The types and numbers of kinesins and dyneins transporting endocytic cargoes modulate their motility and response to tau

Organelles and vesicular cargoes are transported by teams of kinesin and dynein motors along microtubules. We isolated endocytic organelles from cells at different stages of maturation and reconstituted their motility along microtubules in vitro. We asked how the sets of motors transporting a cargo determine its motility and response to the microtubule-associated protein tau. Here, we find that phagosomes move in both directions along microtubules, but the directional bias changes during maturation. Early phagosomes exhibit retrograde-biased transport while late phagosomes are directionally unbiased. Correspondingly, early and late phagosomes are bound by different numbers and combinations of kinesins -1, -2, -3, and dynein. Tau stabilizes microtubules and directs transport within neurons. While single-molecule studies show that tau differentially regulates the motility of kinesins and dynein in vitro, less is known about its role in modulating the trafficking of endogenous cargoes transported by their native teams of motors. Previous studies showed that tau preferentially inhibits kinesin motors, which biases late phagosome transport towards the microtubule minus-end. Here, we show that tau strongly inhibits long-range, dynein-mediated motility of early phagosomes. Tau reduces forces generated by teams of dynein motors on early phagosomes and accelerates dynein unbinding under load. Thus, cargoes differentially respond to tau, where dynein-complexes on early phagosomes are more sensitive to tau inhibition than those on late phagosomes. Mathematical modeling further explains how small changes in the number of kinesins and dynein on cargoes impact the net directionality but also that cargoes with different sets of motors respond differently to tau.

The N-terminal disease-associated R5L Tau mutation increases microtubule shrinkage rate due to disruption of microtubule-bound Tau patches

Regulation of the neuronal microtubule cytoskeleton is achieved through the coordination of microtubule-associated proteins (MAPs). MAP-Tau, the most abundant MAP in the axon, functions to modulate motor motility, participate in signaling cascades, as well as directly mediate microtubule dynamics. Tau misregulation is associated with a class of neurodegenerative diseases, known as tauopathies, including progressive supranuclear palsy, Pick's disease, and Alzheimer's disease. Many disease-associated mutations in Tau are found in the C-terminal microtubule-binding domain. These mutations decrease microtubule-binding affinity and are proposed to reduce microtubule stability, leading to disease. N-terminal disease-associated mutations also exist, but the mechanistic details of their downstream effects are not as clear. Here, we investigate the effect of the progressive supranuclear palsy–associated N-terminal R5L mutation on Tau-mediated microtubule dynamics using an in vitro reconstituted system. We show that the R5L mutation does not alter Tau interactions with tubulin by fluorescence correlation spectroscopy. Using total internal reflection fluorescence microscopy, we determined that the R5L mutation has no effect on microtubule growth rate, catastrophe frequency, or rescue frequency. Rather, the R5L mutation increases microtubule shrinkage rate. We determine this is due to disruption of Tau patches, larger order Tau complexes known to form on the GDP-microtubule lattice. Altogether, these results provide insight into the role of Tau patches in mediating microtubule dynamics and suggesting a novel mechanism by which mutations in the N-terminal projection domain reduce microtubule stability.

Specific phosphorylation of microtubule-associated protein 2c by extracellular signal-regulated kinase reduces interactions at its Pro-rich regions

Microtubule-associated protein 2 (MAP2) is an important neuronal target of extracellular signal-regulated kinase 2 (ERK2) involved in Raf signaling pathways, but mechanistic details of MAP2 phosphorylation are unclear. Here, we used NMR spectroscopy to quantitatively describe the kinetics of phosphorylation of individual serines and threonines in the embryonic MAP2 variant MAP2c. We carried out real-time monitoring of phosphorylation to discover major phosphorylation sites that were not identified in previous studies relying on specific antibodies. Our comparison with phosphorylation of MAP2c by a model cyclin-dependent kinase CDK2 and with phosphorylation of the MAP2c homolog Tau revealed differences in phosphorylation profiles that explain specificity of regulation of biological functions of MAP2c and Tau. To probe the molecular basis of the regulatory effect of ERK2, we investigated the interactions of phosphorylated and unphosphorylated MAP2c by NMR with single-residue resolution. As ERK2 phosphorylates mostly outside the regions binding microtubules, we studied the binding of proteins other than tubulin, namely regulatory subunit RIIα of cAMP-dependent protein kinase (PKA), adaptor protein Grb2, Src homology domain 3 of tyrosine kinases Fyn and Abl, and ERK2 itself. We found ERK2 phosphorylation interfered mostly with binding to proline-rich regions of MAP2c. Furthermore, our NMR experiments in SH-SY5Y neuroblastoma cell lysates showed that the kinetics of dephosphorylation are compatible with in-cell NMR studies and that residues targeted by ERK2 and PKA are efficiently phosphorylated in the cell lysates. Taken together, our results provide a deeper characterization of MAP2c phosphorylation and its effects on interactions with other proteins.

Cytoskeleton and Associated Proteins: Pleiotropic JNK Substrates and Regulators

This review extensively reports data from the literature concerning the complex relationships between the stress-induced c-Jun N-terminal kinases (JNKs) and the four main cytoskeleton elements, which are actin filaments, microtubules, intermediate filaments, and septins. To a lesser extent, we also focused on the two membrane-associated cytoskeletons spectrin and ESCRT-III. We gather the mechanisms controlling cytoskeleton-associated JNK activation and the known cytoskeleton-related substrates directly phosphorylated by JNK. We also point out specific locations of the JNK upstream regulators at cytoskeletal components. We finally compile available techniques and tools that could allow a better characterization of the interplay between the different types of cytoskeleton filaments upon JNK-mediated stress and during development. This overview may bring new important information for applied medical research.

