Fodrin: axonally transported polypeptides associated with the internal periphery of many cells

Binding of HeLa spectrin to a specific HeLa membrane fraction

From 30-40 g of Hela-S3 cells grown in suspension, 0.25-0.50 mg of spectrin has been purified by conventional biochemical procedures starting from a low ionic strength extraction at alkaline pH of crude Hela membranes. Hela spectrin consists in its native form of a tetramer alpha 2 beta 2 of two high molecular weight polypeptides (240,000 and 230,000 daltons). Three different populations of Hela membranes depleted of both spectrin and actin have been prepared on discontinuous sucrose gradients. Surprisingly, spectrin will reassociate with only the heavier membrane fraction. This reassociation is specific for Hela spectrin, since three other purified Hela proteins as well as human erythrocyte spectrin do not reassociate under the same conditions. This binding is not due to the presence of traces of actin still present in the membrane fraction since two Hela actin-binding proteins (filamin I and II) do not show any significant binding to this fraction. The nature of the membrane-binding site for Hela spectrin is discussed.

Intracellular movement of fodrin

Fodrin is an actin/calmodulin-binding protein with similarities to spectrin (erythrocytes) and TW 260/240 (brush border). It is concentrated beneath the plasma membranes of neurons and other cells. We have observed translocations of fodrin in both neurons and lymphocytes. Newly synthesized, radiolabeled fodrin moves down axons at a maximum velocity (about 50 mm/day) that is slower than the most rapidly axonally transported proteins (group I). A portion of fodrin appears to move more slowly at velocities (1-10 mm/day) resembling those of actin and myosin (group IV) and tubulin and neurofilament proteins (group V). In lymphocytes, when certain surface antigens are induced by cross-linking agents to migrate to one pole of the cell and form a cap, fodrin redistributes beneath the membrane and forms a subcap. The movements of fodrin in lymphocyte capping and in the axonal transport of group IV polypeptides have certain similarities. In both cases, the redistribution of fodrin is accompanied by concomitant redistributions of actin, myosin, and calmodulin, and both processes proceed at similar velocities. We consider the possibilities that these two processes are related, both being driven by a submembrane force-generating system comprising in part actin, myosin, and fodrin, and that fodrin serves to link various organelles or proteins to this system.

Expression of the beta subunit of spectrin in nonerythroid cells

Antibodies raised against electrophoretically purified chicken erythrocyte beta subunit of spectrin, called "beta-spectrin," have been used to demonstrate the presence of an immunoreactive form of this polypeptide in nonerythroid tissues. Immunoautoradiography shows that, in chicken erythrocytes, this antiserum reacts with beta-spectrin (Mr 220,000) and another polypeptide (Mr 230,000) that, by two-dimensional tryptic peptide analysis, shows extensive homology with beta-spectrin but not with the alpha subunit of spectrin, called "alpha-spectrin." Immunoautoradiography and immunoprecipitation of various chicken tissues with this antiserum shows that either one variant or both variants of beta-spectrin are expressed. Indirect immunofluorescence reveals that the antiserum reacts with a plasma membrane-associated component of erythroid and some nonerythroid cells. Particularly strong fluorescence is observed in skeletal and cardiac muscle cells where beta-spectrin appears to form a grid-like network along the inner surface of the sarcolemma. The noncoordinated distribution of alpha- and beta-spectrin variants indicates that their expression may be tailored to the functional requirements of the plasma membrane in different cells.

Solubilization and partial purification of protein kinase systems from brain membranes that phosphorylate calspectin. A spectrin-like calmodulin-binding protein (fodrin)

In brain tissue a spectrin-like calmodulin-binding protein calspectin, or fodrin, is concentrated in a synaptosome fraction, where most of the calspectin is associated with the synaptic membranes. This endogenous calspectin was phosphorylated by protein kinase system(s) associated with the membranes. Here, we report the solubilization and partial purification of the membrane-associated calspectin kinase activity. The activity was resolved on a gel filtration column into two fractions, peaks I and II having estimated Mr of 800 000 and 88 000. The activity of peak I was dependent on the presence of both Ca2+ and calmodulin. Peak II revealed a basal activity in the absence of Ca2+ and calmodulin, which was stimulated 2-fold by addition of Ca2+. Calmodulin had no effect on the peak II activity.

