Interaction of microtubule proteins with phospholipid vesicles

Dimeric Tubulin Modifies Mechanical Properties of Lipid Bilayer, as Probed Using Gramicidin A Channel

Using the gramicidin A channel as a molecular probe, we show that tubulin binding to planar lipid membranes changes the channel kinetics-seen as an increase in the lifetime of the channel dimer-and thus points towards modification of the membrane's mechanical properties. The effect is more pronounced in the presence of non-lamellar lipids in the lipid mixture used for membrane formation. To interpret these findings, we propose that tubulin binding redistributes the lateral pressure of lipid packing along the membrane depth, making it closer to the profile expected for lamellar lipids. This redistribution happens because tubulin perturbs the lipid headgroup spacing to reach the membrane's hydrophobic core via its amphiphilic α-helical domain. Specifically, it increases the forces of repulsion between the lipid headgroups and reduces such forces in the hydrophobic region. We suggest that the effect is reciprocal, meaning that alterations in lipid bilayer mechanics caused by membrane remodeling during cell proliferation in disease and development may also modulate tubulin membrane binding, thus exerting regulatory functions. One of those functions includes the regulation of protein-protein interactions at the membrane surface, as exemplified by VDAC complexation with tubulin.

Interfacing Microfluidics with Negative Stain Transmission Electron Microscopy

A microfluidic platform is presented for preparing negatively stained grids for use in transmission electron microscopy (EM). The microfluidic device is composed of glass etched with readily fabricated features that facilitate the extraction of the grid poststaining and maintains the integrity of the sample. Utilization of this device simultaneously reduced environmental contamination on the grids and improved the homogeneity of the heavy metal stain needed to enhance visualization of biological specimens as compared to conventionally prepared EM grids. This easy-to-use EM grid preparation device provides the basis for future developments of systems with more integrated features, which will allow for high-throughput and dynamic structural biology studies.

Tau binds to lipid membrane surfaces via short amphipathic helices located in its microtubule-binding repeats

Tau is a microtubule-associated protein that is genetically linked to dementia and linked to Alzheimer's disease via its presence in intraneuronal neurofibrillary tangle deposits, where it takes the form of aggregated paired helical and straight filaments. Although the precise mechanisms by which tau contributes to neurodegeneration remain unclear, tau aggregation is commonly considered to be a critical component of tau-mediated pathogenicity. Nevertheless, the context in which tau aggregation begins in vivo is unknown. Tau is enriched in membrane-rich neuronal structures such as axons and growth cones, and can interact with membranes both via intermediary proteins and directly via its microtubule-binding domain (MBD). Membranes efficiently facilitate tau aggregation in vitro, and may therefore provide a physiologically relevant context for nucleating tau aggregation in vivo. Furthermore, tau-membrane interactions may potentially play a role in tau's poorly understood normal physiological functions. Despite the potential importance of direct tau-membrane interactions for tau pathology and physiology, the structural mechanisms that underlie such interactions remain to be elucidated. Here, we employ electron spin resonance spectroscopy to investigate the secondary and long-range structural properties of the MBD of three-repeat tau isoforms when bound to lipid vesicles and membrane mimetics. We show that the membrane interactions of the tau MBD are mediated by short amphipathic helices formed within each of the MBD repeats in the membrane-bound state. To our knowledge, this is the first detailed elucidation of helical tau structure in the context of intact lipid bilayers. We further show, for the first time (to our knowledge), that these individual helical regions behave as independent membrane-binding sites linked by flexible connecting regions. These results represent the first (to our knowledge) detailed structural view of membrane-bound tau and provide insights into potential mechanisms for membrane-mediated tau aggregation. Furthermore, the results may have implications for the structural basis of tau-microtubule interactions and microtubule-mediated tau aggregation.

Plasma membrane--cortical cytoskeleton interactions: a cell biology approach with biophysical considerations

From a biophysical standpoint, the interface between the cell membrane and the cytoskeleton is an intriguing site where a "two-dimensional fluid" interacts with an exceedingly complex three-dimensional protein meshwork. The membrane is a key regulator of the cytoskeleton, which not only provides docking sites for cytoskeletal elements through transmembrane proteins, lipid binding-based, and electrostatic interactions, but also serves as the source of the signaling events and molecules that control cytoskeletal organization and remolding. Conversely, the cytoskeleton is a key determinant of the biophysical and biochemical properties of the membrane, including its shape, tension, movement, composition, as well as the mobility, partitioning, and recycling of its constituents. From a cell biological standpoint, the membrane-cytoskeleton interplay underlies--as a central executor and/or regulator--a multitude of complex processes including chemical and mechanical signal transduction, motility/migration, endo-/exo-/phagocytosis, and other forms of membrane traffic, cell-cell, and cell-matrix adhesion. The aim of this article is to provide an overview of the tight structural and functional coupling between the membrane and the cytoskeleton. As biophysical approaches, both theoretical and experimental, proved to be instrumental for our understanding of the membrane/cytoskeleton interplay, this review will "oscillate" between the cell biological phenomena and the corresponding biophysical principles and considerations. After describing the types of connections between the membrane and the cytoskeleton, we will focus on a few key physical parameters and processes (force generation, curvature, tension, and surface charge) and will discuss how these contribute to a variety of fundamental cell biological functions.

