cDNA cloning, sequencing and chromosome mapping of a non-erythroid spectrin, human alpha-fodrin

c-Src binds alpha II spectrin's Src homology 3 (SH3) domain and blocks calpain susceptibility by phosphorylating Tyr1176

Spectrin is a ubiquitous heterodimeric scaffolding protein that stabilizes membranes and organizes protein and lipid microdomains on both the plasma membrane and intracellular organelles. Phosphorylation of beta-spectrin on Ser/Thr is well recognized. Less clear is whether alpha-spectrin is phosphorylated in vivo and whether spectrin is phosphorylated on tyrosine (pTyr). We affirmatively answer both questions. In cultured Madin-Darby canine kidney cells, alphaII spectrin undergoes in vivo tyrosine phosphorylation. Enhancement of the steady state level of pTyr-modified alphaII spectrin by vanadate, a phosphatase inhibitor, implies a dynamic balance between alphaII spectrin phosphorylation and dephosphorylation. Recombinant peptides containing the Src homology 3 domain of alphaII spectrin (but not the Src homology 3 domain of alphaI spectrin) bind specifically to phosphorylated c-Src in Madin-Darby canine kidney cell lysates, suggesting that this kinase is responsible for its in vivo phosphorylation. pTyr-modified alphaII spectrin is resistant to maitotoxin-induced cleavage by mu-calpain in vivo. In vitro studies of recombinant alphaII spectrin peptides representing repeats 9-12 identify two sites of pTyr modification. The first site is at Tyr(1073), a residue immediately adjacent to a region encoded by alternative exon usage (insert 1). The second site is at Tyr(1176). This residue flanks the major site of cleavage by the calcium-dependent protease calpain, and phosphorylation of Tyr(1176) by c-Src reduces the susceptibility of alphaII spectrin to cleavage by mu-calpain. Calpain cleavage of spectrin, activated by Ca(2+) and calmodulin, contributes to diverse cellular processes including synaptic remodeling, receptor-mediated endocytosis, apoptosis, and the response of the renal epithelial cell to ischemic injury. Tyrosine phosphorylation of alphaII spectrin now would appear to also mediate these events. The spectrin skeleton thus forms a point of convergence between kinase/phosphatase and Ca(2+)-mediated signaling cascades.

Synaptic scaffolding proteins in rat brain. Ankyrin repeats of the multidomain Shank protein family interact with the cytoskeletal protein alpha-fodrin

The postsynaptic density is the ultrastructural entity containing the neurotransmitter reception apparatus of excitatory synapses in the brain. A recently identified family of multidomain proteins termed Src homology 3 domain and ankyrin repeat-containing (Shank), also known as proline-rich synapse-associated protein/somatostatin receptor-interacting protein, plays a central role in organizing the subsynaptic scaffold by interacting with several synaptic proteins including the glutamate receptors. We used the N-terminal ankyrin repeats of Shank1 and -3 to search for interacting proteins by yeast two-hybrid screening and by affinity chromatography. By cDNA sequencing and mass spectrometry the cytoskeletal protein alpha-fodrin was identified as an interacting molecule. The interaction was verified by pull-down assays and by coimmunoprecipitation experiments from transfected cells and brain extracts. Mapping of the interacting domains of alpha-fodrin revealed that the highly conserved spectrin repeat 21 is sufficient to bind to the ankyrin repeats. Both interacting partners are coexpressed widely in the rat brain and are colocalized in synapses of hippocampal cultures. Our data indicate that the Shank1 and -3 family members provide multiple independent connections between synaptic glutamate receptor complexes and the cytoskeleton.

Monoclonal antibodies to alphaI spectrin Src homology 3 domain associate with macropinocytic vesicles in nonerythroid cells

Spectrins represent a family of membrane-associated proteins responsible for membrane flexibility and cell shape in erythrocytes, and probably in most nonerythroid cells. Spectrin functions as a tetramer consisting of two heterodimers each containing two subunits termed alpha and beta. In humans, alphaI and alphaII spectrins but not beta spectrins are characterized by the presence of an Src homology 3 (SH3) domain. As a tool to investigate the function of spectrin SH3 domains we derived several monoclonal antibodies (mAb) to the recombinant human alphaI or alphaII spectrin SH3 domain. Immunostaining using these monoclonal antibodies indicated expression of alphaI spectrin in cell bodies and alphaII spectrin in neurites of granule neurons in mouse primary cerebellar cultures. Monoclonal antibodies reactive to alphaI spectrin SH3 domain indicated expression of a protein(s) containing an alphaI-like SH3 domain in cytoplasmic vesicular-like structures in GFAP-positive cells in these cultures. In NIH 3T3 fibroblasts, these antibodies label macropinocytic vesicles. Together, these data and Western blotting results suggest expression of at least three spectrin-SH3 domain antibody-reactive proteins.

