A new model for microtubule-associated protein (MAP)-induced microtubule assembly. The Pro-rich region of MAP4 promotes nucleation of microtubule assembly in vitro

Effects of three microtubule-associated proteins (MAP2, MAP4, and Tau) on microtubules' physical properties and neurite morphology

The physical properties of cytoskeletal microtubules have a multifaceted effect on the expression of their cellular functions. A superfamily of microtubule-associated proteins, MAP2, MAP4, and tau, promote the polymerization of microtubules, stabilize the formed microtubules, and affect the physical properties of microtubules. Here, we show differences in the effects of these three MAPs on the physical properties of microtubules. When microtubule-binding domain fragments of MAP2, tau, and three MAP4 isoforms were added to microtubules in vitro and observed by fluorescence microscopy, tau-bound microtubules showed a straighter morphology than the microtubules bound by MAP2 and the three MAP4 isoforms. Flexural rigidity was evaluated by the shape of the teardrop pattern formed when microtubules were placed in a hydrodynamic flow, revealing that tau-bound microtubules were the least flexible. When full-length MAPs fused with EGFP were expressed in human neuroblastoma (SH-SY5Y) cells, the microtubules in apical regions of protrusions expressing tau were straighter than in cells expressing MAP2 and MAP4. On the other hand, the protrusions of tau-expressing cells had the fewest branches. These results suggest that the properties of microtubules, which are regulated by MAPs, contribute to the morphogenesis of neurites.

MAP4 as a New Candidate in Cardiovascular Disease

Microtubule and mitochondrial dysfunction have been implicated in the pathogenesis of cardiovascular diseases (CVDs), including cardiac hypertrophy, fibrosis, heart failure, and hypoxic/ischemic related heart dysfunction. Microtubule dynamics instability leads to disrupted cell homeostasis and cell shape, decreased cell survival, and aberrant cell division and cell cycle, while mitochondrial dysfunction contributes to abnormal metabolism and calcium flux, increased cell death, oxidative stress, and inflammation, both of which causing cell and tissue dysfunction followed by CVDs. A cytosolic skeleton protein, microtubule-associated protein 4 (MAP4), belonging to the family of microtubule-associated proteins (MAPs), is widely expressed in non-neural cells and possesses an important role in microtubule dynamics. Increased MAP4 phosphorylation results in microtubule instability. In addition, MAP4 also expresses in mitochondria and reveals a crucial role in maintaining mitochondrial homeostasis. Phosphorylated MAP4 promotes mitochondrial apoptosis, followed by cardiac injury. The aim of the present review is to highlight the novel role of MAP4 as a potential candidate in multiple cardiovascular pathologies.

A nematode microtubule-associated protein, PTL-1, closely resembles its mammalian counterparts in overall molecular architecture

The mammalian microtubule-associated proteins (MAPs), MAP2, MAP4, and τ, are structurally similar and considered to be evolutionarily related. The primary structure of a nematode MAP, PTL-1, also reportedly resembles those of the MAPs, but only in a small portion of the molecule. In this study, we elucidated the overall domain organization of PTL-1, using a molecular dissection technique. Firstly, we isolated nematode microtubules and proved that the recombinant PTL-1 binds to nematode and porcine microtubules with similar affinities. Then, the recombinant PTL-1 was genetically dissected to generate four shorter polypeptides, and their microtubule-binding and assembly promoting activities were assessed, using porcine microtubules and tubulin. PTL-1 was found to consist of two parts, microtubule-binding and projection domains, with the former further divided into three functionally distinct subdomains. The molecular architecture of PTL-1 was proved to be quite analogous to its mammalian counterparts, MAP2, MAP4, and τ, strongly supporting their evolutionary relationships.

