An analogue of the erythroid membrane skeletal protein 4.1 in nonerythroid cells

A novel neuron-enriched homolog of the erythrocyte membrane cytoskeletal protein 4.1

We report the molecular cloning and characterization of 4.1N, a novel neuronal homolog of the erythrocyte membrane cytoskeletal protein 4.1 (4.1R). The 879 amino acid protein shares 70, 36, and 46% identity with 4.1R in the defined membrane-binding, spectrin-actin-binding, and C-terminal domains, respectively. 4.1N is expressed in almost all central and peripheral neurons of the body and is detected in embryonic neurons at the earliest stage of postmitotic differentiation. Like 4.1R, 4.1N has multiple splice forms as evidenced by PCR and Western analysis. Whereas the predominant 4.1N isoform identified in brain is approximately 135 kDa, a smaller 100 kDa isoform is enriched in peripheral tissues. Immunohistochemical studies using a polyclonal 4.1N antibody revealed several patterns of neuronal staining, with localizations in the neuronal cell body, dendrites, and axons. In certain neuronal locations, including the granule cell layers of the cerebellum and dentate gyrus, a distinct punctate-staining pattern was observed consistent with a synaptic localization. In primary hippocampal cultures, mouse 4.1N is enriched at the discrete sites of synaptic contact, colocalizing with the postsynaptic density protein of 95 kDa (a postsynaptic marker) and glutamate receptor type 1 (an excitatory postsynaptic marker). By analogy with the roles of 4.1R in red blood cells, 4.1N may function to confer stability and plasticity to the neuronal membrane via interactions with multiple binding partners, including the spectrin-actin-based cytoskeleton, integral membrane channels and receptors, and membrane-associated guanylate kinases.

A novel neuron-enriched homolog of the erythrocyte membrane cytoskeletal protein 4.1

We report the molecular cloning and characterization of 4.1N, a novel neuronal homolog of the erythrocyte membrane cytoskeletal protein 4.1 (4.1R). The 879 amino acid protein shares 70, 36, and 46% identity with 4.1R in the defined membrane-binding, spectrin-actin-binding, and C-terminal domains, respectively. 4.1N is expressed in almost all central and peripheral neurons of the body and is detected in embryonic neurons at the earliest stage of postmitotic differentiation. Like 4.1R, 4.1N has multiple splice forms as evidenced by PCR and Western analysis. Whereas the predominant 4.1N isoform identified in brain is approximately 135 kDa, a smaller 100 kDa isoform is enriched in peripheral tissues. Immunohistochemical studies using a polyclonal 4.1N antibody revealed several patterns of neuronal staining, with localizations in the neuronal cell body, dendrites, and axons. In certain neuronal locations, including the granule cell layers of the cerebellum and dentate gyrus, a distinct punctate-staining pattern was observed consistent with a synaptic localization. In primary hippocampal cultures, mouse 4.1N is enriched at the discrete sites of synaptic contact, colocalizing with the postsynaptic density protein of 95 kDa (a postsynaptic marker) and glutamate receptor type 1 (an excitatory postsynaptic marker). By analogy with the roles of 4.1R in red blood cells, 4.1N may function to confer stability and plasticity to the neuronal membrane via interactions with multiple binding partners, including the spectrin-actin-based cytoskeleton, integral membrane channels and receptors, and membrane-associated guanylate kinases.

Evidence for the association of protein 4.1 immunoreactive forms with neurofibrillary tangles in Alzheimer's disease brains

The formation of neurofibrillary tangles (NFTs) and paired-helical filaments (PHFs) in Alzheimer's disease (AD) reflects a major disorganization of the cytoskeleton. The role of the neuronal membrane skeleton in the development of these abnormalities has not previously been investigated. In this study, we used 9 antibodies raised against the erythrocyte membrane skeleton protein 4.1 (P4.1) for immunocytochemical and immunoblot analyses to investigate whether or not the brain homologues of this protein were constituents of NFTs or PHFs. Our results show that 7 of the 9 monospecific antibodies against the human and pig erythrocyte P4.1 stained NFTs in the prefrontal cortex and hippocampus of AD brains. The P4.1 antibodies used here did not cross-react with tau protein isolated from AD brain, and preabsorption of these antibodies with tau protein did not cause loss of NFT staining. In age-matched control brains, these P4.1 antibodies stained neuronal cell bodies or nuclei. Six of the antibodies also stained isolated NFTs but the SDS-insoluble NFTs were immunostained only by two of the P4.1 antibodies. By using inositol hexaphosphate affinity chromatography and immunoblot analysis, we identified a 68-kDa protein as the most likely brain analogue of P4.1. When SDS-extracted proteins from the isolated NFTs were immunoblotted, a 50-kDa band was immunostained. The 68-kDa and 50-kDa proteins were not stained by tau protein and neurofilament subunit NF-H antibodies, that strongly stained NFTs. We conclude that brain protein 4.1 isoform(s) are constituents of NFTs in AD.

Radixin is a novel member of the band 4.1 family

Radixin is an actin barbed-end capping protein which is highly concentrated in the undercoat of the cell-to-cell adherens junction and the cleavage furrow in the interphase and mitotic phase, respectively (Tsukita, Sa., Y. Hieda, and Sh. Tsukita. 1989 a.J. Cell Biol. 108:2369-2382; Sato, N., S. Yonemura, T. Obinata, Sa. Tsukita, and Sh. Tsukita. 1991. J. Cell Biol. 113:321-330). To further understand the structure and functions of the radixin molecule, we isolated and sequenced the cDNA clones encoding mouse radixin. Direct peptide sequencing of radixin and immunological analysis with antiserum to a fusion protein were performed to confirm that the protein encoded by these clones is identical to radixin. The composite cDNA is 4,241 nucleotides long and codes for a 583-amino acid polypeptide with a calculated molecular mass of 68.5 kD. Sequence analysis has demonstrated that mouse radixin shares 75.3% identity with human ezrin, which was reported to be a member of the band 4.1 family. We then isolated the cDNA encoding mouse ezrin. Sequence analysis and Northern blot analysis revealed that radixin and ezrin are similar but distinct (74.9% identity), leading us to conclude that radixin is a novel member of the band 4.1 family. In erythrocytes the band 4.1 protein acts as a key protein in the association of short actin filaments with a plasma membrane protein (glycophorin), together with spectrin. Therefore, the sequence similarity between radixin and band 4.1 protein described in this study favors the idea that radixin plays a crucial role in the association of the barbed ends of actin filaments with the plasma membrane in the cell-to-cell adherens junction and the cleavage furrow.

