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

Microtubule elongation along actin filaments induced by microtubule-associated protein 4 contributes to the formation of cellular protrusions

Actin-microtubule crosstalk is implicated in the formation of cellular protrusions, but the mechanism remains unclear. In this study, we examined the regulation of cell protrusion involving a ubiquitously expressed microtubule-associated protein (MAP) 4, and its superfamily proteins, neuronal MAP2 and tau. Fluorescence microscopy revealed that these MAPs bound to F-actin and microtubules simultaneously, and formed F-actin/microtubule hybrid bundles. The hybrid bundle-forming activity was in the order of MAP2 > MAP4 ≫ tau. Interestingly, the microtubule assembly-promoting activity of MAP4 and MAP2, but not of tau, was upregulated by their interaction with F-actin. When MAP4 was overexpressed in NG108-15 cells, the number of cell processes and maximum process length of each cell increased significantly by 28% and 30%, respectively. Super-resolution microscopy revealed that 95% of microtubules in cell processes colocalized with F-actin, and MAP4 was always found in their vicinity. These results suggest that microtubule elongation along F-actin induced by MAP4 contributes to the formation of cellular protrusions. Since MAP4, MAP2 and tau had different crosstalk activity between F-actin and microtubules, it is likely that the functional differentiation of these MAPs is a driving force for neural evolution, causing significant changes in cell morphology.

Human MAP Tau Based Targeted Cytolytic Fusion Proteins

Some of the most promising small molecule toxins used to generate antibody drug conjugates (ADCs) include anti-mitotic agents (e.g., auristatin and its derivatives) which are designed to attack cancerous cells at their most vulnerable state during mitosis. We were interested in identifying a human cystostatic protein eventually showing comparable activities and allowing the generation of corresponding targeted fully human cytolytic fusion proteins. Recently, we identified the human microtubule associated protein tau (MAP tau), which binds specifically to tubulin and modulates the stability of microtubules, thereby blocking mitosis and presumably vesicular transport. By binding and stabilizing polymerized microtubule filaments, MAP tau-based fusion proteins skew microtubule dynamics towards cell cycle arrest and apoptosis. This biological activity makes rapidly proliferating cells (e.g., cancer and inflammatory cells) an excellent target for MAP tau-based targeted treatments. Their superior selectivity for proliferating cells confers additional selectivity towards upregulated tumor-associated antigens at their surface, thereby preventing off-target related toxicity against normal cells bearing tumor-associated antigens at physiologically normal to low levels. In this review, we highlight recent findings on MAP tau-based targeted cytolytic fusion proteins reported in preclinical immunotherapeutic studies.

The Microtubule-Associated Protein Tau and Its Relevance for Pancreatic Beta Cells

Structural and biochemical alterations of the microtubule-associated protein tau (MAPT) are associated with degenerative disorders referred to as tauopathies. We have previously shown that MAPT is present in human islets of Langerhans, human insulinomas, and pancreatic beta-cell line models, with biophysical similarities to the pathological MAPT in the brain. Here, we further studied MAPT in pancreatic endocrine tissue to better understand the mechanisms that lead to functional dysregulation of pancreatic beta cells. We found upregulation of MAPT protein expression in human insulinomas when compared to human pancreatic islets of Langerhans and an imbalance between MAPT isoforms in insulinomas tissue. We cloned one 3-repeat domain MAPT and transduced this into a beta-cell derived rodent cell line Rin-5F. Proliferation experiments showed higher growth rates and metabolic activities of cells overexpressing MAPT protein. We observed that a MAPT overexpressing cell line demonstrates altered insulin transcription, translation, and insulin secretion rates. We found the relative insulin secretion rates were significantly decreased in a MAPT overexpressing cell line and these findings could be confirmed using partial MAPT knock-down cell lines. Our findings support that MAPT may play an important role in insulin granule trafficking and indicate the importance of balanced MAPT phosphorylation and dephosphorylation for adequate insulin release.