The domain of unknown function 4005 (DUF4005) in an Arabidopsis IQD protein functions in microtubule binding

The dynamic responses of microtubules (MTs) to internal and external signals are modulated by a plethora of microtubule-associated proteins (MAPs). In higher plants, many plant-specific MAPs have emerged during evolution as advantageous to their sessile lifestyle. Some members of the IQ67 domain (IQD) protein family have been shown to be plant-specific MAPs. However, the mechanisms of interaction between IQD proteins and MTs remain elusive. Here we demonstrate that the domain of unknown function 4005 (DUF4005) of the Arabidopsis IQD family protein ABS6/AtIQD16 is a novel MT-binding domain. Cosedimentation assays showed that the DUF4005 domain binds directly to MTs in vitro. GFP-labeled DUF4005 also decorates all types of MT arrays tested in vivo. Furthermore, we showed that a conserved stretch of 15 amino acid residues within the DUF4005 domain, which shares sequence similarity with the C-terminal MT-binding domain of human MAP Kif18A, is required for the binding to MTs. Transgenic lines overexpressing the DUF4005 domain displayed a spectrum of developmental defects, including spiral growth and stunted growth at the organismal level. At the cellular level, DUF4005 overexpression caused defects in epidermal pavement cell and trichome morphogenesis, as well as abnormal anisotropic cell elongation in the hypocotyls of dark-grown seedlings. These data establish that the DUF4005 domain of ABS6/AtIQD16 is a new MT-binding domain, overexpression of which perturbs MT homeostasis in plants. Our findings provide new insights into the MT-binding mechanisms of plant IQD proteins.

Klp2 and Ase1 synergize to maintain meiotic spindle stability during metaphase I

The spindle apparatus segregates bi-oriented sister chromatids during mitosis but mono-oriented homologous chromosomes during meiosis I. It has remained unclear if similar molecular mechanisms operate to regulate spindle dynamics during mitosis and meiosis I. Here, we employed live-cell microscopy to compare the spindle dynamics of mitosis and meiosis I in fission yeast cells and demonstrated that the conserved kinesin-14 motor Klp2 plays a specific role in maintaining metaphase spindle length during meiosis I but not during mitosis. Moreover, the maintenance of metaphase spindle stability during meiosis I requires the synergism between Klp2 and the conserved microtubule cross-linker Ase1, as the absence of both proteins causes exacerbated defects in metaphase spindle stability. The synergism is not necessary for regulating mitotic spindle dynamics. Hence, our work reveals a new molecular mechanism underlying meiotic spindle dynamics and provides insights into understanding differential regulation of meiotic and mitotic events.

The disordered N-terminus of HDAC6 is a microtubule-binding domain critical for efficient tubulin deacetylation

Histone deacetylase 6 (HDAC6) is a multidomain cytosolic enzyme having tubulin deacetylase activity that has been unequivocally assigned to the second of the tandem catalytic domains. However, virtually no information exists on the contribution of other HDAC6 domains on tubulin recognition. Here, using recombinant protein expression, site-directed mutagenesis, fluorimetric and biochemical assays, microscale thermophoresis, and total internal reflection fluorescence microscopy, we identified the N-terminal, disordered region of HDAC6 as a microtubule-binding domain and functionally characterized it to the single-molecule level. We show that the microtubule-binding motif spans two positively charged patches comprising residues Lys-32 to Lys-58. We found that HDAC6-microtubule interactions are entirely independent of the catalytic domains and are mediated by ionic interactions with the negatively charged microtubule surface. Importantly, a crosstalk between the microtubule-binding domain and the deacetylase domain was critical for recognition and efficient deacetylation of free tubulin dimers both in vitro and in vivo Overall, our results reveal that recognition of substrates by HDAC6 is more complex than previously appreciated and that domains outside the tandem catalytic core are essential for proficient substrate deacetylation.

Independent tubulin binding and polymerization by the proline-rich region of Tau is regulated by Tau's N-terminal domain

Tau is an intrinsically disordered, microtubule-associated protein that has a role in regulating microtubule dynamics. Despite intensive research, the molecular mechanisms of Tau-mediated microtubule polymerization are poorly understood. Here we used single-molecule fluorescence to investigate the role of Tau's N-terminal domain (NTD) and proline-rich region (PRR) in regulating interactions of Tau with soluble tubulin. We assayed both full-length Tau isoforms and truncated variants for their ability to bind soluble tubulin and stimulate microtubule polymerization. We found that Tau's PRR is an independent tubulin-binding domain that has tubulin polymerization capacity. In contrast to the relatively weak interactions with tubulin mediated by sites distributed throughout Tau's microtubule-binding region (MTBR), resulting in heterogeneous Tau: tubulin complexes, the PRR bound tubulin tightly and stoichiometrically. Moreover, we demonstrate that interactions between the PRR and MTBR are reduced by the NTD through a conserved conformational ensemble. On the basis of these results, we propose that Tau's PRR can serve as a core tubulin-binding domain, whereas the MTBR enhances polymerization capacity by increasing the local tubulin concentration. Moreover, the NTD appears to negatively regulate tubulin-binding interactions of both of these domains. The findings of our study draw attention to a central role of the PRR in Tau function and provide mechanistic insight into Tau-mediated polymerization of tubulin.

