Microtubule-associated proteins

Complexing of the CD-3 subunit by a monoclonal antibody activates a microtubule-associated protein 2 (MAP-2) serine kinase in Jurkat cells

Treatment of Jurkat T-cells with anti-CD-3 monoclonal antibodies resulted in the rapid and transient activation of a serine kinase which utilized the microtubule-associated protein, MAP-2, as a substrate in vitro. The kinase was also activated on treatment of Jurkat cells with phytohaemagglutinin, but with a different time course. The activation of the MAP-2 kinase by anti-CD-3 antibodies was dose-dependent, with maximal activity observed at concentrations of greater than 500 ng/ml. Normal human E-rosette-positive T-cells also exhibited induction of MAP-2 kinase activity during anti-CD-3 treatment. The enzyme was optimally active in the presence of 2 mM-Mn2+; lower levels of activity were observed with Mg2+, even at concentrations up to 20 mM. The kinase was partially purified by passage over DE-52 Sephacel with the activity eluting as a single peak at 0.25 M-NaCl. The molecular mass was estimated to be 45 kDa by gel filtration. The activation of the MAP-2 kinase was probably due to phosphorylation of this enzyme as treatment with alkaline phosphatase diminished its activity. These data demonstrate that the stimulation of T-cells through the CD-3 complex results in the activation of a novel serine kinase which may be critically involved in signal transduction in these cells.

Regulation of microtubule-associated protein 2 (MAP2) mRNA expression during rat brain development

The expression of MAP2 during rat brain development was studied by using specific antibodies and cDNA probes. MAP2 cDNAs were isolated from a rat brain lambda gt11 library, and their identity was confirmed by the reactivity of their fusion proteins with several independent monoclonal antibodies that recognize MAP2. Northern blot analyses of the RNA prepared from whole brains, cerebral cortex, hypothalamus, brain stem, olfactory bulbs, and cerebellum showed that the levels of MAP2 mRNA increase during the initial phase of development, reach a maximum between postnatal weeks 2 and 3, and then decrease in the adult. The time course and the kinetics of this change varied between different brain regions and appeared to reflect the pattern of morphological changes in these regions. RNA blots were also analyzed with beta-tubulin and beta-actin cDNA probes to ensure the quality and the quantity of the RNA. The levels of MAP2 mRNA and protein showed similar changes during the initial part of brain development and suggested a transcriptional control. However, while MAP2 protein levels remained high throughout development, MAP2 mRNA levels decreased in adulthood. We suggest that the increased stability of the MAP2 molecule may be a contributing factor in the developmental regulation of steady-state levels of MAP2.

Localization and immunogenicity of tubulin in the filarial nematodes Brugia malayi and B. pahangi

Tubulin was identified in the filarial nematodes Brugia malayi and B. pahangi by several approaches. Initially, a monoclonal antibody (6D8) was selected for its unusual binding to B. malayi microfilariae in indirect immunofluorescence assays: 6D8 showed granular, heterogeneously dispersed fluorescence on fixed parasites but did not bind to unfixed microfilariae. The microfilarial sheath did not bind 6D8, although it did bind fluoresceinated wheatgerm agglutinin. By Western blotting against microfilarial sonicate, 6D8 reacted with a 50,000-55,000 mol. wt protein, and also bound to purified chicken brain beta-tubulin. Additionally, this monoclonal antibody reacted with a recombinant fusion protein expressed by a clone (Bpa-7) originally isolated from an adult B. pahangi cDNA expression library by its reaction with chronic human filariasis serum. This clone encodes a small 40 amino acid C-terminal segment corresponding to residues 409-449 of beta-tubulin, and shows complete amino acid sequence homology with vertebrate beta-tubulin from 409 to 430 but 55% divergence (six amino acid substitutions, four insertions and one deletion) from human and chicken beta-tubulin over positions 431-449 at the C terminus. Antibody to both parasite and vertebrate (chicken) tubulin was found in filarial infection sera, with higher levels of autoreactive antibody apparent in amicrofilaraemic individuals. Immunogold electron microscopy was then used to localize beta-tubulin in B. malayi microfilariae and adult worms. Tubulin was shown not to be exposed on the microfilarial sheath or in the cuticle of either stage, but was found to be abundant in the somatic tissues. In microfilariae, 6D8 bound myofibril structures under the hypodermal layer, and also bound within cell nuclei. In the adult stage, tubulin was associated with muscle blocks, as well as the intestinal brush border and the embryonic uterine microfilariae.

Distribution of phosphate-independent MAP2 epitopes revealed with monoclonal antibodies in microwave-denatured human nervous system tissues

In contrast with results obtained in experimental animals, antibodies to microtubule associated protein-2 (MAP2) preferentially label abnormal structures in human nervous system tissue samples, but the normal sites at which MAP2 is expressed are not well-defined. To determine the distribution of MAP2 in the human central (CNS) and peripheral (PNS) nervous systems, we prepared monoclonal antibodies (MAbs) specific to MAP2, and compared the localization of this MAP in postmortem bovine and human tissues as well as in several human neural cell lines that express either neurofilament (NF) or glial filament (GF) proteins. Eight MAbs specific for phosphate-independent epitopes in bovine and human MAP2 were obtained, and those that performed well in tissues produced immunoreactivity confined to the somatodendritic domain of neurons in bovine and human CNS and PNS tissues. Other neural cells (e.g. astrocytes) did not express MAP2 immunoreactivity using these MAbs. Postmortem delays of less than 24 h prior to tissue denaturation did not affect the distribution of MAP2 immunoreactivity. However, microwave denaturation of these tissues preserved MAP2 immunoreactivity better than fixation with Bouin's solution or formalin. Microwave treatment also improved the immunoreactivity of several MAbs for NF and GF proteins. Finally, MAP2 was not detected in human neural cell lines that express NF (2) or GF (1) proteins. We conclude that microwave denaturation provides an effective means to preserve the immunoreactivity of normal human neuronal cytoskeletal proteins, and that this method of tissue denaturation allows the normal distribution of MAP2 to be defined in postmortem samples of human CNS and PNS tissues.