Emerging roles of apoptotic microtubules during the execution phase of apoptosis

Apoptosis is a genetically programmed energy-dependent process of cell demise, characterized by specific morphological and biochemical events in which the activation of caspases has an essential role. During apoptosis the cytoskeleton participates actively in characteristic morphological rearrangements of the dying cell. This reorganisation has been assigned mainly to actinomyosin ring contraction, while microtubule and intermediate filaments are depolymerized at early stages of apoptosis. However, recent reports have showed that microtubules are reformed during the execution phase of apoptosis organizing an apoptotic microtubule network (AMN). AMN is organized behind plasma membrane, forming a cortical structure. Apoptotic microtubules repolymerization takes place in many cell types and under different apoptotic inducers. It has been hypothesized that AMN is critical for maintaining plasma membrane integrity and cell morphology during the execution phase of apoptosis. AMN disorganization leads apoptotic cells to secondary necrosis and the release of potential toxic molecules which can damage neighbor cells and promotes inflammation. Therefore, AMN formation during physiological apoptosis or in pathological apoptosis induced by anti-cancer treatments is essential for tissue homeostasis and the prevention of additional cell damage and inflammation.

Microtubule-associated protein (MAP) 4 interacts with microtubules in an intrinsically disordered manner

We previously used nuclear magnetic resonance (NMR) to analyze the structure of a synthetic tricosapeptide corresponding to an active site of microtubule-associated protein 4 (MAP4). To further the structural analysis, we have constructed a minimal active domain fragment of MAP4, encompassing the entire active site, and obtained its NMR spectra. The secondary structure prediction using partially assigned NMR data suggested that the fragment is largely unfolded. Two other independent techniques also demonstrated its unfolded nature, indicating that MAP4 belongs to the class of intrinsically disordered proteins (IDPs). The NMR spectra of the fragment-microtubule mixture revealed that the fragment binds to the microtubule using multiple binding sites, apparently contradicting our previous quantitative studies. Given that MAP4 is intrinsically disordered, we propose a mechanism in which any one of the binding sites is active at a time, which is one of the typical interaction mechanisms proposed for IDPs.

The emerging role of Arf/Arl small GTPases in cilia and ciliopathies

Once overlooked as an evolutionary vestige, the primary cilium has recently been the focus of intensive studies. Mounting data show that this organelle is a hub for various signaling pathways during vertebrate embryonic development and pattern formation. However, how cilia form and how cilia execute the sensory function still remain poorly understood. Cilia dysfunction is correlated with a wide spectrum of human diseases, now termed ciliopathies. Various small GTPases, including the members in Arf/Arl, Rab, and Ran subfamilies, have been implicated in cilia formation and/or function. Here we review and discuss the role of one particular group of small GTPase, Arf/Arl, in the context of cilia and ciliopathy.

Microtubule-associated protein 4 binds to actin filaments and modulates their properties

We previously reported that an isoform of microtubule-associated protein 4 (MAP4) is localized to the distal area of developing neurites, where microtubules are relatively scarce, raising the possibility that MAP4 interacts with another major cytoskeletal component, actin filaments. In the present study, we examined the in vitro interaction between MAP4 and actin filaments, using bacterially expressed MAP4 and its truncated fragments. Sedimentation assays revealed that MAP4 and its microtubule-binding domain fragments bind to actin filaments under physiological conditions. The apparent dissociation constant and the binding stoichiometry of the fragments to actin were about 0.1 µm and 1 : 3 (MAP4/actin), respectively. Molecular dissection studies revealed that the actin-binding site on MAP4 is situated at the C-terminal part of the proline-rich region, where the microtubule-binding site is also located. Electron microscopy revealed that the MAP4-bound actin filaments become straighter and longer and that the number of actin bundles increases with greater concentrations of added MAP4 fragment, indicating that MAP4 binding alters the properties of the actin filaments. A multiple sequence alignment of the proline-rich regions of MAP4 and tau revealed two putative actin-binding consensus sequences.

