Biochemical analysis of potential sites for protein 4.1-mediated anchoring of the spectrin-actin skeleton to the erythrocyte membrane

A Review on Adducin from Functional to Pathological Mechanisms: Future Direction in Cancer

Adducin (ADD) is a family of membrane skeleton proteins including ADD1, ADD2, and ADD3 that are encoded by distinct genes on different chromosomes. Adducin is primarily responsible for the assembly of spectrin-actin network that provides physical support to the plasma membrane and mediates signal transduction in various cellular physiological processes upon regulation by protein kinase C-dependent and calcium/calmodulin-dependent pathways. Abnormal phosphorylation, genetic variations, and alternative splicing of adducin may contribute to alterations in cellular functions involved in pathogenic processes. These alterations are associated with a wide range of diseases including cancer. This paper begins with a discussion on how adducin partakes in the structural formation of membrane skeleton, its regulation, and related functional characteristics, followed by a review on the pathogenesis of hypertension, biliary atresia, and cancer with respect to increased disease susceptibility mediated by adducin polymorphism and/or dysregulation. Given the functional diversity of adducin in different cellular compartments, we aim to provide a knowledge base whereby its pathophysiological roles can be better understood. More importantly, we aim to provide novel insights that may be of significance in turning the adducin model to clinical application.

Identification of contact sites between ankyrin and band 3 in the human erythrocyte membrane

The red cell membrane is stabilized by a spectrin/actin-based cortical cytoskeleton connected to the phospholipid bilayer via multiple protein bridges. By virtue of its interaction with ankyrin and adducin, the anion transporter, band 3 (AE1), contributes prominently to these bridges. In a previous study, we demonstrated that an exposed loop comprising residues 175-185 of the cytoplasmic domain of band 3 (cdB3) constitutes a critical docking site for ankyrin on band 3. In this paper, we demonstrate that an adjacent loop, comprising residues 63-73 of cdB3, is also essential for ankyrin binding. Data that support this hypothesis include the following. (1) Deletion or mutation of residues within the latter loop abrogates ankyrin binding without affecting cdB3 structure or its other functions. (2) Association of cdB3 with ankyrin is inhibited by competition with the loop peptide. (3) Resealing of the loop peptide into erythrocyte ghosts alters membrane morphology and stability. To characterize cdB3-ankyrin interaction further, we identified their interfacial contact sites using molecular docking software and the crystal structures of D(3)D(4)-ankyrin and cdB3. The best fit for the interaction reveals multiple salt bridges and hydrophobic contacts between the two proteins. The most important ion pair interactions are (i) cdB3 K69-ankyrin E645, (ii) cdB3 E72-ankyrin K611, and (iii) cdB3 D183-ankyrin N601 and Q634. Mutation of these four residues on ankyrin yielded an ankyrin with a native CD spectrum but little or no affinity for cdB3. These data define the docking interface between cdB3 and ankyrin in greater detail.

Protein 4.2 binds to the carboxyl-terminal EF-hands of erythroid alpha-spectrin in a calcium- and calmodulin-dependent manner

Spectrin and protein 4.1 cross-link F-actin protofilaments into a network called the membrane skeleton. Actin and 4.1 bind to one end of beta-spectrin. The adjacent end of alpha-spectrin, called the EF-domain, is calmodulin-like, with calcium-dependent and calcium-independent EF-hands. It has no known function. However, the sph(1J)/sph(1J) mouse has very fragile red cells and lacks the last 13 amino acids in the EF-domain, suggesting the domain is critical for skeletal integrity. Using pulldown binding assays, we find the alpha-spectrin EF-domain either alone or incorporated into a mini-spectrin binds native and recombinant protein 4.2 at a previously identified region of 4.2 (G(3) peptide). Native 4.2 binds with an affinity comparable with other membrane skeletal interactions (K(d) = 0.30 microM). EF-domains bearing the sph(1J) mutation are inactive. Binding of protein 4.2 to band 3 (K(d) = 0.45 microM) does not interfere with the spectrin-4.2 interaction. Spectrin-4.2 binding is amplified by micromolar concentrations of Ca(2+) (but not Mg(2+)) by three to five times. Calmodulin also binds to the EF-domain (K(d) = 17 microM), and Ca(2+)-calmodulin blocks Ca(2+)-dependent binding of protein 4.2 but not Ca(2+)-independent binding. The data suggest that protein 4.2 is located near protein 4.1 at the spectrin-actin junctions. Because proteins 4.1 and 4.2 also bind to band 3, the erythrocyte anion channel, we suggest that one or both of these proteins cause a portion of band 3 to localize near the spectrin-actin junctions and provide another point of attachment between the membrane skeleton and the lipid bilayer.