Development and characterization of antibodies specific to caspase-3-produced alpha II-spectrin 120 kDa breakdown product: marker for neuronal apoptosis

Alpha II-spectrin (alpha-fodrin) is a demonstrated endogenous substrate for caspase-3 in neurons undergoing unscheduled apoptotic death. We have previously identified the caspase cleavage site that yields the distinctive 120 kDa spectrin breakdown product (SBDP120) as (DSLD(1478)*SVEAL). Here, by using a synthetic peptide (NH(2)-SVEALC) mimicking the neo-N-terminal of SBDP120 as antigen, we report the development of chicken antibodies that specifically recognize the SBDP120 generated by in vitro caspase-3 digestion of bovine alpha-spectrin on Western blot. These anti-SBDP120 antibodies recognize SBDP120 generated by two apoptotic challenges (staurosporine, EGTA) to human neuroblastoma SH-SY5Y cells. Yet they neither react with intact alpha-spectrin nor its other fragments on Western blots. These anti-SBDP120 work equally well in detecting SBDP120 generated in rat cerebellar granule neurons undergoing potassium withdrawal-induced apoptosis. In immunocytochemical studies, these antibodies also specifically stained apoptotic SH-SY5Y or CGN's undergoing apoptosis in a caspase- inhibitor-sensitive manner. These anti-SBDP120s might become powerful markers for apoptotic neurons in various neurological or neurodegenerative conditions in vivo.

Two populations of beta-spectrin in rat skeletal muscle

We use immunoblotting, immunoprecipitation, and centrifugation in sucrose density gradients to show that the product of the erythrocyte beta-spectrin gene in rat skeletal muscle (muscle beta-spectrin) is present in two states, one associated with fodrin, and another that is not associated with any identifiable spectrin or fodrin subunit. Immunofluorescence studies indicate that a significant amount of beta-spectrin without alpha-fodrin is present in the myoplasm of some muscle fibers, and, more strikingly, at distinct regions of the sarcolemma. These results suggest that alpha-fodrin and muscle beta-spectrin associate in muscle in situ, but that some muscle beta-spectrin without a paired alpha-subunit forms distinct domains at the sarcolemma.

Association of hepatic system A amino acid transporter with the membrane-cytoskeletal proteins ankyrin and fodrin

System A activity is a highly regulated mechanism for the active transport of zwitterionic amino acids into mammalian cells. Monoclonal antibodies generated against a previously unidentified rat liver plasma membrane-associated protein were shown to immunoprecipitate solubilized System A transport activity. The immunoreactive protein was later determined by immunoblotting and peptide microsequencing to be rat liver alpha-fodrin (non-erythroid spectrin). Antibody against ankyrin, a protein that often serves as a bridge between integral membrane proteins and fodrin, also immunoprecipitated System A transport activity. Fractionation of solubilized plasma membrane proteins on sucrose gradients revealed that the System A transporter co-migrated as a complex with fodrin and ankyrin, even in the presence of detergent and urea. In contrast, the System N amino acid transporter does not co-migrate with ankyrin and fodrin, nor does the anti-fodrin antibody immunoprecipitate System N activity. The present data are the first to demonstrate an association between an organic solute transporter and the membranocytoskeletal proteins ankyrin and fodrin.

Brain spectrin: of mice and men

This article reviews our current knowledge of the structure of alpha spectrins and beta spectrins in the brain, as well as their location and expression within neural tissue. We discuss the known protein interactions of brain spectrin isoforms, and then describe results that suggest an important role for spectrin (alpha SpII sigma 1/beta SpII sigma 1) in the Ca(2+)-regulated release of neurotransmitters. Evidence that supports a role for spectrin in the docking of synaptic vesicles to the presynaptic plasma membrane and as a Ca2+ sensor protein that unclamps the fusion machinery is described, along with the Casting the Line model, which summarizes the information. We finish with a discussion of the value of spectrin and ankyrin-deficient mouse models in deciphering spectrin function in neural tissue.

Brain alpha erythroid spectrin: identification, compartmentalization, and beta spectrin associations

Using isoform and subunit specific antibodies we have determined the presence, localization, and beta spectrin associations of alpha erythroid spectrin, alpha SpI sigma*, as well as alpha non-erythroid spectrin, alpha SpII sigma 1, in mouse brain. Peptide specific antibodies against unique sequences within the beta SpII sigma 1, non-erythroid beta spectrin isoform, and within beta SpI sigma 1, erythrocyte beta spectrin isoform were used to compare the immunolocalization of beta spectrin subunit isoforms with that of alpha spectrin subunit isoforms and to immunoprecipitate spectrin tetramers in order to identify the subunit components by immunoblot analysis. The specificity and sensitivity of antibodies for isoform specific alpha and beta subunits was determined by immunodot and immunoblot methods. Immunohistochemical analyses indicated that beta SpI sigma 2 is located in neuronal somata and dendrites in mouse cerebellum. beta SpII sigma 1 is located in the medullary layer, chiefly composed of axonal tracts. Parallel immunohistochemical analysis with antibodies for the alpha and beta spectrin isoforms revealed that antibodies specific for the alpha subunit of erythrocyte spectrin (alpha SpI sigma 1) localized antigen to the somata and dendrites of cerebellar granule cell neurons, a pattern similar to that for the localization of the erythroid beta subunit (beta SpI sigma 2). In contrast antibodies specific for the non-erythroid alpha subunit (alpha SpII sigma 1) localized antigen to axons in the cerebellum corresponding to the pattern for the non-erythroid beta subunit (beta SpII sigma 1). The distinct localization of antigens by antisera which recognize either the alpha subunit of red blood cell spectrin or the alpha subunit of non-erythroid brain spectrin, together with the correspondence of their localization with appropriate beta subunits, clearly indicate that brain contains at least two species of spectrin each with distinct alpha and beta subunits. Immunoprecipitation experiments of cerebellar extracts using beta spectrin peptide specific antibodies followed by immunoblotting analysis confirmed the association of an erythroid alpha subunit isoform with a beta erythroid subunit isoform, as well as the association of non-erythroid alpha and beta subunits. In addition the immunoblot analysis of the immunoprecipitated material suggested there are minor populations of various hybrid tetramers in brain consisting of mixed erythroid and non-erythroid subunits. In summary these data collectively demonstrate that in mouse brain there are at least two alpha spectrin subunits, one erythroid alpha SpI sigma* and one non-erythroid alpha SpII sigma 1; these associate with an erythroid beta SpI sigma 1, and a non-erythroid beta SpII sigma 1 in the cerebellum of mouse.(ABSTRACT TRUNCATED AT 400 WORDS)

