Phosphorylation of tubulin enhances its interaction with membranes

Nonequilibrium Amyloid Polymers Exploit Dynamic Covalent Linkage to Temporally Control Charge-Selective Catalysis

Extant proteins exploit thermodynamically activated negatively charged coenzymes and hydrotropes to temporally access mechanistically important conformations that regulate vital biological functions, from metabolic reactions to expression modulation. Herein, we show that a short amyloid peptide can bind to a small molecular coenzyme by exploiting reversible covalent linkage to polymerize and access catalytically proficient nonequilibrium amyloid microphases. Subsequent hydrolysis of the activated coenzyme leads to depolymerization, realizing a variance of the surface charge of the assembly as a function of time. Such temporal change of surface charge dynamically modulates catalytic activities of the transient assemblies as observed in highly evolved modern-day biocatalysts.

Molecular Evolution of Tubulins in Diatoms

Microtubules are formed by α- and β-tubulin heterodimers nucleated with γ-tubulin. Tubulins are conserved eukaryotic proteins. Previously, it was shown that microtubules are involved in diatom silica frustule morphogenesis. Diatom frustules are varied, and their morphology is species-specific. Despite the attractiveness of the problem of elucidating the molecular mechanisms of genetically programmed morphogenesis, the structure and evolution of diatom tubulins have not been studied previously. Based on available genomic and transcriptome data, we analyzed the phylogeny of the predicted amino acid sequences of diatom α-, β- and γ-tubulins and identified five groups for α-tubulins, six for β-tubulins and four for γ-tubulins. We identified characteristic amino acids of each of these groups and also analyzed possible posttranslational modification sites of diatom tubulins. According to our results, we assumed what changes occurred in the diatom tubulin structures during their evolution. We also identified which tubulin groups are inherent in large diatom taxa. The similarity between the evolution of diatom tubulins and the evolution of diatoms suggests that molecular changes in α-, β- and γ-tubulins could be one of the factors in the formation of a high morphological diversity of diatoms.

The tubulin code and its role in controlling microtubule properties and functions

Microtubules are core components of the eukaryotic cytoskeleton with essential roles in cell division, shaping, motility and intracellular transport. Despite their functional heterogeneity, microtubules have a highly conserved structure made from almost identical molecular building blocks: the tubulin proteins. Alternative tubulin isotypes and a variety of post-translational modifications control the properties and functions of the microtubule cytoskeleton, a concept known as the 'tubulin code'. Here we review the current understanding of the molecular components of the tubulin code and how they impact microtubule properties and functions. We discuss how tubulin isotypes and post-translational modifications control microtubule behaviour at the molecular level and how this translates into physiological functions at the cellular and organism levels. We then go on to show how fine-tuning of microtubule function by some tubulin modifications can affect homeostasis and how perturbation of this fine-tuning can lead to a range of dysfunctions, many of which are linked to human disease.

Bioinformatic search of plant microtubule-and cell cycle related serine-threonine protein kinases

A bioinformatic search was carried for plant homologues of human serine-threonine protein kinases involved in regulation of cell division and microtubule protein phosphorylation (SLK, PAK6, PAK7, MARK1, MAST2, TTBK1, TTBK2, AURKA, PLK1, PLK4 and PASK). A number of SLK, MAST2 and AURKA plant homologues were identified. The closest identified homologue of human AURKA kinase was a protein of unknown function, A7PY12/GSVIVT00026259001 from Vitis vinifera (herein named as "STALK", Serine-Threonine Aurora-Like Kinase). Analysis of STALK's three-dimensional structure confirmed its relationship to the subgroup of AURKA-like protein kinases.

Trypanosoma cruzi: in vitro phosphorylation of tubulin by a protein kinase CK2-like enzyme

One predominant 55-kDa polypeptide was phosphorylated in vitro in Trypanosoma cruzi homogenates prepared from three differentiation stages: epimastigotes, trypomastigotes, and spheromastigotes. Anti-alpha and anti-beta tubulin monoclonal antibodies immunoprecipitated the phosphorylated 55-kDa polypeptide from epimastigote extracts. Phosphoserine was the only residue phosphorylated in vitro in the 55-kDa polypeptide and in immunoprecipitated alpha tubulin. The phosphorylation of both the 55-kDa polypeptide and exogenously added casein was inhibited with GTP, heparin, and 2,3-bisphosphoglycerate in a dose-dependent manner, indicating the involvement of a CK2-like protein kinase. Moreover, when tubulin was isolated from an epimastigote homogenate by ultracentrifugation, followed by DEAE-Sephacel chromatography, a protein kinase that phosphorylated tubulin and casein co-purified with this cytoskeletal component. This result suggests an association between tubulin and its corresponding protein kinase in T. cruzi.

