Structural basis for tubulin recognition by cytoplasmic linker protein 170 and its autoinhibition

Targeting the tubulin C-terminal tail by charged small molecules

The disordered tubulin C-terminal tail (CTT), which possesses a higher degree of heterogeneity, is the target for the interaction of many proteins and cellular components. Compared to the seven well-described binding sites of microtubule-targeting agents (MTAs) that localize on the globular tubulin core, tubulin CTT is far less explored. Therefore, tubulin CTT can be regarded as a novel site for the development of MTAs with distinct biochemical and cell biological properties. Here, we designed and synthesized linear and cyclic peptides containing multiple arginines (RRR), which are complementary to multiple acidic residues in tubulin CTT. Some of them showed moderate induction and promotion of tubulin polymerization. The most potent macrocyclic compound 1f was found to bind to tubulin CTT and thus exert its bioactivity. Such RRR containing compounds represent a starting point for the discovery of tubulin CTT-targeting agents with therapeutic potential.

EML2-S constitutes a new class of proteins that recognizes and regulates the dynamics of tyrosinated microtubules

Tubulin post-translational modifications (PTMs) alter microtubule properties by affecting the binding of microtubule-associated proteins (MAPs). Microtubule detyrosination, which occurs by proteolytic removal of the C-terminal tyrosine from ɑ-tubulin, generates the oldest known tubulin PTM, but we lack comprehensive knowledge of MAPs that are regulated by this PTM. We developed a screening pipeline to identify proteins that discriminate between Y- and ΔY-microtubules and found that echinoderm microtubule-associated protein-like 2 (EML2) preferentially interacts with Y-microtubules. This activity depends on a Y-microtubule interaction motif built from WD40 repeats. We show that EML2 tracks the tips of shortening microtubules, a behavior not previously seen among human MAPs in vivo, and influences dynamics to increase microtubule stability. Our screening pipeline is readily adapted to identify proteins that specifically recognize a wide range of microtubule PTMs.

The CLIP-170 N-terminal domain binds directly to both F-actin and microtubules in a mutually exclusive manner

The cooperation between the actin and microtubule (MT) cytoskeletons is important for cellular processes such as cell migration and muscle cell development. However, a full understanding of how this cooperation occurs has yet to be sufficiently developed. The MT plus-end tracking protein CLIP-170 has been implicated in this actin-MT coordination by associating with the actin-binding signaling protein IQGAP1 and by promoting actin polymerization through binding with formins. Thus far, the interactions of CLIP-170 with actin were assumed to be indirect. Here, we demonstrate using high-speed cosedimentation assays that CLIP-170 can bind to filamentous actin (F-actin) directly. We found that the affinity of this binding is relatively weak but strong enough to be significant in the actin-rich cortex, where actin concentrations can be extremely high. Using CLIP-170 fragments and mutants, we show that the direct CLIP-170-F-actin interaction is independent of the FEED domain, the region that mediates formin-dependent actin polymerization, and that the CLIP-170 F-actin-binding region overlaps with the MT-binding region. Consistent with these observations, in vitro competition assays indicate that CLIP-170-F-actin and CLIP-170-MT interactions are mutually exclusive. Taken together, these observations lead us to speculate that direct CLIP-170-F-actin interactions may function to reduce the stability of MTs in actin-rich regions of the cell, as previously proposed for MT end-binding protein 1.

In-Cell Structural Biology by NMR: The Benefits of the Atomic Scale

In-cell structural biology aims at extracting structural information about proteins or nucleic acids in their native, cellular environment. This emerging field holds great promise and is already providing new facts and outlooks of interest at both fundamental and applied levels. NMR spectroscopy has important contributions on this stage: It brings information on a broad variety of nuclei at the atomic scale, which ensures its great versatility and uniqueness. Here, we detail the methods, the fundamental knowledge, and the applications in biomedical engineering related to in-cell structural biology by NMR. We finally propose a brief overview of the main other techniques in the field (EPR, smFRET, cryo-ET, etc.) to draw some advisable developments for in-cell NMR. In the era of large-scale screenings and deep learning, both accurate and qualitative experimental evidence are as essential as ever to understand the interior life of cells. In-cell structural biology by NMR spectroscopy can generate such a knowledge, and it does so at the atomic scale. This review is meant to deliver comprehensive but accessible information, with advanced technical details and reflections on the methods, the nature of the results, and the future of the field.

