Alternative primary structures in the transmembrane domain of the chicken erythroid anion transporter

An isoform-specific mutation in the protein 4.1 gene results in hereditary elliptocytosis and complete deficiency of protein 4.1 in erythrocytes but not in nonerythroid cells

Multiple protein 4.1 isoforms are expressed in a variety of tissues through complex alternative pre-mRNA splicing events, one function of which is to regulate use of two alternative translation initiation signals. Late erythroid cells express mainly the downstream initiation site for synthesis of prototypical 80-kD isoforms; nonerythroid cells in addition use an upstream site to encode higher molecular mass isoform(s). In this study, we examined the effects of a 5' gene rearrangement in a family with hereditary elliptocytosis and complete deficiency of erythrocyte 4.1 protein on 4.1 isoform expression in erythroid vs. nonerythroid cells. Patient 4.1 mRNAs from reticulocytes, fibroblasts, and B lymphocytes were amplified by reverse transcriptase/polymerase chain reaction techniques and shown to exhibit a 318-nucleotide deletion that encompasses the downstream AUG, but leaves intact the upstream AUG. Immunoblot analysis revealed a total deficiency of 4.1 in patient red cells and a selective deficiency of 80-kD isoform(s) but not high molecular weight 4.1 in patient nonerythroid cells. Thus, the 4.1 gene mutation in this family produces an isoform-specific deficiency that is manifested clinically in tissue-specific fashion, such that red cells are affected but other cell types are unaffected because of tissue-specific differences in RNA splicing and translation initiation.

Amino acid sequence of band-3 protein from rainbow trout erythrocytes derived from cDNA

In this report we present the first complete band-3 cDNA sequence of a poikilothermic lower vertebrate. The primary structure of the anion-exchange protein band 3 (AE1) from rainbow trout erythrocytes was determined by nucleotide sequencing of cDNA clones. The overlapping clones have a total length of 3827 bp with a 5'-terminal untranslated region of 150 bp, a 2754 bp open reading frame and a 3'-untranslated region of 924 bp. Band-3 protein from trout erythrocytes consists of 918 amino acid residues with a calculated molecular mass of 101 827 Da. Comparison of its amino acid sequence revealed a 60-65% identity within the transmembrane spanning sequence of band-3 proteins published so far. An additional insertion of 24 amino acid residues within the membrane-associated domain of trout band-3 protein was identified, which until now was thought to be a general feature only of mammalian band-3-related proteins.

Deletion in erythrocyte band 3 gene in malaria-resistant Southeast Asian ovalocytosis

Southeast Asian ovalocytosis (SAO) is a hereditary condition that is widespread in parts of Southeast Asia. The ovalocytic erythrocytes are rigid and resistant to invasion by various malarial parasites. We have previously found that the underlying defect in SAO involves band 3 protein, the major transmembrane protein, which has abnormal structure and function. We now report two linked mutations in the erythrocyte band 3 gene in SAO: (i) a deletion of codons 400-408 and (ii) a substitution, A----G, in the first base of codon 56 leading to substitution of Lys-56 by Glu-56. The first defect leads to a deletion of nine amino acids in the boundary of cytoplasmic and membrane domains of band 3. This defect has been detected in all 30 ovalocytic subjects from Malaysia, the Philippines, and two unrelated coastal regions of Papua New Guinea, whereas it was absent in all 30 controls from Southeast Asia and 20 subjects of different ethnic origin from the United States. The Lys-56----Glu substitution has likewise been found in all SAO subjects. However, it has also been detected in 5 of the 50 control subjects, suggesting that it represents a linked polymorphism. We conclude that the deletion of codons 400-408 in the band 3 gene constitutes the underlying molecular defect in SAO.

