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

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.

βIII-Tubulin Structural Domains Regulate Mitochondrial Network Architecture in an Isotype-Specific Manner

βIII-tubulin is a neuronal microtubule protein that is aberrantly expressed in epithelial cancers. The microtubule network is implicated in regulating the architecture and dynamics of the mitochondrial network, although the isotype-specific role for β-tubulin proteins that constitute this microtubule network remains unclear. High-resolution electron microscopy revealed that manipulation of βIII-tubulin expression levels impacts the volume and shape of mitochondria. Analysis of the structural domains of the protein identifies that the C-terminal tail of βIII-tubulin, which distinguishes this protein from other β-tubulin isotypes, significantly contributes to the isotype-specific effects of βIII-tubulin on mitochondrial architecture. Mass spectrometry analysis of protein–protein interactions with β-tubulin isotypes identifies that βIII-tubulin specifically interacts with regulators of mitochondrial dynamics that may mediate these functional effects. Advanced quantitative dynamic lattice light sheet imaging of the mitochondrial network reveals that βIII-tubulin promotes a more dynamic and extended reticular mitochondrial network, and regulates mitochondrial volume. A regulatory role for the βIII-tubulin C-terminal tail in mitochondrial network dynamics and architecture has widespread implications for the maintenance of mitochondrial homeostasis in health and disease.

Role of Mitochondria-Cytoskeleton Interactions in the Regulation of Mitochondrial Structure and Function in Cancer Stem Cells

Despite the promise of cancer medicine, major challenges currently confronting the treatment of cancer patients include chemoresistance and recurrence. The existence of subpopulations of cancer cells, known as cancer stem cells (CSCs), contributes to the failure of cancer therapies and is associated with poor clinical outcomes. Of note, one of the recently characterized features of CSCs is augmented mitochondrial function. The cytoskeleton network is essential in regulating mitochondrial morphology and rearrangement, which are inextricably linked to its functions, such as oxidative phosphorylation (OXPHOS). The interaction between the cytoskeleton and mitochondria can enable CSCs to adapt to challenging conditions, such as a lack of energy sources, and to maintain their stemness. Cytoskeleton-mediated mitochondrial trafficking and relocating to the high energy requirement region are crucial steps in epithelial-to-mesenchymal transition (EMT). In addition, the cytoskeleton itself interplays with and blocks the voltage-dependent anion channel (VDAC) to directly regulate bioenergetics. In this review, we describe the regulation of cellular bioenergetics in CSCs, focusing on the cytoskeleton-mediated dynamic control of mitochondrial structure and function.

Tubulin-VDAC Interaction: Molecular Basis for Mitochondrial Dysfunction in Chemotherapy-Induced Peripheral Neuropathy

Tubulin is a well-established target of microtubule-targeting agents (MTAs), a widely used class of chemotherapeutic drugs. Yet, aside from their powerful anti-cancer efficiency, MTAs induce a dose-limiting and debilitating peripheral neurotoxicity. Despite intensive efforts in the development of neuroprotective agents, there are currently no approved therapies to effectively manage chemotherapy-induced peripheral neuropathy (CIPN). Over the last decade, attempts to unravel the pathomechanisms underlying the development of CIPN led to the observation that mitochondrial dysfunctions stand as a common feature associated with axonal degeneration. Concomitantly, mitochondria emerged as crucial players in the anti-cancer efficiency of MTAs. The findings that free dimeric tubulin could be associated with mitochondrial membranes and interact directly with the voltage-dependent anion channels (VDACs) located in the mitochondrial outer membrane strongly suggested the existence of an interplay between both subcellular compartments. The biological relevance of the interaction between tubulin and VDAC came from subsequent in vitro studies, which found dimeric tubulin to be a potent modulator of VDAC and ultimately of mitochondrial membrane permeability to respiratory substrates. Therefore, one of the hypothetic mechanisms of CIPN implies that MTAs, by binding directly to the tubulin associated with VDAC, interferes with mitochondrial function in the peripheral nervous system. We review here the foundations of this hypothesis and discuss them in light of the current knowledge. A focus is set on the molecular mechanisms behind MTA interference with dimeric tubulin and VDAC interaction, the potential relevance of tubulin isotypes and availability as a free dimer in the specific context of MTA-induced CIPN. We further highlight the emerging interest for VDAC and its interacting partners as a promising therapeutic target in neurodegeneration.

Non-canonical processes that shape the cell migration landscape

Migration is a vital, intricate, and multi-faceted process that involves the entire cell, entails the integration of multiple external cues and, at times, necessitates high-level coordination among fields of cells that can be physically attached or not, depending on the physiological setting. Recent advances have highlighted the essential role of cellular components that have not been traditionally considered when studying cell migration. This review details how much we recently learned by studying the role of intermediate filaments, the nucleus, extracellular vesicles, and mitochondria during cell migration.

Sequence diversity of tubulin isotypes in regulation of the mitochondrial voltage-dependent anion channel

The microtubule protein tubulin is a heterodimer comprising α/β subunits, in which each subunit features multiple isotypes in vertebrates. For example, seven α-tubulin and eight β-tubulin isotypes in the human tubulin gene family vary mostly in the length and primary sequence of the disordered anionic carboxyl-terminal tails (CTTs). The biological reason for such sequence diversity remains a topic of vigorous enquiry. Here, we demonstrate that it may be a key feature of tubulin's role in regulation of the permeability of the mitochondrial outer membrane voltage-dependent anion channel (VDAC). Using recombinant yeast α/β-tubulin constructs with α-CTTs, β-CTTs, or both from various human tubulin isotypes, we probed their interactions with VDAC reconstituted into planar lipid bilayers. A comparative study of the blockage kinetics revealed that either α-CTTs or β-CTTs block the VDAC pore and that the efficiency of blockage by individual CTTs spans 2 orders of magnitude, depending on the CTT isotype. β-Tubulin constructs, notably β3, blocked VDAC most effectively. We quantitatively described these experimental results using a physical model that accounted only for the number and distribution of charges in the CTT, and not for the interactions between specific residues on the CTT and VDAC pore. Based on these results, we speculate that the effectiveness of VDAC regulation by tubulin depends on the predominant tubulin isotype in a cell. Consequently, the fluxes of ATP/ADP through the channel could vary significantly, depending on the isotype, thus suggesting an intriguing link between VDAC regulation and the diversity of tubulin isotypes present in vertebrates.