Developmental change of alpha-spectrin mRNA in the rat brain

Spectrin is a cytoskeletal protein considered to be a major component of intracellular cohesion. Using an in situ hybridization approach, we have investigated the developmental expression of the mRNA encoding the alpha-subunit of rat brain spectrins, from birth to adulthood. alpha-Subunit mRNA is detectable at birth, in brain areas with perinatal neurogenesis, such as the cerebral cortex, hippocampus, thalamus, and olfactory bulb. alpha-Brain-spectrin mRNA increases gradually during the first postnatal days to reach a plateau between the second and the third week of life. In the young adult brain, the level of alpha-brain spectrin mRNA decreased globally. This spacio-temporal distribution argues for the involvement of the mRNA in the synthesis of both the erythroid and non-erythroid brain spectrin isoforms. We have focused our attention on the hippocampal formation and the cerebellum. In both regions, in situ hybridization signal variations are superimposable with neuronal maturation gradients. This pattern of variation, coupled with the known interaction of brain spectrins with other cytoskeletal proteins, agrees with the notion that brain spectrins may be involved in neuronal differentiation by way of the cytoskeletal lattice organization.

Beta heavy-spectrin has a restricted tissue and subcellular distribution during Drosophila embryogenesis

The components of the membrane skeleton play an important role in maintaining membrane structure during the dynamic changes in cell shape that characterize development. beta Heavy-spectrin is a unique beta-spectrin from Drosophila melanogaster that is closer in size (M(r) = 430 × 10(3)) to dystrophin than to other beta-spectrin members of the spectrin/alpha-actinin/dystrophin gene super-family. Here we establish that both the subcellular localization of the beta Heavy-spectrin protein and the tissue distribution of beta Heavy-spectrin transcript accumulation change dramatically during embryonic development. Maternally loaded protein is uniformly distributed around the plasma membrane of the egg. During cellularization it is associated with the invaginating furrow canals and in a region of the lateral membranes at the apices of the forming cells (apicolateral). During gastrulation the apicolateral staining remains and is joined by a new apical cap, or plate, of beta Heavy-spectrin in areas where morphogenetic movements occur. These locations include the ventral and cephalic furrows and the posterior midgut invagination. Thus, dynamic rearrangement of the subcellular distribution of the protein is precisely coordinated with changes in cell shape. Zygotic message and protein accumulate after the germ band is fully extended, in the musculature, epidermis, hindgut, and trachea of the developing embryo. beta Heavy-spectrin in the epidermis, hindgut, and trachea is apically localized, while the protein in the somatic and visceral musculature is not obviously polarized. The distribution of beta Heavy-spectrin suggests roles in establishing an apicolateral membrane domain that is known to be rich in intercellular junctions and in establishing a unique membrane domain associated with contractile processes.

Two cellular proteins that bind to wild-type but not mutant p53

p53 is a tumor-suppressor protein that can activate and repress transcription. Using the yeast two-hybrid system, we identified two previously uncharacterized human proteins, designated 53BP1 and 53BP2, that bind to p53. 53BP1 shows no significant homology to proteins in available databases, whereas 53BP2 contains two adjacent ankyrin repeats and a Src homology 3 domain. In vitro binding analyses indicate that both of these proteins bind to the central domain of p53 (residues 80-320) required for site-specific DNA binding. Consistent with this finding, p53 cannot bind simultaneously to 53BP1 or 53BP2 and to a DNA fragment containing a consensus p53 binding site. Unlike other cellular proteins whose binding to p53 has been characterized, both 53BP1 and 53BP2 bind to the wild-type but not to two mutant p53 proteins identified in human tumors, suggesting that binding is dependent on p53 conformation. The characteristics of these interactions argue that 53BP1 and 53BP2 are involved in some aspect of p53-mediated tumor suppression.

The complete amino acid sequence for brain beta spectrin (beta fodrin): relationship to globin sequences

The amino acid sequence of mouse brain beta spectrin (beta fodrin), deduced from the nucleotide sequence of complementary DNA clones, reveals that this non-erythroid beta spectrin comprises 2363 residues, with a molecular weight of 274,449 Da. Brain beta spectrin contains three structural domains and we suggest the position of several functional domains including f-actin, synapsin I, ankyrin and spectrin self association sites. Analysis of deduced amino acid sequences indicated striking homology and similar structural characteristics of brain beta spectrin repeats beta 11 and beta 12 to globins. In vitro analysis has demonstrated that heme is capable of specific attachment to brain spectrin, suggesting possible new functions in electron transfer, oxygen binding, nitric oxide binding or heme scavenging.