Mechanism of microtubule plus-end tracking by the plant-specific SPR1 protein and its development as a versatile plus-end marker

Microtubules are cytoskeletal polymers that perform diverse cellular functions. The plus ends of microtubules promote polymer assembly and disassembly and connect the microtubule tips to other cellular structures. The dynamics and functions of microtubule plus ends are governed by microtubule plus end-tracking proteins (+TIPs). Here we report that the Arabidopsis thaliana SPIRAL1 (SPR1) protein, which regulates directional cell expansion, is an autonomous +TIP. Using in vitro reconstitution experiments and total internal reflection fluorescence microscopy, we demonstrate that the conserved N-terminal region of SPR1 and its GGG motif are necessary for +TIP activity whereas the conserved C-terminal region and its PGGG motif are not. We further show that the N- and C-terminal regions, either separated or when fused in tandem (NC), are sufficient for +TIP activity and do not significantly perturb microtubule plus-end dynamics compared with full-length SPR1. We also found that exogenously expressed SPR1-GFP and NC-GFP label microtubule plus ends in plant and animal cells. These results establish SPR1 as a new type of intrinsic +TIP and reveal the utility of NC-GFP as a versatile microtubule plus-end marker.

Tau isoform-specific stabilization of intermediate states during microtubule assembly and disassembly

The microtubule (MT)-associated protein tau regulates the critical growing and shortening behaviors of MTs, and its normal activity is essential for neuronal development and maintenance. Accordingly, aberrant tau action is tightly associated with Alzheimer's disease and is genetically linked to several additional neurodegenerative diseases known as tauopathies. Although tau is known to promote net MT growth and stability, the precise mechanistic details governing its regulation of MT dynamics remain unclear. Here, we have used the slowly-hydrolyzable GTP analog, guanylyl-(α,β)-methylene-diphosphonate (GMPCPP), to examine the structural effects of tau at MT ends that may otherwise be too transient to observe. The addition of both four-repeat (4R) and three-repeat (3R) tau isoforms to pre-formed GMPCPP MTs resulted in the formation of extended, multiprotofilament-wide projections at MT ends. Furthermore, at temperatures too low for assembly of bona fide MTs, both tau isoforms promoted the formation of long spiral ribbons from GMPCPP tubulin heterodimers. In addition, GMPCPP MTs undergoing cold-induced disassembly in the presence of 4R tau (and to a much lesser extent 3R tau) also formed spirals. Finally, three pathological tau mutations known to cause neurodegeneration and dementia were differentially compromised in their abilities to stabilize MT disassembly intermediates. Taken together, we propose that tau promotes the formation/stabilization of intermediate states in MT assembly and disassembly by promoting both longitudinal and lateral tubulin-tubulin contacts. We hypothesize that these activities represent fundamental aspects of tau action that normally occur at the GTP-rich ends of GTP/GDP MTs and that may be compromised in neurodegeneration-causing tau variants.

MAP7 regulates organelle transport by recruiting kinesin-1 to microtubules

Microtubule-associated proteins (MAPs) regulate microtubule polymerization, dynamics, and organization. In addition, MAPs alter the motility of kinesin and dynein to control trafficking along microtubules. MAP7 (ensconsin, E-MAP-115) is a ubiquitous MAP that organizes the microtubule cytoskeleton in mitosis and neuronal branching. MAP7 also recruits kinesin-1 to microtubules. To understand how the activation of kinesin-1 by MAP7 regulates the motility of organelles transported by ensembles of kinesin and dynein, we isolated organelles and reconstituted their motility in vitro In the absence of MAP7, isolated phagosomes exhibit approximately equal fractions of plus- and minus-end-directed motility along microtubules. MAP7 causes a pronounced shift in motility; phagosomes move toward the plus-end ∼80% of the time, and kinesin teams generate more force. To dissect MAP7-mediated regulation of kinesin-driven transport, we examined its effects on the motility and force generation of single and teams of full-length kinesin-1 motors. We find that MAP7 does not alter the force exerted by a single kinesin-1 motor, but instead increases its binding rate to the microtubule. For ensembles of kinesin, a greater number of kinesin motors are simultaneously engaged and generating force to preferentially target organelles toward the microtubule plus-end.

Tau repeat regions contain conserved histidine residues that modulate microtubule-binding in response to changes in pH

Tau, a member of the MAP2/tau family of microtubule-associated proteins, stabilizes and organizes axonal microtubules in healthy neurons. In neurodegenerative tauopathies, tau dissociates from microtubules and forms neurotoxic extracellular aggregates. MAP2/tau family proteins are characterized by three to five conserved, intrinsically disordered repeat regions that mediate electrostatic interactions with the microtubule surface. Here, we used molecular dynamics, microtubule-binding experiments, and live-cell microscopy, revealing that highly-conserved histidine residues near the C terminus of each microtubule-binding repeat are pH sensors that can modulate tau-microtubule interaction strength within the physiological intracellular pH range. We observed that at low pH (<7.5), these histidines are positively charged and interact with phenylalanine residues in a hydrophobic cleft between adjacent tubulin dimers. At higher pH (>7.5), tau deprotonation decreased binding to microtubules both in vitro and in cells. Electrostatic and hydrophobic characteristics of histidine were both required for tau-microtubule binding, as substitutions with constitutively and positively charged nonaromatic lysine or uncharged alanine greatly reduced or abolished tau-microtubule binding. Consistent with these findings, tau-microtubule binding was reduced in a cancer cell model with increased intracellular pH but was rapidly restored by decreasing the pH to normal levels. These results add detailed insights into the intracellular regulation of tau activity that may be relevant in both normal and pathological conditions.