Novel function of ceramide for regulation of mitochondrial ATP release in astrocytes

We reported that amyloid β peptide (Aβ₄₂) activated neutral SMase 2 (nSMase2), thereby increasing the concentration of the sphingolipid ceramide in astrocytes. Here, we show that Aβ₄₂ induced mitochondrial fragmentation in wild-type astrocytes, but not in nSMase2-deficient cells or astrocytes treated with fumonisin B1 (FB1), an inhibitor of ceramide synthases. Unexpectedly, ceramide depletion was concurrent with rapid movements of mitochondria, indicating an unknown function of ceramide for mitochondria. Using immunocytochemistry and super-resolution microscopy, we detected ceramide-enriched and mitochondria-associated membranes (CEMAMs) that were codistributed with microtubules. Interaction of ceramide with tubulin was confirmed by cross-linking to N-[9-(3-pent-4-ynyl-3-H-diazirine-3-yl)-nonanoyl]-D-erythro-sphingosine (pacFACer), a bifunctional ceramide analog, and binding of tubulin to ceramide-linked agarose beads. Ceramide-associated tubulin (CAT) translocated from the perinuclear region to peripheral CEMAMs and mitochondria, which was prevented in nSMase2-deficient or FB1-treated astrocytes. Proximity ligation and coimmunoprecipitation assays showed that ceramide depletion reduced association of tubulin with voltage-dependent anion channel 1 (VDAC1), an interaction known to block mitochondrial ADP/ATP transport. Ceramide-depleted astrocytes contained higher levels of ATP, suggesting that ceramide-induced CAT formation leads to VDAC1 closure, thereby reducing mitochondrial ATP release, and potentially motility and resistance to Aβ₄₂ Our data also indicate that inhibiting ceramide generation may protect mitochondria in Alzheimer's disease.

Structural features and lipid binding domain of tubulin on biomimetic mitochondrial membranes

Dimeric tubulin, an abundant water-soluble cytosolic protein known primarily for its role in the cytoskeleton, is routinely found to be associated with mitochondrial outer membranes, although the structure and physiological role of mitochondria-bound tubulin are still unknown. There is also no consensus on whether tubulin is a peripheral membrane protein or is integrated into the outer mitochondrial membrane. Here the results of five independent techniques-surface plasmon resonance, electrochemical impedance spectroscopy, bilayer overtone analysis, neutron reflectometry, and molecular dynamics simulations-suggest that α-tubulin's amphipathic helix H10 is responsible for peripheral binding of dimeric tubulin to biomimetic "mitochondrial" membranes in a manner that differentiates between the two primary lipid headgroups found in mitochondrial membranes, phosphatidylethanolamine and phosphatidylcholine. The identification of the tubulin dimer orientation and membrane-binding domain represents an essential step toward our understanding of the complex mechanisms by which tubulin interacts with integral proteins of the mitochondrial outer membrane and is important for the structure-inspired design of tubulin-targeting agents.

Quantitative proteomics identifies myoferlin as a novel regulator of A Disintegrin and Metalloproteinase 12 in HeLa cells

A Disintegrin and Metalloproteinase 12 (ADAM12) is expressed significantly higher in multiple tumors than in normal tissues and has been used as a prognostic marker for the evaluation of cancer progression. Although several ADAM12 substrates have been identified biochemically and its proteolytic function has been explored, the upstream regulators and the interacting proteins have not been systematically investigated. Here, we use immunoprecipitation and mass spectrometry (MS)-based quantitative proteomic approaches to identify 28 interacting partners for the long form of ADAM12 (ADAM12-L) in HeLa cells. Proteins that regulate cell proliferation, invasion, and epithelial to mesenchymal transition are among the identified ADAM12-interacting proteins. Further biochemical experiments discover that the protein level and the stability of ADAM12 are upregulated by one of its interacting proteins, myoferlin. In addition, myoferlin also increases the proteolytic activity of ADAM12, leading to the reduction of an ADAM12 substrate, E-cadherin. This result implies that ADAM12 and its interacting proteins might converge to certain signaling pathways in the regulation of cancer cell progression. The information obtained here might be useful in the development of new strategies for modulating cell proliferation and invasion involved in the regulation between ADAM12 and its interacting partners. MS data are available via ProteomeXchange with identifier PXD003560.

BIOLOGICAL SIGNIFICANCE

Regulation of the proliferation and invasion of cancer cells is important in cancer treatment. ADAM12 has been found to play important roles in regulating these processes and identification of its interacting partners will improve our understanding of its biological functions and provide basis for functional modulation. Through mass spectrometry-based quantitative proteomic approaches, we identify the interacting partners for ADAM12 in a human cancer cell line and find many proteins that are involved in the proliferation and invasion of cancer cells. A novel regulator, myoferlin, of ADAM12 is discovered and this protein increases ADAM12 expression level, stability, and its enzymatic activity, leading to the reduction of its substrate, E-cadherin, which plays important roles in the regulation of cell adhesion and tumor metastasis. This result provides a connection for two highly expressed proteins in cancer cells and may shed light on the regulation of their biological functions in cancer progression.