Cloning of cDNAs encoding a rabbit renal brush border membrane protein immunologically related to band 3. Sequence similarity with microsomal dipeptidase

Distinct anion transport processes have been identified in the mammalian renal proximal tubule, but none of the responsible proteins or genes have been isolated. A 43 kDa rabbit microvillus membrane protein that is immunologically related to the erythroid anion exchanger (band 3) was a candidate for a renal anion transporter. To examine the structural relationship with band 3, we cloned cDNAs encoding the 43 kDa protein. The 43 kDa band-3-like protein was purified, and a novel sequence of 24 amino acids was obtained from the N-terminus. Degenerate oligonucleotides were synthesized based on this sequence, and the polymerase chain reaction with single-sided specificity was used to amplify and clone a 1330 bp cDNA from rabbit renal cortex. Additional overlapping 272 bp and 1123 bp cDNAs were obtained by synthesizing and screening a rabbit renal cortical cDNA library. The composite sequence was 1483 bp, terminated with (A)16, and was similar in size to the principal transcript expressed in rabbit renal cortex. The single long open reading frame was predicted to encode a protein composed of 410 amino acids with a molecular mass of 45,193 Da; 15 amino acids predicted to reside at the N-terminus were absent in the mature protein and may constitute a signal peptide. There was only limited sequence similarity with human erythroid band 3. Rather, the sequence was highly similar to microsomal dipeptidase, including the presence of a signal peptide and a consensus sequence for covalent linkage to glycosylphosphatidylinositol. In summary, the 43 kDa protein from rabbit renal cortex that is recognized by a monospecific antibody to erythroid band 3 is most likely a microvillus membrane dipeptidase.

Band 3 protein degradation by calpain is enhanced in erythrocytes of old people

Band 3 protein is a major erythrocyte transmembrane glycoprotein. We compared the degradation of band 3 protein by calpain I (a cytoplasmic, micromolar-Ca2(+)-requiring thiol proteinase) in the cells from old individuals (greater than 70 years old) to that in the cells from young ones (20-30 years old). In the young, little degradation of band 3 protein occurred in calpain-treated erythrocyte ghosts. In the old, significant band 3 protein degradation was found in erythrocyte ghosts treated similarly. The difference between young and old in the susceptibility of band 3 protein to calpain was retained in membrane vesicles (membranes stripped of peripheral proteins by NaOH) and in chymotrypsin-generated 60 kDa fragment (CH-60). The isolated N-terminal cytoplasmic 43 kDa fragment was degraded by calpain to a similar extent in old and in young. The separated 17 kDa membrane domain of the CH-60 and the trypsin-generated C-terminal 55 kDa membrane-spanning fragment were not degraded by calpain I in the young, nor in the old. Thus the N-terminal cytoplasmic domain is the domain degraded by calpain I. Enhanced sensitivity in the old is observed in intact band 3 protein and in CH-60, the isolated cytoplasmic domain being equally susceptible in young and old. The observed age-related enhanced sensitivity to calpain is consistent with the presence of modifications in band 3 protein and alterations in the association with the calpain-calpastatin system. Band 3 protein has several important functions, with modifications in the protein having implications for altered cell behaviour in the old individual.

A 45-kDa protein antigenically related to band 3 is selectively expressed in kidney mitochondria

Anion exchange similar to that catalyzed by erythrocyte band 3 occurs across many nonerythroid cell membranes. To identify anion-exchange proteins structurally related to band 3, we immunoblotted rabbit kidney medullary membrane fractions with anti-band 3 antibodies. Immunoblots using antibodies to the cytoplasmic domain of band 3 revealed cross-reactive proteins in the plasma membrane fraction only. In contrast, two monoclonal antibodies against band 3 membrane domain labeled a 45-kDa protein; further immunoblotting and immunogold studies of membrane fractions and kidney sections using one of the anti-membrane domain antibodies showed that labeling was strongest in mitochondria of H(+)-secreting collecting duct cells. Tissue-to-tissue expression of the 45-kDa mitochondrial protein was variable: kidney medulla greater than heart greater than kidney cortex much greater than liver. We conclude that a 45-kDa protein with immunological cross-reactivity to the erythrocyte band 3 membrane domain is expressed in mitochondria in a highly cell-specific fashion and speculate that the protein may play a role in mitochondrial anion transport.