Novel roles for erythroid Ankyrin-1 revealed through an ENU-induced null mouse mutant

Insights into the role of ankyrin-1 (ANK-1) in the formation and stabilization of the red cell cytoskeleton have come from studies on the nb/nb mice, which carry hypomorphic alleles of Ank-1. Here, we revise several paradigms established in the nb/nb mice through analysis of an N-ethyl-N-nitrosourea (ENU)-induced Ank-1-null mouse. Mice homozygous for the Ank-1 mutation are profoundly anemic in utero and most die perinatally, indicating that Ank-1 plays a nonredundant role in erythroid development. The surviving pups exhibit features of severe hereditary spherocytosis (HS), with marked hemolysis, jaundice, compensatory extramedullary erythropoiesis, and tissue iron overload. Red cell membrane analysis reveals a complete loss of ANK-1 protein and a marked reduction in beta-spectrin. As a consequence, the red cells exhibit total disruption of cytoskeletal architecture and severely altered hemorheologic properties. Heterozygous mutant mice, which have wild-type levels of ANK-1 and spectrin in their RBC membranes and normal red cell survival and ultrastructure, exhibit profound resistance to malaria, which is not due to impaired parasite entry into RBC. These findings provide novel insights into the role of Ank-1, and define an ideal model for the study of HS and malarial resistance.

Bioinformatic characterization of the SynCAM family of immunoglobulin-like domain-containing adhesion molecules

SynCAM 1 (synaptic cell adhesion molecule 1, alternatively named Tslc1 and nectin-like protein 3) belongs to the immunoglobulin superfamily and is an adhesion molecule that operates in a variety of important contexts. Exemplary are its roles in adhesion at synapses in the central nervous system and as tumor suppressor. Here, I describe a family of genes homologous to SynCAM 1 comprising four genes found solely in vertebrates. All SynCAM genes encode proteins with three immunoglobulin-like domains of the V-set, C1-set, and I-set subclasses. Comparison of genomic with cDNA sequences provides their exon-intron structure. Alternative splicing generates isoforms of SynCAM proteins, and diverse SynCAM 1 and 2 isoforms are created in an extracellular region rich in predicted O-glycosylation sites. Protein interaction motifs in the cytosolic sequence are highly conserved among all four SynCAM proteins, indicating their critical functional role. These findings aim to facilitate the understanding of SynCAM genes and provide the framework to examine the physiological functions of this family of vertebrate-specific adhesion molecules.

Topographical significance of membrane skeletal component protein 4.1 B in mammalian organs

The polarized architecture of epithelial cells is a fundamental determinant of cell structures and functions. Both formation and orientation of proper epithelial polarity are needed for cell-cell or cell-matrix adhesion, signal transduction and cytoskeletal interactions of multimolecular complexes at apical, lateral and basal cell membranes. These cell membrane domains are usually segregated by some junctional complexes. Recent molecular genetic studies on the anchor structure between myelin sheaths and axons have indicated the specific molecular organization for polarization of axolemma and the myelin sheaths at paranodes, termed 'septate-like junctions'. It was also speculated that other mammalian organs may use a similar junctional system. The protein 4.1 B was originally found to be localized in paranodes and juxtaparanodes of myelinated nerve fibers. Our recent immunohistochemical studies on protein 4.1B have indicated its significance for the cell-cell and/or cell-matrix adhesion in various rodent organs. The protein 4.1 family of proteins have been supposed to possess variable molecular domains relating to cell adhesion, ion balance, receptor responses and signal transduction. Therefore, more precise studies on the molecular structure and the functional domains of protein 4.1B, as well as on its changes under physiological and pathological conditions, may provide a clue for organogenesis in various mammalian organs.

Red blood cell blood group antigens: structure and function

Red blood cell (RBC) blood group antigens are polymorphic, inherited, carbohydrate or protein structures located on the extracellular surface of the RBC membrane. They contribute to the architecture of the RBC membrane, and their individual function(s) are being slowly revealed. The biological qualities assigned to these RBC membrane structures are based on observed physiological alteration in RBCs that lack the component, by documenting similarities in its protein sequence (predicted from the nucleotide sequence of the gene) to proteins of known function and by extrapolation to identified functional homologues in other cells. The varied roles of RBC antigens include membrane structural integrity, the transport of molecules through the membrane, as receptors for extracellular ligands, adhesion molecules, enzymes, complement components and regulators, and in glycocalyx formation.