Assembly properties of altered beta-tubulin polypeptides containing disrupted autoregulatory domains

beta-Tubulin synthesis in eucaryotic cells is subject to control by an autoregulatory posttranscriptional mechanism in which the first four amino acids of the beta-tubulin polypeptide act either directly or indirectly to control the stability of beta-tubulin mRNA. To investigate the contribution of this amino-terminal domain to microtubule assembly and dynamics, we introduced a series of deletions encompassing amino acids 2 to 5 of a single mammalian beta-tubulin isotype, M beta 1. Constructs carrying such deletions were inserted into an expression vector, and the ability of the altered polypeptide to coassemble into microtubules was tested by using an anti-M beta 1-specific antibody. We show that the M beta 1 beta-tubulin polypeptide was competent for coassembly into microtubules in transient transfection experiments and in stably transfected cell lines when it lacked either amino acid 2 or amino acids 2 and 3. The capacity of these mutant beta-tubulins to coassemble into polymerized microtubules was only slightly diminished relative to that of unaltered beta-tubulin, and their expression did not influence the viability or growth properties of cell lines carrying these deletions. However, more extensive amino-terminal deletions either severely compromised or abolished the capacity for coassembly. In analogous experiments in which alterations were introduced into the amino-terminal domain of a mammalian alpha-tubulin isotype, M alpha 4, deletion of amino acid 2 did not affect the ability of the altered polypeptide to coassemble, although removal of additional amino-terminal residues essentially abolished the capacity for competent coassembly. The stability of the altered assembly-competent alpha- and beta-tubulin polypeptides was measured in pulse-chase experiments and found to be indistinguishable from the stability of the corresponding unaltered polypeptides. An assembly-competent M alpha 4 polypeptide carrying a deletion encompassing the 12 carboxy-terminal amino acids also had a half-life indistinguishable from that of the wild-type alpha-tubulin molecule. These data suggest that the universally conserved amino terminus of beta-tubulin acts largely in a regulatory role and that the carboxy-terminal domain of alpha-tubulin is not essential for coassembly in mammalian cells in vivo.

Tau consists of a set of proteins with repeated C-terminal microtubule-binding domains and variable N-terminal domains

Tau proteins consist of a family of proteins, heterogeneous in size, which associate with microtubules in vivo and are induced during neurite outgrowth. In humans, tau is one of the major components of the pathognomonic neurofibrillary tangles in Alzheimer's disease brain. Screening of a cDNA library prepared from bovine brain led to the isolation of several cDNA clones encoding tau proteins with different N termini and differing by insertions or deletions, suggesting differential splicing of the tau transcripts. One of the N-terminal domains and the repeated C-terminal domain of the encoded tau proteins are recognized by polyclonal antibodies to bovine tau. The bovine tau proteins are highly homologous to murine and human tau, especially within the repeated C-terminal domain. Compared with murine and human tau, bovine tau contains the insertion of three longer segments, one of which is an additional characteristic repeat. Portions of tau proteins generated by in vitro translation were used to show that these repeats represent tubulin-binding domains, two of which are sufficient to bind to microtubules assembled from purified tubulin in the presence of taxol.

The regulation of protein polymerization

Reversible polymerization reactions are essential for many cellular functions. This review briefly discusses some of the mechanisms for controlling the rate and extent of these processes.

F-actin affinity chromatography: technique for isolating previously unidentified actin-binding proteins

We have developed stable and easy to use filamentous actin (F-actin) affinity-chromatography columns that selectively purify proteins that bind to actin filaments from cell extracts. Most traditional assays for actin-associated proteins screen for their effects on actin polymerization or actin filament crosslinking. Because our technique requires only actin-filament binding, it can identify additional types of proteins involved in the function of the actin cytoskeleton. By chromatographing extracts of several types of cells on these columns, we show that known actin-binding proteins are selectively retained as a subset of a larger group of actin-binding proteins that have not been identified previously.

Interacting proteins identified by genetic interactions: a missense mutation in alpha-tubulin fails to complement alleles of the testis-specific beta-tubulin gene of Drosophila melanogaster

In this paper we demonstrate that failure to complement between mutations at separate loci can be used to identify genes that encode interacting structural proteins. A mutation (nc33) identified because it failed to complement mutant alleles of the gene encoding the testis-specific beta 2-tubulin of Drosophila melanogaster (B2t) did not map to the B2t locus. We show that this second-site noncomplementing mutation is a missense mutation in alpha-tubulin that results in substitution of methionine in place of valine at amino acid 177. Because alpha- and beta-tubulin form a heterodimer, our results suggest that the genetic interaction, failure to complement, is based on the structural interaction between the protein products of the two genes. Although the nc33 mutation failed to complement a null allele of B2t (B2tn), a deletion of the alpha-tubulin gene to which nc33 mapped complemented B2tn. Thus, the failure to complement appears to require the presence of the altered alpha-tubulin encoded by the nc33 allele, which may act as a structural poison when incorporated into either the tubulin heterodimer or microtubules.

Cloning of a cDNA encoding MAP1B in rat brain: regulation of mRNA levels during development

This article describes the isolation of a microtubule-associated protein 1B (MAP1B) cDNA clone from a rat brain lambda gt11 library and the study of MAP1B mRNA expression during brain development. On Northern blots, the cDNA hybridized with an mRNA of greater than 10 kilobases which was present only in the brain. The identity of the cDNA was confirmed by the characterization of the antiserum against the fusion protein, and also by comparing both the original antibody and the anti-fusion protein antiserum with a panel of well-studied monoclonal antibodies against different forms of MAP1 and MAP2. The regulation of MAP1B mRNA during development was studied in whole brain, cerebral cortex, hypothalamus, brainstem, and olfactory bulbs. The steady-state levels of MAP1B mRNA in all tissues examined were relatively low in the adult compared to developing brains. This decrease varied in different brain regions, and its time course appeared to coincide with the pattern of postnatal developmental and morphological events. The developmental patterns of the MAP1B mRNA and protein in the brain were similar, suggesting that expression of this protein is under transcriptional control. The RNA blots were also probed with beta-actin and beta-tubulin to compare the levels of MAP1B mRNA with other cytoskeletal elements and as controls for the quality of the RNA.