A pool of beta-tubulin is hyperphosphorylated at serine residues in Alzheimer disease brain

In Alzheimer disease (AD) brain, activities of protein phosphatase (PP)-2A/PP-1 which are known to be associated with microtubules are compromised and are probably a cause of neurofibrillary degeneration through hyperphosphorylation of microtubule proteins. In the present study, an increase of approximately 11 pmol phosphate/microg protein in 100,000 x g pellet from AD compared with age-matched control brains was found. Tau protein, which is hyperphosphorylated in AD can only account for approximately 4 pmol phosphate/microg protein, suggesting the presence of non-tau hyperphosphorylated proteins in the diseased brain. Western blot analysis with phosphoserine antibodies revealed a approximately 54 kDa non-tau protein to be significantly hyperphosphorylated in AD compared with age-matched control cases in the particulate fraction. The approximately 54 kDa protein was purified by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis and identified as beta-tubulin by immunolabeling with specific antibodies, mass spectrometry analysis and by N-terminal amino acid sequencing. The purified protein was hyperphosphorylated at serine residues in AD.

Immunocytochemical localization of metabotropic glutamate receptor type 1 alpha and tubulin in rat brain

The distribution of mGlu1 alpha receptor and tubulin was immunocytochemically examined in the rat cerebellar cortex and primary rat cortical neurons at both immunofluorescence and electron microscopic level. In cryosections from rat cerebellar cortex mGlu1 alpha receptor immunoreactivity was expressed in cell bodies and dendrites of Purkinje and basket cells of the cerebellar molecular layer. Tubulin immunoreactivity was concentrated in the dendritic tree of the cerebellar molecular layer, as well as in the granule cell layer. In primary rat cortical neurons, both proteins colocalized throughout the proximal and distal dendrites of these cells. At the electron microscopic level, the receptor was present in dendritic shafts and dendritic spines of Purkinje cells at perisynaptic sites of asymmetrical synapses. Immunoreactivity corresponding to tubulin was associated with the plasma membrane of dendritic shafts of Purkinje cells, as well as throughout its cytoplasm as part of the cytoskeletal components. Interestingly, double labeling for both proteins reveals an association of tubulin with mGlu1 alpha receptor at the plasma membrane level of dendritic shafts of Purkinje cells. This suggests that tubulin interacts with mGlu1 alpha receptor and may be involved in the anchoring of the receptor to the plasma membrane.

Metabotropic glutamate receptor type 1alpha and tubulin assemble into dynamic interacting complexes

Metabotropic glutamate receptors (mGlu receptors) are coupled to G-protein second messenger pathways and modulate glutamate neurotransmission in the brain, where they are targeted to specific synaptic locations. Very recently, we identified tubulin as an interacting partner of the mGlu(1alpha) receptor in rat brain. Using BHK-570 cells permanently expressing the receptor we have shown that this interaction occurs predominantly with soluble tubulin, following its translocation to the plasma membrane. In addition, treatment of the cells with the agonist quisqualic acid induce tubulin depolymerization and its translocation to the plasma membrane. Immunofluorescence detection of both the receptor and tubulin in agonist-treated cells reveals a disruption of the microtubule network and an increased clustering of the receptor. Collectively these data demonstrate that the mGlu(1alpha) receptor interacts with soluble tubulin and that this association can take place at the plasma membrane.