Mechanically sensitive, nonselective cation channels

Mechanically sensitive channels (MSCs) are ubiquitous in plant and animal cells. They respond primarily to membrane tension, thus making them good transducers for forces derived from osmotic or hydraulic gradients and shear stress. They may also be modulated by membrane voltage and various ligands. MSCs are most commonly cation selective, passing calcium as well as monovalent ions, but some are K+ selective, and a few are anion selective. MSCs occur at a density of about 0.2-5 per microns2. The universal distribution and biophysical properties of MSCs make them the ideal mechanotransducers in a wide variety of cellular processes.

Developmental expression of brain beta-spectrin isoform messenger RNAs

We have investigated the expression of brain beta SpIIa and beta SpIb (previously referred to as the beta-subunits of brain spectrin (240/235) and brain spectrin (240/235E), respectively) during mouse brain development. The 9 kb transcript which encodes beta SpIIa is present in fetal mouse brain tissue and increases to a maximal level in a 30-day-old mouse. There is a coordinate accumulation of the 7.8 kb alpha SpIIa mRNA (with beta SpIIa) during mouse brain development. The coordinate expression of alpha SpIIa and beta SpIIa at the mRNA and protein level allows formation of (alpha SpIIa/beta SpIIa)2 tetramers (brain spectrin(240/235)) early in premitotic neuronal development; and avoids turnover of unassembled alpha and beta-subunits. An 11 kb transcript which encodes beta SpIb is not produced in embryonic tissue, and is first seen in a 6-day-old mouse. The protein translation products beta SpIIa and beta SpIb have previously been demonstrated by our laboratory to first appear in fetal mouse brain tissue and at postnatal day 6-8, respectively [J. Neurosci., 7 (1987) 864-874]. The expression of beta SpIb mRNA on postnatal day 6-8, and the appearance of brain spectrin(240/235E) in postmitotic and postmigratory neurons of the cerebellum at this same time; suggests that brain spectrin(240/235E) is involved in differentiated functions of the neuron (formation of cell-cell contacts, formation of dendritic processes and postsynaptic contacts). Thus, the data from the present study demonstrates that the expression of these two neuronal beta-spectrin isoforms is regulated at the level of mRNA expression.

Characterization of calcium binding to spectrins

Calcium binding to brain and erythrocyte spectrins was studied at physiological ionic strength by a calcium overlay assay and aqueous two-phase partitioning. When the spectrins were immobilized on nylon membranes by slot blotting, the overlay assay showed that even though both spectrins bound 45Ca2+, the brain protein displayed much greater affinity for calcium ions than erythrocyte spectrin did. Since the observed binding was weaker than that displayed by calmodulin under similar conditions, the overlay assay results indicated that the binding must be weaker than 1 microM. The phase partition experiments showed that there are at least two sites for calcium on brain spectrin and that calcium binding to one of these sites is reduced significantly by magnesium ions. From the partition isotherm, the dissociation constants were estimated as 50 microM for the Mg(2+)-independent site and 150 microM for the Mg(2+)-dependent site. The phase partition results also showed that erythrocyte spectrin bound calcium ions at least 1 order of magnitude weaker. By examining calcium binding to slot-blotted synthetic peptides, we identified two binding sites in brain spectrin. One mapped to the second putative calcium binding site (EF-hand) in alpha-spectrin and the other to the 36 amino acid residue long insert in domain 11. In addition, a tryptic fragment derived from the C-terminal of erythrocyte alpha-spectrin, which contained the two postulated EF-hands, also bound calcium. These findings suggest that the calcium signal system may also involve direct binding of calcium to spectrin beside known calcium modulators such as calmodulin and calpain.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification and characterization of a novel cytoskeleton-associated pp60src substrate

Transformation of cells by the src oncogene results in elevated tyrosine phosphorylation of two related proteins, p80 and p85 (p80/85). Immunostaining with specific monoclonal antibodies revealed a striking change of subcellular localization of p80/85 in src-transformed cells. p80/85 colocalizes with F-actin in peripheral extensions of normal cells and rosettes (podosomes) of src-transformed cells. Sequence analysis of cDNA clones encoding p80/85 revealed an amino-terminal domain composed of six copies of a direct tandem repeat, each repeat containing 37 amino acids, a carboxyl-terminal SH3 domain, and an interdomain region composed of a highly charged acidic region and a region rich in proline, serine, and threonine. The multidomain structure of p80/85 and its colocalization with F-actin in normal and src-transformed cells suggest that these proteins may associate with components of the cytoskeleton and contribute to organization of cell structure.

Identification and characterization of a novel cytoskeleton-associated pp60src substrate

Transformation of cells by the src oncogene results in elevated tyrosine phosphorylation of two related proteins, p80 and p85 (p80/85). Immunostaining with specific monoclonal antibodies revealed a striking change of subcellular localization of p80/85 in src-transformed cells. p80/85 colocalizes with F-actin in peripheral extensions of normal cells and rosettes (podosomes) of src-transformed cells. Sequence analysis of cDNA clones encoding p80/85 revealed an amino-terminal domain composed of six copies of a direct tandem repeat, each repeat containing 37 amino acids, a carboxyl-terminal SH3 domain, and an interdomain region composed of a highly charged acidic region and a region rich in proline, serine, and threonine. The multidomain structure of p80/85 and its colocalization with F-actin in normal and src-transformed cells suggest that these proteins may associate with components of the cytoskeleton and contribute to organization of cell structure.