The microtubule-associated protein EML3 regulates mitotic spindle assembly by recruiting the Augmin complex to spindle microtubules

In all eukaryotes, a functional mitotic spindle is essential for distributing duplicated chromosomes into daughter cells. Mitotic spindle assembly involves highly ordered arrangement of microtubules (MTs). The Augmin protein complex recruits γ-tubulin ring complex (γ-TuRC) to MTs and thereby promotes MT-based MT nucleation and mitotic spindle assembly. However, several factors that may promote Augmin recruitment to MTs remain unknown. Here, we show that echinoderm microtubule-associated protein-like 3 (EML3), an MT-associated protein, facilitates binding between MTs and Augmin/γ-TuRC and recruiting the latter to MTs for proper mitotic spindle assembly and kinetochore-MT connections. Using immunofluorescence microscopy, live-cell imaging, and immunoprecipitation assays, we found that EML3 recruits Augmin/γ-TuRC to the MTs to enhance MT-based MT nucleation in both spindle and small acentrosomal asters. We also noted that the EML3-mediated recruitment is controlled by cyclin-dependent kinase 1 (CDK1), which phosphorylated EML3 at Thr-881 and promoted its binding to Augmin/γ-TuRC. RNAi-mediated EML3 knockdown in HeLa cells reduced spindle localization of Augmin/γ-TuRC, which resulted in abnormal spindle assembly and caused kinetochore-MT misconnection. The introduction of exogenous WT or a Thr-881 phosphorylation mimic EML3 variant into the EML3 knockdown cells restored normal Augmin/γ-TuRC localization and spindle assembly. The EML3 knockdown also affected the spindle assembly checkpoint, delaying chromosome congression and cell division. Taken together, our results indicate that EML3 regulates mitotic spindle assembly and the kinetochore-MT connection by regulating MT-based MT nucleation and recruiting Augmin/γ-TuRC to MTs.

Mapping multivalency in the CLIP-170-EB1 microtubule plus-end complex

Cytoplasmic linker protein 170 (CLIP-170) is a microtubule plus-end factor that links vesicles to microtubules and recruits the dynein-dynactin complex to microtubule plus ends. CLIP-170 plus-end localization is end binding 1 (EB1)-dependent. CLIP-170 contains two N-terminal cytoskeleton-associated protein glycine-rich (CAP-Gly) domains flanked by serine-rich regions. The CAP-Gly domains are known EB1-binding domains, and the serine-rich regions have also been implicated in CLIP-170's microtubule plus-end localization mechanism. However, the determinants in these serine-rich regions have not been identified. Here we elucidated multiple EB1-binding modules in the CLIP-170 N-terminal region. Using isothermal titration calorimetry and size-exclusion chromatography, we mapped and biophysically characterized these EB1-binding modules, including the two CAP-Gly domains, a bridging SXIP motif, and a unique array of divergent SXIP-like motifs located N-terminally to the first CAP-Gly domain. We found that, unlike the EB1-binding mode of the CAP-Gly domain in the dynactin-associated protein p150^Glued, which dually engages the EB1 C-terminal EEY motif as well as the EB homology domain and sterically occludes SXIP motif binding, the CLIP-170 CAP-Gly domains engage only the EEY motif, enabling the flanking SXIP and SXIP-like motifs to bind the EB homology domain. These multivalent EB1-binding modules provided avidity to the CLIP-170-EB1 interaction, likely clarifying why CLIP-170 preferentially binds EB1 rather than the α-tubulin C-terminal EEY motif. Our finding that CLIP-170 has multiple non-CAP-Gly EB1-binding modules may explain why autoinhibition of CLIP-170 GAP-Gly domains does not fully abrogate its microtubule plus-end localization. This work expands our understanding of EB1-binding motifs and their multivalent networks.

The microtubule-associated protein HURP recruits the centrosomal protein TACC3 to regulate K-fiber formation and support chromosome congression

Kinetochore fibers (K-fibers) are microtubule bundles attached to chromosomes. Efficient K-fiber formation is required for chromosome congression, crucial for faithful chromosome segregation in cells. However, the mechanisms underlying K-fiber formation before chromosome biorientation remain unclear. Depletion of hepatoma up-regulated protein (HURP), a RanGTP-dependent microtubule-associated protein localized on K-fibers, has been shown to result in low-efficiency K-fiber formation. Therefore, here we sought to identify critical interaction partners of HURP that may modulate this function. Using co-immunoprecipitation and bimolecular fluorescence complementation assays, we determined that HURP interacts directly with the centrosomal protein transforming acidic coiled coil-containing protein 3 (TACC3), a centrosomal protein, both in vivo and in vitro through the HURP^1-625 region. We found that HURP is important for TACC3 function during kinetochore microtubule assembly at the chromosome region in prometaphase. Moreover, HURP regulates stable lateral kinetochore attachment and chromosome congression in early mitosis by modulation of TACC3. These findings provide new insight into the coordinated regulation of K-fiber formation and chromosome congression in prometaphase by microtubule-associated proteins.