Characterization of isoforms of protein 4.1 present in the nucleus

Although protein 4.1 was originally identified as an element of the erythrocyte membrane skeleton, its presence in most mammalian cell types is now well described. Antibodies raised against erythrocyte protein 4.1 or synthetic peptides corresponding to the spectrin-actin-binding domain of protein 4.1 react with plasma membranes and, unexpectedly, nuclei of different cell types. Nuclear staining was further confirmed in isolated nuclei prepared from rat liver and human leukaemic cell lines. Immunoblot analysis of subcellular fractions derived from these cells revealed three prominent proteins, of 80, 135 and 145 kDa. The structural relationship of the high-molecular-mass proteins with erythrocyte protein 4.1 was demonstrated by peptide mapping. These results indicate that mammalian nucleated cells contain several isoforms of erythrocyte protein 4.1 and that some high-molecular-mass forms may primarily reside in the nucleus.

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.

Toad urinary bladder epithelial cells contain an analogue of cytoskeletal protein 4.1

Epithelial cell polarity and vectorial transport require cytoskeletal proteins that maintain local cell membrane structure and mediate cytoplasmic vesicle movement. The cytoskeleton of leaky epithelia, such as the intestinal mucosa and renal proximal tubule cells, has been extensively studied. However, cytoskeletal studies in tight epithelia such as the mammalian collecting duct and toad urinary bladder generally have been confined to ultrastructural investigation. Recent research in nonepithelial cell types has identified an interesting family of cytoskeletal proteins. Present in multiple cell types, these protein 4.1 analogues share a number of similar functional characteristics, yet are structurally diverse. They are multiply phosphorylated by several different kinases, and phosphorylation regulates their associations with other cytoskeletal constituents, integral membrane components, and cytoplasmic vesicles. Using a combination of immunochemical and immunofluorescent techniques, we have demonstrated that toad bladder epithelial cells contain a 65-kDa analogue of human erythrocyte protein 4.1. Toad bladder epithelial cell protein 4.1 is structurally similar to its erythrocyte counterpart and is phosphorylated. This protein 4.1 species is present throughout the toad bladder granular cell cytoplasm, suggesting that it participates in multiple granular cell functions.

Presence and localization of proteins immunologically related to erythrocyte protein 4.1 in human skin

Analogues of human erythrocyte protein 4.1 have been examined in the human skin by immunochemical techniques using anti-human erythrocyte protein 4.1 antibodies. Immunoblot analysis revealed that human epidermis contains 4.1-like proteins of 80 kDa and 78 kDa that cross react with anti-protein 4.1 antibodies. Analysis with immunofluorescence microscopy revealed that the plasma membrane of the human epidermal keratinocyte was stained intensively in the basal cells, whereas spinous cells were moderately stained. It is noted that eccrine sweat gland cells and ductal cells were also stained in the peripheral cytoplasma. Taken together, these results demonstrate that 4.1-like proteins are present in human epidermal keratinocytes, eccrine sweat gland cells and ductal cells. The present findings enable us to suggest that a membrane skeletal protein lattice might exist in these cells.

Diversity in motile responses of human neutrophil granulocytes: functional meaning and cytoskeletal basis

Different agonists induce motility and shape changes, but only a specific polarized shape is correlated with directed migration. An intact and dynamic actin network appears to be important for motility and migration. Motility is usually associated with an increased level of F-actin, and a specific location of F-actin into surface protrusions. For locomotion, a specific location of F-actin, rather than a large net increase in F-actin appears to be of importance. Three major groups of responses can be distinguished on the basis of the type of shape changes, functional activity and organization of F-actin. 1. Agents capable of polarizing cells, such as chemotactic peptides, and microtubule-disassembling agents elicit, at appropriate concentrations, a marked chemokinetic response, but little if any fluid pinocytosis. F-actin shows a polar location, being concentrated mainly in the protrusions at the leading front. Chemotactic peptide also induces an increase in the level of F-actin and cytoskeleton-associated actin. It is, however, not clear if front-tail polarity and locomotion, induced by chemotactic peptide after longer time of stimulation, correlate with an actual increase in the level of cytoskeleton-associated actin. 2. Activators of protein kinase C such as PMA and diacylglycerols, induce nonpolar cells with surface projections. PMA and diacylglycerols stimulate pinocytosis substantially. All three agents tend to inhibit locomotion or chemotaxis as an immediate response. They also increase the percentage of cytoskeletal actin, and induce an enrichment of F-actin in surface projections. 3. Circus movement may occur in response to D20. These cells show little or no stimulation of locomotion or pinocytosis. Thus the functional significance of this motor response remains to be elucidated. We conclude that different agonists can induce motility and shape changes, but not necessarily chemotaxis. Only a polarized shape is correlated with directed locomotion. An intact and dynamic actin network appears to be important for motility including locomotion. Motility is usually associated with an increased level of F-actin, and a specific location of F-actin into surface protrusions. The actin-associated proteins alpha-Actinin, myosin and actin-binding protein appear also to be important for pseudopod formation. For locomotion, a specific location of F-actin, rather than a large net increase in F-actin may be of importance.

Molecular analysis of insertion/deletion mutations in protein 4.1 in elliptocytosis. I. Biochemical identification of rearrangements in the spectrin/actin binding domain and functional characterizations

Protein 4.1 (80 kD) interacts with spectrin and short actin filaments to form the erythrocyte membrane skeleton. Mutations of spectrin and protein 4.1 are associated with elliptocytosis or spherocytosis and anemia of varying severity. We analyzed two mutant protein 4.1 molecules associated with elliptocytosis: a high molecular weight 4.1 (95 kD) associated with mild elliptocytosis without anemia, and a low molecular weight 4.1 (two species at 68 and 65 kD) associated with moderate elliptocytosis and anemia. 4.1(95) was found to contain a approximately 15-kD insertion adjacent to the spectrin/actin binding domain comprised, at least in part, of repeated sequence. 4.1(68/65) was found to lack the entire spectrin-actin binding domain. The mechanical stability of erythrocyte membranes containing 4.1(95) was identical to that of normal membranes, consistent with the presence of an intact spectrin-actin binding domain in protein 4.1. In contrast, membranes containing 4.1(68/65) have markedly reduced mechanical stability as a result of deleting the spectrin-actin binding domain. The mechanical stability of these membranes was improved following reconstitution with normal 4.1. These studies have thus enabled us to establish the importance of the spectrin-actin binding domain in regulating the mechanical stability of the erythrocyte membrane.