Isolation and structural properties of a high-molecular-weight actin-binding protein (filamin-like protein) in hog thyroid gland

A high-molecular-weight protein has been isolated from hog thyroid gland. This protein, with a molecular weight of 475,000 determined by ultracentrifugation and gel filtration, is a complex of two polypeptides with apparent molecular weights of 250,000 and 240,000. It may be related to filamin-like proteins by its physicochemical properties and its immunogenic cross-reactivity towards gizzard filamin antibodies. Furthermore it interacts with F-actin in a stoichiometry of 1 mol of high-molecular-weight protein/approximately 12-14 mol actin monomer allowing microfilament association, as shown by electron microscopy.

Actin polymerization induced by calspectin, a calmodulin-binding spectrin-like protein

We have purified from a membrane fraction of bovine brain a calmodulin-binding protein (calspectin) that shares a number of properties with erythrocyte spectrin: It has a heterodimeric structure with Mr 240 000 and 235 000 and binds to (dimeric form) or crosslinks (tetrameric form) F-actin. We show that calspectin (tetramer) is capable of inducing the polymerization of G-actin to actin filaments by increasing nucleation under conditions where actin alone polymerizes at a much slower rate. Thus, brain calspectin behaves in the same manner as erythrocyte spectrin, supporting the idea that, in conjunction with actin oligomers it comprises the cytoskeletal meshwork underlying the cytoplasmic surface of the nerve cell.

Proteins involved in the attachment of actin to the plasma membrane

Proteins that may be involved in two types of actin-membrane association are discussed. The first set includes alpha-actinin, vinculin, fimbrin and a new cytoskeletal protein that are all concentrated in adhesion plaques, those regions of cultured fibroblasts where bundles of actin microfilaments terminate and where the plasma membrane comes close to the underlying substrate. The properties of non-muscle alpha-actinin suggest that it functions to cross-link actin filaments and thereby stabilize microfilament bundles rather than functioning in their attachment to the membrane. Fimbrin also appears to be involved in bundling of filaments rather than in attachment. In contrast, vinculin binds to the ends of actin filaments in vitro and is probably the best candidate for a role in the attachment of actin to membranes at the adhesion plaque. The discovery of a new protein, 215k, of unknown function, in the adhesion plaque suggests that many more proteins remain to be identified in this region. Attachment of actin filaments to other regions of the plasma membrane is also considered and a protein is described that seems to be a spectrin in brain and other tissues. The brain protein resembles erythrocyte spectrin in its physical properties, in binding actin, in being associated with cell membranes and in cross-reacting immunologically. We suggest that the brain protein and erythrocyte spectrin both belong to a family of related proteins (the spectrins) which function in the attachment of actin to membranes in many different cell types.

Nonerythrocyte spectrins: actin-membrane attachment proteins occurring in many cell types

The properties of brain fodrin have been analyzed and compared with those of erythrocyte spectrin. Both proteins consist of high molecular weight polypeptide doublets on SDS polyacrylamide gels and in solution behave as very large asymmetric molecules. Both proteins show a characteristic increase in sedimentation coefficient in the presence of 20 mM KCl. Antibodies against the brain protein cross-react with erythrocyte spectrin and cross-react with similar high molecular weight doublet polypeptides in SDS polyacrylamide gels of other cell types and plasma membrane preparations. Both proteins bind actin. The brain protein and erythrocyte spectrin show specific and competitive binding to erythrocyte membranes and this binding is inhibited by antibodies against erythrocyte ankyrin. Several of these properties distinguish these proteins from the class of high molecular weight actin-binding proteins that includes filamin and macrophage actin-binding protein. We conclude that together with erythrocyte spectrin, the brain protein and equivalent, immunologically related proteins in other cell types belong to a single class of proteins with the common function of attachment of actin to plasma membranes. Based on the structural and functional similarities, the name spectrin would seem appropriate for this whole class of proteins.