Biochemical identification of alpha-fodrin and protein 4.1 in human keratinocytes

The mature erythrocyte has a cytoskeleton of less complexity than that of nucleated cells and has been elucidated in greater detail. Two of its major components are the heterodimeric protein spectrin and protein 4.1. We report here our isolation from human keratinocytes of immunoreactive forms of both protein 4.1 and of alpha-fodrin, the extra-erythrocytic form of alpha-spectrin. These keratinocyte proteins are approximately 125 kD and 240 kD in size, respectively. We also have isolated clones containing alpha-fodrin and protein 4.1 sequences from a human keratinocyte cDNA library. These sequences confirm the active transcription in keratinocytes of the alpha-fodrin and protein 4.1 genes. Both alpha-fodrin and protein 4.1 mRNA are detectable by Northern blot analysis in human keratinocytes, where their abundance appears not to be regulated by calcium concentration in the medium.

The use of irradiation and fusion gene transfer (IFGT) hybrids to isolate DNA clones from human chromosome region 9q33-q34

We have generated somatic cell hybrids containing fragments of human chromosome arm 9q by an irradiation and fusion technique. No selection for human material was imposed, but of 23 clones analyzed most contained human DNA sequences and many contained multiple fragments of the human chromosome arm. A hybrid that appears to contain only two small fragments of human DNA from the regions of q33 and q34 has been used as a source from which to clone probes specific to those areas of the chromosome.

Structure and evolution of the actin crosslinking proteins

The actin crosslinking proteins exhibit marked diversity in size and shape and crosslink actin filaments in different ways. Amino acid sequence analysis of many of these proteins has provided clues to the origin of their diversity. Spectrin, alpha-actinin, ABP-120, ABP-280, fimbrin, and dystrophin share a homologous sequence segment that is implicated as the common actin binding domain. The remainder of each protein consists of repetitive and non-repetitive sequence segments that have been shuffled and multiplied in evolution to produce a variety of proteins that are related in function and in composition, but that differ significantly in structure.

The exon-intron organization of the human erythrocyte alpha-spectrin gene

The human erythrocyte alpha-spectrin gene which spans 80 kbp has been cloned from human genomic DNA as overlapping lambda recombinants. The exon-intron junctions were identified and the exons mapped. The gene is encoded by 52 exons whose sizes range from 684 bp to the smallest of 18 bp. The donor and acceptor splice site sequences match the splice site consensus sequences, with the exception of one splice site where a donor sequence begins with -GC. The size and location of exons do not correlate with the 106-amino-acid repeat, except in three locations where the surrounding codons are conserved as well. The lack of correspondence between exons and 106-amino-acid repeat is interpreted to reflect the appearance of a spectrin-like gene from a minigene early in the evolution of eukaryotes. Since current evidence indicates that introns were present in genes before the divergence of prokaryotes and eukaryotes, it is possible that the original distribution of introns within the minigene has been lost by the random deletion of introns from the spectrin gene.

Patterns of spectrin transcripts in erythroid and non-erythroid cells

Spectrin is the major protein of the membrane erythrocyte skeleton. More recently, homologous but non-identical spectrins (fodrins) were also found in various non-erythroid tissues. Spectrin mRNA in erythroid and various non-erythroid cells was examined by direct hybridization with human alpha-spectrin, beta-spectrin (erythroid spectrins), and alpha-fodrin (non-erythroid spectrin) cDNA probes. Northern blot analysis of poly (A)+ RNA revealed a distinct pattern of expression in erythroid vs. non-erythroid cells. Erythroid cells from early erythroblasts to reticulocyte stage expressed two mRNA species of beta-spectrin, whereas they expressed only a single species of alpha-spectrin, and no alpha-fodrin mRNA. In contrast, non-erythroid cells (platelets, myeloid cells, liver, muscle, heart, cerebellum, and eye lens) expressed either no alpha-spectrin mRNA or a different molecular weight transcript(s) of this gene, and a single species of alpha-fodrin mRNA. Additionally, they also expressed from none to multiple species of beta-spectrin, and these were of different molecular size(s) from that found in erythroid cells (with the exception of platelets). Transcripts of non-erythroid spectrin, alpha-fodrin, were found as a single copy only in non-erythroid tissues. Human and murine erythroleukemia cells expressed both erythroid spectrin transcripts in addition to alpha-fodrin and raise the possibility that erythroid progenitors may have the potential to express both erythroid and non-erythroid species. These data indicated that several mRNA species of beta-spectrin could be detected in both erythroid and some non-erythroid cells. Whether multiple spectrin peptides could also be found with functional heterogeneity is unclear. However, in each case, the pattern combination observed appeared to be tissue-specific.