Membrane skeleton protein 4.1 in developing Xenopus: expression in postmitotic cells of the retina

Membrane skeleton protein 4.1 plays a key role in modulating the interactions of spectrin, actin, and integral membrane proteins in erythroid and nonerythroid cells. We have investigated its structure and expression during embryonic development of Xenopus laevis. An analysis of the complete 2758-nucleotide sequence and predicted translation of 801 amino acids (85.5 kDa) of X. laevis oocyte protein 4.1 reveals that, within overlapping regions, oocyte protein 4.1 is 74% identical to a composite amino acid sequence of human erythroid and lymphoid protein 4.1 and has an identity similar to that of amino acid motifs variably expressed in either human erythroid or lymphoid protein 4.1 S1 nuclease protection analysis demonstrates the presence of a single species of protein 4.1 transcript in embryos. Antibodies produced against X. laevis protein 4.1 fusion protein recognize two bands of 180 and 115 kDa on Western blots of X. laevis embryos and retina and, using immunocytochemical techniques, label the developing retina most intensely. In vitro transcription of a cDNA construct fully encoding X. laevis protein 4.1 yields a synthetic mRNA which, when translated in vitro, produces a polypeptide that comigrates on SDS-polyacrylamide gels with the 115-kDa form of embryos and retina. Protein 4.1 is found exclusively in photoreceptors following the terminal mitosis of retinal neurons. When retinal synaptogenesis is complete, protein 4.1 is also expressed in the inner retina. In adult amphibian retinas, protein 4.1 is detected in photoreceptors, bipolar cells, and ganglion cell axons. As these cell types have previously been shown to express spectrin, actin, and ankyrin, it is likely that the membrane skeleton of erythrocytes and retinal cells share functional similarities.

Heterogeneity of mRNA and protein products arising from the protein 4.1 gene in erythroid and nonerythroid tissues

Immunologically cross-reactive isoforms of the cytoskeletal element protein 4.1 have been identified in many tissues in which they exhibit heterogeneity of molecular weight, abundance, and intracellular localization. To examine the basis for isoform production in erythroid and nonerythroid tissues, we have compared the structure and expression of cDNAs isolated from human erythroid and nonerythroid sources. We have encountered cDNAs representing many distinct mRNA sequences. These exhibit complete nucleotide sequence homology along most of their lengths. Differences were confined to five sequence blocks designated Motifs I-V, which were present or absent in each mRNA moiety. Motif I was expressed only in erythroid cells; it encodes 21 amino acids in a well-characterized spectrin/actin binding domain. Motif II, located near the COOH terminus of the 80-kD "erythroid" protein 4.1 molecule is present in the vast majority of transcripts from both erythroid and nonerythroid cells. Motifs IV and V alter the 5' untranslated region: simultaneous insertion of Motif IV and deletion of Motif V in the untranslated region inserts a new initiator methionine and establishes a contiguous open reading frame encoding a novel 135-kD protein 4.1 molecule. By immunochemical analysis we have identified the longer isoform in cells. Our results are most consistent with tissue-specific alternative mRNA splicing of transcripts of the protein 4.1 gene to yield numerous isoforms. These isoforms exhibit tissue specificity and alter strategic portions of the molecule. Moreover, we describe a novel high molecular weight form of protein 4.1 that arises by splicing events which allow translation at an upstream site.

Characterization and cytoskeletal association of a major cell surface glycoprotein, GP 140, in human neutrophils

The binding of specific ligands to neutrophil cell surface receptors and the association of these receptors with the cytoskeleton may represent an essential step in activation. To identify surface proteins that are linked to the cytoskeleton during activation, neutrophil 125I-surface labeled plasma membranes were extracted with Triton X-100, and the soluble and insoluble (cytoskeleton) fractions analyzed by SDS-PAGE and autoradiography. The major cell surface proteins recruited to the cytoskeleton after activation with Con A, FMLP, zymosan-activated serum, or immune complexes possessed a relative molecular mass in the range of 80 to 13 kD. In addition to these proteins, WGA stimulates the recruitment of a 140-kD protein (GP 140) to the cytoskeletal fraction. That GP 140 is a WGA-binding protein was verified by Western blotting and WGA-Sepharose affinity chromatography. The Coomassie blue staining pattern of the WGA cytoskeletal fraction revealed major protein bands at apparent molecular weights of greater than 200 (approximately 250, 240, 235), 200, 115, 82/78 (a doublet), 56, 43, 36, and 18 kD. Labeling cells with 32PO4 before WGA treatment indicated that the cytoskeletal proteins with molecular weights of 115, 82/78, and 72 kD, and a 40-kD detergent soluble protein, are phosphorylated during activation. The 78 kD cytoskeletal phosphoprotein co-migrates with the lower subunit of erythrocyte (RBC) band 4.1 and shows strong cross-reactivity with RBC anti-band 4.1 antibody. Phosphorylation of cytoskeletal proteins like 4.1 may be involved in the regulation of interactions between GP 140 and the actin-containing cytoskeleton. Unlike the C3bi receptor, GP 140 is a major surface component of unactivated PMNs, has no stoichiometrically related 95-kD subunit, and has two isoforms with pIs in the range of 6.4 to 6.6. Under conditions that result in an increased expression of the C3bi receptor (such as treatment with the Ca2+ ionophore A23187), the amount of GP 140 on the PMN cell surface appears to be significantly reduced. The interaction of GP 140 with the cytoskeleton during activation suggests that GP 140 may play an important role in neutrophil functional responses.