Metabolic and structural integrity of magnetic nanoparticle-loaded primary endothelial cells for targeted cell therapy

AIM

To successfully translate magnetically mediated cell targeting from bench to bedside, there is a need to systematically assess the potential adverse effects of magnetic nanoparticles (MNPs) interacting with 'therapeutic' cells. Here, we examined in detail the effects of internalized polymeric MNPs on primary rat endothelial cells' structural intactness, metabolic integrity and proliferation potential.

MATERIALS & METHODS

The intactness of cytoskeleton and organelles was studied by fluorescent confocal microscopy, flow cytometry and high-resolution respirometry.

RESULTS

MNP-loaded primary endothelial cells preserve intact cytoskeleton and organelles, maintain normal rate of proliferation, calcium signaling and mitochondria energy metabolism.

CONCLUSION

This study provides supportive evidence that MNPs at doses necessary for targeting did not induce significant adverse effects on structural integrity and functionality of primary endothelial cells - potential cell therapy vectors.

The oviducal protein, heat-shock 70-kDa protein 8, improves the long-term survival of ram spermatozoa during storage at 17°C in a commercial extender

Poor fertility rates are often observed when fresh ram semen stored in conventional extenders is used for cervical artificial insemination (AI). Heat-shock 70-kDa protein 8 (HSPA8), found within the oviduct, prolongs boar, ram and bull sperm survival at body temperatures in vitro. Here, we aimed to determine whether supplementing extenders (INRA-96 and RSD-1) with HSPA8 (4 µg mL⁻¹) would improve their performance in maintaining freshly collected ram sperm viability and sperm nuclear DNA integrity during storage over 48 h at 17°C. Sperm function was assessed at 1, 6, 24 and 48h and this experiment was repeated using 25 × 10⁶ and 800 × 10⁶ spermatozoa mL⁻¹. INRA96 supplemented with HSPA8 maintained sperm viability significantly better than INRA96 alone at both sperm concentrations. However, sperm nuclear DNA fragmentation (DF) increased significantly during storage using the higher sperm concentration, irrespective of the extender and the protein treatment used. Increasing levels of sperm nuclear DF over time could explain why poor fertility rates are often observed following cervical AI using stored ram semen. However, further research is required to ascertain whether supplementing the commercially available INRA96 extender with HSPA8 will improve fertility rates following cervical AI using stored ram semen.

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.

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.

VDAC inhibition by tubulin and its physiological implications

Regulation of mitochondrial outer membrane (MOM) permeability has dual importance: in normal metabolite and energy exchange between mitochondria and cytoplasm, and thus in control of respiration, and in apoptosis by release of apoptogenic factors into the cytosol. However, the mechanism of this regulation involving the voltage-dependent anion channel (VDAC), the major channel of MOM, remains controversial. For example, one of the long-standing puzzles was that in permeabilized cells, adenine nucleotide translocase is less accessible to cytosolic ADP than in isolated mitochondria. Still another puzzle was that, according to channel-reconstitution experiments, voltage regulation of VDAC is limited to potentials exceeding 30mV, which are believed to be much too high for MOM. We have solved these puzzles and uncovered multiple new functional links by identifying a missing player in the regulation of VDAC and, hence, MOM permeability - the cytoskeletal protein tubulin. We have shown that, depending on VDAC phosphorylation state and applied voltage, nanomolar to micromolar concentrations of dimeric tubulin induce functionally important reversible blockage of VDAC reconstituted into planar phospholipid membranes. The voltage sensitivity of the blockage equilibrium is truly remarkable. It is described by an effective "gating charge" of more than ten elementary charges, thus making the blockage reaction as responsive to the applied voltage as the most voltage-sensitive channels of electrophysiology are. Analysis of the tubulin-blocked state demonstrated that although this state is still able to conduct small ions, it is impermeable to ATP and other multi-charged anions because of the reduced aperture and inversed selectivity. The findings, obtained in a channel reconstitution assay, were supported by experiments with isolated mitochondria and human hepatoma cells. Taken together, these results suggest a previously unknown mechanism of regulation of mitochondrial energetics, governed by VDAC interaction with tubulin at the mitochondria-cytosol interface. Immediate physiological implications include new insights into serine/threonine kinase signaling pathways, Ca(2+) homeostasis, and cytoskeleton/microtubule activity in health and disease, especially in the case of the highly dynamic microtubule network which is characteristic of cancerogenesis and cell proliferation. In the present review, we speculate how these findings may help to identify new mechanisms of mitochondria-associated action of chemotherapeutic microtubule-targeting drugs, and also to understand why and how cancer cells preferentially use inefficient glycolysis rather than oxidative phosphorylation (Warburg effect). This article is part of a Special Issue entitled: VDAC structure, function, and regulation of mitochondrial metabolism.

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.

The N-terminal domain of 2',3'-cyclic nucleotide 3'-phosphodiesterase harbors a GTP/ATP binding site

The interaction between 2',3'-cyclic nucleotide 3'-phosphodiesterase and guanine/adenine nucleotides was investigated. The binding of purine nucleotides to 2',3'-cyclic nucleotide 3'-phosphodiesterase was revealed by both direct and indirect methods. In fact, surface plasmon resonance experiments, triphosphatase activity measurements, and fluorescence experiments revealed that 2',3'-cyclic nucleotide 3'-phosphodiesterase binds purine nucleotide triphosphates with an affinity higher than that displayed for diphosphates; on the contrary, the affinity for both purine monophosphates and pyrimidine nucleotides was negligible. An interpretation of biological experimental data was achieved by a building of 2',3'-cyclic nucleotide 3'-phosphodiesterase N-terminal molecular model. The structural elements responsible for nucleotide binding were identified and potential complexes between the N-terminal domain of CNP-ase and nucleotide were analyzed by docking simulations. Therefore, our findings suggest new functional and structural property of the N-terminal domain of CNPase.