Binding and phosphorylation of tubulin by G protein-coupled receptor kinases

Although the beta-adrenergic receptor kinase (betaARK) mediates agonist-dependent phosphorylation and desensitization of G protein-coupled receptors, recent studies suggest additional cellular functions. During our attempts to identify novel betaARK interacting proteins, we found that the cytoskeletal protein tubulin could specifically bind to a betaARK-coupled affinity column. In vitro analysis demonstrated that betaARK and G protein-coupled receptor kinase-5 (GRK5) were able to stoichiometrically phosphorylate purified tubulin dimers with a preference for beta-tubulin and, under certain conditions, the betaIII-isotype. Examination of the GRK/tubulin binding characteristics revealed that tubulin dimers and assembled microtubules bind GRKs, whereas the catalytic domain of betaARK contains the primary tubulin binding determinants. In vivo interaction of GRK and tubulin was suggested by the following: (i) co-purification of betaARK with tubulin from brain tissue; (ii) co-immunoprecipitation of betaARK and tubulin from COS-1 cells; and (iii) co-localization of betaARK and GRK5 with microtubule structures in COS-1 cells. In addition, GRK-phosphorylated tubulin was found preferentially associated with the microtubule fraction during in vitro assembly assays suggesting potential functional significance. These results suggest a novel link between the cytoskeleton and GRKs that may be important for regulating GRK and/or tubulin function.

The G protein-coupled receptor kinase 2 is a microtubule-associated protein kinase that phosphorylates tubulin

The G protein-coupled receptor kinase 2 (GRK2) is a serine/threonine kinase that phosphorylates and desensitizes agonist-occupied G protein-coupled receptors (GPCRs). Here we demonstrate that GRK2 is a microtubule-associated protein and identify tubulin as a novel GRK2 substrate. GRK2 is associated with microtubules purified from bovine brain, forms a complex with tubulin in cell extracts, and colocalizes with tubulin in living cells. Furthermore, an endogenous tubulin kinase activity that copurifies with microtubules has properties similar to GRK2 and is inhibited by anti-GRK2 monoclonal antibodies. Indeed, GRK2 phosphorylates tubulin in vitro with kinetic parameters very similar to those for phosphorylation of the agonist-occupied beta2-adrenergic receptor, suggesting a functionally relevant role for this phosphorylation event. In a cellular environment, agonist occupancy of GPCRs, which leads to recruitment of GRK2 to the plasma membrane and its subsequent activation, promotes GRK2-tubulin complex formation and tubulin phosphorylation. These findings suggest a novel role for GRK2 as a GPCR signal transducer mediating the effects of GPCR activation on the cytoskeleton.

Interaction of cytoskeletal proteins with membrane lipids

Rapid and significant progress has been made in understanding lipid/protein interactions involving cytoskeletal components and the plasma membrane. Covalent and noncovalent lipid modifications of cytoskeletal proteins mediate their interaction with lipid bilayers. The application of biophysical techniques such as differential scanning colorimetry, neutron reflection, electron spin resonance, CD spectroscopy, nuclear magnetic resonance, and hydrophobic photolabeling, allow various folding stages of proteins during electrostatic adsorption and hydrophobic insertion into lipid bilayers to be analyzed. Reconstitution of proteins into planar lipid films and liposomes help to understand the architecture of biological interfaces. During signaling events at plasma membrane interfaces, lipids are important for the regulation of catalytic protein functions. Protein/lipid interactions occur selectively and with a high degree of specificity and thus have to be considered as physiologically relevant processes with gaining impact on cell functions.

Tubulin post-translational modifications--enzymes and their mechanisms of action

This review describes the enzymes responsible for the post-translational modifications of tubulin, including detyrosination/tyrosination, acetylation/deacetylation, phosphorylation, polyglutamylation, polyglycylation and the generation of non-tyrosinatable alpha-tubulin. Tubulin tyrosine-ligase, which reattaches tyrosine to detyrosinated tubulin, has been extensively characterized and its gene sequenced. Enzymes such as tubulin-specific carboxypeptidase and alpha-tubulin acetyltransferase, required, respectively, for detyrosination and acetylation of tubulin, have yet to be purified to homogeneity and examined in defined systems. This has produced some conflicting results, especially for the carboxypeptidase. The phosphorylation of tubulin by several different types of kinases has been studied in detail but drawing conclusions is difficult because many of these enzymes modify proteins other than their actual substrates, an especially pertinent consideration for in vitro experiments. Tubulin phosphorylation in cultured neuronal cells has proven to be the best model for evaluation of kinase effects on tubulin/microtubule function. There is little information on the enzymes required for polyglutamylation, polyglycylation, and production of non-tyrosinatable tubulin, but the available data permit interesting speculation of a mechanistic nature. Clearly, to achieve a full appreciation of tubulin post-translational changes the responsible enzymes must be characterized. Knowing when the enzymes are active in cells, if soluble or polymerized tubulin is the preferred substrate and the amino acid residues modified by each enzyme are all important. Moreover, acquisition of purified enzymes will lead to cloning and sequencing of their genes. With this information, one can manipulate cell genomes in order to either modify key enzymes or change their relative amounts, and perhaps reveal the physiological significance of tubulin post-translational modifications.