Multisite phosphorylation of microtubule-associated protein 2 (MAP-2) in rat brain: peptide mapping distinguishes between cyclic AMP-, calcium/calmodulin-, and calcium/phospholipid-regulated phosphorylation mechanisms

Microtubule-associated protein 2 (MAP-2), a cytoskeletal protein of 280 kilodalton that is highly enriched in dendrites and neuronal perikarya, is subject to both cyclic AMP-, calcium/calmodulin-, and calcium/phospholipid-regulated phosphorylation when incubated with [gamma-32P]ATP in vitro. We have analyzed the different sites in MAP-2 phosphorylated by these three kinases in fresh or boiled cytosol from different regions of the rat brain, in particular the olfactory bulb, where only one form (MAP-2B) is present, and the cerebral cortex, where both forms (MAP-2A and MAP-2B) are equally enriched. Cyclic AMP-dependent protein kinase and calcium/calmodulin-dependent protein kinase II phosphorylated four common phosphorylation sites, as well as a number of distinct sites that were unique to each enzyme. Calcium/phospholipid-dependent protein kinase phosphorylated a minimum of 15 sites, only one of which appeared to be shared with the other protein kinases. Only serine residues were phosphorylated by cyclic AMP-dependent and calcium/phospholipid-dependent protein kinases, while both serine and threonine residues were phosphorylated by calcium/calmodulin-dependent protein kinase II. No differences were observed in the peptide maps of phospho-MAP-2 prepared from different brain regions. These results emphasize the complexity of the phosphorylation systems that may regulate the function of MAP-2 in situ.

Tyrosination state of tubulin and the activity of tubulin:tyrosine ligase and tubulin carboxypeptidase in the developing retina of the chick

The tyrosination state of tubulin and the enzymes involved in the tubulin tyrosination/detyrosination cycle--tubulin:tyrosine ligase and tubulin carboxypeptidase--were determined in chick retina during development. The amount of tyrosinable (tyrosinated plus detyrosinated) tubulin increased approximately 110% from embryonic day 7 to 14. Then it decreased, and by day 19 it was similar to the value on day 7. This result did not change after hatching, at least up to day 20. The proportion of tyrosinated and detyrosinated tubulin significantly changed with the development of the animal. At embryonic day 7, these tubulin species were at a proportion of 70 and 30%, respectively, and after hatching, the values inverted, to 30 and 70%, respectively. This change did not correlate with the activity of the ligase relative to that of the carboxypeptidase, as measured in vitro. This observation suggested that a change in the turnover rate of microtubules, in the proportion of assembled and nonassembled tubulin pools, or in both had occurred. Coincident with the last possibility, the proportion of assembled tubulin was found to increase during the development of the animal. This finding suggests that the tyrosination state of tubulin may be determined, at least in part, by the assembly state.

Tau consists of a set of proteins with repeated C-terminal microtubule-binding domains and variable N-terminal domains

Tau proteins consist of a family of proteins, heterogeneous in size, which associate with microtubules in vivo and are induced during neurite outgrowth. In humans, tau is one of the major components of the pathognomonic neurofibrillary tangles in Alzheimer's disease brain. Screening of a cDNA library prepared from bovine brain led to the isolation of several cDNA clones encoding tau proteins with different N termini and differing by insertions or deletions, suggesting differential splicing of the tau transcripts. One of the N-terminal domains and the repeated C-terminal domain of the encoded tau proteins are recognized by polyclonal antibodies to bovine tau. The bovine tau proteins are highly homologous to murine and human tau, especially within the repeated C-terminal domain. Compared with murine and human tau, bovine tau contains the insertion of three longer segments, one of which is an additional characteristic repeat. Portions of tau proteins generated by in vitro translation were used to show that these repeats represent tubulin-binding domains, two of which are sufficient to bind to microtubules assembled from purified tubulin in the presence of taxol.

Tubulin and MAP2 regulate the PCSL phosphatase activity. A possible new role for microtubular proteins

Tubulin can stimulate specifically the aryl phosphatase activity of the low-Mr polycation-stimulated (PCSL) phosphatase, measured as p-nitrophenyl phosphatase activity, or using reduced carboxamidomethylated and maleylated (RCM) lysozyme, phosphorylated on tyrosyl residues, as a substrate. This stimulation is independent of the degree of polymerization of tubulin (A50 = 60 nM) and is due to an increase in Vmax. It is mechanistically different from the ATP-induced activation and resistant to heat and trypsin treatment. Chymotrypsin destroys the stimulatory effect of tubulin. The polycation-stimulated phosphorylase phosphatase activity is inhibited by tubulin, probably by a polycation/polyanion interaction. The microtubule-associated protein, MAP2, is inhibitory to the p-nitrophenyl phosphatase activity and tubulin can eliminate this inhibitory effect. MAP2 also inhibits the polycation-stimulated phosphorylase phosphatase activity.

Interacting proteins identified by genetic interactions: a missense mutation in alpha-tubulin fails to complement alleles of the testis-specific beta-tubulin gene of Drosophila melanogaster

In this paper we demonstrate that failure to complement between mutations at separate loci can be used to identify genes that encode interacting structural proteins. A mutation (nc33) identified because it failed to complement mutant alleles of the gene encoding the testis-specific beta 2-tubulin of Drosophila melanogaster (B2t) did not map to the B2t locus. We show that this second-site noncomplementing mutation is a missense mutation in alpha-tubulin that results in substitution of methionine in place of valine at amino acid 177. Because alpha- and beta-tubulin form a heterodimer, our results suggest that the genetic interaction, failure to complement, is based on the structural interaction between the protein products of the two genes. Although the nc33 mutation failed to complement a null allele of B2t (B2tn), a deletion of the alpha-tubulin gene to which nc33 mapped complemented B2tn. Thus, the failure to complement appears to require the presence of the altered alpha-tubulin encoded by the nc33 allele, which may act as a structural poison when incorporated into either the tubulin heterodimer or microtubules.