Identification of an unusual deletion within homologous repeats of human reticulocyte beta-spectrin and probable peptide polymorphism

We screened two different human reticulocyte cDNA libraries with beta-spectrin(beta Sp)-specific polyclonal antibodies and with our original radiolabeled human BSP cDNA probe (encoding beta Sp). Of the 20 independent clones, the largest had about a 2.5-kb insert corresponding to the deduced amino acid (aa) sequence of the beta-7 to beta-14 repetitive segments. Among these segments, segment 12 was 7 aa shorter than the other repetitive segments. We showed that this truncation was not a result of (i) cloning artifact, (ii) alternate splicing, or (iii) common genomic polymorphism by additional examination of 14 individual human chromosomes. Recently, another laboratory described the BSP nucleotide (nt) sequence overlapping partially with our sequence. These overlapping sequences were homologous with the exception of two nt differences at the positions 1342 and 1514. The discrepancy at nt 1342 changes the His to Arg. This newly derived probe has been used to find an additional example of BSP restriction fragment length polymorphism.

Cloning of a 67-kD neutrophil oxidase factor with similarity to a noncatalytic region of p60c-src

Chronic granulomatous diseases (CGDs) are characterized by recurrent infections resulting from impaired superoxide production by a phagocytic cell, nicotinamide adenine dinucleotide phosphate (reduced) (NADPH) oxidase. Complementary DNAs were cloned that encode the 67-kilodalton (kD) cytosolic oxidase factor (p67), which is deficient in 5% of CGD patients. Recombinant p67 (r-p67) partially restored NADPH oxidase activity to p67-deficient neutrophil cytosol from these patients. The p67 cDNA encodes a 526-amino acid protein with acidic middle and carboxyl-terminal domains that are similar to a sequence motif found in the noncatalytic domain of src-related tyrosine kinases. This motif was recently noted in phospholipase C-gamma, nonerythroid alpha-spectrin (fodrin), p21ras-guanosine triphophatase-activating protein (GAP), myosin-1 isoforms, yeast proteins cdc-25 and fus-1, and the 47-kD phagocyte oxidase factor (p47), which suggests the possibility of common regulatory features.

Molecular cloning of human protein 4.2: a major component of the erythrocyte membrane

Protein 4.2 (P4.2) comprises approximately 5% of the protein mass of human erythrocyte (RBC) membranes. Anemia occurs in patients with RBCs deficient in P4.2, suggesting a role for this protein in maintaining RBC stability and integrity. We now report the molecular cloning and characterization of human RBC P4.2 cDNAs. By immunoscreening a human reticulocyte cDNA library and by using the polymerase chain reaction, two cDNA sequences of 2.4 and 2.5 kilobases (kb) were obtained. These cDNAs differ only by a 90-base-pair insert in the longer isoform located three codons downstream from the putative initiation site. The 2.4- and 2.5-kb cDNAs predict proteins of approximately 77 and approximately 80 kDa, respectively, and the authenticity was confirmed by sequence identity with 46 amino acids of three cyanogen bromide-cleaved peptides of P4.2. Northern blot analysis detected a major 2.4-kb RNA species in reticulocytes. Isolation of two P4.2 cDNAs implies existence of specific regulation of P4.2 expression in human RBCs. Human RBC P4.2 has significant homology with human factor XIII subunit a and guinea pig liver transglutaminase. Sequence alignment of P4.2 with these two transglutaminases, however, revealed that P4.2 lacks the critical cysteine residue required for the enzymatic crosslinking of substrates.

Identification of the calmodulin binding domain of alpha-fodrin and implications for folding

A cDNA clone producing a protein that binds calmodulin has been isolated from a mouse macrophage library. The cDNA was sequenced and identified as coding for fodrin. By deleting part of the sequence, the calmodulin binding domain was located. The site is situated on repeat 11 of fodrin probably on its extra arm. This part of the sequence exhibits great similarity to other calmodulin binding proteins. Analysis of the sequence and spatial structure of calmodulin revealed a domain which is quite complementary to the sequence identified on fodrin. These results provide a new insight into the structure of fodrin and consequently into the structure of proteins of the spectrin family. A model for the general folding of these molecules is proposed, involving a simple three-layer folding. The structure was further corroborated by analysis of charge distribution in the vicinity of the calmodulin binding site. The folding we propose is in good agreement with digestion experiments and explains observations in diseases resulting from mutations of human spectrin.

The complete sequence of Drosophila alpha-spectrin: conservation of structural domains between alpha-spectrins and alpha-actinin

We report the complete sequence of Drosophila alpha-spectrin and show that it is similar to vertebrate nonerythroid spectrins. As in vertebrates, the alpha subunit consists of two large domains of repetitive sequence (segments 1-9 and 11-19) separated by a short nonrepetitive sequence (segment 10). The 106-residue repetitive segments are defined by a consensus sequence of 54 residues. Chicken alpha-spectrin (Wasenius, V.-M., M. Saraste, P. Salven, M. Eramaa, L. Holm, V.-P. Lehto. 1989. J. Cell Biol. 108:79-93) shares 50 of these consensus positions. Through comparison of spectrin and alpha-actinin sequences, we describe a second lineage of spectrin segments (20 and 21) that differs from the 106-residue segments by an 8-residue insertion and by lack of many of the consensus residues. We present a model of spectrin evolution in which the repetitive lineage of spectrin segments and the nonrepetitive lineage of segments found in spectrin and alpha-actinin arose by separate multiplication events.