Phosphorylation in vivo of chick brain microtubule-associated phospholipids

Microtubules were prepared by temperature-dependent cycles of assembly/disassembly from chick brain labeled in vivo with 32Pi and the distribution of labeled phospholipids extracted from cold-insoluble and soluble microtubular protein fractions was analyzed by thin-layer and paper chromatography. While 32P-labeling was associated with all of the phospholipids identified after 2-D TLC, it was found that all of the relatively high radioactivity associated with phosphatidylserine (PS) was in fact associated with a minor co-migrating component which was subsequently identified as phosphatidylinositol(PI) by three independent separation procedures. It was estimated that the relative specific radioactivity in PI was several-fold higher than that associated with other microtubule-associated phospholipids. Additional experiments, in which the protein components of once-cycled microtubules were fractionated by gel permeation chromatography, provided evidence that the 36S component containing ring-like tubulin oligomers (36S) appears to be selectively associated with phospholipid components that were specifically enriched in 32P-PI. The possible significance of these findings is discussed in relation to the effects of phospholipids on microtubule dynamics and to the function of microtubules in their interactions with membranes.

Human T lymphocyte activation induces tyrosine phosphorylation of alpha-tubulin and its association with the SH2 domain of the p59fyn protein tyrosine kinase

A glutathione-S-transferase-src-homology domain 2 (GST-SH2) fusion protein was employed to identify molecules interacting with the protein tyrosine kinase p59fyn. Among several proteins which bound to the fyn SH2 domain in lysates of human Jurkat T lymphocytes, alpha- and beta-tubulin were identified by N-terminal sequencing. Further analysis established that alpha-tubulin exists as a tyrosine-phosphorylated protein in Jurkat cells, where it interacts with p59fyn, but not with p56lck. By contrast, in untransformed resting human T lymphocytes alpha-tubulin is not detectable as a tyrosine phosphorylated protein. However, following T cell activation, it becomes rapidly phosphorylated on tyrosine residues and subsequently associates with the SH2 domain of fyn. Interestingly, constitutively tyrosine-phosphorylated alpha-tubulin that is able to interact with the fyn-SH2 domain is expressed in peripheral blood T lymphoblasts isolated from leukemic patients in the absence of external stimulation.

Biochemical and electron microscopy analysis of the endotoxin binding to microtubules in vitro

The mechanisms involved in cellular activation and damage by bacterial endotoxins are not completely defined. In particular, there is little information about possible intracellular targets of endotoxins. Recently, the participation of a microtubule associated protein in endotoxin actions on macrophages has been suggested. In the present work, we have studied the effect of E. coli lipopolysaccharide on the polymerization of microtubular protein in vitro. Electrophoretic analysis of the polymerization mixtures showed that the endotoxin inhibited the polymerization when present at high concentrations. At lower concentrations, LPS selectively displaced the microtubule associated protein MAP-2 from the polymerized microtubules. Electron microscopy showed that LPS binds to microtubules of tubulin + MAPs and to microtubules of purified tubulin (without MAPs) polymerized with taxol. Gel filtration experiments confirmed the binding of LPS to tubulin, and by ligand blot assays an interaction LPS-MAP-2 was detected. The ability of LPS to interact with microtubular proteins suggests a possible participation of microtubules on the cellular effects of endotoxins.

Microtubule functions

Microtubules, with intermediate filaments and microfilaments, are the components of the cell skeleton which determinates the shape of a cell. Microtubules are involved in different functions including the assembly of mitotic spindle, in dividing cells, or axon extension, in neurons. In the first case, microtubules are highly dynamic, while in the second case microtubules are quite stable, suggesting that microtubule with different physical properties (stability) are involved in different functions. Thus, to understand the mechanisms of microtubule functions it is very important to understand microtubule dynamics. Historically, tubulin, the main component of microtubules, was first characterized as the major component of the mitotic spindle that binds to colchicine. Afterwards, it was found that tubulin is particularly more abundant in brain than in other tissues. Therefore, the roles of microtubules in mitosis, and in neurons, have been more extensively analyzed and, in this review, these roles will be discussed.