Ankyrin links fodrin to the alpha subunit of Na,K-ATPase in Madin-Darby canine kidney cells and in intact renal tubule cells

In nonerythroid cells the distribution of the cortical membrane skeleton composed of fodrin (spectrin), actin, and other proteins varies both temporally with cell development and spatially within the cell and on the membrane. In monolayers of Madin-Darby canine kidney (MDCK) cells, it has previously been shown that fodrin and Na,K-ATPase are codistributed asymmetrically at the basolateral margins of the cell, and that the distribution of fodrin appears to be regulated posttranslationally when confluence is achieved (Nelson, W. J., and P. I. Veshnock. 1987. J. Cell Biol. 104:1527-1537). The molecular mechanisms underlying these changes are poorly understood. We find that (a) in confluent MDCK cells and intact kidney proximal tubule cells, Na,K-ATPase, fodrin, and analogues of human erythrocyte ankyrin are precisely colocalized in the basolateral domain at the ultrastructural level. (b) This colocalization is only achieved in MDCK cells after confluence is attained. (c) Erythrocyte ankyrin binds saturably to Na,K-ATPase in a molar ratio of approximately 1 ankyrin to 4 Na,K-ATPase's, with a kD of 2.6 microM. (d) The binding of ankyrin to Na,K-ATPase is inhibited by the 43-kD cytoplasmic domain of erythrocyte band 3. (e) 125I-labeled ankyrin binds to the alpha subunit of Na,K-ATPase in vitro. There also appears to be a second minor membrane protein of approximately 240 kD that is associated with both erythrocyte and kidney membranes that binds 125I-labeled ankyrin avidly. The precise identity of this component is unknown. These results identify a molecular mechanism in the renal epithelial cell that may account for the polarized distribution of the fodrin-based cortical cytoskeleton.

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.

Abolition of actin-bundling by phosphorylation of human erythrocyte protein 4.9

Protein 4.9, first identified as a component of the human erythrocyte membrane skeleton, binds to and bundles actin filaments. Protein 4.9 is a substrate for various kinases, including a cyclic AMP(cAMP)-dependent one, in vivo and in vitro. We show here that phosphorylation of protein 4.9 by the catalytic subunit of cAMP-dependent protein kinase reversibly abolishes its actin-bundling activity, but phosphorylation by protein kinase C has no such effect. A quantitative immunoassay showed that human erythrocytes contain 43,000 trimers of protein 4.9 per cell, which is equivalent to one trimer for each actin oligomer in these red blood cells. As analogues of protein 4.9 have been identified together with analogues of other erythroid skeletal proteins in non-erythroid tissues of numerous vertebrates, phosphorylation and dephosphorylation of protein 4.9 may be the basis for a mechanism that regulates actin bundling in many cells.

Modulation of erythrocyte membrane material properties by Ca2+ and calmodulin. Implications for their role in regulation of skeletal protein interactions

Skeletal proteins of the red blood cell apparently play an important role in regulating membrane material properties of deformability and stability. However, the role of various intracellular constituents in regulating membrane properties has not been clearly defined. To determine whether Ca2+ and calmodulin might play a role in this regulation, we measured the membrane stability and deformability of resealed ghosts prepared in the presence of varying concentrations of Ca2+ and calmodulin (CaM). For membranes resealed in the presence of Ca2+ and physiologic concentrations of CaM (2-8 microM), membrane stability decreased with increasing Ca2+ concentrations (greater than 1.0 microM). Moreover, Ca2+ and CaM-induced alterations in membrane stability were completely reversible. In the absence of CaM, an equivalent decrease in membrane stability was seen only when Ca2+ concentration was two orders of magnitude higher (greater than 100 microM). Calmodulin did not alter membrane stability in the absence of Ca2+. Compared with these changes in membrane stability, membrane deformability decreased only at Ca2+ concentrations greater than 100 microM, and calmodulin had no effect on Ca2+-induced decrease in membrane deformability. Examination of the effects of Ca2+ and CaM on various membrane interactions have enabled us to suggest that spectrin-protein 4.1-actin interaction may be one of the targets for the effect of Ca2+ and CaM. These results imply that Ca2+ and calmodulin can regulate membrane stability through modulation of skeletal protein interactions, and that these protein interactions are of a dynamic nature on intact membranes.

Selective expression of an erythroid-specific isoform of protein 4.1

We have conducted comparative analysis of nucleotide sequences encoding erythroid and lymphoid protein 4.1 isoforms. The lymphoid protein 4.1 isoforms exhibit several nucleotide sequence motifs that appear to be either inserted into or deleted from the mRNA sequence by alternative splicing of a common mRNA precursor. One of these motifs, located within the spectrin-actin binding domain, is found only in erythroid cells and is specifically produced during erythroid cell maturation. The selective expression of the alternatively spliced mRNA during erythroid maturation implies the existence of a lineage-specific splicing mechanism whose activity is triggered by terminal maturation.

Antisense RNA inhibits expression of membrane skeleton protein 4.1 during embryonic development of Xenopus

Plasmids expressing partial-length sense or antisense protein 4.1 RNA were microinjected into fertilized Xenopus eggs. Nuclease protection assays reveal that antisense protein 4.1 RNA lead to the specific loss of endogenous protein 4.1 transcripts after midblastula transition, with no effect on the levels of three unrelated transcripts. As a control, we show that this dramatic loss of endogenous protein 4.1 transcripts is blocked when fertilized eggs receive a second injection of plasmids that express partial-length sense protein 4.1 RNA. Immunocytochemistry of tadpole embryos with antibodies monospecific for protein 4.1 demonstrates that the antisense protein 4.1 RNA blocks the normal expression of protein 4.1 in embryos and interferes with the normal interdigitation of the photoreceptor outer segments with the pigment epithelium layer in the retina. These data suggest that reduced expression of a single membrane skeleton protein is sufficient to perturb normal cellular interactions of the retina.

A heparin-binding protein involved in inhibition of smooth-muscle cell proliferation

A heparin-binding protein was isolated from bovine uteri and purified to homogeneity. This protein appears as a double band of approx. 78 kDa in SDS/polyacrylamide-gel electrophoresis and has an isoelectric point of 5.2. The binding of heparin to this protein is saturable. No other glycosaminoglycan from mammalian tissue, such as hyaluronic acid, chondroitin sulphate, dermatan sulphate or keratan sulphate, binds to the 78 kDa protein. Dextran sulphate binds in a non-saturable fashion. Certain heparan sulphate polysaccharide structures are required for binding to the 78 kDa protein. Some proteoheparan sulphates, such as endothelial cell-surface proteoheparan sulphate, show only weak interaction with the 78 kDa protein in contrast with a basement-membrane proteoheparan sulphate from HR-9 cells. Antibodies against the 78 kDa protein inhibit binding of proteoheparan [35S]sulphate from basement membranes to smooth-muscle cells. Conventional antibodies, Fab fragments and some monoclonal antibodies, inhibit smooth-muscle cell proliferation in a similar range as that observed for heparin. The protein was detected in a variety of tissues and cells but not in blood cells. A possible role of this protein as a receptor for heparin or heparan sulphate and its function in the control of the arterial wall structure are discussed.