Identification of quantitative trait loci that modify the severity of hereditary spherocytosis in wan, a new mouse model of band-3 deficiency

Defects in red blood cell (RBC) membrane skeleton components cause hereditary spherocytosis (HS). Clinically, HS varies significantly even among individuals with identical gene defects, illustrating the profound effects of genetic background on disease severity. We exploited a new spontaneous mouse model, wan, which arose on the inbred C3H/HeJ strain, to identify quantitative trait loci (QTL) that modify the HS phenotype. Homozygous wan mice have severe HS due to a complete deficiency of erythroid band 3. A QTL analysis of RBC count, hemoglobin, hematocrit, mean corpuscular volume (MCV), and mean corpuscular hemoglobin content (MCHC) was performed in wan/wan mice from an F2 intercross between C3H/HeJ+/wan and CAST/Ei+/+ F1 hybrids. Hematologic and survival data from C3H, CAST/Ei F2 wan homozygotes support the hypothesis that genetic modifiers significantly influence the band-3 null HS phenotype. Significant QTL were identified for the MCV trait only, suggesting that RBC membrane characteristics are a target for modifier gene action. The most significant quantitative trait locus, Hsm1 (hereditary spherocytosis modifier 1), localizes to mouse Chromosome 12 and is dominant. The peak LOD score was obtained with a marker for Spnb1 encoding erythroid β-spectrin, an obvious candidate gene. (Blood. 2004;103: 3233-3240)

Hereditary elliptocytosis: spectrin and protein 4.1R

Hereditary elliptocytosis (HE) is a common disorder of erythrocyte shape, occurring especially in individuals of African and Mediterranean ancestry, presumably because elliptocytes confer some resistance to malaria. The principle lesion in HE is mechanical weakness or fragility of the erythrocyte membrane skeleton due to defects in alpha-spectrin, beta-spectrin, or protein 4.1. Numerous mutations have been described in the genes encoding these proteins, including point mutations, gene deletions and insertions, and mRNA processing defects. Several mutations have been identified in a number of individuals on the same genetic background, suggesting a "founder effect." The majority of HE patients are asymptomatic, but some may experience hemolytic anemia, splenomegaly, and intermittent jaundice.

Distinct distribution of specific members of protein 4.1 gene family in the mouse nephron

BACKGROUND

Protein 4.1 is an adapter protein that links the actin cytoskeleton to various transmembrane proteins. These 4.1 proteins are encoded by four homologous genes, 4.1R, 4.1G, 4.1N, and 4.1B, which undergo complex alternative splicing. Here we performed a detailed characterization of the expression of specific 4.1 proteins in the mouse nephron.

METHODS

Distribution of renal 4.1 proteins was investigated by staining of paraformaldehyde-fixed mouse kidney sections with antibodies highly specific for each 4.1 protein. Major 4.1 splice forms, amplified from mouse kidney marathon cDNA, were expressed in transfected COS-7 cells in order to assign species of known exon composition to proteins detected in kidney.

RESULTS

A 105 kD 4.1R splice form, initiating at ATG-2 translation initiation site and lacking exon 16, but including exon 17B, was restricted to thick ascending limb of Henle's loop. A 95 kD 4.1N splice form, lacking exons 15 and 17D, was expressed in either descending or ascending thin limb of Henle's loop, distal convoluted tubule, and all regions of the collecting duct system. A major 108 kD 4.1B splice form, initiating at a newly characterized ATG translation initiation site, and lacking exons 15, 17B, and 21, was present only in Bowman's capsule and proximal convoluted tubule (PCT). There was no expression of 4.1G in kidney.

CONCLUSION

Distinct distribution of 4.1 proteins along the nephron suggests their involvement in targeting of selected transmembrane proteins in kidney epithelium and, therefore, in regulation of specific kidney functions.

CASK and protein 4.1 support F-actin nucleation on neurexins

Rearrangements of the actin cytoskeleton are involved in a variety of cellular processes from locomotion of cells to morphological alterations of the cell surface. One important question is how local interactions of cells with the extracellular space are translated into alterations of their membrane organization. To address this problem, we studied CASK, a member of the membrane-associated guanylate kinase homologues family of adaptor proteins. CASK has been shown to bind the erythrocyte isoform of protein 4.1, a class of proteins that promote formation of actin/spectrin microfilaments. In neurons, CASK also interacts via its PDZ domain with the cytosolic C termini of neurexins, neuron-specific cell-surface proteins. We now show that CASK binds a brain-enriched isoform of protein 4.1, and nucleates local assembly of actin/spectrin filaments. These interactions can be reconstituted on the cytosolic tail of neurexins. Furthermore, CASK can be recovered with actin filaments prepared from rat brain extracts, and neurexins are recruited together with CASK and protein 4.1 into these actin filaments. Thus, analogous to the PDZ-domain protein p55 and glycophorin C at the erythrocyte membrane, a similar complex comprising CASK and neurexins exists in neurons. Our data suggest that intercellular junctions formed by neurexins, such as junctions initiated by beta-neurexins with neuroligins, are at least partially coupled to the actin cytoskeleton via an interaction with CASK and protein 4.1.