Microtubule-binding proteins from carrot : I. Initial characterization and microtubule bundling

Microtubules (MTs) participate in several processes of fundamental importance to growth and development in higher plants, yet little is known about the proteins with which they associate. Information about these molecules is important because they probably play a role in mediating functional and structural differences between various MT arrays. As a first step in gaining insight into this problem, we have isolated, from suspension-cultured cells of carrot (Daucus carota L.), non-tubulin proteins which bind to and affect microtubules (MTs) in vitro. These proteins were isolated using taxol-stabilized neuronal MTs as an affinity substrate. They cause MT bundling at substoichiometric concentrations, support the assembly of tubulin in vitro, and at low concentrations, decorate single MTs in a periodic fashion. The bundled MTs formed in vitro share similarities with those seen in situ in a variety of plant cells, including a center-center spacing of 34 nm, cold stability, resistance to anti-microtubule drugs, and sensitivity to calcium. The bundling activity is specific; other cationic proteins, as well as poly-L-lysine, do not behave in a similar manner. The bundling activity is insensitive to ATP. By assaying bundling activity with dark-field microscopy and employing standard biochemical procedures, a small number of polypeptides involved in the bundling process were identified. Affinity-isolated antibodies to one of these polypeptides (Mr=76000) were found to co-localize with MTs in the cortical array of protoplasts. Our findings are discussed with reference to the importance of these proteins in the cell and to their relationship to microtubule-associated proteins in other eukaryotes.

Characteristics of microtubules at the different stages of neuronal differentiation and maturation

The developing nervous system has proved to be a very powerful tool to analyze how MT are involved in basic biological processes such as cell proliferation, cell migration, cell shaping, and transport. A better knowledge of the basic events occurring during neurogenesis also affords us the possibility of establishing the basis of experiments and trying to solve unanswered and important questions. Despite the considerable value of cell culture, we need to use more discrete regions of the developing brain in situ in order to analyze the MT and their modifications into cells developing their "natural" environment. One major problem remains the question of the mode of assembly and disassembly, that is, the behavior of MT in living cells. Dynamic instability and/or treadmilling are accurate interpretations of the dynamics of MT at least in vitro or in cell culture, but we do need more information on what happens in situ and in vitro. One of the main tasks of cell biologists is to devise satisfactory tests to approach this fundamental question. In this view, pharmacological manipulation of embryos treated in whole-embryo culture systems might be a possible way. Microtubules are ubiquitous cell components. However, the extensive heterogeneity of MAP and tubulin in the CNS confers on the neurons a wide range of capabilities of assembly of these proteins and suggests that the neuron has a unique potential of a relation between MT composition and cell function. We have seen that each major event during neurogenesis is related to a specific series of modifications of the MT components. It remains to be determined if there is a causal or just a correlative relationship between the appearance of specific isotypes and the occurrence of specific events and/or functions. We have also to determine the exact spatial and temporal relations among the different isotypes of MT proteins, tubulin, and MAP. Is there a close correspondence between a tubulin and a MAP isotype? Can the appearance of one isotype of tubulin influence the appearance and the assembly of a specific MAP, or vice versa? Recent results obtained with the Tyr- and Glu-MT shed light on these questions and suggest a whole series of possibilities for cells to modulate the structure, behavior, and function of MT in specific domains of the neuron or in specific regions of the brain, by only a minute modification of the molecule of tubulin. Microtubule protein heterogeneity raises also a number of questions.(ABSTRACT TRUNCATED AT 400 WORDS)

Binding of microtubule-associated proteins (MAPs) to rat brain mitochondria: a comparative study of the binding of MAP2, its microtubule-binding and projection domains, and tau proteins

Two major brain microtubule-associated proteins (MAPs), MAP2 and tau, were found to be able to bind to purified rat brain mitochondria. The apparent dissociation constants of the binding of thermostable 32P-labeled MAP2 and tau are 0.9 +/- 0.04 x 10(-7) and 3.8 +/- 0.7 x 10(-7) M, respectively. 32P-labeled MAP2 and tau bound to the mitochondria can be displaced by phosphorylated, nonradioactive MAP2. The binding parameters of MAP2 prepared without heat treatment and those of the thermostable MAP2 were of the same order of magnitude. Microtubule-binding and projection domains of MAP2 were obtained by chymotryptic digestion of rat brain microtubules (Vallee, Proc. Natl. Acad. Sci. USA, 77:3206-3210, 1980). Displacement studies with these two domains show that MAP2 bound to mitochondria can be displaced by the microtubule-binding domain, whereas the projection domain does not displace MAP2. The two domains of MAP2 bind to the mitochondria with similar affinity constants; however, the Bmax for the projection domain was 10 times and 35 times lower than the Bmax of the binding of the intact MAP2 and the microtubule-binding domain, respectively. Chymotryptic digestion of MAP2 bound to the mitochondria yielded peptide fragments with molecular masses similar to those obtained by the digestion of MAP2 bound to the microtubules. The fragments corresponding to the projection domain were released into the extramitochondrial supernatant, whereas the fragments originating from the microtubule-binding domain remained bound to the mitochondria. These results suggest that MAP2 binds to mitochondria preferentially via its microtubule-binding domain.

Plectin: general overview and appraisal of its potential role as a subunit protein of the cytomatrix

Plectin has recently been identified as a widespread and abundant cytoplasmic protein of mammalian cells. In this article the available data on plectin are reviewed, focusing on plectin's occurrence and localization in various cell types and tissues, its biochemical characterization, and its molecular interaction partners. Furthermore, the putative role of this protein has a multifunctional connecting link of the cytomatrix and its structural as well as functional relationship to other cytoskeletal proteins is discussed. It is concluded that plectin is potentially the most versatile crosslinking element of the cytomatrix reported to date.