Increased calmodulin affects cell morphology and mRNA levels of cytoskeletal protein genes

We have previously described stable mouse C127 cell lines in which a CaM mini-gene has been expressed in a bovine papilloma virus-based expression vector (Rasmussen and Means: EMBO J. 6:3961-3968, 1987). Elevation of CaM to levels five-fold higher than in control cells caused an acceleration in cell cycle progression by reducing the length of the G1 period. When these cell lines were originally isolated it was observed that cells in which CaM levels were increased had a flattened morphology. In this study we have examined the localization of actin, vimentin, and tubulin in these cells as compared to the BPV-transformed control cell line in order to determine if changes in shape were accompanied by differences in the cytoskeletal organization. Cell-cycle-dependent changes in the levels of mRNAs for histone H4, glyceraldehyde-3-phosphate dehydrogenase, beta-actin, vimentin, and beta-tubulin have also been examined. Our results indicate that increased CaM causes differences in the organization of microfilaments, intermediate filaments, and microtubules and that these changes are accompanied by selective differences in the cell-cycle-dependent expression of some mRNAs. Elevated CaM was also correlated with a reduced stability of beta-tubulin mRNA. These studies indicate that CaM has pleiotropic effects on cell function and suggest that stable cell lines with altered CaM levels may provide a useful model system for understanding the molecular basis of CaM-dependent regulation of cellular processes.

Experimental manipulation of compaction of the mouse embryo alters patterns of protein phosphorylation

Compaction, occurring at the eight-cell stage of mouse development, is the process of cell flattening and polarisation by which cellular asymmetry is first established. Changes in the pattern of protein phosphorylation have been correlated with this early event of development (TL Bloom, J McConnell: Mol Reprod Dev 26:199-210, 1990). In the study reported here, groups of embryos were treated in ways known to affect particular features of compaction and were then labeled with [32P]orthophosphate; the phosphoproteins obtained were examined following electrophoresis in one and two dimensions. Four-cell embryos were treated with protein synthesis inhibitors, which advance cell flattening. This treatment resulted in only minor differences from the phosphoprotein profile of untreated four-cell embryos. Inhibition of protein synthesis at the eight-cell stage has little effect on cell flattening or polarisation. However, some phosphoproteins that are observed normally in eight-cell but not in four-cell embryos were no longer detectable if labeling took place in the presence of protein synthesis inhibitors. Eight-cell embryos incubated in phorbol 12-myristate 13-acetate, which disrupts various features of compaction, showed a relative increase in the phosphorylation of a group of phosphoprotein spots associated with the eight-cell but not with the four-cell stage. Embryos incubated in Ca2(+)-free medium, which prevents intercellular flattening and delays polarisation, showed a relative decrease in the phosphorylation of the same group of phosphoprotein spots. The behaviour of these phosphoproteins may therefore be correlated with some of the features of compaction.

Changes in protein phosphorylation associated with compaction of the mouse preimplantation embryo

In order to investigate the role of protein phosphorylation in the early differentiative events of mouse preimplantation development, timed groups of embryos of various stages were incubated in medium containing [32P]orthophosphate and harvested immediately after labelling or following a chase period. The phosphoproteins obtained were separated by electrophoresis in one and two dimensions. While some of the phosphoproteins found were common to all the stages examined, the detection of many depended on both the combination of pulse-labelling and chase periods used and on the developmental stage examined. Some phosphoproteins were only found in compacted 8-cell embryos, a correlation which suggests a possible link with the post-translational mechanisms which underlie compaction.

The phosphorylation of the integral membrane (M1) protein of influenza virus

The phosphorylation of the internal and integral membrane (M1) protein of influenza virus was studied. Four points can be made based on the data: (1) The M1 contains at least two moles of phosphate per mole of M1. (2) Phosphorylation of M1 is conserved between influenza A, B and C viruses. Other characteristics of the M1 are also conserved, such as solubility in organic solvent, heterogeneity and ability to partition into lipid vesicles. (3) M1 is phosphorylated in cells infected with a vaccinia recombinant (vP273) containing only the gene of M1, either as a result of a vaccinia virus associated kinase or a cellular one. (4) The phosphate is located within or in close proximity to the major stretch of neutral and hydrophobic amino acids found in M1, as determined by analyzing cyanogen bromide fragments.

