passively cross the lipid bilayer of cell membranes, all cells require a mechanism for sulfate influx and efflux to ensure an optimal supply of sulfate in the body. The class of proteins involved in moving sulfate into or out of cells is called sulfate transporters. To date, numerous sulfate transporters have been identified in tissues and cells from many origins. These include the renal sulfate transporters NaSi-1 and sat-1, the ubiquitously expressed diastrophic dysplasia sulfate transporter DTDST, the intestinal sulfate transporter DRA that is linked to congenital chloride diarrhea, and the erythrocyte anion exchanger AE1. These transporters have only been isolated in the last 10-15 years, and their physiological roles and contributions to body sulfate homeostasis are just now beginning to be determined. This review focuses on the structural and functional properties of mammalian sulfate transporters and highlights some of regulatory mechanisms that control their expression in vivo, under normal physiological and pathophysiological states.

Erythroid band 3 variants and disease

This review describes some of the naturally occurring band 3 (AEI) variants and their association with disease. Southeast Asian Ovalocytic (SAO) band 3, an inactive and misfolded protein, is probably only maintained in certain populations because it provides protection against the cerebral form of malaria. Many mutations that cause instability of band 3, either at the mRNA or protein level, result in hereditary spherocytosis (HS). Some polymorphisms alter amino acid residues in the extracellular loops of band 3 and are associated with blood group antigens. A truncated form of AEI is expressed in kidney cells and certain AEI mutations are associated with distal renal tubular acidosis (dRTA). The molecular basis of these variants and their effect on the structure and function of band 3 are discussed. The association between band 3 and glycophorin A (GPA) and the structure/function changes of band 3 in the absence of GPA are also described.