Nucleotide sequence of the fruA gene, encoding the fructose permease of the Rhodobacter capsulatus phosphotransferase system, and analyses of the deduced protein sequence

The nucleotide sequence of the fruA gene, the terminal gene in the fructose operon of Rhodobacter capsulatus, is reported. This gene codes for the fructose permease (molecular weight, 58,575; 578 aminoacyl residues), the fructose enzyme II (IIFru) of the phosphoenolpyruvate-dependent phosphotransferase system. The deduced aminoacyl sequence of the encoded gene product was found to be 55% identical throughout most of its length with the fructose enzyme II of Escherichia coli, with some regions strongly conserved and others weakly conserved. Sequence comparisons revealed that the first 100 aminoacyl residues of both enzymes II were homologous to the second 100 residues, suggesting that an intragenic duplication of about 300 nucleotides had occurred during the evolution of IIFru prior to divergence of the E. coli and R. capsulatus genes. The protein contains only two cysteyl residues, and only one of these residues is conserved between the two proteins. This residue is therefore presumed to provide the active-site thiol group which may serve as the phosphorylation site. IIFru was found to exhibit regions of homology with sequenced enzymes II from other bacteria, including those specific for sucrose, beta-glucosides, mannitol, glucose, N-acetylglucosamine, and lactose. The degree of evolutionary divergence differed for different parts of the proteins, with certain transmembrane segments exhibiting high degrees of conservation. The hydrophobic domain of IIFru was also found to be similar to several uniport and antiport transporters of animals, including the human and mouse insulin-responsive glucose facilitators. These observations suggest that the mechanism of transmembrane transport may be similar for permeases catalyzing group translocation and facilitated diffusion.

The membrane domain of the human erythrocyte anion transport protein. Epitope mapping of a monoclonal antibody defines the location of a cytoplasmic loop near the C-terminus of the protein

We have used synthetic peptides to study the location of the amino acid sequences in the human erythrocyte anion transport protein (band 3) which are recognized by four murine monoclonal antibodies, BRIC 130, 132, 154 and 155. These antibodies are known to react with epitopes in the protein which are on the cytoplasmic side of the membrane. The results suggest that the amino acid residues important for the reaction of BRIC 130 and BRIC 154/155 are located within amino acids 899-908 and 895-901 respectively in the cytoplasmic tail of the protein. The BRIC 132 epitope is located within amino acid residues 813-824. This is part of a surface loop in the protein which probably extends from residue 814 to residue 832 and is located on the cytoplasmic side of the membrane. These results provide direct evidence for the topographical location of a sequence in a poorly understood region of the protein.

Cloning and characterization of band 3, the human erythrocyte anion-exchange protein (AE1)

The human erythrocyte anion-exchange protein (band 3 or AE1) was cloned from a fetal liver cDNA library. Three overlapping clones, encompassing 3637 nucleotides, were analyzed in detail. These encode a 911-amino acid protein (Mr 101,791) and detect a single 4.7-kilobase species in human reticulocyte RNA. The corresponding gene is located on chromosome 17. The protein is similar in structure to other anion exchangers and is divided into three regions: a hydrophilic, cytoplasmic domain that interacts with a variety of membrane and cytoplasmic proteins (residues 1-403); a hydrophobic, transmembrane domain that forms the anion antiporter (residues 404-882); and an acidic, C-terminal domain of unknown function (residues 883-911). The N-terminal domain contains several conserved sections (e.g., residues 57-86, 102-164, 219-347, and 375-403), some of which may contribute to binding sites for ankyrin, protein 4.1, or protein 4.2. The membrane domain is highly conserved with the exception of a single segment (residues 543-567) that contains several sites for cleavage of the protein by extracellular proteases. Based on hydropathy analyses and the wealth of available topographical and functional data, a model is proposed in which the protein crosses the membrane 14 times.