Direct interaction of Bcl-2 proteins with tubulin

A direct interaction between tubulin and several pro-apoptotic and anti-apoptotic members of the Bcl-2 family has been demonstrated by effects on the assembly of microtubules from pure rat brain tubulin. Bcl-2, Bid, and Bad inhibit assembly sub-stoichiometrically, whereas peptides from Bak and Bax promote tubulin polymerization at near stoichiometric concentrations. These opposite effects on microtubule assembly are mutually antagonistic. The BH3 homology domains, common to all members of the family, are involved in the interaction with tubulin but do not themselves affect polymerization. Pelleting experiments with paclitaxel-stabilized microtubules show that Bak is associated with the microtubule pellet, whereas Bid remains primarily with the unpolymerized fraction. These interactions require the presence of the anionic C-termini of alpha- and beta-tubulin as they do not occur with tubulin S in which the C-termini have been removed. While in no way ruling out other pathways, such direct associations are the simplest potential regulatory mechanism for apoptosis resulting from disturbances in microtubule or tubulin function.

Regulation of Bim by TCR signals in CD4/CD8 double-positive thymocytes

Bim, a BH3-only Bcl-2 family member, is required for apoptosis of thymocytes in response to negative selection signals. Regulation of the apoptotic activity of Bim during negative selection is not understood. In this study we demonstrate that in murine thymocytes undergoing apoptosis in response to anti-CD3epsilon injection, levels of Bim protein expression do not change. In immature thymocytes, Bim is associated with mitochondria before stimulation and is not regulated by a change in subcellular localization during apoptosis. We also show that Bim(EL) is rapidly phosphorylated in thymocytes in response to CD3epsilon cross-linking both in vivo and in vitro, and that phosphorylation is sustained for at least 24 h. Analysis of MHC-deficient mice shows that phosphorylation of Bim occurs in CD4/CD8 double-positive thymocytes and does not depend on activation of mature T cells. We also find that TCR cross-linking on thymocytes induces an increase in the proportion of Bcl-x(L) bound to Bim at late time points. Our results favor a model in which strong TCR signals regulate the apoptotic activity of Bim by phosphorylation and subsequent changes in binding to Bcl-x(L) in immature thymocytes.

[Ca2+]i signaling between mitochondria and endoplasmic reticulum in neurons is regulated by microtubules. From mitochondrial permeability transition pore to Ca2+-induced Ca2+ release

The positioning and dynamics of organelles depend on membrane-cytoskeleton interactions. Mitochondria relocate along microtubules (MT), but it is not clear whether MT have direct effects on mitochondrial function. Using two-photon microscopy and the mitochondrial fluorescent dyes rhodamine 123 and Rhod-2, we showed that Taxol and nocodazole, which correspondingly stabilize and disrupt MT, decreased potential and Ca(2+) in the mitochondria of brain stem pre-Botzinger complex neurons. Without changing basal cytoplasmic Ca(2+) ([Ca(2+)](i)), Taxol promoted the generation of [Ca(2+)](i) spikes in dendrites. These spikes were abolished after blockade of Ca(2+) influx and after depletion of internal Ca(2+) stores, indicating the involvement of Ca(2+)-induced Ca(2+) release. Nocodazole decreased mitochondrial potential and [Ca(2+)](m) and produced a long lasting increase in [Ca(2+)](i). MT-acting drugs depolarized single immobilized mitochondria and released previously stored Ca(2+). All of these effects were inhibited by pretreatment with blockers of mitochondrial permeability transition pore (mPTP), cyclosporin A, and 2-aminoethoxydiphenyl borate. Induction of mPTP by Taxol and nocodazole was confirmed by using a calcein/Co(2+) imaging technique. Electron and optical microscopy revealed tubulin bound to mitochondria. Mitochondria, MT, and endoplasmic reticulum (ER) showed strong co-localization, the degree of which decreased after MT were disrupted. We propose that changes in the structure of MT by Taxol and nocodazole promote the induction of mPTP. Subsequent Ca(2+) efflux stimulates the Ca(2+) release from the ER that drives spontaneous [Ca(2+)](i) transients. Thus, close positioning of mitochondria to the ER as determined by MT can be essential for the local [Ca](i) signaling in neurons.

A-kinase anchor protein 84/121 are targeted to mitochondria and mitotic spindles by overlapping amino-terminal motifs

A-kinase anchor proteins (AKAPs) assemble multi-enzyme signaling complexes in proximity to substrate/effector proteins, thus directing and amplifying membrane-generated signals. S-AKAP84 and AKAP121 are alternative splicing products with identical NH(2) termini. These AKAPs bind and target protein kinase A (PKA) to the outer mitochondrial membrane. Tubulin was identified as a binding partner of S-AKAP84 in a yeast two-hybrid screen. Immunoprecipitation and co-sedimentation experiments in rat testis extracts confirmed the interaction between microtubules and S-AKAP84. In situ immunostaining of testicular germ cells (GC2) shows that AKAP121 concentrates on mitochondria in interphase and on mitotic spindles during M phase. Purified tubulin binds directly to S-AKAP84 but not to a deletion mutant lacking the mitochondrial targeting domain (MT) at residues 1-30. The MT is predicted to form a highly hydrophobic alpha-helical wheel that might also mediate interaction with tubulin. Disruption of the wheel by site-directed mutagenesis abolished tubulin binding and reduced mitochondrial attachment of an MT-GFP fusion protein. Some MT mutants retain tubulin binding but do not localize to mitochondria. Thus, the tubulin-binding motif lies within the mitochondrial attachment motif. Our findings indicate that S-AKAP84/AKAP121 use overlapping targeting motifs to localize signaling enzymes to mitochondrial and cytoskeletal compartments.