TRPV1-tubulin complex: involvement of membrane tubulin in the regulation of chemotherapy-induced peripheral neuropathy

Existence of microtubule cytoskeleton at the membrane and submembranous regions, referred as 'membrane tubulin' has remained controversial for a long time. Since we reported physical and functional interaction of Transient Receptor Potential Vanilloid Sub Type 1 (TRPV1) with microtubules and linked the importance of TRPV1-tubulin complex in the context of chemotherapy-induced peripheral neuropathy, a few more reports have characterized this interaction in in vitro and in in vivo condition. However, the cross-talk between TRPs with microtubule cytoskeleton, and the complex feedback regulations are not well understood. Sequence analysis suggests that other than TRPV1, few TRPs can potentially interact with microtubules. The microtubule interaction with TRPs has evolutionary origin and has a functional significance. Biochemical evidence, Fluorescence Resonance Energy Transfer analysis along with correlation spectroscopy and fluorescence anisotropy measurements have confirmed that TRPV1 interacts with microtubules in live cell and this interaction has regulatory roles. Apart from the transport of TRPs and maintaining the cellular structure, microtubules regulate signaling and functionality of TRPs at the single channel level. Thus, TRPV1-tubulin interaction sets a stage where concept and parameters of 'membrane tubulin' can be tested in more details. In this review, I critically analyze the advancements made in biochemical, pharmacological, behavioral as well as cell-biological observations and summarize the limitations that need to be overcome in the future.

Shedding of phosphatidylserine from developing erythroid cells involves microtubule depolymerization and affects membrane lipid composition

Phosphatidylserine (PS), which is normally localized in the cytoplasmic leaflet of the membrane, flip-flops to the external leaflet during aging of, or trauma to, cells. A fraction of this PS undergoes shedding into the extracellular milieu. PS externalization and shedding change during maturation of erythroid cells and affect the functioning, senescence and elimination of mature RBCs. Several lines of evidence suggest dependence of PS shedding on intracellular Ca concentration as well as on interaction between plasma membrane phospholipids and microtubules (MTs), the key components of the cytoskeleton. We investigated the effect of Ca flux and MT assembly on the distribution of PS across, and shedding from, the membranes of erythroid precursors. Cultured human and murine erythroid precursors were treated with the Ca ionophore A23187, the MT assembly enhancer paclitaxel (Taxol) or the inhibitor colchicine. PS externalization and shedding were measured by flow cytometry and the cholesterol/phospholipids in RBC membranes and supernatants, by ¹H-NMR. We found that treatment with Taxol or colchicine resulted in a marked increase in PS externalization, while shedding was increased by colchicine but inhibited by Taxol. These results indicate that PS externalization is mediated by Ca flux, and PS shedding by both Ca flux and MT assembly. The cholesterol/phospholipid ratio in the membrane is modified by PS shedding; we now show that it was increased by colchicine and A23187, while taxol had no effect. In summary, the results indicate that the Ca flux and MT depolymerization of erythroid precursors mediate their PS externalization and shedding, which in turn changes their membrane composition.

Membrane lipid composition regulates tubulin interaction with mitochondrial voltage-dependent anion channel

Elucidating molecular mechanisms by which lipids regulate protein function within biological membranes is critical for understanding the many cellular processes. Recently, we have found that dimeric αβ-tubulin, a subunit of microtubules, regulates mitochondrial respiration by blocking the voltage-dependent anion channel (VDAC) of mitochondrial outer membrane. Here, we show that the mechanism of VDAC blockage by tubulin involves tubulin interaction with the membrane as a critical step. The on-rate of the blockage varies up to 100-fold depending on the particular lipid composition used for bilayer formation in reconstitution experiments and increases with the increasing content of dioleoylphosphatidylethanolamine (DOPE) in dioleoylphosphatidylcholine (DOPC) bilayers. At physiologically low salt concentrations, the on-rate is decreased by the charged lipid. The off-rate of VDAC blockage by tubulin does not depend on the lipid composition. Using confocal fluorescence microscopy, we compared tubulin binding to the membranes of giant unilamellar vesicles (GUVs) made from DOPC and DOPC/DOPE mixtures. We found that detectable binding of the fluorescently labeled dimeric tubulin to GUV membranes requires the presence of DOPE. We propose that prior to the characteristic blockage of VDAC, tubulin first binds to the membrane in a lipid-dependent manner. We thus reveal a new potent regulatory role of the mitochondrial lipids in control of the mitochondrial outer membrane permeability and hence mitochondrial respiration through tuning VDAC sensitivity to blockage by tubulin. More generally, our findings give an example of the lipid-controlled protein-protein interaction where the choice of lipid species is able to change the equilibrium binding constant by orders of magnitude.