Alpha-actinins, calspectin (brain spectrin or fodrin), and actin participate in adhesion and movement of growth cones

We have used biochemical and immunocytochemical techniques to investigate the possible involvement of membrane cytoskeletal elements such as alpha-actinin, calspectin (brain spectrin or fodrin), and actin in growth cone activities. During NGF-induced differentiation of PC12 cells, alpha-actinin increased in association with neurite outgrowth and was predominantly distributed throughout the entire growth cone and the distal portion of neurites. Filopodial movements were sensitive to Ca2+ flux. Two types of alpha-actinin, with Ca2(+)-sensitive and -insensitive actin binding abilities, were identified in the differentiated cells. Ca2(+)-sensitive alpha-actinin and actin filaments were concentrated in filopodia. The Ca2(+)-insensitive protein was distributed from the body of the growth cone to the distal portion of neurites, corresponding to the substratum-adhesive sites. The location of calspectin in growth cones was similar to that of the Ca2(+)-insensitive alpha-actinin. These results are consistent with the hypothesis that Ca2(+)-sensitive alpha-actinin and actin filaments are involved in Ca2(+)-dependent filopodial movement and Ca2(+)-insensitive alpha-actinin and calspectin are associated with adhesion of growth cones.

Structural analysis of homologous repeated domains in alpha-actinin and spectrin

The amino acid sequences of chick and slime mould alpha-actinin each contain four repeats of approximately 122 residues. These repeats are homologous to the 18-22 repeats, each of approximately 106 residues, found in the alpha and beta subunits of spectrin and fodrin, and to the multiple repeats of approximately 110 residues found in the Duchenne muscular dystrophy protein (dystrophin). The repeats correspond to the elongated rod-like portion of these molecules. We present a multiple sequence alignment of 21 repeats from this superfamily (8 alpha-actinin and 13 spectrin/fodrin), based on optimal pairwise alignments, from which a characteristic consensus pattern of amino acid types is deduced. Trp 46 is invariant in all but one repeat, and physicochemical classes of amino acids are conserved at 25 other positions. Secondary structure prediction on both the alpha-actinin and spectrin repeats taken together with the distribution of proline residues in the sequences, strongly suggest that each repeated domain consists of a four-helix structure. Our predictions differ significantly from previous three-helix models based on analyses of fewer sequences. To determine possible interdomain regions, sites of limited proteolysis of the native chick alpha-actinin dimer were determined and located in the amino acid sequence. The majority of these sites were in corresponding positions in different repeats within a segment predicted as a long helix. We propose a model, consistent with the overall dimensions of the rod-like portions of the molecules, in which these long, probably interrupted helices, link adjacent domains.

Cloning and deletion mutagenesis using direct protein-protein interaction on an expression vector. Identification of the calmodulin binding domain of alpha-fodrin

We have screened a lambda gt11 library, constructed with mouse macrophage cDNA, in order to isolate clones that code for calmodulin binding proteins. We have developed a new approach for this purpose using radioactive calmodulin (produced by genetic engineering) to detect fusion proteins that interact with this protein with high affinity. A cDNA clone that codes for mouse macrophage fodrin was isolated, sequenced and identified. By deleting part of the sequence the calmodulin binding domain was located on the fodrin sequence. The site is situated on repeat 11 of fodrin and probably on the extra arm of this repeat. The method we developed is widely applicable to site-directed mutagenesis of interacting proteins.

Contributions of the beta-subunit to spectrin structure and function

The three avian spectrins that have been characterized consist of a common alpha-subunit (240 kD) paired with an isoform-specific beta-subunit from either erythrocyte (220 or 230 kD), brain (235 kD), or intestinal brush border (260 kD). Analysis of avian spectrins, with their naturally occurring "subunit replacement" has proved useful in assessing the relative contribution of each subunit to spectrin function. In this study we have completed a survey of avian spectrin binding properties and present morphometric analysis of the relative flexibility and linearity of various avian and human spectrin isoforms. Evidence is presented that, like its mammalian counterpart, avian brain spectrin binds human erythroid ankyrin with low affinity. Cosedimentation analysis demonstrates that 1) avian erythroid protein 4.1 stimulates spectrin-actin binding of both mammalian and avian erythrocyte and brain spectrins, but not the TW 260/240 isoform, 2) calpactin I does not potentiate actin binding of either TW 260/240 or brain spectrin, and 3) erythrocyte adducin does not stimulate the interaction of TW 260/240 with actin. In addition, a morphometric analysis of rotary-shadow images of spectrin isoforms, individual subunits, and reconstituted complexes from isolated subunits was performed. This analysis revealed that the overall flexibility and linearity of a given spectrin heterodimer and tetramer is largely determined by the intrinsic rigidity and linearity of its beta-spectrin subunit. No additional rigidity appears to be imparted by noncovalent associations between the subunits. The scaled flexural rigidity of the most rigid spectrin analyzed (human brain) is similar to that reported for F-actin.

Functional diversity among spectrin isoforms

The purpose of this review on spectrin is to examine the functional properties of this ubiquitous family of membrane skeletal proteins. Major topics include spectrin-membrane linkages, spectrin-filament linkages, the subcellular localization of spectrins in various cell types and a discussion of major functional differences between erythroid and nonerythroid spectrins. This includes a summary of studies from our own laboratories on the functional and structural comparison of avian spectrin isoforms which are comprised of a common alpha subunit and a tissue-specific beta subunit. Consequently, the observed differences among these spectrins can be assigned to differences in the properties of the beta subunits.