The cytoskeletal system of nucleated erythrocytes. II. presence of a high molecular weight calmodulin-binding protein

Calmodulin was detected in dogfish erythrocyte lysates by means of phosphodiesterase activation. Anucleate dogfish erythrocyte cytoskeletons bound calmodulin. Binding of calmodulin was calcium-dependent, concentration-dependent, and saturable. Cytoskeletons consisted of a marginal band of microtubules containing primarily tubulin, and trans-marginal band material containing actin and spectrinlike proteins. Dogfish erythrocyte ghosts and cytoskeletons were found to contain a calcium-dependent calmodulin-binding protein, CBP, by two independent techniques: (a) 125I-calmodulin binding to cytoskeletal proteins separated by SDS PAGE, and (b) in situ azidocalmodulin binding in whole anucleate ghosts and cytoskeletons. CBP, with an apparent molecular weight of 245,000, co-migrated with the upper band of human and dogfish erythrocyte spectrin. CBP was present in anucleate ghosts devoid of marginal bands and absent from isolated marginal bands. CBP therefore appears to be localized in the trans-marginal band material and not in the marginal band. Similarities between CBP and high molecular weight calmodulin-binding proteins from mammalian species are discussed.

Interactions between actin filaments and between actin filaments and membranes in quick-frozen and deeply etched hair cells of the chick ear

Replicas of the apical surface of hair cells of the inner ear (vestibular organ) were examined after quick freezing and rotary shadowing. With this technique we illustrate two previously undescribed ways in which the actin filaments in the stereocilia and in the cuticular plate are attached to the plasma membrane. First, in each stereocilium there are threadlike connectors running from the actin filament bundle to the limiting membrane. Second, many of the actin filaments in the cuticular plate are connected to the apical cell membrane by tiny branched connecting units like a "crow's foot." Where these "feet" contact the membrane there is a small swelling. These branched "feet" extend mainly from the ends of the actin filaments but some connect the lateral surfaces of the actin filaments as well. Actin filaments in the cuticular plate are also connected to each other by finer filaments, 3 nm in thickness and 74 +/- 14 nm in length. Interestingly, these 3-nm filaments (which measure 4 nm in replicas) connect actin filaments not only of the same polarity but of opposite polarities as documented by examining replicas of the cuticular plate which had been decorated with subfragment 1 (S1) of myosin. At the apicolateral margins of the cell we find two populations of actin filaments, one just beneath the tight junction as a network, the other at the level of the zonula adherens as a ring. The latter which is quite substantial is composed of actin filaments that run parallel to each other; adjacent filaments often show opposite polarities, as evidenced by S1 decoration. The filaments making up this ring are connected together by the 3-nm connectors. Because of the polarity of the filaments this ring may be a "contractile" ring; the implications of this is discussed.

Brain spectrin, a membrane-associated protein related in structure and function to erythrocyte spectrin

An immunoreactive analogue of erythrocyte spectrin has been purified from brain membranes. This protein co-sediments with and cross-links actin filaments, associates with spectrin-binding sites on erythrocyte membranes, and has been visualized by rotary shadowing as an extended, flexible rod. The brain spectrin comprises 3% of the total membrane protein, and may have a major role in mediating linkage of actin to membranes.

Vimentin in the central nervous system. A study of the mesenchymal-type intermediate filament-protein in Wallerian degeneration and in postnatal rat development by two-dimensional gel electrophoresis

Intermediate filament proteins were identified by two-dimensional gel electrophoresis in urea extracts of rat optic nerves undergoing Wallerian degeneration and in cytoskeletal preparations of rat brain and spinal cord during postnatal development. The glial fibrillary acidic (GFA) protein and vimentin were the major optic nerve proteins following Wallerian degeneration. Vimentin was a major cytoskeletal component of newborn central nervous system (CNS) and then progressively decreased until it became barely identifiable in mature brain and spinal cord. The decrease of vimentin occurred concomitantly with an increase in GFA protein. A protein with the apparent molecular weight of 61,000 and isoelectric point of 5.6 was identified in both cytoskeletal preparations of brain and spinal cord, and in urea extracts of normal optic nerves. The protein disappeared together with the polypeptides forming the neurofilament triplet in degenerated optic nerves.