Multiple phosphorylated variants of the high molecular mass subunit of neurofilaments in axons of retinal cell neurons: characterization and evidence for their differential association with stationary and moving neurofilaments

The 200-kD subunit of neurofilaments (NF-H) functions as a cross-bridge between neurofilaments and the neuronal cytoskeleton. In this study, four phosphorylated NF-H variants were identified as major constituents of axons from a single neuron type, the retinal ganglion cell, and were shown to have characteristics with different functional implications. We resolved four major Coomassie Blue-stained proteins with apparent molecular masses of 197, 200, 205, and 210 kD on high resolution one-dimensional SDS-polyacrylamide gels of mouse optic axons (optic nerve and optic tract). Proteins with the same electrophoretic mobilities were radiolabeled within retinal ganglion cells in vivo after injecting mice intravitreally with [35S]methionine or [3H]proline. Extraction of the radiolabeled protein fraction with 1% Triton X-100 distinguished four insoluble polypeptides (P197, P200, P205, P210) with expected characteristics of NF-H from two soluble neuronal polypeptides (S197, S200) with few properties of neurofilament proteins. The four Triton-insoluble polypeptides displayed greater than 90% structural homology by two-dimensional alpha-chymotryptic iodopeptide map analysis and cross-reacted with four different monoclonal and polyclonal antibodies to NF-H by immunoblot analysis. Each of these four polypeptides advanced along axons primarily in the Group V (SCa) phase of axoplasmic transport. By contrast, the two Triton-soluble polypeptides displayed only a minor degree of alpha-chymotryptic peptide homology with the Triton-insoluble NF-H forms, did not cross-react with NF-H antibodies, and moved primarily in the Group IV (SCb) wave of axoplasmic transport. The four NF-H variants were generated by phosphorylation of a single polypeptide. Each of these polypeptides incorporated 32P when retinal ganglion cells were radiolabeled in vivo with [32P]orthophosphate and each cross-reacted with monoclonal antibodies specifically directed against phosphorylated epitopes on NF-H. When dephosphorylated in vitro with alkaline phosphatase, the four variants disappeared, giving rise to a single polypeptide with the same apparent molecular mass (160 kD) as newly synthesized, unmodified NF-H. The NF-H variants distributed differently along optic axons. P197 predominated at proximal axonal levels; P200 displayed a relatively uniform distribution; and P205 and P210 became increasingly prominent at more distal axonal levels, paralleling the distribution of the stationary neurofilament network.(ABSTRACT TRUNCATED AT 400 WORDS)

Detection of tubulin-binding proteins by an overlay assay

A blot overlay technique to detect tubulin-binding proteins has been developed. It involves the fractionation of the putative tubulin-binding proteins by gel electrophoresis, their transfer to nitrocellulose paper, and the incubation of the nitrocellulose sheet with purified tubulin. The proteins which bind tubulin are recognized by monoclonal anti-tubulin antibodies. Proteins which associate with microtubules through a direct interaction with tubulin are easily detected by this procedure, which has been tested for both known microtubule-associated proteins and newly reported tubulin-binding proteins.

Sertoli cell processes have axoplasmic features: an ordered microtubule distribution and an abundant high molecular weight microtubule-associated protein (cytoplasmic dynein)

Microtubules in the cytoplasm of rat Sertoli cell stage VI-VIII testicular seminiferous epithelium were studied morphometrically by electron microscopy. The Sertoli cell microtubules demonstrated axonal features, being largely parallel in orientation and predominantly spaced one to two microtubule diameters apart, suggesting the presence of microtubule-bound spacer molecules. Testis microtubule-associated proteins (MAPs) were isolated by a taxol, salt elution procedure. Testis MAPs promoted microtubule assembly, but to a lesser degree than brain MAPs. High molecular weight MAPs, similar in electrophoretic mobilities to brain MAP-1 and MAP-2, were prominent components of total testis MAPs, though no shared immunoreactivity was detected between testis and brain high molecular weight MAPs using both polyclonal and monoclonal antibodies. Unlike brain high molecular weight MAPs, testis high molecular weight MAPs were not heat stable. Testis MAP composition, studied on postnatal days 5, 10, 15, and 24 and in the adult, changed dramatically during ontogeny. However, the expression of the major testis high molecular weight MAP, called HMW-2, was constitutive and independent of the development of mature germ cells. The Sertoli cell origin of HMW-2 was confirmed by identifying this protein as the major MAP found in an enriched Sertoli cell preparation and in two rat models of testicular injury characterized by germ cell depletion. HMW-2 was selectively released from testis microtubules by ATP and co-purified by sucrose density gradient centrifugation with MAP-1C, a neuronal cytoplasmic dynein. The inhibition of the microtubule-activated ATPase activity of HMW-2 by vanadate and erythro-(2-hydroxy-3-nonyl)adenine and its proteolytic breakdown by vanadate-dependent UV photocleavage confirmed the dynein-like nature of HMW-2. As demonstrated by this study, the neuronal and Sertoli cell cytoskeletons share morphological, structural and functional properties.

Tau proteins: the molecular structure and mode of binding on microtubules

Tau is a family of closely related proteins (55,000-62,000 mol wt) which are contained in the nerve cells and copolymerize with tubulin to induce the formation of microtubules in vitro. All information so far has indicated that tau is closely apposed to the microtubule lattice, and there was no indication of domains projecting from the microtubule polymer lattice. We have studied the molecular structure of the tau factor and its mode of binding on microtubules using the quick-freeze, deep-etch method (QF.DE) and low angle rotary shadowing technique. Phosphocellulose column-purified tubulin from porcine brain was polymerized with tau and the centrifuged pellets were processed by QF.DE. We observed periodic armlike elements (18.7 +/- 4.8 nm long) projecting from the microtubule surface. Most of the projections appeared to cross-link adjacent microtubules. We measured the longitudinal periodicity of tau projections on the microtubules and found it to match the 6-dimer pattern better than the 12-dimer pattern. The stoichiometry of tau versus tubulin in preparations of tau saturated microtubules was 1:approximately 5.0 (molar ratio). Tau molecules adsorbed on mica took on rodlike forms (56.1 +/- 14.1 nm long). Although both tau and MAP1 are contained in axons, competitive binding studies demonstrated that the binding sites of tau and MAP1A on the microtubule surfaces are most distinct, although they may partially overlap.

Large microtubule-associated protein of T. brucei has tandemly repeated, near-identical sequences

The parasitic protozoon Trypanosoma brucei contains a highly organized membrane skeleton, consisting of a dense array of parallel, singlet microtubules that are laterally interconnected and that are also in tight contact with the overlying cell membrane. A high molecular weight, heat-stable protein from this membrane skeleton was isolated that is localized along the microtubules. Protease digestion experiments and sequencing of a cloned gene segment showed that most of the protein is built up by more than 50 nearly identical tandem repeats with a periodicity of 38 amino acids.