Functionally specific binding regions of microtubule-associated protein 2c exhibit distinct conformations and dynamics

Microtubule-associated protein 2c (MAP2c) is a 49-kDa intrinsically disordered protein regulating the dynamics of microtubules in developing neurons. MAP2c differs from its sequence homologue Tau in the pattern and kinetics of phosphorylation by cAMP-dependent protein kinase (PKA). Moreover, the mechanisms through which MAP2c interacts with its binding partners and the conformational changes and dynamics associated with these interactions remain unclear. Here, we used NMR relaxation and paramagnetic relaxation enhancement techniques to determine the dynamics and long-range interactions within MAP2c. The relaxation rates revealed large differences in flexibility of individual regions of MAP2c, with the lowest flexibility observed in the known and proposed binding sites. Quantitative conformational analyses of chemical shifts, small-angle X-ray scattering (SAXS), and paramagnetic relaxation enhancement measurements disclosed that MAP2c regions interacting with important protein partners, including Fyn tyrosine kinase, plectin, and PKA, adopt specific conformations. High populations of polyproline II and α-helices were found in Fyn- and plectin-binding sites of MAP2c, respectively. The region binding the regulatory subunit of PKA consists of two helical motifs bridged by a more extended conformation. Of note, although MAP2c and Tau did not differ substantially in their conformations in regions of high sequence identity, we found that they differ significantly in long-range interactions, dynamics, and local conformation motifs in their N-terminal domains. These results highlight that the N-terminal regions of MAP2c provide important specificity to its regulatory roles and indicate a close relationship between MAP2c's biological functions and conformational behavior.

The disorderly conduct of Hsc70 and its interaction with the Alzheimer's-related Tau protein

Hsp70 chaperones bind to various protein substrates for folding, trafficking, and degradation. Considerable structural information is available about how prokaryotic Hsp70 (DnaK) binds substrates, but less is known about mammalian Hsp70s, of which there are 13 isoforms encoded in the human genome. Here, we report the interaction between the human Hsp70 isoform heat shock cognate 71-kDa protein (Hsc70 or HSPA8) and peptides derived from the microtubule-associated protein Tau, which is linked to Alzheimer's disease. For structural studies, we used an Hsc70 construct (called BETA) comprising the substrate-binding domain but lacking the lid. Importantly, we found that truncating the lid does not significantly impair Hsc70's chaperone activity or allostery in vitro Using NMR, we show that BETA is partially dynamically disordered in the absence of substrate and that binding of the Tau sequence GKVQIINKKG (with a K_D = 500 nm) causes dramatic rigidification of BETA. NOE distance measurements revealed that Tau binds to the canonical substrate-binding cleft, similar to the binding observed with DnaK. To further develop BETA as a tool for studying Hsc70 interactions, we also measured BETA binding in NMR and fluorescent competition assays to peptides derived from huntingtin, insulin, a second Tau-recognition sequence, and a KFERQ-like sequence linked to chaperone-mediated autophagy. We found that the insulin C-peptide binds BETA with high affinity (K_D < 100 nm), whereas the others do not (K_D > 100 μm). Together, our findings reveal several similarities and differences in how prokaryotic and mammalian Hsp70 isoforms interact with different substrate peptides.

Structural basis for disassembly of katanin heterododecamers

The reorganization of microtubules in mitosis, meiosis, and development requires the microtubule-severing activity of katanin. Katanin is a heterodimer composed of an ATPase associated with diverse cellular activities (AAA) subunit and a regulatory subunit. Microtubule severing requires ATP hydrolysis by katanin's conserved AAA ATPase domains. Whereas other AAA ATPases form stable hexamers, we show that katanin forms only a monomer or dimers of heterodimers in solution. Katanin oligomers consistent with hexamers of heterodimers or heterododecamers were only observed for an ATP hydrolysis-deficient mutant in the presence of ATP. X-ray structures of katanin's AAA ATPase in monomeric nucleotide-free and pseudo-oligomeric ADP-bound states revealed conformational changes in the AAA subdomains that explained the structural basis for the instability of the katanin heterododecamer. We propose that the rapid dissociation of katanin AAA oligomers may lead to an autoinhibited state that prevents inappropriate microtubule severing or that cyclical disassembly into heterodimers may critically contribute to the microtubule-severing mechanism.

Annexins A2 and A6 interact with the extreme N terminus of tau and thereby contribute to tau's axonal localization

During neuronal development, the microtubule-associated protein tau becomes enriched in the axon, where it remains concentrated in the healthy brain. In tauopathies such as Alzheimer's disease, tau redistributes from the axon to the somatodendritic compartment. However, the cellular mechanism that regulates tau's localization remains unclear. We report here that tau interacts with the Ca²⁺-regulated plasma membrane-binding protein annexin A2 (AnxA2) via tau's extreme N terminus encoded by the first exon (E1). Bioinformatics analysis identified two conserved eight-amino-acids-long motifs within E1 in mammals. Using a heterologous yeast system, we found that disease-related mutations and pseudophosphorylation of Tyr-18, located within E1 but outside of the two conserved regions, do not influence tau's interaction with AnxA2. We further observed that tau interacts with the core domain of AnxA2 in a Ca²⁺-induced open conformation and interacts also with AnxA6. Moreover, lack of E1 moderately increased tau's association rate to microtubules, consistent with the supposition that the presence of the tau-annexin interaction reduces the availability of tau to interact with microtubules. Of note, intracellular competition through overexpression of E1-containing constructs reduced tau's axonal enrichment in primary neurons. Our results suggest that the E1-mediated tau-annexin interaction contributes to the enrichment of tau in the axon and is involved in its redistribution in pathological conditions.