Immunohistochemical localization of a membrane-associated, 4.1-like protein in the rat visual cortex during postnatal development

Expression and localization of a membrane-associated protein, an analog of erythrocyte protein 4.1, in the visual cortex were immunohistochemically studied in the rat, ranging in age from newborn to adult. In the adult, dendrites and somas of layer V pyramidal cells were stained by the antiprotein 4.1 antibody. In most of these immunoreactive neurons, the plasma membrane seemed to be preferentially stained. Neurons located in layers II and III of the cortex were only faintly stained, and those in layers IV and VI were not stained. At birth, the immunoreactivity was already present in pyramidal cells located in the upper part of the cortical subplate. Immature neurons located in the cortical plate were not stained by the antibody, suggesting that the 4.1-like protein is expressed only in the neurons that have differentiated or are differentiating. At postnatal days 2-8, immunoreactive neurons were dramatically increased in layers V and VI and intense labeling was seen at the apical dendrites of layer V pyramidal cells. Most of the stained processes of these and other neurons showed a sign of rapid dendritic growth, i.e., growth cones and filopidia. At days 10-17, the basal dendrites of pyramidal cells in layers II and III became detectable, although still slender. At days 20-37, these dendrites in layers II, III, and V became intensely immunoreactive, and dendritic spines were visualized by the antibody. Throughout all the ages, axons of neurons and neuroglia were not stained by the antibody. Also, most of the neurons in layer IV of the cortex were not immunoreactive. These results suggest that the 4.1-like protein is abundantly expressed in growing parts of the dendrites and spines. A hypothesis that this protein may play a role in synaptic plasticity in the developing visual cortex is discussed.

Expression of specific isoforms of protein 4.1 in erythroid and non-erythroid tissues

Protein 4.1 in red cells is an important submembrane linking protein that binds to spectrin actin complexes at one end of its structure and to transmembrane proteins, such as glycophorin, at the other. Protein 4.1 thus contributes to the strength and flexibility of the erythrocyte membrane, a fact dramatically exemplified by the appearance of hereditary hemolytic anemias in patients with absent or abnormal protein 4.1. Recently, protein 4.1 forms have been discovered in many non-erythroid tissues. Their intracellular locations raise the possibility that these isoforms might have different functions. We have thus conducted comparative analysis of erythroid and non-erythroid protein 4.1 forms by cloning and sequencing erythroid and lymphoid protein 4.1 cDNAs. The lymphoid protein 4.1 isoforms exhibit at least five nucleotide sequence motifs that appear to be either inserted or deleted relative to the erythroid mRNA sequence by alternative splicing of a common mRNA precursor. One of these motifs, located within the spectrin-actin binding domain, is found only in erythroid cells and is specifically produced during erythroid cell maturation. The selective expression of this alternatively spliced mRNA during erythroid maturation implies the existence of a lineage specific splicing mechanism whose activity is triggered by terminal maturation. Two motifs alter the 5' untranslated region of the "prototypical" erythroid mRNA in such a way as to permit synthesis of a novel larger isoform. This form appears to localize preferentially in the nucleus. We thus conclude that a single gene gives rise to multiple protein 4.1 isoforms with potentially diverse locations and functions.

Regulated expression of multiple chicken erythroid membrane skeletal protein 4.1 variants is governed by differential RNA processing and translational control

Protein 4.1 is an extrinsic membrane protein that facilitates the interaction of spectrin and actin in the erythroid membrane skeleton and exists as several structurally related polypeptides in chickens. The ratio of protein 4.1 variants is developmentally regulated during terminal differentiation of chicken erythroid and lenticular cells. To examine the mechanisms by which multiple chicken protein 4.1 variants are differentially expressed, we have isolated cDNA clones specific for chicken erythroid protein 4.1. We show that a single protein 4.1 gene gives rise to multiple 6.6-kilobase mRNAs by differential RNA processing. Furthermore, the ratios of protein 4.1 mRNAs change during chicken embryonic erythropoiesis. We observe a quantitative difference in variant ratios when protein 4.1 is synthesized in vivo or in a rabbit reticulocyte lysate in vitro. Our results show that the expression of multiple protein 4.1 polypeptides is regulated at the levels of translation and RNA processing.

300-kD subsynaptic protein copurifies with acetylcholine receptor-rich membranes and is concentrated at neuromuscular synapses

Acetylcholine receptor-rich membranes from the electric organ of Torpedo californica are enriched in the four different subunits of the acetylcholine receptor and in two peripheral membrane proteins at 43 and 300 kD. We produced monoclonal antibodies against the 300-kD protein and have used these antibodies to determine the location of the protein, both in the electric organ and in skeletal muscle. Antibodies to the 300-kD protein were characterized by Western blots, binding assays to isolated membranes, and immunofluorescence on tissue. In Torpedo electric organ, antibodies to the 300-kD protein stain only the innervated face of the electrocytes. The 300-kD protein is on the intracellular surface of the postsynaptic membrane, since antibodies to the 300-kD protein bind more efficiently to saponin-permeabilized, right side out membranes than to intact membranes. Some antibodies against the Torpedo 300-kD protein cross-react with amphibian and mammalian neuromuscular synapses, and the cross-reacting protein is also highly concentrated on the intracellular surface of the post-synaptic membrane.