Plasma membrane tubulin

The association of tubulin with the plasma membrane comprises multiple levels of penetration into the bilayer: from integral membrane protein, to attachment via palmitoylation, to surface binding, and to microtubules attached by linker proteins to proteins in the membrane. Here we discuss the soundness and weaknesses of the chemical and biochemical evidence marshaled to support these associations, as well as the mechanisms by which tubulin or microtubules may regulate functions at the plasma membrane.

Folding of the repeat domain of tau upon binding to lipid surfaces

The microtubule-associated protein tau is impacted in neurodegeneration and dementia through its deposition in the form of paired helical filaments in Alzheimer's disease neurofibrillary tangles and through mutations linking it to the autosomal dominant disorder frontotemporal dementia with Parkinsonism. When isolated in solution tau is intrinsically unstructured and does not fold, while the conformation of the protein in the microtubule-bound state remains uncharacterized. Here we show that the repeat region of tau, which has been reported both to mediate tau microtubule interactions and to constitute the proteolysis-resistant core of disease-associated tau aggregates, associates with lipid micelles and vesicles and folds into an ordered structure upon doing so. In addition to providing the first structural insights into a folded state of tau, our results support a role for lipid membranes in mediating tau function and tau pathology.

Mechanisms of tubulin modification by phosphatidylcholine hydroperoxides

The interaction of lipid peroxides with cellular proteins has been postulated to contribute to cellular aging. A potential target for such effects is tubulin, the building block of microtubules. We examined the concentration-dependent effects of phosphatidylcholine hydroperoxides on the ability of tubulin to polymerize into microtubules. The results demonstrated that even very low concentrations of peroxides were sufficient to interfere with the tubulin and, therefore, the microtubule function. Decreased tubulin activity (as measured by tubulin GTPase activity) showed correlation with the modification of methionine and cysteine in tubulin and a change in the tubulin conformational state as indicated by fluorescence and ultraviolet spectroscopic measurements. As no effect on electric conductivity was observed, indicating that modulation of ionic binding was not involved, the interaction mechanism may be a hydrophobic one.

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.

Posttranslational modification of tubulin by palmitoylation: I. In vivo and cell-free studies

It is well established that microtubules interact with intracellular membranes of eukaryotic cells. There is also evidence that tubulin, the major subunit of microtubules, associates directly with membranes. In many cases, this association between tubulin and membranes involves hydrophobic interactions. However, neither primary sequence nor known posttranslational modifications of tubulin can account for such an interaction. The goal of this study was to determine the molecular nature of hydrophobic interactions between tubulin and membranes. Specifically, I sought to identify a posttranslational modification of tubulin that is found in membrane proteins but not in cytoplasmic proteins. One such modification is the covalent attachment of the long chain fatty acid palmitate. The possibility that tubulin is a substrate for palmitoylation was investigated. First, I found that tubulin was palmitoylated in resting platelets and that the level of palmitoylation of tubulin decreased upon activation of platelets with thrombin. Second, to obtain quantities of palmitoylated tubulin required for protein structure analysis, a cell-free system for palmitoylation of tubulin was developed and characterized. The substrates for palmitoylation were nonpolymerized tubulin and tubulin in microtubules assembled with the slowly hydrolyzable GTP analogue guanylyl-(alpha, beta)-methylene-diphosphonate. However, tubulin in Taxol-assembled microtubules was not a substrate for palmitoylation. Likewise, palmitoylation of tubulin in the cell-free system was specifically inhibited by the antimicrotubule drugs Colcemid, podophyllotoxin, nocodazole, and vinblastine. These experiments identify a previously unknown posttranslational modification of tubulin that can account for at least one type of hydrophobic interaction with intracellular membranes.

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.

Reduction in microtubule dynamics in vitro by brain microtubule-associated proteins and by a microtubule-associated protein-2 second repeated sequence analogue

Microtubule-associated protein (MAP) binding to assembled microtubules (MTs) can be reduced by the addition of polyglutamate without significant MT depolymerization or interference with MT elongation reactions. Ensuing polymer length redistribution in MAP-depleted MTs occurs on a time scale characteristic of that observed with MAP-free MTs. The redistribution phase occurs even in the absence of mechanical shearing and without appreciable effects from end-to-end annealing, as indicated by the time course of incremental changes in polymer length and MT number concentration. We also observed higher rates of MT length redistribution when the [MAP]/[tubulin] ratio was decreased. Together, these results demonstrate that MT length redistribution rates are greatly influenced by MAP content, and the data are compatible with the dynamic instability model. We also found that a peptide analogue corresponding to the second repeated sequence in the MT-binding region of MAP-2 can also markedly retard MT length redistribution kinetics, a finding that accords with the ability of this peptide to promote tubulin polymerization in the absence of MAPs and to displace MAP-2 from MTs. These results provide further evidence that MAPs can modulate MT assembly/disassembly dynamics and that peptide analogues can mimic the action of intact MAPs without the need for three contiguous repeated sequences in the MT-binding region.