The calcium-binding protein ALG-2 regulates protein secretion and trafficking via interactions with MISSL and MAP1B proteins

Mobilization of intracellular calcium is essential for a wide range of cellular processes, including signal transduction, apoptosis, and vesicular trafficking. Several lines of evidence have suggested that apoptosis-linked gene 2 (ALG-2, also known as PDCD6), a calcium-binding protein, acts as a calcium sensor linking calcium levels with efficient vesicular trafficking, especially at the endoplasmic reticulum (ER)-to-Golgi transport step. However, how ALG-2 regulates these processes remains largely unclear. Here, we report that MAPK1-interacting and spindle-stabilizing (MISS)-like (MISSL), a previously uncharacterized protein, interacts with ALG-2 in a calcium-dependent manner. Live-cell imaging revealed that upon a rise in intracellular calcium levels, GFP-tagged MISSL (GFP-MISSL) dynamically relocalizes in a punctate pattern and colocalizes with ALG-2. MISSL knockdown caused disorganization of the components of the ER exit site, the ER-Golgi intermediate compartment, and Golgi. Importantly, knockdown of either MISSL or ALG-2 attenuated the secretion of secreted alkaline phosphatase (SEAP), a model secreted cargo protein, with similar reductions in secretion by single- and double-protein knockdowns, suggesting that MISSL and ALG-2 act in the same pathway to regulate the secretion process. Furthermore, ALG-2 or MISSL knockdown delayed ER-to-Golgi transport of procollagen type I. We also found that ALG-2 and MISSL interact with microtubule-associated protein 1B (MAP1B) and that MAP1B knockdown reverts the reduced secretion of SEAP caused by MISSL or ALG-2 depletion. These results suggest that a change in the intracellular calcium level plays a role in regulation of the secretory pathway via interaction of ALG-2 with MISSL and MAP1B.

Microtubule plus-end tracking of end-binding protein 1 (EB1) is regulated by CDK5 regulatory subunit-associated protein 2

Microtubules are polar cytoskeleton filaments that extend via growth at their plus ends. Microtubule plus-end-tracking proteins (+TIPs) accumulate at these growing plus ends to control microtubule dynamics and attachment. The +TIP end-binding protein 1 (EB1) and its homologs possess an autonomous plus-end-tracking mechanism and interact with other known +TIPs, which then recruit those +TIPs to the growing plus ends. A major +TIP class contains the SXIP (Ser-X-Ile-Pro, with X denoting any amino acid residue) motif, known to interact with EB1 and its homologs for plus-end tracking, but the role of SXIP in regulating EB1 activities is unclear. We show here that an interaction of EB1 with the SXIP-containing +TIP CDK5 regulatory subunit-associated protein 2 (CDK5RAP2) regulates several EB1 activities, including microtubule plus-end tracking, dynamics at microtubule plus ends, microtubule and α/β-tubulin binding, and microtubule polymerization. The SXIP motif fused with a dimerization domain from CDK5RAP2 significantly enhanced EB1 plus-end-tracking and microtubule-polymerizing and bundling activities, but the SXIP motif alone failed to do so. An SXIP-binding-deficient EB1 mutant displayed significantly lower microtubule plus-end tracking than the wild-type protein in transfected cells. These results suggest that EB1 cooperates with CDK5RAP2 and perhaps other SXIP-containing +TIPs in tracking growing microtubule tips. We also generated plus-end-tracking chimeras of CDK5RAP2 and the adenomatous polyposis coli protein (APC) and found that overexpression of the dimerization domains interfered with microtubule plus-end tracking of their respective SXIP-containing chimeras. Our results suggest that disruption of SXIP dimerization enables detailed investigations of microtubule plus-end-associated functions of individual SXIP-containing +TIPs.

Quantitative mapping of microtubule-associated protein 2c (MAP2c) phosphorylation and regulatory protein 14-3-3ζ-binding sites reveals key differences between MAP2c and its homolog Tau

Microtubule-associated protein 2c (MAP2c) is involved in neuronal development and is less characterized than its homolog Tau, which has various roles in neurodegeneration. Using NMR methods providing single-residue resolution and quantitative comparison, we investigated molecular interactions important for the regulatory roles of MAP2c in microtubule dynamics. We found that MAP2c and Tau significantly differ in the position and kinetics of sites that are phosphorylated by cAMP-dependent protein kinase (PKA), even in highly homologous regions. We determined the binding sites of unphosphorylated and phosphorylated MAP2c responsible for interactions with the regulatory protein 14-3-3ζ. Differences in phosphorylation and in charge distribution between MAP2c and Tau suggested that both MAP2c and Tau respond to the same signal (phosphorylation by PKA) but have different downstream effects, indicating a signaling branch point for controlling microtubule stability. Although the interactions of phosphorylated Tau with 14-3-3ζ are supposed to be a major factor in microtubule destabilization, the binding of 14-3-3ζ to MAP2c enhanced by PKA-mediated phosphorylation is likely to influence microtubule-MAP2c binding much less, in agreement with the results of our tubulin co-sedimentation measurements. The specific location of the major MAP2c phosphorylation site in a region homologous to the muscarinic receptor-binding site of Tau suggests that MAP2c also may regulate processes other than microtubule dynamics.