The small GTPases ARL-13 and ARL-3 coordinate intraflagellar transport and ciliogenesis

Intraflagellar transport (IFT) machinery mediates the bidirectional movement of cargos that are required for the assembly and maintenance of cilia. However, little is known about how IFT is regulated in vivo. In this study, we show that the small guanosine triphosphatase (GTPase) adenosine diphosphate ribosylation factor-like protein 13 (ARL-13) encoded by the Caenorhabditis elegans homologue of the human Joubert syndrome causal gene ARL13B, localizes exclusively to the doublet segment of the cilium. arl-13 mutants have shortened cilia with various ultrastructural deformities and a disrupted association between IFT subcomplexes A and B. Intriguingly, depletion of ARL-3, another ciliary small GTPase, partially suppresses ciliogenesis defects in arl-13 mutants by indirectly restoring binding between IFT subcomplexes A and B. Rescue of arl-13 mutants by ARL-3 depletion is mediated by an HDAC6 deacetylase-dependent pathway. Thus, we propose that two conserved small GTPases, ARL-13 and ARL-3, coordinate to regulate IFT and that perturbing this balance results in cilia deformation.

The apoptotic microtubule network preserves plasma membrane integrity during the execution phase of apoptosis

It has recently been shown that the microtubule cytoskeleton is reformed during the execution phase of apoptosis. We demonstrate that this microtubule reformation occurs in many cell types and under different apoptotic stimuli. We confirm that the apoptotic microtubule network possesses a novel organization, whose nucleation appears independent of conventional gamma-tubulin ring complex containing structures. Our analysis suggests that microtubules are closely associated with the plasma membrane, forming a cortical ring or cellular "cocoon". Concomitantly other components of the cytoskeleton, such as actin and cytokeratins disassemble. We found that colchicine-mediated disruption of apoptotic microtubule network results in enhanced plasma membrane permeability and secondary necrosis, suggesting that the reformation of a microtubule cytoskeleton plays an important role in preserving plasma membrane integrity during apoptosis. Significantly, cells induced to enter apoptosis in the presence of the pan-caspase inhibitor z-VAD, nevertheless form microtubule-like structures suggesting that microtubule formation is not dependent on caspase activation. In contrast we found that treatment with EGTA-AM, an intracellular calcium chelator, prevents apoptotic microtubule network formation, suggesting that intracellular calcium may play an essential role in the microtubule reformation. We propose that apoptotic microtubule network is required to maintain plasma membrane integrity during the execution phase of apoptosis.

Phosphorylated vasodilator-stimulated phosphoprotein is localized on mitotic spindles of the gastric cancer cell line SGC-7901

AIM: To elucidate the localization of vasodilator stimulated phosphoprotein (VASP), a cytoskeletal organizing protein and a substrate of protein kinases A and G in mitotic gastric cancer cells.

METHODS: Immunofluorescence microscopy was used to observe the localization of α-tubulin, VASP and Ser157 phosphorylated VASP (p-VASP) in interphase of mitotic gastric cancer of the cell line SGC-7901.

RESULTS: Immunofluorescence staining showed that p-VASP but not VASP was co-localized with α-tubulin on spindle poles and fibers in prophase, metaphase and anaphase of the mitotic process of the gastric cancer cell line SGC-7901. H89, an inhibitor of protein kinases A and G, had no effect on the localization of p-VASP on the spindles.

CONCLUSION: VASP may play a role in assembling and stabilizing the mitotic spindle of cells, and phosphorylation of the protein is the precondition for it to exert this function.

Differences in the regulation of microtubule stability by the pro-rich region variants of microtubule-associated protein 4

We have recently reported a neural variant of microtubule-associated protein 4 with a short pro-rich region (MAP4-SP). Here, we show that the neural MAP4 has reduced microtubule-stabilizing activity, compared to the ubiquitous MAP4 with a long pro-rich region (MAP4-LP), both in vitro and in vivo. Fluorescence recovery after photobleaching analyses revealed that the interaction of MAP4-SP with the microtubules is very rapid, with a half-time of fluorescence recovery of 7 +/- 2.36 s, compared to 19.5 +/- 3.03 s in case of MAP4-LP. The dynamic interaction of MAP4-SP with microtubules in neural cells may contribute to the dynamic behaviors of extending neurites.