Dynamic behavior of endoplasmic reticulum in living cells

Endoplasmic reticulum (ER) was studied by fluorescence microscopy of living CV-1 cells treated with the fluorescent carbocyanine dye DiOC6(3). Using video recording and image processing techniques, several distinct forms of highly localized movements of ER were documented, categorized, and analyzed in terms of mechanism and structural implications. These include tubule branching, ring closure, and sliding. These localized movements have been observed to generate the basic elements of ER: linear tubules, polygonal reticulum, and triple junctions. We propose that as such they act as the mechanism for constructing the polygonal lattice of interconnected membrane tubules that constitutes ER. The nature of these movements suggests possible involvement of the cytoskeleton, and, in view of the close correlations in the distributions of ER and microtubules, and the accompanying paper (Dabora and Sheetz), it is possible that microtubules may play a role in generating ER motility and in constructing and maintaining the ER network in living cells.

The characterization of phospholipids associated with microtubules, purified tubulin and microtubule associated proteins in vitro

1. Significant levels of total phospholipid phosphate were detected in highly purified microtubule protein preparations. 2. While the phospholipid profiles of total microtubule proteins and microtubule-associated proteins showed both similarities and differences to that of a whole brain homogenate, purified tubulin was associated only with phospholipids that were not detectable in the latter. 3. Phosphatidyl ethanolamine, found exclusively in a fraction of microtubule associated proteins, stimulated microtubule assembly in vitro.

Phosphorylation of tubulin enhances its interaction with membranes

Tubulin, the main component of intracellular microtubules, is also a major protein in subcellular membrane preparations and can interact with biological and artificial membranes in vitro. Of particular interest is the association of tubulin with postsynaptic junctional lattices enriched in a polypeptide of relative molecular mass (Mr) 50,000 (50K), recently identified as the major subunit of the calmodulin-dependent protein kinase. Phosphorylation of tubulin with a calmodulin-dependent protein kinase similar to that found in postsynaptic densities inhibits its ability to self-assemble into microtubules in a reversible fashion. This involves the phosphorylation of residues in its 4K carboxy-terminal region, a domain that seems to regulate its self-assembly. The results presented here suggest that the phosphorylation of tubulin with this kinase enhances its ability to interact with membranes. This effect is reversible.

Interaction of tubulin with octyl glucoside and deoxycholate. 1. Binding and hydrodynamic studies

Tubulin purified from calf brain cytoplasm, normally a compact water-soluble dimer, is able to interact with the mild detergents octyl glucoside (a minimum of 60 detergent molecules) and deoxycholate (95 +/- 8 molecules). Binding is cooperative and approaches saturation below the critical micelle concentration of the amphiphiles. Binding is accompanied by a quenching of the intrinsic protein fluorescence, but no spectral shape changes indicating denaturation such as in the case of sodium dodecyl sulfate are observed. Glycerol, which is known to be preferentially excluded from the tubulin domain and to favor the folded and associated forms of this protein, inhibits the binding of the mild detergents. Octyl glucoside induces a rapidly equilibrating tubulin self-association reaction characterized by a bimodal sedimentation velocity profile with boundaries at approximately 5 and 12 S. Full dissociation of this detergent restores the normal sedimentation behavior to 90% of the protein. Binding of deoxycholate slows the sedimentation velocity of tubulin from s(0)20,w = 5.6 +/- 0.2 S to s(0)20,w = 4.8 +/- 0.3 S. Measurements of the molecular weight of the tubulin-deoxycholate complex indicate an increase from 100,000 to 143,000 +/- 5,000. The diffusion rate consistently decreases from (5.3 +/- 0.5) X 10(-7) to (3.8 +/- 0.2) X 10(-7) cm2 S-1. This is most simply interpreted as an expansion of the undissociated tubulin dimer upon detergent binding (a change in the frictional ratio, f/f min, from 1.35 to 1.86). It is concluded that tubulin shows a reversible transition between the water-soluble state and amphipathic detergent-bound forms which constitute a model system of tubulin-membrane interactions.

Diabetic rat serum has an increased capacity to inhibit brain microtubule formation in vitro

The assembly of pig brain microtubule proteins was measured in vitro in the presence of serum from control rats and rats that had been rendered diabetic with 50 mg/kg streptozotocin 14 d previously. Control serum inhibited total microtubule assembly and increased the lag time before assembly commenced. Serum from diabetic animals was significantly more potent in both respects. The effect on lag time was reproduced in a predominantly albumin-containing fraction of serum that had been fractionated by affinity chromatography. Glycosylation of rat albumin in vitro led to an increase in its ability to increase polymerization lag time, but the concentration of albumin required was greater than that found in the serum fractions. The results indicate that diabetic serum contains factors that can adversely affect microtubule formation and that part of this effect may be caused by the presence of glycosylated albumin. This phenomenon may underlie some of the complications associated with diabetes.