Beta spectrin bestows protein 4.1 sensitivity on spectrin-actin interactions

The ability of protein 4.1 to stimulate the binding of spectrin to F-actin has been compared by cosedimentation analysis for three avian (erythrocyte, brain, and brush border) and two mammalian (erythrocyte and brain) spectrin isoforms. Human erythroid protein 4.1 stimulated actin binding of all spectrins except the brush border isoform (TW 260/240). These results suggested that the beta subunit determined the protein 4.1 sensitivity of the heterodimer, since all avian alpha subunits are encoded by a single gene. Tissue-specific posttranslational modification of the alpha subunit was excluded by examining the properties of hybrid spectrins composed of the purified alpha subunit from avian erythrocyte or brush border spectrin and the beta subunit of human erythrocyte spectrin. A hybrid composed of avian brush border alpha and human erythroid beta spectrin ran on nondenaturing gels as a discrete band, migrating near human erythroid spectrin tetramers. The actin-binding activity of this hybrid was stimulated by protein 4.1, while either chain alone was devoid of activity. Therefore, although both subunits were required for actin binding, the sensitivity of the spectrin-actin interaction to protein 4.1 is a property uniquely bestowed on the heterodimer by the beta subunit. The singular insensitivity of brush border spectrin to stimulation by erythroid protein 4.1 was also consistent with the absence of proteins in avian intestinal epithelial cells which were immunoreactive with polyclonal antisera sensitive to all of the known avian and human erythroid 4.1 isoforms.

Calpactins: two distinct Ca++-regulated phospholipid- and actin-binding proteins isolated from lung and placenta

Three forms of calpactin, the 36,000 Mr Ca++-binding cytoskeletal protein, were isolated in large amounts from bovine lung and human placenta using cycles of calcium-dependent precipitation followed by solubilization with EGTA-containing buffers. Calpactin-I as a tetramer of heavy (36 kD) and light (11 kD) chains was the predominant form of calpactin isolated, however milligram amounts of the calpactin-I heavy chain monomer and calpactin-II, a related but distinct molecule, were also isolated by this method. Calpactin-II was characterized in some detail and found to bind two Ca++ ions with Kd's of 10 microM in the presence of phosphatidylserine. Both calpactin-I and -II were found to aggregate liposomes at micromolar Ca++ concentrations, suggesting that at least two phospholipid-binding sites are present on these molecules. Both calpactin monomers bind to and bundle actin filament at high (1 mM) but not low (less than 1 microM) Ca++ concentrations. Amino-terminal sequence analysis of a lower molecular mass variant of calpactin-II revealed that this protein was the previously identified human "lipocortin" molecule. Antibodies were elicited to calpactin-I and -II and the cell and subcellular distribution of each was compared. Calpactin-II was only present at high levels in tissues (lung, placenta) which contained high levels of calpactin-I. Other tissues (intestine) contained high calpactin-I and undetectable levels of calpactin-II. Double-label immunofluorescence microscopy on human fibroblasts revealed that, like calpactin-I, calpactin-II is present in a submembraneous reticular network, although the distribution of the two calpactins is not identical.

Lymphoma Thy-1 glycoprotein is linked to the cytoskeleton via a 4.1-like protein

In this study we have found that the phosphoprotein doublet of 68,000 and 65,000 daltons (68/65 kD) in mouse T-lymphoma cells shares several structural and functional similarities with erythrocyte band 4.1. Our evidence for identifying the 68/65-kD doublet as a lymphoma 4.1-like protein is as follows: it displays an immunological cross-reactivity with anti-erythrocyte band 4.1 antibody; it exhibits a Svedberg unit of sedimentation coefficient of 4 S; it is phosphorylated in the presence of phorbol ester (phorbol-12-O-tetradecanoylphorbol-13-acetate) and its phosphorylation requires Ca2+; it is phosphorylated primarily at serine residues; and it can bind directly to fodrin (a spectrin-like actin-binding protein). In addition, this lymphoma 4.1-like protein can be both colocalized and coisolated with the major T-lymphocyte-specific glycoprotein, Thy-1 (gp 25). Therefore, all of these results strongly suggest that the lymphoma 4.1-like protein (68/65-kD doublet) may play a pivotal role in linking the Thy-1 (gp 25) glycoprotein to fodrin which, in turn, binds to the actin filaments that are responsible for recruiting Thy-1 antigens into cap structures.

Molecular cloning of protein 4.1, a major structural element of the human erythrocyte membrane skeleton

Protein 4.1 is an important structural protein that is expressed in erythroid and in a variety of non-erythroid tissues. In mammalian erythrocytes, it plays a key role in regulating membrane physical properties of mechanical stability and deformability by stabilizing spectrin-actin interaction. We report here the molecular cloning and characterization of human erythrocyte protein 4.1 cDNA and the complete amino acid sequence of the protein derived from the nucleotide sequence. Probes prepared from the cloned erythrocyte protein 4.1 cDNA hybridized with distinct mRNA species from a wide variety of non-erythroid tissues, including brain, liver, placenta, pancreas, and intestine, implying substantial homology between erythroid and non-erythroid protein 4.1. The availability of cloned erythrocyte protein 4.1 cDNA should facilitate the study of the functional characteristics of this protein in erythroid as well as non-erythroid cells.

Isolation of bovine aortic endothelial cell plasma membranes: identification of membrane-associated cytoskeletal proteins

The plasma membrane of bovine aortic endothelium was isolated, characterized, and found to contain at least four membrane-associated cytoskeletal proteins. Exposure of the plasma membranes to salt media (up to 1M KCl) resulted in the release of 30% of the total plasma membrane-associated proteins and extraction with 1% Triton X-100, 60%. At least four heavily glycosylated bands (185-, 165-, 150-, and 130,000 mol-wt) were evident. The Triton-insoluble pellet fraction contained several major polypeptides (30-, 43-, 58-, and 240,000 mol-wt), two of which were identified by immunoblotting as cytoplasmic actin (43,000 mol-st) and vimentin (58,000 mol-wt). Strikingly, vimentin and a 240,000 mol-wt polypeptide were routinely present in approximately a mole ratio of 4:1 in more than 60% of the plasma membrane preparations. We also report the presence of a 2.1-like and a 4.1-like protein associated with plasma membranes. The 2.1-like protein demonstrated similar solubilities and apparent molecular weight (210,000) as erythroid protein 2.1. Likewise, the endothelial 4.1-like protein exhibited similar solubilities and apparent molecular weight as erythroid protein 4.1. Immunofluorescence staining of fixed and permeabilized cultures with anti-2.1 antibodies showed a fibrillar pattern. In contrast, cells stained with anti-protein 4.1 were brightly fluorescent, bearing both a diffuse and punctate pattern. This paper presents several novel observations pertaining to the composition of bovine aortic endothelial cell plasma membranes, namely: the presence of two erythroid-like cytoskeletal polypeptides; the presence of vimentin and a 240,000 mol-wt polypeptide in a 4:1 mole ratio in more than 60% of the plasma membrane preparations and the co-elution in a 4:1 mol ratio with a protein perturbant; and the inability to release actin from the plasma membrane preparations, suggesting the association of actin with other molecules in the plasma membrane preparation.