The cytoskeleton in myelinated axons: a freeze-etch replica study

The organization of the cytoskeleton in myelinated axons of the rat has been analyzed without chemical fixation in replicas of deep-etched materials after rapid freezing. Freeze-etch replicas of trigeminal nerves provided three-dimensional views of the well-developed cytoskeleton inside axons. In these preparations, the axonal cytoskeleton was seen to be composed of longitudinally-oriented microtubules and neurofilaments which were interconnected by slender strands. Such strands also connected membranous organelles with microtubules and neurofilaments. After Triton X-100 extraction, the neurofilament-associated interconnecting strands (cross-linking filaments) persisted, indicating that they are not artifactual products of soluble protein condensation during freeze-etching. In non-extracted axons many granular structures were closely associated with cytoskeletal components. These granular elements were not seen after Triton treatment. These findings, together with fluorographic analyses, suggest that the granular structures may represent. These findings, together with fluorographic analyses, suggest that the granular structures may represent slowly transported "soluble proteins' in axoplasm. This freeze-etch replica study, without any chemical fixation, substantiates the reality of the axonal cytoskeleton which is directly involved in the axonal transport. Furthermore, using this approach ultrastructural evidence was obtained of the close association of membranous structures with the cytoskeleton.

Erythroid spectrin, brain fodrin, and intestinal brush border proteins (TW-260/240) are related molecules containing a common calmodulin-binding subunit bound to a variant cell type-specific subunit

Spectrin, fodrin, and TW-260/240 form a group of structurally and functionally similar but not identical high molecular weight actin-binding proteins from chicken erythrocytes, brain tissue, or intestinal epithelial brush borders. Immunological data and one-dimensional peptide maps of the separated subunits suggest that a common (Mr 240,000) and a variant (Mr 220,000, 235,000, or 260,000) subunit account for the three different heterodimers. These results are in line with the related but distinct morphology of the three proteins observed in micrographs of rotary-shadowed molecules and the finding that the common (Mr 240,000) subunit seems to account for the calcium-dependent calmodulin-binding activity displayed by the three proteins. The possible functions of spectrin-like molecules in nonerythroid cells are discussed.

Cross-linker system between neurofilaments, microtubules, and membranous organelles in frog axons revealed by the quick-freeze, deep-etching method

The elaborate cross-connections among membranous organelles (MO), microtubules (MT), and neurofilaments (NF) were demonstrated in unifixed axons by the quick-freeze, deep-etch, and rotary-shadowing method. They were categorized into three groups: NF-associated cross-linker, MT-associated cross-bridges, and long cross-links in the subaxolemmal space. Other methods were also employed to make sure that the observed cross-connections in the unfixed axons were not a result of artifactual condensation or precipitation of soluble components or salt during deep-etching. Axolemma were permeablized either chemically (0.1% saponin) or physically (gentle homogenization), to allow egress of their soluble components from the axon; or else the axons were washed with distilled water after fixation. After physical rupture of the axolemma or saponin treatment, most of the MO remained intact. MT were stabilized by adding taxol in the incubation medium. Axons prepared by these methods contained many longitudinally oriented NF connected to each other by numerous fine cross-linkers (4-6 nm in diameter, 20-50 nm in length). Two specialized regions were apparent within the axons: one composed of fascicles of MT linked with each other by fine cross-bridges; the other was in the subaxolemmal space and consisted of actinlike filaments and a network of long cross-links (50-150 nm) which connected axolemma and actinlike filaments with NF and MT. F-actin was localized to the subaxolemmal space by the nitrobenzooxadiazol phallacidin method. MO were located mainly in these two specialized regions and were intimately associated with MT via fine short (10-20 nm in length) cross-bridges. Cross-links from NF to MO and MT were also common. All these cross-connections were observed after chemical extraction or physical rupture of the axon; however, these procedures removed granular materials which were attached to the filaments in the fresh unextracted axons. The cross-connections were also found in the axons washed with distilled water after fixation. I conclude that the cross- connections are real structures while the granular material is composed of soluble material, probably protein in nature.