Truncation of the projection domain of MAP4 (microtubule-associated protein 4) leads to attenuation of microtubule dynamic instability

MAP4, a ubiquitous heat-stable MAP, is composed of an asymmetric structure common to the heat-stable MAPs, consisting of an N-terminal projection (PJ) domain and a C-terminal microtubule (MT)-binding (MTB) domain. Although the MTB domain has been intensively studied, the role of the PJ domain, which protrudes from MT-wall and does not bind to MTs, remains unclear. We investigated the roles of the PJ domain on the dynamic instability of MTs by dark-field microscopy using various PJ domain deletion constructs of human MAP4 (PJ1, PJ2, Na-MTB and KDM-MTB). There was no obvious difference in the dynamic instability between the wtMAP4 and any fragments at 0.1 microM, the minimum concentration required to stabilize MTs. The individual MTs stochastically altered between polymerization and depolymerization phases with similar profiles of length change as had been observed in the presence of MAP2 or tau. We also examined the effects at the increased concentrations of 0.7 microM, and found that in some cases the dynamic instability was almost entirely attenuated. The length of both the polymerization and depolymerization phases decreased and "pause-phases" were occasionally observed, especially in the case of PJ1, PJ2 or Na-MTB. No obvious change was observed in the increased concentration of wtMAP4 and KDM-MTB. Additionally, the profiles of MT length change were quite different in 0.7 microM PJ2. Relatively rapid and long depolymerization phases were sometimes observed among quite slow length changes. Perhaps, this unusual profile could be due to the uneven distribution of PJ2 along the MT lattice. These results indicate that the PJ domain of MAP4 participates in the regulation of the dynamic instability.

Identification of a Neural Cell Specific Variant of Microtubule-Associated Protein 4

The microtubule-binding domain of MAP4, a ubiquitous microtubule-associated protein, contains a region rich in proline and basic residues (proline-rich region). We searched the bovine adrenal gland for MAP4 isoforms, and identified a novel variant lacking 72 consecutive amino acid residues within the proline-rich region, as compared with the full-length MAP4. The amino acid sequence of the missing region was highly conserved (about 85% identity/similarity) among the corresponding regions of bovine, human, mouse, and rat MAP4, which suggested the functional significance of this region. A comparison of the genomic sequence with the cDNA sequence revealed that the missing region is encoded by a single exon. A MAP4 variant cDNA homologous to the bovine form was also detected in rat cells, suggesting that the new variant can be generated by alternative splicing, not only in bovine but also in other mammalian species. The mRNA expression of the novel isoform was restricted to the brain and the adrenal medulla, suggesting that this isoform is specific to a certain cell type. Using a bacterially expressed fragment corresponding to the microtubule-binding domain of the novel isoform, we analyzed its in vitro characteristics. The fragment induced microtubule assembly and bound to preformed microtubules, but the activities were slightly lower than those of the conventional MAP4 fragment, which carries the full-length proline-rich region. The microtubules assembled in the presence of the fragment failed to be bundled. Instead, a constant spacing between neighboring microtubules was observed.