In vitro studies of the biosynthesis of brain tubulin on membranes

Membrane elements in brain tissue contain relatively large amounts of alpha- and beta-tubulin (FIGURES 2 and 3). We have investigated the subcellular sites of tubulin biosynthesis in order to determine the origin of this membrane-associated tubulin. Free and membrane-bound polysomes from rat forebrain were separated by differential centrifugation, and the products of translation from these polysome populations were analyzed by 2DGE (FIGURES 4 and 6). Alpha- and beta-tubulin subunits were synthesized by the free polysome population (FIGURES 4 and 5A and B). The membrane-bound polysome fraction synthesized a protein with similar (but not identical) characteristics to alpha-tubulin (denoted as "MB" in FIGURE 6), including isoelectric point, molecular weight, peptide map, and copurification with microtubules after aggregation-disaggregation. Tubulin subunits synthesized in vitro by free polysomes could associate posttranslationally with a microsome fraction (FIGURE 7A). The association of the tubulin translation products with membranes was not disrupted by high salt; the associated tubulin, however, was susceptible to proteolytic digestion, with the exception of one of the beta-tubulin subunits (FIGURE 7B). There was an identical protease-resistant beta-tubulin subunit among the native proteins of the smooth microsome fractions. Our data is consistent with the conclusion that at least one beta subunit of membrane-associated tubulin is synthesized by free polysomes and becomes posttranslationally added to membrane structures. It is unlikely that a cotranslational mechanism is responsible, in which there is a signal-mediated insertion of a growing polypeptide chain to membrane. Our results, however, are consistent with a "membrane trigger" mechanism proposed by Wickner in which the membrane lipid bilayer triggers the folding of a polypeptide into a configuration that allows integral membrane insertion. The association of tubulin with membranes may also be secondary to the interaction of hydrophobic elements. The amino acid sequence of beta tubulin is known to contain several hydrophobic domains. Tubulin can be incorporated into phospholipid vesicles and various subcellular membrane elements. In our studies, in vitro synthesized tubulin from free polysome was found to be purified by hydrophobic affinity chromatography with ethane-sepharose (FIGURE 8). Thus, the hydrophobic characteristics of newly synthesized tubulin could be partially responsible for the posttranslational association of tubulin subunit with membranes. Native tubulin in a soluble fraction of CNS tissue was not purified by hydrophobic affinity chromatography.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for a tubulin-containing lipid-protein structural complex in ciliary membranes

The proteins and lipids of the scallop gill ciliary membrane may be reassociated through several cycles of detergent solubilization, detergent removal, and freeze-thaw, without significant change in overall protein composition. Membrane proteins and lipids reassociate to form vesicles of uniform, discrete density classes under a variety of reassociation conditions involving detergent removal and concentration. Freed of the solubilizing detergent during equilibrium centrifugation, a protein-lipid complex equilibrates to a position on a sucrose density gradient characteristic of the original membrane density. When axonemal tubulin is solubilized by dialysis, mixed with 2:1 lecithin/cholesterol dissolved in Nonidet P-40, freed of detergent, and reconstituted by freeze-thaw, vesicles of a density essentially equal to pure lipid result. If the lipid fraction is derived through chloroform-methanol extraction of natural ciliary membranes, a moderate increase in density occurs upon reconstitution, but the protein is adsorbed and most is removed by a simple low ionic strength wash, in contrast to vesicles reconstituted from membrane proteins where even high salt extraction causes no loss of protein. The proteins of the ciliary membrane dissolve with constant composition, regardless of the type, concentration, or efficiency of detergent. Analytical ultracentrifugation demonstrates that monodisperse mixed micelles form at high detergent concentrations, but that membranes are dispersed to large sedimentable aggregates by Nonidet P-40 even at several times the critical micelle concentration, which suggests reasons for the efficacy of certain detergent for the production of ATP-reactivatable cell models. In extracts freed of detergent, structured polydisperse particles, but not membrane vesicles, are seen in negative staining; vesicles form upon concentration of the extract. Membrane tubulin is not in a form that will freely undergo electrophoresis, even in the presence of detergent above the critical micelle concentration. All chromatographic attempts to separate membrane tubulin from other membrane proteins have failed; lipid and protein are excluded together by gel filtration in the presence of high concentrations of detergent. These observations support the idea that a relatively stable lipid-protein complex exists in the ciliary membrane and that in this complex membrane tubulin is tightly associated with lipids and with a number of other proteins.