2',3'-Cyclic nucleotide 3'-phosphodiesterase: a membrane-bound, microtubule-associated protein and membrane anchor for tubulin

2',3'-Cyclic nucleotide-3'-phosphodiesterase (CNP) is firmly associated with tubulin from brain tissue and FRTL-5 thyroid cells as demonstrated by copolymerization with microtubules through several warm/cold cycles, the presence of CNP activity in purified tubulin preparations, and identical behavior during various extraction procedures. CNP acts as a microtubule-associated protein in promoting microtubule assembly at low mole ratios. This activity resides in the C terminus of the enzyme, which, by itself, promotes microtubule assembly at higher mole ratios. Phosphorylation of CNP interferes with its assembly-promoting activity, as does deletion of the C terminus, which leads to abnormal microtubule distribution in the cell. Submembranous colocalization of the proteins and CNP-dependent microtubule organization suggest that CNP is a membrane-bound microtubule-associated protein that can link tubulin to membranes and may regulate cytoplasmic microtubule distribution.

Tubulinlike protein from Spirochaeta bajacaliforniensis

Tubulin proteins are the fundamental subunits of all polymeric microtubule-based eukaryotic structures. Long, hollow structures each composed of 13 protofilaments as revealed by electron microscopy, microtubules (240 angstroms in diameter) are nearly ubiquitous in eukaryotes. These proteins have been the subject of intense biochemical and biophyiscal interest since the early 1970s and are of evolutionary interest as well. If tubulin-based structures (i.e., neurotubules, mitotic spindle tubules, centrioles, kinetosomes, axonemes, etc.) evolved from spirochetes by way of motility symbioses, tubulin homologies with spirochete proteins should be detectable. Tubulin proteins are widely thought to be limited to eukaryotes. Yet both azotobacters and spirochetes have shown immunological cross-reactivity with antitubulin antibodies. In neither of these studies was tubulin isolated nor any specific antigen identified as responsible for the immunoreactivity. Furthermore, although far less uniform in structure than eukaryotic microtubules, various cytoplasmic fibers and tubules (as seen by electron microscopy) have been reported in several types of prokaryotes (e.g., Spirochaeta; large termite spirochetes; treponemes; cyanobacteria; and Azotobacter. This work forms a part of our long-range study of the possible prokaryotic origin of tubulin and microtubules. Spirochetes are helically shaped gram-negative motile prokaryotes. They differ from all other bacterial in that the position of their flagella is periplasmic: their flagella lie between the inner and outer membranes of the gram-negative cell wall. Some of the largest spirochetes have longitudinally aligned 240 angstrom microtubules. Unfortunately, in spite of many attempts, all of the larger spirochetes (family Pillotaceae) with well-defined cytoplasmic tubules and antitubulin immunoreactivity are not cultivable. However, a newly described spirochete species (Spirochaeta bajacaliforniensis) possessing cytoplasmic fibers displays antitubulin immunoreactivity in whole-cell preparations. Since preliminary observations suggested that Spirochaeta bajacaliforniensis proteins may be related to eukaryotic tubulins, their characterization was undertaken. Brain tubulin can be purified by utilizing its ability to polymerize at warm temperatures and to depolymerize in the cold. After several cycles of sedimentation and redissolution the microtubule fraction is comprised of 75% tubulin and 20% high molecular mass microtubule-associated proteins (MAPs). In this paper we report that components of cell lysates, prepared from a spirochete that contains cytoplasmic fibers (Spirochaeta bajacaliforniensis), also exhibit the property of temperature-dependent cyclical sedimentation. Additionally we report the identification and characterization of the polypeptide responsible for cross-reactivity with antitubulin antiserum.

Plasma membrane localization of palmitoylated tubulin

PC12 pheochromocytoma cells incorporate [(3)H]palmitic acid into tubulin in a time- and cell-density-dependent manner. The plasma membrane-enriched fraction contains most of the radioactivity of the membrane pellet. While palmitoylated tubulin is found in both the cytoplasm and particulate fraction, the bulk of [(3)H]palmitic acid bound to tubulin is present in the crude membrane pellet and the tubulin extracted from the plasma membrane is more heavily palmitoylated than that extracted from endoplasmic reticulum. Detergent-extracted tubulin from plasma membrane is, to a large extent, polymerization competent; a substantial fraction, increasing as a function of labeling time, is not hydroxylamine-labile. The requirement for detergent extraction, the accompanying changes in tubulin properties and the present findings of preferential incorporation of labeled tubulin into plasma membranes, make it clear that direct incorporation of tubulin into the plasma membrane can occur.

Dynamic interaction between soluble tubulin and C-terminal domains of N-methyl-D-aspartate receptor subunits

The cytoplasmic C-terminal domains (CTs) of the NR1 and NR2 subunits of the NMDA receptor have been implicated in its anchoring to the subsynaptic cytoskeleton. Here, we used affinity chromatography with glutathione S-transferase-NR1-CT and -NR2B-CT fusion proteins to identify novel binding partner(s) of these NMDA receptor subunits. Upon incubation with rat brain cytosolic protein fraction, both NR1-CT and NR2B-CT, but not glutathione S-transferase, specifically bound tubulin. The respective fusion proteins also bound tubulin purified from brain, suggesting a direct interaction between the two binding partners. In tubulin polymerization assays, NR1-CT and NR2B-CT significantly decreased the rate of microtubule formation without destabilizing preformed microtubules. Moreover, only minor fractions of either fusion protein coprecipitated with the newly formed microtubules. Consistent with these findings, ultrastructural analysis of the newly formed microtubules revealed a limited association only with the CTs of the NR1 and NR2B. These data suggest a direct interaction of the NMDA receptor channel subunit CTs and tubulin dimers or soluble forms of tubulin. The efficient modulation of microtubule dynamics by the NR1 and NR2 cytoplasmic domains suggests a functional interaction of the receptor and the subsynaptic cytoskeletal network that may play a role during morphological adaptations, as observed during synaptogenesis and in adult CNS plasticity.