A model of synaptic memory: a CaMKII/PP1 switch that potentiates transmission by organizing an AMPA receptor anchoring assembly

Ca2+/calmodulin-dependent protein kinase II (CaMKII) is localized in the postsynaptic density (PSD) and is necessary for LTP induction. Much has been learned about the autophosphorylation of CaMKII and its dephosphorylation by PSD protein phosphatase-1 (PP1). Here, we show how the CaMKII/PP1 system could function as an energy-efficient, bistable switch that could be activated during LTP induction and remain active despite protein turnover. We also suggest how recently discovered binding interactions could provide a structural readout mechanism: the autophosphorylated state of CaMKII binds tightly to the NMDAR and forms, through CaMKII-actinin-actin-(4.1/SAP97) linkages, additional sites for anchoring AMPARs at synapses. The proposed model has substantial experimental support and elucidates principles by which a local protein complex could produce stable information storage and readout.

Modulation of erythrocyte band 4.1 binding by volume expansion

Erythrocyte band 4.1 is an important protein in the control and maintenance of the cytoskeleton. Skate erythrocyte band 3, the anion exchanger, appears to play a pivotal role in the regulation of volume-stimulated solute efflux during volume expansion. Because band 4.1 interacts with band 3, we tested whether their interaction might change during volume expansion. Skate red blood cells were volume-expanded in either hypotonic media (one-half osmolarity) or were swollen under isoosmotic conditions by inclusion of ethylene glycol or ammonium chloride in the medium. Microsomal membranes isolated from red cells under volume expanded conditions demonstrated a significant decrease in the amount of band 4.1 bound to band 3. In unstimulated cells, approximately one third of the binding of band 4.1 occurred to band 3. This binding was characterized as being sensitive to competition by the peptide IRRRY. The majority of band 4.1 is bound to glycophorin (as demonstrated in other species), and this binding does not change during volume expansion. The alteration in band 4.1:band 3 interaction occurs within 5 min after volume expansion and is transient, returning to near normal interaction within 60 min. Two drugs that promote band 3 oligomerization, pyridoxal-5'-phosphate and DIDS, also decreased band 4.1 interaction with band 3. Band 4.1 and ankyrin binding to band 3 may be reciprocally related as high-affinity ankyrin binding sites to band 3 observed under volume-expanded conditions are decreased by inclusion of band 4.1 in the binding reactions. J. Exp. Zool. 289:177-183, 2001.

Blood groups and their function

The function(s) assigned to red blood cell membrane components is based on an observed effect in the red cells that lack the component, comparison of the protein sequence (predicted from the nucleotide sequence of the gene) to proteins of known function, and extrapolation of function of the component in other cells. The functions are varied and include membrane structure, transport, receptor, adhesion, enzyme activity, complement components, complement regulation and glycocalyx formation. Several components have more than one function.

Molecular characterization of a new member of the protein 4.1 family (brain 4.1) in rat brain

In addition to the well-known erythroid 4.1 gene, two human genes (KIAA0338 and 4.1G) have recently been identified as members of the protein 4.1 family of genes. We compared the expression levels of these three genes and found that the KIAA0338 gene was predominantly expressed in human brain. To further characterize this novel protein 4.1, called brain 4.1, we isolated rat brain 4.1 cDNA and analyzed its gene products in rat brain. The results indicated that the mRNA and protein products of the brain 4.1 gene were more abundant in brain compared to any other tissues examined. The brain 4.1 mRNA appeared as multiple bands with estimated sizes of 3.9 kb, 6.2 kb and 8.7 kb on RNA blotting analysis, and was found to consist of various alternative forms as reported previously for the erythroid 4. 1 gene. As for the brain 4.1 gene product, many isoforms discernible by immunoblotting analysis were also observed depending on the tissue type and the brain region. The existence of multiple forms of the brain 4.1 implies that it has multiple and diverse functions like the erythroid 4.1 gene product.