Functional analysis of microtubule-binding domain of bovine MAP4

Bovine microtubule-associated protein 4 (MAP4) consists of an amino-terminal projection domain and a carboxyl-terminal microtubule-binding domain. The carboxyl-terminal domain of MAP4 is further divided into three subdomains: a region rich in proline and basic residues (Pro-rich region), a region containing four repeats of an assembly-promoting (AP) sequence, which consists of 22 amino acid residues (AP sequence region), and a hydrophobic tail region (Tail region). The subdomain structure of MAP4 microtubule binding domain is similar to those of other MAPs (MAP2 and tau). In order to study the function of each subdomain per se of bovine MAP4 microtubule-binding domain, we purified a series of truncated fragments of MAP4, expressed in Escherichia coil. Binding affinity of the PA4T fragment (containing the Pro-rich region, the AP sequence region and the Tail region) is only four times higher than that of the A4T fragment (containing the AP sequence region and the Tail region), while the microtubule nucleating activity of the PA4T fragment is far greater. We propose that the Pro-rich region promotes the nucleation of microtubule assembly. The A4 fragment (corresponding to the AP sequence region) stimulated the assembly of tubulin into coldstable amorphous aggregates. The AP sequence region of MAP4 failed to promote microtubule assembly. On the other hand, the fragment has an activity to stimulate microtubule elongation. The function of the MAP4 Tail region is not clear at present. The A4T fragment (containing the AP sequence region and the Tail region) promote both microtubule nucleation and elongation step, but the A4 fragment only promotes microtubule elongation, suggesting that the Tail region is indispensable for the nucleation step. However, the fragment containing only the Tail region could not bind to microtubule. Although MAP4 was considered to be long, thin and flexible molecule, never the Tail region may contribute to be the proper folding of MAP4, and/or may interact with other molecules. We concluded that both the Pro-rich region and the AP sequence region take part in the promotion of tubulin polymerization, and that the former is important for the lateral protofilament-protofilament interaction, and the latter is important for the longitudinal affinity between each tubulin dimer in a protofilament.

The number of repeat sequences in microtubule-associated protein 4 affects the microtubule surface properties

The microtubule-binding domain of MAP4, a ubiquitous microtubule-associated protein, contains a Repeat region with tandemly organized repeat sequences. In this study, we focused on the variations of the Repeat region, and searched for MAP4 isoforms with diverse Repeat region organizations. We successfully isolated four types of MAP4 cDNAs, which differed from each other in both the number and the arrangement of the repeat sequences, from a single source (bovine adrenal gland). To examine the functional differences among the isoforms, we prepared the microtubule-binding domain polypeptides of three of the four isoforms, and examined their activities. The isoform fragments showed similar degrees of microtubule assembly promoting activity and microtubule binding affinity. This result suggested that the Repeat region variation is not important for the control of microtubule dynamics, which is believed to be the main function of MAPs. On the other hand, the microtubule bundle-forming activity differed among the isoform fragments. The bundle formation was augmented by increasing the number of repeat sequences in the fragments. Based on these results, we propose the hypothesis that the role of the MAP4 isoforms is to regulate the surface charge of microtubules.

Different protofilament-dependence of the microtubule binding between MAP2 and MAP4

To see a molecular basis of the difference in the microtubule binding between MAP2 and MAP4, we compared the binding of them onto microtubule and Zinc-sheet in the presence of various concentrations of NaCl. The Zinc-sheet is the lateral association of protofilaments arranged in an antiparallel fashion with alternatively exposed opposite surfaces, so that binding requiring adjacent protofilaments is restricted. While the salt-dependence of the MAP2 desorption was not altered between these tubulin polymers, MAP4 dissociated from Zinc-sheet at lower concentrations of NaCl than from microtubule. These results suggest that single protofilament is sufficient for microtubule binding of MAP2 as observed by Al-Bassam et al. [J. Cell Biol. 157 (2002) 1187], but MAP4 appeared to interact with adjacent protofilaments during microtubule-binding. Weakened binding on Zinc-sheets was also observed in the projection domain-deletion mutants of MAP4, so that the difference in the protofilament-dependence would lie in the relatively conserved microtubule-binding domain.

The projection domain of MAP4 suppresses the microtubule-bundling activity of the microtubule-binding domain