Microtubules are involved in early hypertrophic responses of myocardium during pressure overload

Mechanical overloading to cardiac muscle causes fetal contractile protein gene expression and acceleration of protein synthesis. Myocyte microtubules might be involved in these pressure overload-induced hypertrophic responses. We assessed c-fos and fetal contractile protein genes such as beta-myosin heavy chain (MHC) and alpha-skeletal actin using Northern blot analysis and quantified total cardiac protein, DNA, and RNA content in the left ventricular myocardium obtained from four groups of rats: sham-operated rats; sham-operated rats treated with colchicine, which depolymerized microtubules; rats in which acute pressure overload was imposed by abdominal aortic constriction for 3 days (AoC); and AoC rats treated with colchicine (AoC + colchicine). Systolic arterial pressure was elevated to a similar degree in AoC and AoC + colchicine rats. c-fos and beta-MHC mRNA levels were significantly upregulated in AoC rats, which was attenuated by microtubule inhibition. Both RNA content and RNA-to-DNA ratio, the index of the protein synthesis capacity, were increased in AoC rats, which effect was also abolished by colchicine. Furthermore, induction of nonfunctioning microtubules by taxol or deuterium oxide exerted the same inhibitory effects. Thus the hypertrophic responses of the myocardium during pressure overload might depend on the integrity of myocyte microtubules.

The role of GTP-binding proteins in mechanochemical movements of microorganisms and their potential to form filamentous structures

Prokaryotic cells contain proteins which form extended chains or multimers that oscillate between monomers and oligomers of varying length. Hydrolysis of nucleoside triphosphates combined with site-specific disposition of substrates and products to monomers and multimers is the driving force of dynamic instability of these molecules. Polymeric structures are connected in some manner to a variety of signaling systems that adhere to the polymeric matrix, including the GTP-binding protein(s), protein kinases and phosphatases, and other proteins or systems that communicate between the cytoplasmic membrane and the cytosol. Flexible organization allowing regulated dynamic movement is one of the key elements in all living cells. In eukaryotic cells actin and tubulin are the two main components of dynamically controlled spatial system. These proteins are noteworthy for their ability to polymerize, reversibly, into filaments or microtubules in association with hydrolysis of ATP or GTP, respectively. As such, they regulate most of the mechanics of cell movement including cell division, cell differentiation, phagocytosis and other dynamic phenomena. Recent evidence revealed that microbial cells create functional domains at specific sites of the cells and can form cytoplasmic tubules and fibers.

Multiple forms of tubulin: different gene products and covalent modifications

Tubulin, the subunit protein of microtubules, is an alpha/beta heterodimer. In many organisms, both alpha and beta exist in numerous isotypic forms encoded by different genes. In addition, both alpha and beta undergo a variety of posttranslational covalent modifications, including acetylation, phosphorylation, detyrosylation, polyglutamylation, and polyglycylation. In this review the distribution and possible functional significance of the various forms of tubulin are discussed. In analyzing the differences among tubulin isotypes encoded by different genes, some appear to have no functional significance, some increase the overall adaptability of the organism to environmental challenges, and some appear to perform specific functions including formation of particular organelles and interactions with specific proteins. Purified isotypes also display different properties in vitro. Although the significance of all the covalent modification of tubulin is not fully understood, some of them may influence the stability of modified microtubules in vivo as well as interactions with certain proteins and may help to determine the functional role of microtubules in the cell. The review also discusses isotypes of gamma-tubulin and puts various forms of tubulin in an evolutionary context.

Microtubule-active drugs suppress the closure of the permeability transition pore in tumour mitochondria

We report the effects of anticancer drugs, inhibitors of microtubule organisation, on the mitochondrial permeability transition pore (PTP) in Ehrlich ascites tumour cells. Taxol (5-20 microM) and colchicine (100-500 microM) prevented closing of the cyclosporin A-sensitive PTP. No taxol or colchicine effects on oxidative phosphorylation were observed in the range of concentrations used. We suggest that either membrane-bound tubulin per se can be part of PTP and/or the attachment of mitochondria to the microtubular network is essential for PTP regulation. The taxol inhibition of PTP closure, mediated through interaction with the cytoskeleton, sheds new light on the cytotoxic properties of this anticancer drug.

Identification and characterization of a tubulin binding protein in rat brain plasma membrane

Studies on the interaction of FITC-tubulin and 125I-tubulin with isolated plasma membrane of neural cells and with primary cultures of neuronal (N) and glial (G) cells of rat brain demonstrate the presence of specific, saturable, high affinity tubulin binding sites in these cells. The positive fluorescence of live unfixed primary cultures of N and G cells following incubation with FITC-tubulin indicate that the tubulin binding sites are located on the outer side of the plasma membrane. Such fluorescence was not observed with FITC-BSA, FITC-conalbumin or freshly dissociated cells from rat tissues or established cell lines. Binding of FITC-tubulin or 125I-tubulin is competed only by tubulin and not by other proteins. Scatchard analysis of the binding of 125I-tubulin to purified plasma membrane indicates very high affinity (Kd = 85 nM) with a Bmax of 7.4 pmol/mg protein. The putative tubulin receptor was partially purified by affinity chromatography on tubulin-sepharose column. Immunoprecipitation of the solubilized tubulin-receptor complex followed by SDS-PAGE analysis and autoradiography, revealed the presence of two components of molecular weights 70 and 45 kDa respectively, presumably representing the two nonidentical subunits of the putative receptor. In conjunction with several recent reports indicating the secretion of high molecular weight proteins from cultured neural cells and the ability of tubulin to modulate adenyl cyclase in synaptic membranes these findings suggest that the binding of exogenous tubulin to sites external to the plasma membrane may be involved in signal transduction.