The Microtubule Plus End Tracking Protein TIP150 Interacts with Cortactin to Steer Directional Cell Migration

Cell migration is orchestrated by dynamic interactions of microtubules with the plasma membrane cortex. How these interactions facilitate these dynamic processes is still being actively investigated. TIP150 is a newly characterized microtubule plus end tracking protein essential for mitosis and entosis (Ward, T., Wang, M., Liu, X., Wang, Z., Xia, P., Chu, Y., Wang, X., Liu, L., Jiang, K., Yu, H., Yan, M., Wang, J., Hill, D. L., Huang, Y., Zhu, T., and Yao, X. (2013) Regulation of a dynamic interaction between two microtubule-binding proteins, EB1 and TIP150, by the mitotic p300/CBP-associated factor (PCAF) orchestrates kinetochore microtubule plasticity and chromosome stability during mitosis. J. Biol. Chem. 288, 15771-15785; Xia, P., Zhou, J., Song, X., Wu, B., Liu, X., Li, D., Zhang, S., Wang, Z., Yu, H., Ward, T., Zhang, J., Li, Y., Wang, X., Chen, Y., Guo, Z., and Yao, X. (2014) Aurora A orchestrates entosis by regulating a dynamic MCAK-TIP150 interaction. J. Mol. Cell Biol. 6, 240-254). Here we show that TIP150 links dynamic microtubules to steer cell migration by interacting with cortactin. Mechanistically, TIP150 binds to cortactin via its C-terminal tail. Interestingly, the C-terminal TIP150 proline-rich region (CT150) binds to the Src homology 3 domain of cortactin specifically, and such an interaction is negatively regulated by EGF-elicited tyrosine phosphorylation of cortactin. Importantly, suppression of TIP150 or overexpression of phospho-mimicking cortactin inhibits polarized cell migration. In addition, CT150 disrupts the biochemical interaction between TIP150 and cortactin in vitro, and perturbation of the TIP150-cortactin interaction in vivo using a membrane-permeable TAT-CT150 peptide results in an inhibition of directional cell migration. We reason that a dynamic TIP150-cortactin interaction orchestrates directional cell migration via coupling dynamic microtubule plus ends to the cortical cytoskeleton.

Crystal Structures of the Human Doublecortin C- and N-terminal Domains in Complex with Specific Antibodies

Doublecortin is a microtubule-associated protein produced during neurogenesis. The protein stabilizes microtubules and stimulates their polymerization, which allows migration of immature neurons to their designated location in the brain. Mutations in the gene that impair doublecortin function and cause severe brain formation disorders are located on a tandem repeat of two doublecortin domains. The molecular mechanism of action of doublecortin is only incompletely understood. Anti-doublecortin antibodies, such as the rabbit polyclonal Abcam 18732, are widely used as neurogenesis markers. Here, we report the generation and characterization of antibodies that bind to single doublecortin domains. The antibodies were used as tools to obtain structures of both domains. Four independent crystal structures of the N-terminal domain reveal several distinct open and closed conformations of the peptide linking N- and C-terminal domains, which can be related to doublecortin function. An NMR assignment and a crystal structure in complex with a camelid antibody fragment show that the doublecortin C-terminal domain adopts the same well defined ubiquitin-like fold as the N-terminal domain, despite its reported aggregation and molten globule-like properties. The antibodies' unique domain specificity also renders them ideal research tools to better understand the role of individual domains in doublecortin function. A single chain camelid antibody fragment specific for the C-terminal doublecortin domain affected microtubule binding, whereas a monoclonal mouse antibody specific for the N-terminal domain did not. Together with steric considerations, this suggests that the microtubule-interacting doublecortin domain observed in cryo-electron micrographs is the C-terminal domain rather than the N-terminal one.

Mitotic Protein CSPP1 Interacts with CENP-H Protein to Coordinate Accurate Chromosome Oscillation in Mitosis

Mitotic chromosome segregation is orchestrated by the dynamic interaction of spindle microtubules with the kinetochores. During chromosome alignment, kinetochore-bound microtubules undergo dynamic cycles between growth and shrinkage, leading to an oscillatory movement of chromosomes along the spindle axis. Although kinetochore protein CENP-H serves as a molecular control of kinetochore-microtubule dynamics, the mechanistic link between CENP-H and kinetochore microtubules (kMT) has remained less characterized. Here, we show that CSPP1 is a kinetochore protein essential for accurate chromosome movements in mitosis. CSPP1 binds to CENP-H in vitro and in vivo. Suppression of CSPP1 perturbs proper mitotic progression and compromises the satisfaction of spindle assembly checkpoint. In addition, chromosome oscillation is greatly attenuated in CSPP1-depleted cells, similar to what was observed in the CENP-H-depleted cells. Importantly, CSPP1 depletion enhances velocity of kinetochore movement, and overexpression of CSPP1 decreases the speed, suggesting that CSPP1 promotes kMT stability during cell division. Specific perturbation of CENP-H/CSPP1 interaction using a membrane-permeable competing peptide resulted in a transient mitotic arrest and chromosome segregation defect. Based on these findings, we propose that CSPP1 cooperates with CENP-H on kinetochores to serve as a novel regulator of kMT dynamics for accurate chromosome segregation.