Diverse effects of beta-tubulin mutations on microtubule formation and function

We have used in vitro mutagenesis and gene replacement to construct five new cold-sensitive mutations in TUB2, the sole gene encoding beta-tubulin in the yeast Saccharomyces cerevisiae. These and one previously isolated tub2 mutant display diverse phenotypes that have allowed us to define the functions of yeast microtubules in vivo. At the restrictive temperature, all of the tub2 mutations inhibit chromosome segregation and block the mitotic cell cycle. However, different microtubule arrays are present in these arrested cells depending on the tub2 allele. One mutant (tub2-401) contains no detectable microtubules, two (tub2-403 and tub2-405) contain greatly diminished levels of both nuclear and cytoplasmic microtubules, one (tub2-104) contains predominantly nuclear microtubules, one (tub2-402) contains predominantly cytoplasmic microtubules, and one (tub2-404) contains prominent nuclear and cytoplasmic microtubule arrays. Using these mutants we demonstrate here that cytoplasmic microtubules are necessary for nuclear migration during the mitotic cell cycle and for nuclear migration and fusion during conjugation; only those mutants that possess cytoplasmic microtubules are able to perform these functions. We also show that microtubules are not required for secretory vesicle transport in yeast; bud growth and invertase secretion occur in cells which contain no microtubules.

Microtubule-associated protein 2 (MAP 2) immunoreactivity in human fetal neocortex

We used a monoclonal antibody to study the immunocytochemical distribution of microtubule-associated protein 2 (MAP 2) in human fetal neocortex between the ages of 16 and 22 weeks gestation. The staining pattern was lamina-specific. Neuronal somata and dendrites in all cortical layers and in the intermediate zone were labelled. Cajal-Retzius cells of layer I, large pyramidal neurons in the inner cortical plate and neurons in the subplate were most strongly immunoreactive. Separate from the underlying cortical plate a thin sheet of small neurons in the inner marginal zone was highlighted by MAP 2 immunoreactivity. The morphologic diversity, density and regional distribution of the interstitial neurons in the subplate was emphasized by MAP 2 staining. In general, the intensity of MAP 2 immunoreactivity in cell somata and dendrites correlated with the degree of neuronal differentiation but the pattern of intracellular staining also varied as a function of laminar position, and presumably cell type.

Heat-stable microtubule protein MAP-1 binds to microtubules and induces microtubule assembly

The microtubule-associated proteins, MAP-1, MAP-2 and tau, have been purified from brain tissue via a new approach using a heating step directly on the homogenate, followed by selective adsorption on calmodulin-Sepharose affinity columns and gel-filtration chromatography. Our results indicate that these MAPs share common biochemical properties, including heat stability, calmodulin binding and promotion of tubulin assembly into microtubules.

Neuronal microtubule-associated proteins in the embryonic avian spinal cord

We have used monoclonal antibodies to study the distribution of three developmentally regulated microtubule-associated proteins-MAP2, MAP5, and tau-during the morphogenesis of the thoracic spinal cord and peripheral nervous system in the quail. MAP5 is the only one of the three that is present in growing motor neuron processes in the day 3 embryo. The low-molecular weight form of MAP2, MAP2c, is found in motor neuron cell bodies at embryonic day 3. At later stages MAP2c appears in axons and in glia; it decreases in abundance between embryonic days 5 and 7. High-molecular weight MAP2 appears in motor neuron cell bodies and spinal cord gray matter at embryonic day 4, and is never encountered in axons. Tau is found in axons, but only at embryonic day 3.5, after they have commenced active extension. The molecular form and patterns of intracellular compartmentalization of each of the microtubule-associated proteins studied is conserved in mammalian and avian neurons. We conclude that MAP5 may be involved in the active growth of neuronal processes, whereas MAP2 and tau are not, and that high-molecular weight MAP2 and tau may stabilize dendritic and axonal processes, respectively.

Iodination of proteins on nitrocellulose blotting paper

A procedure is described for radioiodination to a high specific activity of proteins immobilized on nitrocellulose membranes. After radioiodination, the proteins can be removed from the nitrocellulose to perform such structural analyses as proteolytic mapping. This method allows the detection of small amounts of protein recognized by immunoblotting and may be used to compare the structure of immunologically related proteins.

Nerve growth factor regulates both the phosphorylation and steady-state levels of microtubule-associated protein 1.2 (MAP1.2)

This study characterizes effects of nerve growth factor (NGF) on the steady-state level and phosphorylation of a high molecular mass microtubule-associated protein in PC12 rat pheochromocytoma cells. Past work showed that NGF significantly raises the relative levels of this phosphoprotein, designated MAP1.2, with a time course similar to that of neurite outgrowth. To study this in greater detail, MAP1.2 in PC12 cell lysates was resolved by SDS-PAGE in gels containing 3.25% acrylamide/4 M urea and identified by comigration with material immunoprecipitated from the lysates by MAP1 antibodies. Quantification by metabolic radiolabeling with [35S]methionine or by silver staining revealed a 3.0-3.5-fold increase in MAP1.2 levels relative to total cell protein after NGF treatment for 2 wk or longer. A partial increase was detectable after 3 d, but not after 2 h of NGF exposure. As measured by incorporation of [32P]phosphate, NGF had a dual effect on MAP1.2. Within 15 min to 2 h, NGF enhanced the incorporation of phosphate into MAP1.2 by two- to threefold relative to total cell phosphoproteins. This value slowly increased thereafter so that by 2 wk or more of NGF exposure, the average enhancement of phosphate incorporation per MAP1.2 molecule was over fourfold. The rapid action of NGF on MAP1.2 could not be mimicked by either epidermal growth factor, a permeant cAMP derivative, phorbol ester, or elevated K+, each of which alters phosphorylation of other PC12 cell proteins. SDS-PAGE revealed multiple forms of MAP1.2 which, based on the effects of alkaline phosphatase on their electrophoretic mobilities, differ, at least in part, in extent of phosphorylation. Before NGF treatment, most PC12 cell MAP1.2 is in more rapidly migrating, relatively poorly phosphorylated forms. After long-term NGF exposure, most is in more slowly migrating, more highly phosphorylated forms. The effects of NGF on the rapid phosphorylation of MAP1.2 and on the long-term large increase in highly phosphorylated MAP1.2 forms could play major functional roles in NGF-mediated neuronal differentiation. Such roles may include effects on microtubule assembly, stability, and cross-linking and, possibly for the rapid effects, nuclear signaling.