Thyroidal stimulation of tubulin and actin in rat brain cytoskeleton

In cultures of neonatal rat brain cells, labeled with 35S-methionine in the presence or absence of triiodothyronine (T3), the hormone promoted a significant enhancement of labeled tubulin and actin in the insoluble fraction (30,000 g pellet) of cell homogenate. To identify the specific sub-cellular fraction associated with this induction, organ cultures of 1 day rat cerebra were labelled with 35S-methionine in the presence and absence of T3 and the insoluble fraction (30,000 g pellet) was subfractionated into mitochondria, plasma membrane and cytoskeleton. Analysis of the labeled proteins by SDS-PAGE, autoradiography and densitometry revealed a T3-induced increase of 50-80% for both tubulin and actin, only in the cytoskeleton fraction without any significant effect on the other fractions. Similar results were obtained when plasma membrane or cytoskeleton were isolated directly from labeled cerebrum by conventional methods instead of fractionating from the 30,000 g pellet. Analysis of relative stimulation of labeled tubulin and actin by T3 in cytoskeleton fraction derived from primary cultures of neuronal (N) and glial (G) cells labeled with 35-methionine show that the stimulatory effect is predominantly on the N cells. Studies on the kinetics of induction of labeled tubulin and actin by T3 in the cytoskeleton fraction prepared from cerebra labeled with 35S-methionine for 2, 8 and 18 hrs revealed no significant difference at 2 hrs; at 8 hrs, an increased incorporation into both tubulin and actin was reproducibly seen in the controls relative to T3-treated samples.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural dynamics of the mitochondrial compartment

The metabolic activities of mitochondria have been extensively characterized. However, there is much less known about the morphogenic changes of the mitochondrial compartment during growth, development and aging of the cell and the consequences of those structural changes on cellular metabolism. There is a growing body of evidence for interactions of mitochondria with cytoskeletal components and changes of mitochondrial structure during development and in response to changing environmental conditions. Segregation and recombination of mitochondrial genomes are also processes dependent upon the dynamic nature of the mitochondrial compartment. These regulatory and structural aspects of mitochondrial compartment dynamics will play an important role in the analysis of mitochondrial function and pathology.

Tyrosinated, detyrosinated and acetylated tubulin isotypes in rat brain membranes. Their proportions in comparison with those in cytosol

The heterogeneity of alpha-tubulin and the relative proportions of the tubulin isotypes were investigated in brain membranes of rats of 1, 25 and 180 days of age by using four anti-alpha-tubulin antibodies: a) the monoclonal DM1A antibody, specific for alpha-tubulin; b) the monoclonal 6-11B-1 antibody, specific for acetylated tubulin; c) a polyclonal antibody (Glu antibody), specific for detyrosinated tubulin; and d) a polyclonal antibody (Tyr antibody), specific for tyrosinated tubulin. We found that rat brain membranes contain the three tubulin isotypes mentioned above. The proportions of tyrosinated and detyrosinated tubulin relative to total alpha-tubulin were somewhat lower in membrane than in cytosol in animals of 25 and 180 days of age. At day one of development, the proportions in membrane were similar to those found in cytosol. With respect to the acetylated form, it was about 20 times higher in membrane than in cytosol at the three ages studied. The proportion of acetylated tubulin was determined in different subcellular fractions: myelin, synaptic vesicles, mitochondria, microsomes, and plasma membrane. While the amount of total tubulin differed between the different subcellular fractions, the proportion of acetylated tubulin relative to total alpha-tubulin was constant and similar to that found in total membranes. The proportion of acetylated tubulin was also investigated in non-neural tissues (kidney, liver and lung). Although values for cytosol were about 10-fold higher than that found in brain cytosol, no detectable values for membranes could be obtained in these organs.

Colchicine inhibition of duodenal absorption of calcium

1. The effect of colchicine on calcium absorption across rat duodenum has been investigated using the single-pass continuous perfusion technique and the two-compartment system model. 2. Perfusing the rat duodenum with 0.1 and 0.5 mM colchicine produced a dose-dependent inhibiting pattern of calcium transport with no effect noted for water transport. 3. Colchicine at 0.5 mM caused a significant decrease in the rate of calcium uptake and in the accumulation capacity of the duodenal cells. 4. Accumulation of calcium in the duodenal strips displayed saturation kinetics with increasing concentration of calcium in the incubation medium. Colchicine at 0.5 mM showed a lower saturation level and decreased the average maximal flux around 46%.