TPX2 Inhibits Eg5 by Interactions with Both Motor and Microtubule

The microtubule-associated protein, TPX2, regulates the activity of the mitotic kinesin, Eg5, but the mechanism of regulation is not established. Using total internal reflection fluorescence microscopy, we observed that Eg5, in extracts of mammalian cells expressing Eg5-EGFP, moved processively toward the microtubule plus-end at an average velocity of 14 nm/s. TPX2 bound to microtubules with an apparent dissociation constant of ∼ 200 nm, and microtubule binding was not dependent on the C-terminal tails of tubulin. Using single molecule assays, we found that full-length TPX2 dramatically reduced Eg5 velocity, whereas truncated TPX2, which lacks the domain that is required for the interaction with Eg5, was a less effective inhibitor at the same concentration. To determine the region(s) of Eg5 that is required for interaction with TPX2, we performed microtubule gliding assays. Dimeric, but not monomeric, Eg5 was differentially inhibited by full-length and truncated TPX2, demonstrating that dimerization or residues in the neck region are important for the interaction of TPX2 with Eg5. These results show that both microtubule binding and interaction with Eg5 contribute to motor inhibition by TPX2 and demonstrate the utility of mammalian cell extracts for biophysical assays.

CLASP2 Has Two Distinct TOG Domains That Contribute Differently to Microtubule Dynamics

CLIP-associated proteins CLASPs are mammalian microtubule (MT) plus-end tracking proteins (+TIPs) that promote MT rescue in vivo. Their plus-end localization is dependent on other +TIPs, EB1 and CLIP-170, but in the leading edge of the cell, CLASPs display lattice-binding activity. MT association of CLASPs is suggested to be regulated by multiple TOG (tumor overexpressed gene) domains and by the serine-arginine (SR)-rich region, which contains binding sites for EB1. Here, we report the crystal structures of the two TOG domains of CLASP2. Both domains consist of six HEAT repeats, which are similar to the canonical paddle-like tubulin-binding TOG domains, but have arched conformations. The degrees and directions of curvature are different between the two TOG domains, implying that they have distinct roles in MT binding. Using biochemical, molecular modeling and cell biological analyses, we have investigated the interactions between the TOG domains and αβ-tubulin and found that each domain associates differently with αβ-tubulin. Our findings suggest that, by varying the degrees of domain curvature, the TOG domains may distinguish the structural conformation of the tubulin dimer, discriminate between different states of MT dynamic instability and thereby function differentially as stabilizers of MTs.

Building the Microtubule Cytoskeleton Piece by Piece

The microtubule (MT) cytoskeleton gives cells their shape, organizes the cellular interior, and segregates chromosomes. These functions rely on the precise arrangement of MTs, which is achieved by the coordinated action of MT-associated proteins (MAPs). We highlight the first and most important examples of how different MAP activities are combined in vitro to create an ensemble function that exceeds the simple addition of their individual activities, and how the Xenopus laevis egg extract system has been utilized as a powerful intermediate between cellular and purified systems to uncover the design principles of self-organized MT networks in the cell.

Writing and Reading the Tubulin Code

Microtubules give rise to intracellular structures with diverse morphologies and dynamics that are crucial for cell division, motility, and differentiation. They are decorated with abundant and chemically diverse posttranslational modifications that modulate their stability and interactions with cellular regulators. These modifications are important for the biogenesis and maintenance of complex microtubule arrays such as those found in spindles, cilia, neuronal processes, and platelets. Here we discuss the nature and subcellular distribution of these posttranslational marks whose patterns have been proposed to constitute a tubulin code that is interpreted by cellular effectors. We review the enzymes responsible for writing the tubulin code, explore their functional consequences, and identify outstanding challenges in deciphering the tubulin code.

Parvulin 17-catalyzed Tubulin Polymerization Is Regulated by Calmodulin in a Calcium-dependent Manner

Recently we have shown that the peptidyl-prolyl cis/trans isomerase parvulin 17 (Par17) interacts with tubulin in a GTP-dependent manner, thereby promoting the formation of microtubules. Microtubule assembly is regulated by Ca(2+)-loaded calmodulin (Ca(2+)/CaM) both in the intact cell and under in vitro conditions via direct interaction with microtubule-associated proteins. Here we provide the first evidence that Ca(2+)/CaM interacts also with Par17 in a physiologically relevant way, thus preventing Par17-promoted microtubule assembly. In contrast, parvulin 14 (Par14), which lacks only the first 25 N-terminal residues of the Par17 sequence, does not interact with Ca(2+)/CaM, indicating that this interaction is exclusive for Par17. Pulldown experiments and chemical shift perturbation analysis with (15)N-labeled Par17 furthermore confirmed that calmodulin (CaM) interacts in a Ca(2+)-dependent manner with the Par17 N terminus. The reverse experiment with (15)N-labeled Ca(2+)/CaM demonstrated that the N-terminal Par17 segment binds to both CaM lobes simultaneously, indicating that Ca(2+)/CaM undergoes a conformational change to form a binding channel between its two lobes, apparently similar to the structure of the CaM-smMLCK(796-815) complex. In vitro tubulin polymerization assays furthermore showed that Ca(2+)/CaM completely suppresses Par17-promoted microtubule assembly. The results imply that Ca(2+)/CaM binding to the N-terminal segment of Par17 causes steric hindrance of the Par17 active site, thus interfering with the Par17/tubulin interaction. This Ca(2+)/CaM-mediated control of Par17-assisted microtubule assembly may provide a mechanism that couples Ca(2+) signaling with microtubule function.