Interaction of the sperm adhesive protein, bindin, with phospholipid vesicles. I. Specific association of bindin with gel-phase phospholipid vesicles

Bindin is a 30,000-mol-wt protein of sea urchin sperm that is responsible for the specific adhesion of the sperm acrosomal process to the vitelline layer covering the egg plasma membrane during fertilization. Sulfated glycoconjugates are believed to be the egg surface receptors for bindin, but the mechanism by which bindin associates with the sperm acrosomal membrane is unknown. Here I report that bindin specifically associates with phospholipid vesicles in vitro. Interaction of the bindin polypeptide with liposomes was found to cause an increase in the density of the liposomes and induce the aggregation of the vesicles. A novel property of this association of bindin with membranes was that it required phospholipids in a gel phase. The interaction of bindin with liposomes was greatly reduced at temperatures above the phase transition temperature. The interaction of bindin with gel-phase vesicles appeared to be reversible, since the aggregated vesicles dissaggregated as the temperature was raised above the phase transition temperature. Association of bindin with the bilayer did not alter the accessibility of the polypeptide to cleavage by trypsin, which suggests that most of the polypeptide chain remains exposed at the surface of the membrane.

Altered tubulin distribution in the hypothalamus of aging female C57BL/6J mice

The effects of age and estradiol on hypothalamic content and distribution of tubulin and calmodulin were examined by radioimmunoassay in ovariectomized C57BL/6J female mice. A small (14%) increase in particulate tubulin, but not soluble tubulin, was found in the hypothalamus of reproductively senescent mice (20 months) compared to young (8 months) controls. This alteration was limited to tubulin; calmodulin content was unaffected by age. Post-castration serum LH levels were lower in old, ovariectomized controls relative to young controls, but physiologic levels of estradiol, achieved by subcutaneous implants, suppressed LH levels in both age groups. In contrast to LH and uterine weight, hypothalamic tubulin and calmodulin were unaffected by estradiol treatment. These results suggest that the negative feedback effect of estradiol on LH secretion is exerted by a mechanism other than redistribution of hypothalamic tubulin or calmodulin, or that changes are restricted to a discrete sub-population of neurons.

Purification and characterization of oocyte cytoplasmic tubulin and meiotic spindle tubulin of the surf clam Spisula solidissima

Assembly-competent tubulin was purified from the cytoplasm of unfertilized and parthogenetically activated oocytes, and from isolated meiotic spindles of the surf clam, Spisula solidissima. At 22 degrees C or 37 degrees C, Spisula tubulin assembled into 48-51-nm macrotubules during the first cycle of polymerization and 25-nm microtubules during the third and subsequent cycles of assembly. Macrotubules were formed from sheets of 26-27 protofilaments helically arranged at a 36 degree angle relative to the long axis of the polymer and were composed of alpha and beta tubulins and several other proteins ranging in molecular weight from 30,000 to 270,000. Third cycle microtubules contained 14-15 protofilaments in cross-section and were composed of greater than 95% alpha and beta tubulins. After three cycles of polymerization at 37 degrees C, unfertilized and activated oocyte tubulin self-assembled into microtubules at a critical concentration (Ccr) of 0.09 mg/ml. At the physiological temperature of 22 degrees C, unfertilized oocyte tubulin assembled into microtubules at a Ccr of 0.36 mg/ml, activated oocyte tubulin assembled at a Ccr of 0.42 mg/ml, and isolated meiotic spindle tubulin assembled at a Ccr of 0.33 mg/ml. The isoelectric points of tubulin from both unfertilized oocytes and isolated meiotic spindles were 5.8 for alpha tubulin and 5.6 for beta tubulin. In addition, one dimensional peptide maps of oocyte and spindle alpha and beta tubulins were very similar, if not identical. These results indicate that unfertilized oocyte tubulin and tubulin isolated from the first meiotic spindle are indistinguishable on the basis of assembly properties, isoelectric focusing, and one dimensional peptide mapping. These results suggest that the transition of tubulin from the quiescent oocyte state to that competent to form spindle microtubules in vivo does not require special modification of tubulin but may involve changes in the availability of microtubule organizing centers or assembly-promoting microtubule-associated proteins.

Tubulin as a molecular component of coated vesicles

Two proteins of 53,000 and 56,000 mol wt have been found to be associated with coated vesicles (CV) purified from bovine brain and chicken liver. These proteins share molecular weights, isoelectric points, and antigenic determinants with alpha- and beta-tubulins purified from bovine brain. Based on SDS PAGE and electron microscopic analysis of controlled pore glass bead exclusion column fractions, both the tubulins and the major CV polypeptide clathrin were found to chromatograph as components of a single kinetic particle. In addition, tubulin and CV antigens assayed by a sensitive enzyme-linked-immunoadsorbent method eluted from the columns with constant stoichiometry. These data provide evidence that tubulin is a molecular component of coated vesicles.

Tubulin is a major protein constituent of bovine brain coated vesicles

A major protein constituent of coated vesicles derived from bovine brain is present as a Mr = 55,000 protein doublet on SDS-polyacrylamide gels. This protein comigrates with purified calf brain tubulin after two-dimensional electrophoresis, binds tubulin antibody and has two-dimensional tryptic peptide maps identical to those of purified tubulin. The results indicate that tubulin is a major protein constituent of coated vesicles. Coated vesicles had no significant effect on the polymerization rate of purified tubulin. Therefore, the physiological relevance of tubulin associated with coated vesicles remains to be determined.