MAP2-mediated in vitro interactions of brain microtubules and their modulation by cAMP

Microtubule-associated proteins (MAPs) are involved in microtubule (MT) bundling and in crossbridges between MTs and other organelles. Previous studies have assigned the MT bundling function of MAPs to their MT-binding domain and its modulation by the projection domain. In the present work, we analyse the viscoelastic properties of MT suspensions in the presence or the absence of cAMP. The experimental data reveal the occurrence of interactions between MT polymers involving MAP2 and modulated by cAMP. Two distinct mechanisms of action of cAMP are identified, which involve on one hand the phosphorylation of MT proteins by the cAMP-dependent protein kinase A (PKA) bound to the end of the N-terminal projection of MAP2, and on the other hand the binding of cAMP to the RII subunit of the PKA affecting interactions between MTs in a phosphorylation-independent manner. These findings imply a role for the complex of PKA with the projection domain of MAP2 in MT-MT interactions and suggest that cAMP may influence directly the density and bundling of MT arrays in dendrites of neurons.

Transitions in microtubule C-termini conformations as a possible dendritic signaling phenomenon

We model the dynamical states of the C-termini of tubulin dimers that comprise neuronal microtubules. We use molecular dynamics and other computational tools to explore the time-dependent behavior of conformational states of a C-terminus of tubulin within a microtubule and assume that each C-terminus interacts via screened Coulomb forces with the surface of a tubulin dimer, with neighboring C-termini and also with any adjacent microtubule-associated protein 2 (MAP2). Each C-terminus can either bind to the tubulin surface via one of the several positively charged regions or can be allowed to explore the space available in the solution surrounding the dimer. We find that the preferential orientation of each C-terminus is away from the tubulin surface but binding to the surface may also take place, albeit at a lower probability. The results of our model suggest that perturbations generated by the C-termini interactions with counterions surrounding a MAP2 may propagate over distances greater than those between adjacent microtubules. Thus, the MAP2 structure is able to act as a kind of biological wire (or a cable) transmitting local electrostatic perturbations resulting in ionic concentration gradients from one microtubule to another. We briefly discuss the implications the current dynamic modeling may have on synaptic activation and potentiation.

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.

Formation of P10 tubular structures during AcMNPV infection depends on the integrity of host-cell microtubules

During infection of insect cells with Autographa californica nucleopolyhedrovirus (AcMNPV), the very late protein P10 forms large fibrillar structures in the cytoplasm and nuclei of infected cells. In this study we have used confocal microscopy in association with a novel P10 antiserum to localise and study P10 in virus-infected cells. P10 was shown to be a component of tubular-like structures that spiralled throughout the cytoplasm and nucleus of AcMNPV-infected cells. These structures were observed to colocalise partly with cortical microtubules. When microtubules were depolymerised with the drug nocodazole, P10 tubules failed to form and the protein appeared concentrated in cytoplasmic foci. For the first time, we provide direct evidence using both antibody pulldown and yeast two-hybrid experiments for the interaction of P10 with host-cell tubulin. It is suggested that this interaction may be a critical factor in AcMNPV-induced cell lysis.

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.

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.

Purification and characterization of a new, ubiquitously distributed class of microtubule-associated protein with molecular mass 250 kDa

A heat-stable microtubule-associated protein (MAP) with relative molecular mass 250 000, termed 250-kDa MAP, was purified from bovine adrenal cortex. It is classified as a MAP subspecies distinct from MAP1, MAP2, tau, and MAP4, as judged from its electrophoretic mobility, heat stability and immunoreactivity. Purified 250-kDa MAP was able to bind to taxol-stabilized microtubules, although it lacked the ability to polymerize purified tubulin into microtubules. Western-blot analysis showed that this MAP was expressed ubiquitously in mammalian tissues. Immunofluorescence microscopy revealed that polyclonal antibodies raised against 250-kDa MAP stained many punctate structures in the cytoplasm of cultured cells. Blurry cytosolic staining was also observed. Judging from the result of nocodazole treatment, the punctate structures were associated with the microtubule network throughout the cytoplasm, while cytosolic 250-kDa MAP colocalized with free tubulin. Under electron microscopy, 250-kDa MAP has the appearance of a hollow sphere of about 12 nm diameter.

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.