Microtubule-associated protein 4 (MAP4), a major MAP expressed in proliferating non-neuronal cells, consists of an N-terminal projection (PJ) domain and a C-terminal microtubule-binding (MTB) domain. The PJ domain of MAP4 is divided into three regions; the N-terminal acidic region (the Na-region), the multiple KDM-repeated sequence region (the KDM-region), and the b-region followed by the MTB domain. To investigate roles of the PJ domain, we prepared three truncated forms of human MAP4 with different PJ domain lengths; PJ1, PJ2 and MTB with deletion of about one-third, two-third and all of the PJ domain, respectively, and examined their effects on bundle formation of microtubules (MTs). MTs polymerized by full length MAP4 were singly distributed as observed by both negative staining electron microscopy and dark field microscopy. MTs with PJ1 were also separated in solution but became pairs when pelleted by centrifugation. PJ2 formed planar two-dimensional bundles consisting of several MTs (the 2D-bundle). MTB induced large bundles of many MTs, tightly packed without space in between (termed the 3D-bundle). To study how the PJ domain decreases the bundle-forming activity of the MTB domain of MAP4, we made three additional deletion-mutants of MAP4, called Na-MTB, KDM-MTB and Na-PJ2. Na-MTB and KDM-MTB, in which the KDM/b-region and both of Na- and b-regions were deleted respectively, were prepared by fusing the Na-region or KDM-region to MTB. Both of Na-MTB and KDM-MTB suppressed the 3D-bundle formation as effectively as PJ2. MTs polymerized with Na-PJ2, the KDM-deletion mutant made by adding the Na-region to PJ2, were singular and did not become bundles. These results indicated that the PJ domain kept individual MTs separated by suppressing the bundle-forming ability of the MTB domain. The suppressive activity of the PJ domain was correlated with the length, but not the amino acid sequence, of the PJ.

Regulation of microtubule-associated proteins

Microtubule-associated proteins (MAPs) function to regulate the assembly dynamics and organization of microtubule polymers. Upstream regulation of MAP activities is the major mechanism used by cells to modify and control microtubule assembly and organization. This review summarizes the functional activities of MAPs found in animal cells and discusses how these MAPs are regulated. Mechanisms controlling gene expression, isoform-specific expression, protein localization, phosphorylation, and degradation are discussed. Additional regulatory mechanisms include synergy or competition between MAPs and the activities of cofactors or binding partners. For each MAP it is likely that regulation in vivo reflects a composite of multiple regulatory mechanisms.

Accompanying protein alterations in malignant cells with a microtubule-polymerizing drug-resistance phenotype and a primary resistance mechanism

Microtubules (MTs) are cytoskeletal components whose structural integrity is mandatory for the execution of many basic cell functions. Utilizing parental and drug-resistant ovarian carcinoma cell lines that have acquired point mutations in beta-tubulin and p53, we studied the level of expression and modification of proteins involved in apoptosis and MT integrity. Extending previous results, we demonstrated phosphorylation of pro-survival Bcl-x(L) in an epothilone-A resistant cell line, correlating it with drug sensitivity to tubulin-active compounds. Furthermore, Mcl-1 protein turned over more rapidly following exposure to tubulin-modifying agents, the stability of Mcl-1 protein paralleling the drug sensitivity profile of the paclitaxel or epothilone-A resistant cell lines. The observed decreases in Mcl-1 were not a consequence of G(2)M arrest, as determined by flow cytometry analysis, which showed prominent levels of Mcl-1 in the absence of any drug treatment in populations enriched in mitotic cells. We also observed that a paclitaxel-resistant cell line expressed Bax at a much lower level than the sensitive parental line [A2780(1A9)], consistent with its mutant p53 status. MT-associated protein-4 (MAP4), whose phosphorylation during specific phases of the cell cycle reduces its MT-polymerizing and -stabilizing capabilities, was phosphorylated in response to drug challenge without a change in expression. Phosphorylation of MAP4 correlated with sensitivity to tubulin-binding drugs and with a dissociation from MTs. We propose that the tubulin mutations, which result in a compromised paclitaxel:tubulin or epothilone:tubulin interaction and paclitaxel or epothilone resistance, indirectly inhibit downstream events that lead to cell death, and this, in turn, may contribute to the drug-resistance phenotype

Visualization of the stop of microtubule depolymerization that occurs at the high-density region of microtubule-associated protein 2 (MAP2)