Identification of three coated vesicle components as alpha- and beta-tubulin linked to a phosphorylated 50,000-dalton polypeptide

Coated vesicles are involved in the intracellular transport of membrane proteins between a variety of membrane compartments. The coats of bovine brain coated vesicles contain at least six polypeptides in addition to an 180,000-dalton polypeptide called clathrin. In this report we show that the 54,000- and 56,000-dalton coated vesicle polypeptides are alpha- and beta-tubulin, determined by immunoblotting and two-dimensional gel electrophoresis. An affinity-purified tubulin antiserum can precipitate coated vesicles. The tubulin polypeptides are tightly associated with a 50,000-dalton coated vesicle polypeptide, which is phosphorylated. The phosphorylated 50,000-dalton polypeptide appears to be related to brain microtubule-associated tau proteins since it can be specifically immunoprecipitated by an affinity-purified antiserum directed against these proteins. In addition, gel filtration experiments indicate that at least a fraction of the 50,000-dalton polypeptide may associate with the 100,000-dalton coated vesicle polypeptide. Since brain is a tissue rich in tubulins, liver coated vesicles were analyzed for the presence of alpha- and beta-tubulin. Like brain coated vesicles, liver coated vesicles also contain an endogenous kinase activity, which phosphorylates polypeptides of the same molecular weights and isoelectric points as the brain coated vesicle 50,000-dalton, tau-like polypeptide, and alpha- and beta-tubulin. The phosphorylated 50,000-dalton polypeptide may link the membrane and contents of coated vesicles with components of the cytoskeleton.

Evidence for tubulin-binding sites on cellular membranes: plasma membranes, mitochondrial membranes, and secretory granule membranes

We describe the interaction of pure brain tubulin with purified membranes specialized in different cell functions, i.e., plasma membranes and mitochondrial membranes from liver and secretory granule membranes from adrenal medulla. We studied the tubulin-binding activity of cellular membranes using a radiolabeled ligand-receptor assay and an antibody retention assay. The tubulin-membrane interaction was time- and temperature-dependent, reversible, specific, and saturable. The binding of tubulin to membranes appears to be specific since acidic proteins such as serum albumin or actin did not interfere in the binding process. The apparent overall affinity constant of the tubulin-membrane interaction ranged between 1.5 and 3.0 X 10(7) M-1; similar values were obtained for the three types of membranes. Tubulin bound to membranes was not entrapped into vesicles since it reacted quantitatively with antitubulin antibodies. At saturation of the tubulin-binding sites, the amount of reversibly bound tubulin represents 5-10% by weight of membrane protein (0.4-0.9 nmol tubulin/mg membrane protein). The high tubulin-binding capacity of membranes seems to be inconsistent with a 1:1 stoichiometry between tubulin and a membrane component but could be relevant to a kind of tubulin assembly. Indeed, tubulin-membrane interaction had some properties in common with microtubule formation: (a) the association of tubulin to membranes increased with the temperature, whereas the dissociation of tubulin-membrane complexes increased by decreasing temperature; (b) the binding of tubulin to membranes was prevented by phosphate buffer. However, the tubulin-membrane interaction differed from tubulin polymerization in several aspects: (a) it occurred at concentrations far below the critical concentration for polymerization; (b) it was not inhibited at low ionic strength and (c) it was colchicine-insensitive. Plasma membranes, mitochondrial membranes, and secretory granule membranes contained tubulin as an integral component. This was demonstrated on intact membrane and on Nonidet P-40 solubilized membrane protein using antitubulin antibodies in antibody retention and radioimmune assays. Membrane tubulin content varied from 2.2 to 4.4 micrograms/mg protein. The involvement of membrane tubulin in tubulin-membrane interactions remains questionable since erythrocyte membranes devoid of membrane tubulin exhibited a low (one-tenth of that of rat liver plasma membranes) but significant tubulin-binding activity. These results show that membranes specialized in different cell functions possess high-affinity, large-capacity tubulin-binding sites...

Identity and Origin of the ATPase activity associated with neuronal microtubules. I. The ATPase activity is associated with membrane vesicles

Microtubule protein purified from brain tissue by cycles of in vitro assembly-disassembly contains ATPase activity that has been postulated to be associated with microtubule-associated proteins (MAPs) and therefore significant for studies of microtubule-dependent motility. In this paper we demonstrate that greater than 90% of the ATPase activity is particulate in nature and may be derived from contaminating membrane vesicles. We also show that the MAPs (MAP-1, MAP-2, and tau factors) and other high molecular weight polypeptides do not contain significant amounts of ATPase activity. These findings do not support the concept of "brain dynein" or of MAPs with ATPase activity.