Primary structure of the brain alpha-spectrin

We have determined the nucleotide sequence coding for the chicken brain alpha-spectrin. It is derived both from the cDNA and genomic sequences, comprises the entire coding frame, 5' and 3' untranslated sequences, and terminates in the poly(A)-tail. The deduced amino acid sequence was used to map the domain structure of the protein. The alpha-chain of brain spectrin contains 22 segments of which 20 correspond to the repeat of the human erythrocyte spectrin (Speicher, D. W., and V. T. Marchesi. 1984. Nature (Lond.). 311:177-180.), typically made of 106 residues. These homologous segments probably account for the flexible, rod-like structure of spectrin. Secondary structure prediction suggests predominantly alpha-helical structure for the entire chain. Parts of the primary structure are excluded from the repetitive pattern and they reside in the middle part of the sequence and in its COOH terminus. Search for homology in other proteins showed the presence of the following distinct structures in these nonrepetitive regions: (a) the COOH-terminal part of the molecule that shows homology with alpha-actinin, (b) two typical EF-hand (i.e., Ca2+-binding) structures in this region, (c) a sequence close to the EF-hand that fulfills the criteria for a calmodulin-binding site, and (d) a domain in the middle of the sequence that is homologous to a NH2-terminal segment of several src-tyrosine kinases and to a domain of phospholipase C. These regions are good candidates to carry some established as well as some yet unestablished functions of spectrin. Comparative analysis showed that alpha-spectrin is well conserved across the species boundaries from Xenopus to man, and that the human erythrocyte alpha-spectrin is divergent from the other spectrins.

Chromosomal location of three spectrin genes: relationship to the inherited hemolytic anemias of mouse and man

Three genetic loci in the mouse affect the synthesis and assembly of the erythrocyte membrane skeleton. The spherocytosis and jaundiced loci affect the membrane skeletal protein known as spectrin. The normoblastosis locus affects the spectrin binding protein called ankyrin. We have obtained genetic data that define the linkage relationships among three spectrin genes and the spherocytosis and jaundiced loci. The erythroid alpha-spectrin gene is tightly linked to the spherocytosis locus on chromosome 1 and the jaundiced locus is on chromosome 12, tightly linked to the erythroid beta-spectrin gene. The brain alpha-spectrin (alpha-fodrin) gene is located on the centromeric end of chromosome 2 and is not closely linked to any previously mapped erythroid or neurological mutation. These results are consistent with the hypothesis that defects in the alpha- and beta-spectrin genes cause the spherocytosis and jaundiced hemolytic anemias in mice. All five loci studied are located within chromosomal segments that are conserved between mouse and man. Analysis of the data from the chromosome 12 study defines a new order for the genes on that chromosome and delineates the largest mouse/human conserved chromosomal segment yet known.

Comparison of nonerythroid alpha-spectrin genes reveals strict homology among diverse species

The spectrins are a family of widely distributed filamentous proteins. In association with actin, spectrins form a supporting and organizing scaffold for cell membranes. Using antibodies specific for human brain alpha-spectrin (alpha-fodrin), we have cloned a rat brain alpha-spectrin cDNA from an expression library. Several closely related human clones were also isolated by hybridization. Comparison of sequences of these and other overlapping nonerythroid and erythroid alpha-spectrin genes demonstrated that the nonerythroid genes are strictly conserved across species, while the mammalian erythroid genes have diverged rapidly. Peptide sequences deduced from these cDNAs revealed that the nonerythroid alpha-spectrin chain, like the erythroid spectrin, is composed of multiple 106-amino-acid repeating units, with the characteristic invariant tryptophan as well as other charged and hydrophobic residues in conserved locations. However, the carboxy-terminal sequence varies markedly from this internal repeat pattern and may represent a specialized functional site. The nonerythroid alpha-spectrin gene was mapped to human chromosome 9, in contrast to the erythroid alpha-spectrin gene, which has previously been assigned to a locus on chromosome 1.

Spectrin and related molecules

This review begins with a complete discussion of the erythrocyte spectrin membrane skeleton. Particular attention is given to our current knowledge of the structure of the RBC spectrin molecule, its synthesis, assembly, and turnover, and its interactions with spectrin-binding proteins (ankyrin, protein 4.1, and actin). We then give a historical account of the discovery of nonerythroid spectrin. Since the chicken intestinal form of spectrin (TW260/240) and the brain form of spectrin (fodrin) are the best characterized of the nonerythroid spectrins, we compare these molecules to RBC spectrin. Studies establishing the existence of two brain spectrin isoforms are discussed, including a description of the location of these spectrin isoforms at the light- and electron-microscope level of resolution; a comparison of their structure and interactions with spectrin-binding proteins (ankyrin, actin, synapsin I, amelin, and calmodulin); a description of their expression during brain development; and hypotheses concerning their potential roles in axonal transport and synaptic transmission.

A monoclonal antibody against a synthetic peptide reveals common structures among spectrins and alpha-actinin

A monoclonal antibody (Mab) against a synthetic peptide, SEDYGKDL, corresponding to one conserved sequence in the chicken alpha-fodrin repeats reacts in immunoblotting with avian alpha-spectrin and alpha-fodrin, both mammalian spectrins and with mammalian alpha-fodrin. This Mab also reacts with alpha-actinin in both chicken and human cells. Our results confirm the previously detected structural homology between spectrins and alpha-actinin and implicate their common evolutionary origin.