Agorins: major structural proteins of the plasma membrane skeleton of P815 tumor cells

Plasma membranes of P815 mastocytoma cells contain a set of proteins that remain selectively insoluble upon extraction of the membranes with Triton X-100, and appear to form a membrane skeletal matrix independent of the filamentous cytoskeletal systems. EGTA treatment of the matrix was found to release approximately 25% of the protein as polypeptides of 70, 69, 38, and 36 kD, all of which appear to be peripheral components associated with the cytoplasmic face of the plasma membrane via divalent cation-dependent interactions. About 75% of the total matrix protein was recovered in the EGTA-insoluble fraction. Actin accounted for approximately 5% of the total protein in the EGTA-insoluble fraction. The rest was accounted for by two novel proteins of 20 and 40 kD which, despite their relatively low molecular weights, do not enter SDS PAGE gels. Together these proteins account for approximately 15% of the total plasma membrane protein, and are thus present in much higher amounts than any other characterized protein of nucleated cell plasma membranes. Based on the extensive associations of these proteins to form very large detergent-insoluble structures, we propose that they may be named agorin I, the 20-kD protein, and agorin II, the 40-kD protein, from the Greek agora meaning assembly. The amount and properties of these proteins and the appearance of the EGTA-insoluble material in thin-section electron micrographs indicate that the agorins are the major structural elements of the membrane matrix, and thus of the putative membrane skeleton.

Limited proteolysis of the erythrocyte membrane skeleton by calcium-dependent proteinases

The action of purified calcium-dependent proteinases on human erythrocyte membrane skeleton proteins has been examined. Preferential cleavage of proteins 4.1 a and b and band 3 and limited cleavage of alpha- and beta-spectrin occur when either calcium-dependent proteinase I or calcium-dependent proteinase II has access to the cytoplasmic side of the ghost membrane skeleton in the presence of calcium. Thus, when these proteinases are incubated with sealed ghosts they do not cleave these proteins. Leupeptin, mersalyl, the specific cellular protein inhibitor of these enzymes, and calcium chelators can inhibit proteolysis of the red cell ghost proteins by Ca2+-dependent proteinases. Each proteinase has also been loaded into erythrocyte ghosts in the absence of calcium at low ionic strength and subsequently trapped inside by resealing the ghosts. The proteinases were activated by incubating these ghosts in the presence of the calcium ionophore A23187 and calcium. Examination of the ghost proteins by electrophoresis demonstrated calcium-dependent proteolysis of Bands 4.1 and 3 and limited cleavage of alpha- and beta-spectrin similar to that observed on proteolysis of the open, leaky ghosts. In the presence of calcium each calcium-dependent proteinase appears to associate with the erythrocyte ghost membrane.

Restoration of normal membrane stability to unstable protein 4.1-deficient erythrocyte membranes by incorporation of purified protein 4.1

Protein 4.1, a principal component of the erythrocyte membrane skeleton, is thought to be important in regulating membrane stability through its interaction with spectrin and actin. A key role for protein 4.1 has been indicated in studies in which deficiency of this protein was shown to result in marked instability of the membrane. In order to obtain direct evidence for the functional role of protein 4.1, we reconstituted protein 4.1-deficient membranes with purified protein 4.1 and showed restoration of membrane stability. Erythrocyte membranes totally and partially deficient in protein 4.1 were reconstituted by exchange hemolysis with various concentrations of purified protein 4.1, and their stability measured using an ektacytometer. Native erythrocyte membranes totally deficient in protein 4.1 were markedly unstable, while those partially deficient had intermediate reductions in membrane stability. Reconstitution with increasing concentrations of purified protein 4.1 resulted in progressive restoration of membrane stability. Near-normal membrane stability could be restored to both totally and partially protein 4.1-deficient membranes. In contrast, the addition of protein 4.1 to resealed membranes did not improve membrane stability. This implies that the added protein 4.1 must have access to the cell interior in order to affect membrane stability. Furthermore, in control experiments, the addition of protein 4.1 to normal membranes did not increase their stability. Also, the addition of purified spectrin and human serum albumin during resealing did not improve stability of protein 4.1-deficient membranes. These results provide direct evidence for the crucial role of protein 4.1 in regulating erythrocyte membrane stability.

The 4.1-like proteins of the bovine lens: spectrin-binding proteins closely related in structure to red blood cell protein 4.1

The superficial cortical fiber cells of the bovine lens contain membrane-associated proteins of 150,000, 80,000, and 78,000 D that cross-react with antisera prepared against red blood cell (RBC) protein 4.1 (Aster, J. C., G. J. Brewer, S. M. Hanash, and H. Maisel, 1984, Biochem. J., 224:609-616). To further study their relationship to protein 4.1, these proteins were immunoprecipitated from detergent extracts of crude lens membranes with purified polyclonal and monoclonal anti-4.1 antibodies and resolved by SDS PAGE. The electrophoretic mobilities of the lens proteins of 80,000 and 78,000 D were found to be identical to bovine RBC protein 4.1a and protein 4.1b, respectively. One- and two-dimensional peptide mapping revealed that a high degree of structural homology exists among all three of the lens 4.1-like proteins and RBC protein 4.1a and protein 4.1b. Despite the large difference in apparent molecular mass, the 150,000-D lens protein showed only minor peptide map differences. A nitrocellulose filter overlay assay showed that all three of the lens 4.1-like proteins bind to RBC and lens spectrins. We conclude that the bovine lens contains proteins of 80,000 and 78,000 D that are highly similar to protein 4.1 in structure and functional capacity. Additionally, the lens also contains a 4.1 isomorph of 150 kD. Analogous to RBC protein 4.1, these proteins may function in the lens by promoting association of spectrin with actin and by playing a role in the coupling of lens cytoskeleton to plasma membrane.