A modified colchicine-binding assay for the measurement of total and microtubule-derived tubulin in rat liver

The usual measurement of liver tubulin by the colchicine-binding assay does not take into account the accelerated decay of the colchicine-binding capacity of tubulin when liver supernatants, especially those containing microtubule-derived tubulin, are incubated at 37 degrees C. This results in marked underestimations. Our findings indicate that this alteration is due to an inhibitor of colchicine-tubulin binding in liver supernatants that is probably extracted from particulate fractions. The inhibitory activity is decreased by dilution of the supernatants and by increasing the concentration of colchicine. However, the former modification decreases the sensitivity of the assay and the latter increases the nonspecific binding of colchicine to liver proteins other than tubulin. Assessment of the decay and correction for it by calculating the initial binding capacity results in complete recovery of brain tubulin from liver supernatants and values for microtubule-derived tubulin that closely correspond to those expected from simultaneous morphometric assessment of liver microtubules by electron microscopy. The modified method also indicates that the fraction of liver tubulin assembled in microtubules is greater than previously reported.

Fc gamma of IgG: a specific agent of destabilization of lipid bilayers containing oleic acid

gamma-Immunoglobulins induce the fusion of oleic acid-containing liposomes into tubular structures. An F(ab')2 preparation does not exert the same influence unless it is several orders of magnitude more concentrated. The same is true for several proteins with an isoelectric point higher or lower than the IgG isoelectric point. The Fc of IgG seems thus to exert a specific destabilizing and fusogenic action on artificial lipid membranes. An hypothesis is presented concerning the mode of action of Fc gamma on lymphocyte membrane which is based on the facts mentioned above and on the existence of a phospholipasic activity of Fc gamma membrane receptor.

Reconstitution of ciliary membranes containing tubulin

Membranes from the gill cilia of the mollusc Aequipecten irradians may be solubilized readily with Nonidet P-40. When the detergent is removed from the solution by adsorption to polystyrene beads, the proteins of the extract remain soluble. However, when the solution is frozen and thawed, nearly all of the proteins reassociate to form membrane vesicles, recruiting lipids from the medium. The membranes equilibrate as a narrow band (d = 1.167 g/cm3) upon sucrose density gradient centrifugation. The lipid composition of reconstituted membranes (1:2 cholesterol:phospholipids) closely resembles that of the original extract, as does the protein content (45%). Ciliary calmodulin is the major extract protein that does not associate with the reconstituted membrane, even in the presence of 1 mM calcium ions, suggesting that it is a soluble matrix component. The major protein of reconstituted vesicles is membrane tubulin, shown previously to differ hydrophobically from axonemal tubulin. The tubulin is tightly associated with the membrane since extraction with 1 mM iodide or thiocyanate leaves a vesicle fraction whose protein composition and bouyant density are unchanged. Subjecting the detergent-free membrane extract to a freeze-thaw cycle in the presence of elasmobranch brain tubulin or forming membranes by warming the extract in the presence of polymerization-competent tubulin yields a membrane fraction with little incorporated brain tubulin. This suggests that ciliary membrane tubulin specifically associates with lipids, whereas brain tubulin preferentially forms microtubules.

Purification and chemical characterization of a W2 protein from brain myelin

Starting from a pellet of beef brain myelin insoluble in chloroform/methanol (2:1, vol/vol)(Wolfgram protein fraction), a pure W2 protein with apparent molecular weight of 52,000 was isolated by a simple preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis method. A comparative chemical analysis was carried out between purified W2 and a standard tubulin. Glutamic acid and arginine were the N-terminals detected. Similar peptide maps and amino acid composition were also found in both proteins. Immunological cross-reactivity was detected when W2 protein was tested against antitubulin serum. These results suggest that W2 protein could have a tubulin-like protein nature that is associated with the myelin membrane and could play a role in the myelination process.

Association between endocrine pancreatic secretory granules and in-vitro-assembled microtubules is dependent upon microtubule-associated proteins

By use of dark-field light microscopy, secretory granules isolated from the anglerfish endocrine pancreas were observed to attach to and release from microtubules assembled in vitro from brain homogenates. Secretory granules only bound to microtubules assembled in the presence of microtubule-associated proteins (MAPs) and not to microtubules assembled from purified tubulin. The addition of a MAP fraction to purified tubulin restored secretory granule binding. The secretory granules were released from MAP-containing microtubules by the addition of Mg-ATP but not by other nucleotides. The number of secretory granules bound to MAP-containing microtubules was increased in the presence of cyclic AMP. In addition to the associations of secretory granules with microtubules, MAP-containing microtubules also associated with each other. These laterally associated microtubules were dispersed by the addition of Mg-ATP. Electron micrographs confirmed that the associations between MAP-containing microtubules and secretory granules as well as the associations of microtubules with one another were mediated by the high molecular weight MAPs known to project from the surface of in-vitro-assembled microtubules.