Different forms of microtubule-associated protein 2 are encoded by separate mRNA transcripts

Brain microtubule-associated protein 2 (MAP2) consists of a pair of high molecular mass (280 kD) polypeptides, MAP2a and MAP2b, and a recently identified 70-kD protein, MAP2c, which is antigenically related to these high molecular mass MAP2's. Using cDNA clones we have analyzed the expression of these three proteins at the nucleic acid level. cDNA probes selective for the high molecular mass MAP2's a and b identified only a 9-kb mRNA, whereas a probe for sequence common to all three MAP2 isoforms, a, b, and c, recognized the 9-kb transcript and additionally a 6-kb mRNA. Southern blot analysis with cDNA probes indicated that there is only one MAP2 gene from which these two distinct mRNAs are derived. The 70-kD MAP2c protein is much more abundant in neurons of developing brain than those of adult tissues. Similarly the expression of the 6-kb MAP2c-related mRNA, is much greater in neonatal than adult rat brain, indicating that the developmental expression of MAP2 is determined by transcriptional regulation from a single MAP2 gene.

A 70-kilodalton microtubule-associated protein (MAP2c), related to MAP2

Microtubule-associated protein 2 (MAP2) from adult brain consists of a pair of high molecular mass (280 kilodaltons) polypeptides, MAP2a and MAP2b. Juvenile brain microtubules also contain a 70-kilodalton protein that cross-reacts with monoclonal antibodies against these high molecular weight MAP2s. We have analyzed the relationship between this 70-kilodalton protein and MAP2 by peptide mapping. Our results show that the 70-kilodalton species bears strong homology to the MAP2 molecules and that it is distinct from the tau MAPs. We propose the name MAP2c for this low molecular weight MAP2 species. MAP2c is developmentally regulated in brain, being more abundant in neonatal tissue than in the adult. In several cell lines, MAP2c is the sole MAP2 species expressed. We examined homogenates from both juvenile brain and MAP2c-containing cell lines for evidence of a protease activity that might be responsible for generating MAP2c from either MAP2a or MAP2b. No such activity was found, suggesting that MAP2c is an independently synthesized MAP2 species some 200 kilodaltons smaller than the previously recognized forms.

A 60-kDa cytoskeletal protein from Trypanosoma brucei brucei can interact with membranes and with microtubules

The cytoskeleton of eukaryotic cells is a major determinant of cellular architecture and of many cellular functions. In addition to or in place of the transcellular cytoskeleton, many eukaryotic cells also contain membrane-associated cytoskeletal structures (membrane skeletons), which are important for cellular structure and function. The membrane skeleton of the parasitic hemoflagellate Trypanosoma brucei consists of a dense array of singlet microtubules (subpellicular microtubules), which are tightly associated to the overlying cell membrane. This study reports the identification of a microtubule-associated protein from Trypanosoma brucei that constitutes a component of the link between this microtubular array and the cell membrane. The protein can bind in vitro both to microtubules and to membrane vesicles or liposomes. Furthermore, it can crosslink microtubules and membrane vesicles, suggesting that it exerts a similar function in the membrane skeleton.

Differential localization of the high- and low-molecular weight variants of MAP2 in the developing retina

Microtubule-associated protein 2 (MAP2) occurs in developing mammalian neuronal tissue as both high (280 kDa)- and low (70 kDa)-molecular weight forms with temporally regulated expression. We have studied the developing avian retina with a monoclonal antibody that recognizes both the high- and low-molecular weight forms of MAP2 and a second monoclonal antibody that recognizes only high-molecular weight MAP2. The developmentally regulated, low-molecular weight protein, MAP2c, has a more widespread distribution in the embryonic avian retina than high-molecular weight MAP2. Our results suggest that MAP2c is the first form of MAP2 to appear in differentiated embryonic retinal neurons, and that the high-molecular weight isoforms of MAP2 appear only later when they may confer stability to neuronal processes.

The primary structure and heterogeneity of tau protein from mouse brain

Tau protein is a family of microtubule binding proteins, heterogeneous in molecular weight, that are induced during neurite outgrowth and are found prominently in neurofibrillary tangles in Alzheimer's disease. The predicted amino acid sequences of two forms of tau protein from mouse brain were determined from complementary DNA clones. These forms are identical in their amino-terminal sequences but differ in their carboxyl-terminal domains. Both proteins contain repeated sequences that may be tubulin binding sites. The sequence suggests that tau is an elongated molecule with no extensive alpha-helical or beta-sheet domains. These complementary DNAs should enable the study of various functional domains of tau and the study of tau expression in normal and pathological states.

Interacting genes that affect microtubule function: the nc2 allele of the haywire locus fails to complement mutations in the testis-specific beta-tubulin gene of Drosophila

A mutation that fails to complement certain alleles of the testis-specific beta 2-tubulin gene (B2t) of Drosophila melanogaster maps to a separate locus, haywire, located at 3-34.4 map units in polytene region 67E3-F3. Second-site non-complementing mutations such as haync2 and B2t alleles could identify genes that encode products that participate in the same functions or that interact in the same structure. Consistent with a structural interaction between the hay gene product and beta 2-tubulin, the genetic interaction between haync2 and B2t requires the presence of the mutant hay gene product; a deficiency for the hay region complements the same alleles of B2t that haync2 fails to complement. haync2 is a recessive male sterile mutation in a genetic background that is wild type at the B2t locus. Homozygous males have defects in meiosis, flagellar elongation and nuclear shaping, the three major microtubule-based processes in which the testis-specific beta 2-tubulin participates. The haync2 allele also has effects outside of spermatogenesis. It is a temperature-sensitive semilethal mutation, and homozygous haync2 females have reduced fertility. These phenotypes are consistent with a role for the haywire gene product in general microtubule function. Analysis of second-site non-complementing mutations such as haync2 offers a genetic tool for analysis of interacting proteins in complex assemblies.