A major calmodulin-binding protein common to various vertebrate tissues

A major calmodulin-binding protein (CaM-BP) of Mr 240,000 was demonstrated in various rat tissues by using a 125I-labeled CaM gel overlay technique. This protein (designated p240) was detected in the particulate fraction and to a lesser extent in the cytosol of all tissues studied. Binding of CaM to p240 was completely dependent on Ca2+. A second, exclusively soluble, CaM-BP (Mr115,000) common to several tissues and a number of other CaM-BPs with a more restricted tissue distribution were also observed by using this technique. CaM binding to p240 occurred in high amounts in plasma membranes from avian erythrocytes but was absent from mammalian erythrocyte membranes. Antibodies prepared against turkey erythrocyte p240 (anti-Tp240) crossreacted with p240 in other tissues. Identity between the proteins recognized by anti-Tp240 and CaM was confirmed by demonstrating that 125I-labeled CaM could bind to p240 specifically immunoprecipitated from either rat brain or turkey erythrocytes by anti-Tp240. The p240 may be related to a previously described actin-binding protein and may represent a major site of action of CaM on the cytoskeleton.

An F-actin- and calmodulin-binding protein from isolated intestinal brush borders has a morphology related to spectrin

A high molecular weight protein from the brush border of chicken intestinal epithelial cells has been purified. This protein (TW 260/240), a complex of two polypeptides with apparent molecular weights of 260,000 and 240,000, accounts for a significant amount of the terminal web organization. TW 260/240 is an F-actin-binding protein that also interacts with calmodulin. Rotary shadowing reveals long flexible rods of double-stranded morphology tightly connected at each end. TW 260/240 is quite distinct from smooth muscle filamin and macrophage actin-binding protein (APB), but, in spite of its higher contour length (265 nm), seems to be related to erythrocyte spectrin (194 nm for the tetramer). Immunofluorescence microscopy with antibodies against TW 260/240 indicates the existence of a submembranous organization distinctly different from that of stress fibers. We have compared TW 260/240 with fodrin, a brain protein known to occur in submembranous organization but not previously characterized in molecular terms. TW 260/240 and fodrin are clearly distinct molecules but are similar in many aspects. Ultrastructural, biochemical and immunological results indicate three distinct classes of rod-like high molecular weight actin-binding proteins, possibly reflected by the prototypes filamin (ABP), spectrin and TW 260/240 (fodrin). The latter group may be responsible for calmodulin control of submembranous microfilament structures in various nonmuscle cells.

In vivo phosphorylation of neurofilament proteins in the central nervous system of immature rat and rabbit

Neurofilament proteins from brain and spinal cord of immature rat (20-35 days of age) and rabbit (15-17) days of age) were prepared by an axonal flotation technique. Examination of rat filament preparations by electron microscopy revealed a preponderance of 10 nm diameter filaments that were usually loosely aggregated although some bundles of more tightly packed filaments were present as well. The neurofilament triplet proteins of the rat and rabbit central nervous system were found to be phosphorylated 24 hr after the intracerebral injection of [32P]orthophosphate when examined by sodium dodecyl sulfate polyacrylamide gel electrophoresis followed by fluorography. Examination of each eluted neurofilament protein from both species showed that [32P]phosphate was retained after reelectrophoresis and fluorography. Evidence is presented that the [32P]phosphate is covalently linked to the purified neurofilament proteins by phosphoester bonds.