Hypothesis: microtubules, a key to Alzheimer disease

Alzheimer disease (AD) is a clinicopathologic syndrome of unknown etiology with numerous abnormalities in neuronal and nonneuronal cells. A review of the literature suggests that a common basic intracellular defect may underlie many of the reported abnormalities. We hypothesize impairment of the microtubule (MT) system as one explanation for the pathogenesis of AD. Evidence in support of the hypothesis includes the following: MTs are ubiquitous and vital cell components, unequally distributed, with the highest concentration in the brain; various abnormalities, including the key neuropathologic lesions, can be explained by impairments of the MT system; and experiments utilizing pharmacologic agents known to disrupt MTs have reproduced certain abnormalities observed in AD. The hypothesis provides a framework for systematic investigations of MTs at the cellular and molecular levels as well as the basis for in vivo diagnostic tests for AD.

Characterization of a cytoskeletal matrix associated with myelin from rat brain

Extraction of rat brain myelin in a buffer containing Triton X-100 yielded a soluble fraction and an insoluble residue that was enriched in cytoskeletal elements. Immunoblot analysis of the detergent-soluble fraction and the insoluble cytoskeletal residue showed that all of the tubulin and more than half of the actin were found within the cytoskeletal fraction. The distribution of myelin-specific proteins was also examined, and revealed that 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNPase) I and most of the myelin basic proteins (MBPs) were equally distributed between both fractions. By contrast, the large MBP (21.5 kDa) and CNPase II (50 kDa) were observed to partition almost entirely with the cytoskeletal fraction. Proteolipid protein was found predominantly in the detergent-soluble fraction, as was DM-20 protein. Analysis of the cytoskeletal fraction by sucrose-density-gradient centrifugation demonstrated that a distinct subset of lipids was tightly bound to the cytoskeletal protein residue. The cytoskeleton-associated lipid was considerably enriched in cerebroside and sphingomyelin by comparison with total myelin lipids. These results indicate that a cytoskeletal matrix is associated with multilamellar myelin, and suggest that this structure may play a fundamental role in myelinogenesis.

Subcellular localization of iodinated thyroid tubulin

Subcellular fractions enriched in mitochondria, plasma membranes, microsomes and Golgi apparatus were obtained from thyroid glands of rats injected with I125. Autoradiography of SDS-polyacrylamide gels revealed the presence of a number of radiolabelled proteins in each membrane fraction. One polypeptide, with the same electrophoretic mobility as brain tubulin, was found in all fractions except the plasma membranes and was immunoprecipitated with commercial anti-tubulin monoclonal antibodies. Hydrolysis of Asp-Pro linkages of I125 labelled tubulin with formic acid indicated that there were iodination sites in both the carboxy terminal one third and the amino terminal two thirds of the molecule. These results, together with the absence of iodinated tubulin from the cytosolic fraction, are consistent with the idea that a population of thyroid membrane tubulin is iodinated at multiple sites either just before or after insertion into intracellular membranes where it may act as an anchorage point for microtubule-membrane interactions.

Role of the growth cone in neuronal differentiation

Nerve growth cones are motile, exploring organelles at the tip of a growing neurite. The growth cone is a highly specialized structure, equipped with a complex machinery for reversible membrane expansion and rapid cytoskeletal reorganization, a machinery required for growth cone motility and neurite elongation. It also contains perception systems that enable the growth cone to respond to external signals, thereby steering the trailing neurite to the correct target. Soluble and substrate bound guidance molecules in the environment modulate growth cone behavior either through direct interaction or classical receptor activation coupled to second messengers. A prominent phosphoprotein of the growth cone is B-50. We propose a role for this growth-associated protein kinase C substrate in signal transduction processes in the growth cone.

Fractionation of Tetrahymena ciliary membranes with triton X-114 and the identification of a ciliary membrane ATPase

Cilia were isolated from Tetrahymena thermophila, extracted with Triton X-114, and the detergent-soluble membrane + matrix proteins separated into Triton X-114 aqueous and detergent phases. The aqueous phase polypeptides include a high molecular mass polypeptide previously identified as a membrane dynein, detergent-soluble alpha and beta tubulins, and numerous polypeptides distinct from those found in axonemes. Integral membrane proteins partition into the detergent phase and include two major polypeptides of 58 and 50 kD, a 49-kD polypeptide, and 5 polypeptides in relatively minor amounts. The major detergent phase polypeptides are PAS-positive and are phosphorylated in vivo. A membrane-associated ATPase, distinct from the dynein-like protein, partitions into the Triton X-114 detergent phase and contains nearly 20% of the total ciliary ATPase activity. The ATPase requires Mg++ or Ca++ and is not inhibited by ouabain or vanadate. This procedure provides a gentle and rapid technique to separate integral membrane proteins from those that may be peripherally associated with the matrix or membrane.