Isolation and characterization of cDNA clones encoding epitopes shared with Alzheimer neurofibrillary tangles

A pool of ten monoclonal antibodies to SDS-insoluble epitopes of Alzheimer neurofibrillary tangles (NFT) was used to screen an adult human brain cDNA expression library. Fourteen clones were isolated, two of which are described. The largest of the clones encodes 80 kD, or approximately 600 amino acids, of microtubule-associated protein 2 (MAP 2). The MAP 2 region encoded by the clone shares at least two epitopes with human tau, another microtubule-associated protein which cross-reacts with NFT. In rat brain mRNA, the MAP 2 cDNA hybridizes to a single transcript of 9.5 kb. In human neuroblastoma mRNA, the MAP 2 cDNA hybridizes, at high stringency, to two transcripts of 9.5 kb and 6 kb. The 6-kb transcript comigrates with the transcript for tau, as detected by a human tau cDNA. The properties of the MAP 2 cDNA suggest that, in humans, MAP 2 and tau have a common domain which may play a role in NFT formation. Another clone isolated with the anti-NFT antibodies shares epitopes, but not nucleic acid homology, with the MAP 2 cDNA. This clone detects a single abundant transcript of 1 kb present in RNA from human neuroblastoma and from several non-neuronal human cell lines. The properties of this cDNA suggest that it encodes a protein other than those previously reported to cross-react with NFT.

Isolation of cDNAs coding for epitopes shared by microtubule-associated proteins and neurofibrillary tangle in Alzheimer's disease

5 cross-hybridizing cDNA clones of sizes 2.2 (3 cDNAs), 1.3 and 0.8 kb corresponding to tau microtubule-associated protein have been isolated from a rat brain lambda gt11 expression library. Antibodies affinity-purified using the fusion protein encoded by the cDNAs were observed to label tau polypeptides on Western blots and Alzheimer's neurofibrillary tangles.

Microtubule-associated proteins bind to 30 kDa and 60 kDa proteins of rat brain mitochondria: visualization by ligand blotting

Axonal transport of mitochondria is a microtubule-associated movement. Microtubule-mitochondria interactions were studied in vitro using organelles isolated from rat brain. Thanks to the ligand blotting method we were able to show two mitochondrial membrane proteins with apparent molecular masses of 30 kDa and 60 kDa that bind microtubule-associated proteins. The binding of the 30 kDa protein has an apparent Kd of 8 x 10(-8) M. Digitonin fractionation of mitochondria reveals a bimodal localization of the 30 kDa and the 60 kDa proteins within the outer membrane. The data suggest that these polypeptides could participate to the interactions observed in situ between microtubules and mitochondria.

The 28,000 Mr microtubule-binding domain of microtubule-associated protein-2 also contains a neurofilament-binding site

We have developed a thrombin proteolytic cleavage procedure to obtain higher yields of the Mr 28,000 microtubule-binding and Mr 240,000 microtubule-projection components of MAP-2. The former is a highly basic component, whereas the latter and intact MAP-2 are acidic polypeptides. Most notably, our studies reveal that this Mr 28,000 fragment binds to neurofilaments, but the Mr 240,000 projection domain fails to interact. These data indicate that microtubules and neurofilaments share a common binding site on high-molecular-weight MAP-2.

A microtubule-associated protein from Xenopus eggs that specifically promotes assembly at the plus-end

We have isolated a protein factor from Xenopus eggs that promotes microtubule assembly in vitro. Assembly promotion was associated with a 215-kD protein after a 1,000-3,000-fold enrichment of activity. The 215-kD protein, termed Xenopus microtubule assembly protein (XMAP), binds to microtubules with a stoichiometry of 0.06 mol/mol tubulin dimer. XMAP is immunologically distinct from the Xenopus homologues to mammalian brain microtubule-associated proteins; however, protein species immunologically related to XMAP with different molecular masses are found in Xenopus neuronal tissues and testis. XMAP is unusual in that it specifically promotes microtubule assembly at the plus-end. At a molar ratio of 0.01 mol XMAP/mol tubulin the assembly rate of the microtubule plus-end is accelerated 8-fold while the assembly rate of the minus-end is increased only 1.8-fold. Under these conditions XMAP promotes a 10-fold increase in the on-rate constant (from 1.4 s-1.microM-1 for microtubules assembled from pure tubulin to 15 s-1.microM-1), and a 10-fold decrease in off-rate constant (from 340 to 34 s-1). Given its stoichiometry in vivo, XMAP must be the major microtubule assembly factor in the Xenopus egg. XMAP is phosphorylated during M-phase of both meiotic and mitotic cycles, suggesting that its activity may be regulated during the cell cycle.

Microtubule assembly in cytoplasmic extracts of Xenopus oocytes and eggs

We have investigated the differences in microtubule assembly in cytoplasm from Xenopus oocytes and eggs in vitro. Extracts of activated eggs could be prepared that assembled extensive microtubule networks in vitro using Tetrahymena axonemes or mammalian centrosomes as nucleation centers. Assembly occurred predominantly from the plus-end of the microtubule with a rate constant of 2 microns.min-1.microM-1 (57 s-1.microM-1). At the in vivo tubulin concentration, this corresponds to the extraordinarily high rate of 40-50 microns.min-1. Microtubule disassembly rates in these extracts were -4.5 microns.min-1 (128 s-1) at the plus-end and -6.9 microns.min-1 (196 s-1) at the minus-end. The critical concentration for plus-end microtubule assembly was 0.4 microM. These extracts also promoted the plus-end assembly of microtubules from bovine brain tubulin, suggesting the presence of an assembly promoting factor in the egg. In contrast to activated eggs, assembly was never observed in extracts prepared from oocytes, even at tubulin concentrations as high as 20 microM. Addition of oocyte extract to egg extracts or to purified brain tubulin inhibited microtubule assembly. These results suggest that there is a plus-end-specific inhibitor of microtubule assembly in the oocyte and a plus-end-specific promoter of assembly in the eggs. These factors may serve to regulate microtubule assembly during early development in Xenopus.