Individual microtubules (MTs) repeat alternating phases of polymerization and depolymerization, a process known as dynamic instability. Microtubule-associated proteins (MAPs) regulate the dynamic instability by increasing the rescue frequency. To explore the influence of MAP2 on in vitro MT dynamics, we correlated the distribution of MAP2 on individual MTs with the dynamic phase changes of the same MTs. MAP2 was modified selectively on its projection region by X-rhodamine iodoacetamide without altering the MT-binding activity. When the labeled MAP2 was added to MTs, the fluorescence was distributed along almost the entire length of individual MTs. However, the inhomogeneity of the distribution gradually became obvious due to the fluorescence bleaching, and the MTs appeared to consist of rapidly bleached portions (RBPs) and slowly bleached portions (SBPs), which were distributed randomly along the MT. By measuring the duration of fluorescence bleaching, the density of MAP2 in SBP was estimated to be approximately 2.5 times higher than the RBP. The average tubulin:MAP2 ratio in SBP was calculated to be 16. When the MT dynamics were observed by dark-field microscopy after determining the MAP2 distribution, rescues were always found to occur only at the SBPs. MTs also displayed intermittent shortening by repeated depolymerization phases separated by pause phases. In these cases, depolymerization phases stopped only at the SBPs. Not every SBP stopped depolymerization, but depolymerization always stopped at an SBP. Taken together, we suggest that there is a minimum density of MAP2 that is necessary to stop depolymerization.

Orbit, a novel microtubule-associated protein essential for mitosis in Drosophila melanogaster

We describe a Drosophila gene, orbit, that encodes a conserved 165-kD microtubule-associated protein (MAP) with GTP binding motifs. Hypomorphic mutations in orbit lead to a maternal effect resulting in branched and bent mitotic spindles in the syncytial embryo. In the larval central nervous system, such mutants have an elevated mitotic index with some mitotic cells showing an increase in ploidy. Amorphic alleles show late lethality and greater frequencies of hyperploid mitotic cells. The presence of cells in the hypomorphic mutant in which the chromosomes can be arranged, either in a circular metaphase or an anaphase-like configuration on monopolar spindles, suggests that polyploidy arises through spindle and chromosome segregation defects rather than defects in cytokinesis. A role for the Orbit protein in regulating microtubule behavior in mitosis is suggested by its association with microtubules throughout the spindle at all mitotic stages, by its copurification with microtubules from embryonic extracts, and by the finding that the Orbit protein directly binds to MAP-free microtubules in a GTP-dependent manner.

Focusing-in on microtubules

A good approximation of the atomic structure of a microtubule has been derived from docking the high-resolution structure of tubulin, solved by electron crystallography, into lower resolution maps of whole microtubules. Some structural interactions with other molecules, including nucleotides, drugs, motor proteins and microtubule-associated proteins, can now be predicted.

Microtubule-binding property of microtubule-associated protein 2 differs from that of microtubule-associated protein 4 and tau

The microtubule-binding domains of microtubule-associated protein (MAP) 2, MAP4, and tau are structurally similar [Aizawa, H., Emori, Y., Murofushi, H., Kawasaki, H., Sakai., H., and Suzuki, K. (1990) J. Biol. Chem. 265, 13849-13855]. To compare the microtubule-binding mechanisms of the three MAPs, we performed a quantitative competition analysis using the three MAPs and the microtubule-binding domain fragment of MAP4 (PA4T fragment). The two-cycled microtubule protein fraction from bovine brain contains MAP1, MAP2, MAP4, and tau. When an excess of the PA4T fragment was added to the microtubule protein fraction, MAP4 and tau were completely released from the microtubules, while MAP1 remained bound. MAP2 was only partially released from the microtubules. The competition between MAP2 and MAP4 was further analyzed using purified MAP2, the PA4T fragment, and tubulin. About half of the MAP2 was still bound to the microtubules, even in the presence of an excess amount of the PA4T fragment. The microtubule-binding mechanisms of MAP2 and MAP4 seem to be different, in spite of their similar primary structures.