Small zone gel chromatography of interacting systems: theoretical and experimental evaluation of elution profiles for kinetically controlled macromolecule-ligand reactions

A phenomenological theory of small zone gel chromatography is elaborated for kinetically controlled macromolecule-ligand interactions. Chromatography, direct binding experiments, and rate measurements are used for successful comparison of experimental behavior with theoretical elution profiles for the interaction of MAP-2 with two synthetic peptide fragments from the C-terminal moieties of alpha- and beta-tubulin subunits. The results of this study provide guidelines for interpretation of experimental small zone elution profiles of total ligand.

Desmin heterogeneity in the main electric organ of Electrophorus electricus

Desmin, the muscle-specific intermediate filament protein was purified from the main electric organ of Electrophorus electricus. It is shown that pure desmin can be separated into 5 isoforms presenting different isoelectric points. These isoforms have similar molecular weight, react with an antibody directed against desmin and generate identical peptides after digestion with protease V8 from Staphylococcus aureus.

Microtubule and plasmalemmal reorganization: acute response to estrogen

The acute ultrastructural effects of estrogen in endometrial epithelial cells were investigated by transmission electron microscopy (TEM), with special reference to the microtubule (MT) apparatus and the luminal surface. Ovariectomized rats anesthetized with pentobarbitol sodium were injected intravenously with estradiol-17 beta (E2 beta), 0.5 micrograms/100 g body wt. At intervals from approximately 30 s to 30 min thereafter, 70-80 nm cross sections of a uterine horn were prepared for TEM. In placebo controls, cytoplasmic MT were conspicuous in length and number, whereas only a minimal population of short microvilli (MV) was evident. In contrast, the specimens subjected to E2 beta for only 35 s showed a significant decrease in MT number and length, with virtually complete depletion of these organelles by approximately 80 s. Concomitantly, the luminal MV exhibited striking enhancement in length and density. Thereafter, these rapid and reciprocal alterations of MT and MV underwent inversion. Thus MT structures began to reappear within 2 min, increasing progressively so that by 30 min their numbers were again substantial, although lengths remained diminished. During the same interval, the initial surge of luminal MV gradually subsided, to near-control appearance by 30 min. These coordinate, reciprocal, and biphasic responses are consistent with biochemical evidences of abrupt membrane perturbation associated with interception of estrogen at its cellular targets. The resultant modification of the intracellular environment may contribute to limited reorganization of cellular architecture and propagation of the hormonal signal.

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.

Tubulin phosphorylation by casein kinase II is similar to that found in vivo

Purified brain tubulin subjected to an exhaustive phosphatase treatment can be rephosphorylated by casein kinase II. This phosphorylation takes place mainly on a serine residue, which has been located at the carboxy-terminal domain of the beta-subunit. Interestingly, tubulin phosphorylated by casein kinase II retains its ability to polymerize in accordance with descriptions by other authors of in vivo phosphorylated tubulin. Moreover, the V8 phosphopeptide patterns of both tubulin phosphorylated in vitro by casein kinase II and tubulin phosphorylated in vivo in N2A cells are quite similar, and different from that of tubulin phosphorylated in vitro by Ca/calmodulin-dependent kinase II. On the other hand, we have found an endogenous casein kinase II-like activity in purified brain microtubule protein that uses GTP and ATP as phosphate donors, is inhibited by heparin, and phosphorylates phosphatase-treated tubulin. Thus it appears that a casein kinase II-like activity should be considered a candidate for the observed phosphorylation of beta-tubulin in vivo in brain or neuroblastoma cells.

Characterization of a membrane-specific tubulin isoform by peptide mapping

A membrane-specific tubulin-like protein, found in preparations of synaptic plasma membranes and brain mitochondria, was analyzed by chemical and proteolytic peptide mapping to determine which part of the molecule was different from cytoplasmic tubulin. The membrane polypeptide was identical to alpha tubulin in the first two-thirds of the molecule containing the amino terminal, as found by peptide mapping. However, some differences were observed in the peptide maps of the carboxy terminal one third of the molecule which includes a domain that is important in the regulation of tubulin self-assembly.