Functional aspects of red cell antigens

Over 250 blood group determinants are known and most of these are located on integral red cell proteins and glycoproteins. The functions of some of these structures are known: Diego (band 3) is the red cell anion exchanger; Kidd, a urea transporter; Colton (aquaporin 1), a water channel; Cromer (DAF) and Knops (CRI), complement regulators; Diego (band 3) and Gerbich (glycophorin C/D) link the red cell membrane and the membrane skeleton. The Duffy glycoprotein is a chemokine receptor that may act as a scavenger for inflammatory mediators in the peripheral blood, but is also exploited as a receptor by Plasmodium vivax merozoites. The functions of some blood group antigens can be speculated upon because of structural similarity to proteins and glycoproteins of known function. For example, the Lutheran, LW, and Ok glycoproteins are members of the immunoglobulin superfamily of receptors and signal transducers, the Rh proteins and related glycoproteins show homology to ammonium transporters, and the Kell glycoprotein resembles a family of endopeptidases. Yet most blood groups systems contain null phenotypes associated with no apparent pathology. If these blood group antigens have important functions, other structures must be able to carry out those functions in their absence. Almost nothing is known of the biological significance of blood group polymorphism.

Characterization of Multiple Isoforms of Protein 4.1R Expressed During Erythroid Terminal Differentiation

In erythrocytes, 80-kD protein 4.1R regulates critical membrane properties of deformability and mechanical strength. However, previously obtained data suggest that multiple isoforms of protein 4.1, generated by alternative pre-mRNA splicing, are expressed during erythroid differentiation. Erythroid precursors use two splice acceptor sites at the 5′ end of exon 2, thereby generating two populations of 4.1 RNA: one that includes an upstream AUG-1 in exon 2′ and encodes high molecular weight isoforms, and another that skips AUG-1 in exon 2′ and encodes 4.1 by initiation at a downstream AUG-2 in exon 4. To begin an analysis of the complex picture of protein 4.1R expression and function during erythropoiesis, we determined the number and primary structure of 4.1R isoforms expressed in erythroblasts. We used reverse-transcription polymerase chain reaction to amplify and clone full-length coding domains from the population of 4.1R cDNA containing AUG-1 and the population excluding AUG-1. We observed an impressive repertoire of 4.1R isoforms that included 7 major and 11 minor splice variants, thus providing the first definitive characterization of 4.1R primary structures in a single-cell lineage. 4.1R isoforms, transfected into COS-7 cells, distributed to the nucleus, cytoplasm, plasma membrane, and apparent centrosome. We confirmed previous studies showing that inclusion of exon 16 was essential for efficient nuclear localization. Unexpectedly, immunochemical analysis of COS-7 cells transfected with an isoform lacking both AUG-1 and AUG-2 documented that a previously unidentified downstream translation initiation codon located in exon 8 can regulate expression of 4.1R. We speculate that the repertoire of primary structure of 4.1R dictates its distinct binding partners and functions during erythropoiesis.

Characterization of Multiple Isoforms of Protein 4.1R Expressed During Erythroid Terminal Differentiation

In erythrocytes, 80-kD protein 4.1R regulates critical membrane properties of deformability and mechanical strength. However, previously obtained data suggest that multiple isoforms of protein 4.1, generated by alternative pre-mRNA splicing, are expressed during erythroid differentiation. Erythroid precursors use two splice acceptor sites at the 5′ end of exon 2, thereby generating two populations of 4.1 RNA: one that includes an upstream AUG-1 in exon 2′ and encodes high molecular weight isoforms, and another that skips AUG-1 in exon 2′ and encodes 4.1 by initiation at a downstream AUG-2 in exon 4. To begin an analysis of the complex picture of protein 4.1R expression and function during erythropoiesis, we determined the number and primary structure of 4.1R isoforms expressed in erythroblasts. We used reverse-transcription polymerase chain reaction to amplify and clone full-length coding domains from the population of 4.1R cDNA containing AUG-1 and the population excluding AUG-1. We observed an impressive repertoire of 4.1R isoforms that included 7 major and 11 minor splice variants, thus providing the first definitive characterization of 4.1R primary structures in a single-cell lineage. 4.1R isoforms, transfected into COS-7 cells, distributed to the nucleus, cytoplasm, plasma membrane, and apparent centrosome. We confirmed previous studies showing that inclusion of exon 16 was essential for efficient nuclear localization. Unexpectedly, immunochemical analysis of COS-7 cells transfected with an isoform lacking both AUG-1 and AUG-2 documented that a previously unidentified downstream translation initiation codon located in exon 8 can regulate expression of 4.1R. We speculate that the repertoire of primary structure of 4.1R dictates its distinct binding partners and functions during erythropoiesis.