Effects of microtubule agents on the spatial and electrical properties of the plasma membrane inChara corallina

The freshwater algaChara corallina Klein ex Willd., em. R.D.W. (=C. australis R.Br.) develops alternating outward (acid) and inward (alkaline) current areas on its surface when illuminated. Exposure of cells to vinblastine, colchicine, or oryzalin caused a reduction in and a shifting of this extracellular current pattern. Removal of these agents from the bathing media resulted in regeneration of the initial current profile. Because these agents all affect tubulin, microtubules may be responsible for orchestrating the transmembrane currents responsible for the acid and alkaline banding phenomenon. Analysis of the membrane potential showed a fast depolarization after vinblastine exposure; however, analysis of the current-voltage curve did not show a change in membrane conductance. A 30-min colchicine treatment decreased the conductance of the plasma membrane with either an hyperor a depolarization in the membrane potential. In contrast, although a 9-h exposure to oryzalin caused a major reduction in the extra-cellular current pattern, only minor changes were observed in the membrane potential and conductance. However, in the presence of oryzalin, the time constants in the light response of the membrane potential increased over this 9-h period. Collectively, these results implicate an involvement of microtubules in spatial control of plasma-membrane transport events inC. corallina.

Interaction of microtubules and microtubule-associated proteins (MAPs) with rat brain mitochondria

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Evidence that plasma membrane fluidity of isolated hepatocytes is modified by exposure to microtubule-depolymerizing drugs

The role of microtubules on membrane fluidity has been investigated on freshly isolated whole rat hepatocytes prepared by the perfusion method and exposed either to the microtubule-depolymerizing drugs colchicine and vincristine or to beta-lumicolchicine, a colchicine analog deprived of biological activity. Exposure of hepatocytes to 6.3 microM colchicine or to 3.0 microM vincristine led to a significant decrease of membrane fluidity as measured by fluorescence polarization of trimethylammoniodiphenylhexatriene (TMA-DPH). No changes were observed in cells exposed to 10.0 microM beta-lumicolchicine. These observations support the hypothesis that the microtubular system plays a role in the modulations of physico-chemical properties of the plasma membrane.

Associations between beta-tubulin and mitochondria in adult isolated heart myocytes as shown by immunofluorescence and immunoelectron microscopy

We have investigated the associations between beta-tubulin and mitochondria in freshly isolated cardiac myocytes from the rat. Beta-tubulin was identified by using monoclonal antibodies for immunofluorescence and high resolution immunogold electron microscopy. In addition, conventional transmission and scanning electron microscopic studies were performed. After chemical stabilization in a formaldehyde solution, the myocytes were shock-frozen at -150 degrees C, cryosectioned at -70 degrees C and subsequently processed for immunohistochemical and immunocytochemical microscopy. A characteristic of the rod shaped myocytes is the presence of a dense network of microtubules in the cytoplasm displaying a pattern of strong anti-beta-tubulin reaction. The complexity of this network however varies considerably among the myocytes reflecting microtubule dynamic instability. Further, our findings demonstrate that the beta-tubulin label in rod cells is confined to the perinuclear and interfibrillar spaces and, therefore, is largely colocalized with the cytoplasmic organelles. In myocytes undergoing severe contracture the distribution of beta-tubulin is entirely restricted to the outer mitochondrial-containing domain. This implies that, in a cell model with marked segregation of the contractile filaments and organelles, mitochondria are codistributed with microtubules in the total absence of desmin intermediate filaments. Moreover, our immunogold preparations demonstrate anti-beta-tubulin labelling in the outer mitochondrial membrane as well as of fibres in close apposition to this membrane. These results indicate the presence of a specific beta-tubulin binding to the outer mitochondrial membrane that probably also involves microtubule based translocators and/or MAPs.

The cytoplasmic matrix of the adrenal chromaffin cells of rats under normal and stressed conditions

In embedment-free electron microscopy with polyethylene glycol embedding and subsequent deembedding, the conventional cytoplasm of the chromaffin cells was revealed to consist of a three-dimensional lattice of microtrabeculae and gives the impression that the chromaffin granules are held in place by the lattice. After the restraint stress, a substantial number of chromaffin cells were almost free of granules, and the microtrabecular lattice was much more compact than that in cytoplasmic regions occupied with remaining granules or increased mitochondria. In immunocytochemistry, actin immunofluorescence was confined to the subplasmalemmal regions, while tubulin and tropomyosin immunofluorescence appeared throughout the entire cytoplasm of normal chromaffin cells. After the stress, the immunofluorescence for actin and tubulin increased in intensity, while that for tropomyosin decreased. Immunogold labelings for actin and tubulin were found mainly on the thinner subplasmalemmal microtrabeculae and the thicker perikaryal ones, respectively, while some were deposited in the form of small aggregates on portions of microtrabeculae. No specific association between the gold labelings for actin or tubulin and the chromaffin granules was found, even in the subplasmalemmal regions. A hypothetical interpretation was proposed in which a more compact lattice of the microtrabeculae in spatial association with a looser lattice represents a gelated state of the cytoplasm. The significance of the gel-sol transition of the cytoplasmic matrix in relation to the secretory mechanism was discussed.

Colchicine analogues that bind reversibly to tubulin define microtubular requirements for newly synthesized protein secretion in rat lacrimal gland

The role of microtubules in 3H-labeled protein secretion in rat lacrimal glands was probed by the use of colchicine and two of its analogues that reversibly bind to tubulin. These analogues were 2-methoxy-5-(2,3,4,4'-trimethoxyphenyl)-2,4,6-cycloheptatriene-1-o ne and 2,3,4,4'-tetramethoxy-1,1'-biphenyl, the latter having been synthesized for these studies. Immunofluorescence revealed that untreated exocrine acinar cells contained an intact microtubule network, which was totally abolished by drug addition. Subsequent drug removal restored the network for the two reversibly binding drugs--more rapidly so for the biphenyl, but this was not the case with colchicine. The protein-secretory process was examined by adding the three drugs at various stages--prepulse incubation, pulse, maturation, apical storage of granules, and discharge under cholinergic stimulation. Comparison with the kinetics of microtubular network restoration, which differed for the two reversibly binding drugs, led to the conclusion that the microtubular system is critical to the maturation phase of secretion.