Monoclonal antibodies to the membrane domain of the human erythrocyte anion transport protein. Localization of the C-terminus of the protein to the cytoplasmic side of the red cell membrane and distribution of the protein in some human tissues

(1) We have prepared murine monoclonal antibodies to the membrane domain of the human erythrocyte anion transport protein (band 3). (2) All of these antibodies react with regions of the protein located at the cytoplasmic surface of the red cell. (3) One of the antibodies reacts with an epitope present on a cytoplasmic loop of the protein located between the C-terminus and a point 168 amino acids from the C-terminus. The other antibodies recognize different epitopes on the C-terminal tail of the protein and the sequences likely to be involved in these epitopes are defined. (4) Our results show that the C-terminus of the red-cell anion transport protein is located on the cytoplasmic side of the red-cell membrane. (5) None of the antibodies inhibited sulphate exchange transport when introduced into resealed red-cell membranes; however, the bivalent form of one of the antibodies reduced the inhibitory potency of 4-acetamido-4'-isothiocyanatostilbene disulphonate on sulphate exchange transport in resealed erythrocyte membranes. (6) Immunostaining of human kidney sections with the antibodies showed strong staining of the basolateral membrane of some but not all of the epithelial cells of distal tubules and the initial connecting segment of collecting tubules. With human liver, only the haematopoeitic cells of fetal liver reacted with all the antibodies.

The scanning model for translation: an update

The small (40S) subunit of eukaryotic ribosomes is believed to bind initially at the capped 5'-end of messenger RNA and then migrate, stopping at the first AUG codon in a favorable context for initiating translation. The first-AUG rule is not absolute, but there are rules for breaking the rule. Some anomalous observations that seemed to contradict the scanning mechanism now appear to be artifacts. A few genuine anomalies remain unexplained.

Two different mRNAs are transcribed from a single genomic locus encoding the chicken erythrocyte anion transport proteins (band 3)

The chicken erythrocyte anion transport protein (band 3 of the erythrocyte cytoskeleton) is a central component taking part in two widely divergent functions of erythroid cells; it is a primary determinant of cytoskeletal architecture and responsible for electroneutral Cl-/HCO3- exchange across the plasma membrane. To analyze interesting aspects of the developmental regulation of this gene, we have cloned the cDNA and genomic counterparts of the erythroid-specific anion transport protein. We show that a single genetic locus for band 3 encodes two different erythroid cell-specific mRNAs, with different translational initiation sites, which predict polypeptides of sizes very close to those observed in vivo. In vitro translation and immune precipitation of synthetic mRNA derived from one putative fully encoding cDNA clone demonstrate that this clone gives rise to a protein which is identical in size and antigenicity to bona fide chicken erythroid band 3.

The complete amino acid sequence of the human erythrocyte membrane anion-transport protein deduced from the cDNA sequence

1. We have isolated cDNA clones corresponding to the red cell membrane anion-transport protein (Band 3). 2. The cDNA clones cover 3475 bases of the mRNA and contain the entire protein-coding region, 150 bases of the 5' untranslated region and part of the 3' non-coding region, but do not extend to the 3' end of the mRNA. 3. The translated protein sequence predicts that the human red cell anion transporter contains 911 amino acids. 4. The availability of the amino acid sequence allows the interpretation of some of the many studies on the chemical and proteolytic modification of the human protein aimed at examining the structure and mechanism of this membrane transport protein.

Two different mRNAs are transcribed from a single genomic locus encoding the chicken erythrocyte anion transport proteins (band 3)

The chicken erythrocyte anion transport protein (band 3 of the erythrocyte cytoskeleton) is a central component taking part in two widely divergent functions of erythroid cells; it is a primary determinant of cytoskeletal architecture and responsible for electroneutral Cl-/HCO3- exchange across the plasma membrane. To analyze interesting aspects of the developmental regulation of this gene, we have cloned the cDNA and genomic counterparts of the erythroid-specific anion transport protein. We show that a single genetic locus for band 3 encodes two different erythroid cell-specific mRNAs, with different translational initiation sites, which predict polypeptides of sizes very close to those observed in vivo. In vitro translation and immune precipitation of synthetic mRNA derived from one putative fully encoding cDNA clone demonstrate that this clone gives rise to a protein which is identical in size and antigenicity to bona fide chicken erythroid band 3.