Mechanisms of cytoskeletal regulation: modulation of aortic endothelial cell protein band 4.1 by the extracellular matrix

The bovine aortic endothelial cell (BAEC) cytoskeleton is a complex structure modulated by many stimuli including release from contact inhibition and various components of the extracellular matrix (ECM). Transduction of information from the ECM to the cell nucleus proceeds via several complex pathways including the cytoskeleton. We have demonstrated the presence of an immunoreactive isoform of the human erythrocyte cytoskeletal protein band 4.1 (4.1) in BAEC. BAEC 4.1 is similar in molecular weight to the erythroid protein by immunoblot analyses and produces a similar pattern of cysteine specific cleavage products consistent with a cluster of cysteine residues previously described in the erythroid molecule. We have also examined the effects of defined ECM proteins on the distributions of cultured BAEC 4.1 and actin filaments (AF) at confluency and following release from contact inhibition. The distribution of 4.1 in BAEC on a plasma fibronectin substrate is complex, having partial codistribution with cytoplasmic AF and a unique perinuclear staining. In contrast, on a collagen type I/III substrate, 4.1 is localized, in part, to peripheral areas of cell-cell contact distinct from the dense peripheral band staining of AF. During migration on this substrate, 4.1 had a filamentous distribution having partial codistribution with AF. Indirect immunofluorescence staining of cross-sections of bovine calf aortae revealed a cortical staining pattern in the aortic endothelial cells with staining noted on the luminal and basolateral aspects of the cells. These data suggest that, in endothelial cells, protein 4.1 is a cortical membrane protein which may function to link actin filaments to other skeletal proteins such as spectrin. These findings also suggest an active role for protein 4.1 in cytoskeletal reorganization events which can occur in response to external stimuli, such as the extracellular matrix or contact with other cells.

Purification of erythrocyte band 4.1 and other cytoskeletal components using hydroxyapatite-Ultrogel

An improved method for purifying erythrocyte band 4.1, the protein which mediates the interaction between spectrin and actin, has been developed. The new procedure, using adsorption chromatography on hydroxylapatite crystals immobilized within a crosslinked agarose gel (HA-Ultrogel), is simple and reproducibly provides a high yield of band 4.1 which is essentially free of protein kinase. Other components eluted from the hydroxylapatite matrix include band 4.9, ankyrin, and a 35,000-Da polypeptide that appears to be glyceraldehyde-3-phosphate dehydrogenase that remains bound to the erythrocyte membrane in 150 mM NaCl.

Co-precipitation of intestinal p36 with a 73-K protein and a high molecular weight factor

p36 is a major substrate of tyrosine kinases that co-localizes with spectrin in nonerythroid cells. Recent studies by Gerke & Weber [14] have shown that p36 can be isolated from intestine by selective extraction with the Ca2+-chelating agent EGTA. We now show that p36 can be re-precipitated by adding free Ca2+ to 1 mM with the co-precipitation of a high molecular weight (MW) factor and a polypeptide of 73K. The 73K protein was purified to apparent homogeneity, rabbit antibodies were raised to it and used in Western blots and immunofluorescence microscopy. The 73K protein is found in a wide range of tissues and is particularly concentrated in fibroblasts, where its distribution partially overlaps that of non-erythroid spectrin.

Pig thyroid spectrin. A membrane-associated protein related in structure and function to brain spectrin

Thyroid spectrin has been obtained pure from pig thyroid glands. This protein, composed of two non-identical polypeptide chains of 240 kDa and 235 kDa, appears to possess the same structural and immunological properties as well as the same calmodulin and actin-binding properties as brain spectrin. Through cross-linking of actin filaments it is a potent gelation factor for F-actin solutions. It represents one of the major protein of the cytoskeleton underlying the thyroid plasma membrane together with myosin, alpha-actinin and actin.

Regulation of the association of membrane skeletal protein 4.1 with glycophorin by a polyphosphoinositide

Many of the physical properties of the erythrocyte membrane appear to depend on the membrane skeleton, which is attached to the membrane through associations with transmembrane proteins. A membrane skeletal protein, protein 4.1, is pivotal in the assembly of the membrane skeleton because of its ability to promote associations between spectrin and actin. Protein 4.1 also binds to the membrane through at least two sites: a high-affinity site on the glycophorins and a site of lower affinity associated with band 3 (ref. 11). The glycophorin-protein 4.1 association has been proposed to be involved in maintenance of cell shape. Here we show that the association between glycophorin and protein 4.1 is regulated by a polyphosphoinositide cofactor. This observation suggests a mechanism which may explain the recently reported dependence of red cell shape on the level of polyphosphoinositides in the membrane.

Mechanisms of cytoskeletal regulation: modulation of membrane affinity in avian brush border and erythrocyte spectrins

The spectrins isolated from chicken erythrocytes and chicken intestinal brush border, TW260/240, share a common alpha subunit and a tissue-specific beta subunit. The ability of these related proteins to bind human erythrocyte inside out vesicles (IOVs) and human erythrocyte ankyrin in vitro have been quantitatively compared with human erythrocyte spectrin. Chicken erythrocyte spectrin binds human IOVs and human ankyrin with affinities nearly identical to that for human erythrocyte spectrin. TW260/240 does not significantly bind to either IOVs or ankyrin. These results demonstrate a remarkable tissue preservation of ankyrin-binding capacity, even between diverse species, and confirm the role of the avian beta-spectrins in modulating this functionality. Avian brush border spectrin may represent a unique spectrin which serves primarily as a filament cross-linker and which does not interact strongly with membrane-associated proteins.

Synapsin I is a spectrin-binding protein immunologically related to erythrocyte protein 4.1

The membrane-associated cytoskeleton is considered to be the apparatus by which cells regulate the properties of their plasma membranes, although recent evidence has indicated additional roles for the proteins of this structure, including an involvement in intracellular transport and exocytosis (see refs 1-3 for review). Of the membrane skeletal proteins, to date only spectrin (fodrin) and ankyrin have been purified and characterized from non-erythroid sources. Protein 4.1 in the red cell is a spectrin-binding protein that enhances the binding of spectrin to actin and can apparently bind to at least one transmembrane protein Immunoreactive forms of 4.1 have been detected in several cell types, including brain. Here we report the purification of brain 4.1 on the basis of its cross-reactivity with erythrocyte 4.1 and spectrin-binding activity. We further show that brain 4.1 is identical to the synaptic vesicle protein, synapsin I, one of the brain's major substrates for cyclic AMP and Ca2+-calmodulin-dependent kinases. Spectrin and synapsin are present in brain homogenates in an approximately 1:1 molar ratio. Although synapsin I has been implicated in synaptic transmission, no activity has been previously ascribed to it.