Overexpression of the microtubule-binding protein CLIP-170 induces a +TIP network superstructure consistent with a biomolecular condensate

Proper regulation of microtubule (MT) dynamics is critical for cellular processes including cell division and intracellular transport. Plus-end tracking proteins (+TIPs) dynamically track growing MTs and play a key role in MT regulation. +TIPs participate in a complex web of intra- and inter- molecular interactions known as the +TIP network. Hypotheses addressing the purpose of +TIP:+TIP interactions include relieving +TIP autoinhibition and localizing MT regulators to growing MT ends. In addition, we have proposed that the web of +TIP:+TIP interactions has a physical purpose: creating a dynamic scaffold that constrains the structural fluctuations of the fragile MT tip and thus acts as a polymerization chaperone. Here we examine the possibility that this proposed scaffold is a biomolecular condensate (i.e., liquid droplet). Many animal +TIP network proteins are multivalent and have intrinsically disordered regions, features commonly found in biomolecular condensates. Moreover, previous studies have shown that overexpression of the +TIP CLIP-170 induces large “patch” structures containing CLIP-170 and other +TIPs; we hypothesized that these structures might be biomolecular condensates. To test this hypothesis, we used video microscopy, immunofluorescence staining, and Fluorescence Recovery After Photobleaching (FRAP). Our data show that the CLIP-170-induced patches have hallmarks indicative of a biomolecular condensate, one that contains +TIP proteins and excludes other known condensate markers. Moreover, bioinformatic studies demonstrate that the presence of intrinsically disordered regions is conserved in key +TIPs, implying that these regions are functionally significant. Together, these results indicate that the CLIP-170 induced patches in cells are phase-separated liquid condensates and raise the possibility that the endogenous +TIP network might form a liquid droplet at MT ends or other +TIP locations.

α-tubulin tail modifications regulate microtubule stability through selective effector recruitment, not changes in intrinsic polymer dynamics

Microtubules are non-covalent polymers of αβ-tubulin dimers. Posttranslational processing of the intrinsically disordered C-terminal α-tubulin tail produces detyrosinated and Δ2-tubulin. Although these are widely employed as proxies for stable cellular microtubules, their effect (and of the α-tail) on microtubule dynamics remains uncharacterized. Using recombinant, engineered human tubulins, we now find that neither detyrosinated nor Δ2-tubulin affect microtubule dynamics, while the α-tubulin tail is an inhibitor of microtubule growth. Consistent with the latter, molecular dynamics simulations show the α-tubulin tail transiently occluding the longitudinal microtubule polymerization interface. The marked differential in vivo stabilities of the modified microtubule subpopulations, therefore, must result exclusively from selective effector recruitment. We find that tyrosination quantitatively tunes CLIP-170 density at the growing plus end and that CLIP170 and EB1 synergize to selectively upregulate the dynamicity of tyrosinated microtubules. Modification-dependent recruitment of regulators thereby results in microtubule subpopulations with distinct dynamics, a tenet of the tubulin code hypothesis.

Genetically encoded live-cell sensor for tyrosinated microtubules

Microtubule cytoskeleton exists in various biochemical forms in different cells due to tubulin posttranslational modifications (PTMs). Tubulin PTMs are known to affect microtubule stability, dynamics, and interaction with MAPs and motors in a specific manner, widely known as tubulin code hypothesis. At present, there exists no tool that can specifically mark tubulin PTMs in living cells, thus severely limiting our understanding of their dynamics and cellular functions. Using a yeast display library, we identified a binder against terminal tyrosine of α-tubulin, a unique PTM site. Extensive characterization validates the robustness and nonperturbing nature of our binder as tyrosination sensor, a live-cell tubulin nanobody specific towards tyrosinated microtubules. Using this sensor, we followed nocodazole-, colchicine-, and vincristine-induced depolymerization events of tyrosinated microtubules in real time and found each distinctly perturbs the microtubule polymer. Together, our work describes a novel tyrosination sensor and its potential applications to study the dynamics of microtubule and their PTM processes in living cells.

Stress-induced phosphorylation of CLIP-170 by JNK promotes microtubule rescue

The stress-induced c-Jun N-terminal kinase (JNK) controls microtubule dynamics by enhancing both microtubule growth and rescues. Here, we show that upon cell stress, JNK directly phosphorylates the microtubule rescue factor CLIP-170 in its microtubule-binding domain to increase its rescue-promoting activity. Phosphomimetic versions of CLIP-170 enhance its ability to promote rescue events in vitro and in cells. Furthermore, while phosphomimetic mutations do not alter CLIP-170’s capability to form comets at growing microtubule ends, both phosphomimetic mutations and JNK activation increase the occurrence of CLIP-170 remnants on the microtubule lattice at the rear of comets. As the CLIP-170 remnants, which are potential sites of microtubule rescue, display a shorter lifetime when CLIP-170 is phosphorylated, we propose that instead of acting at the time of rescue occurrence, CLIP-170 would rather contribute in preparing the microtubule lattice for future rescues at these predetermined sites.

Tubulin mutations in neurodevelopmental disorders as a tool to decipher microtubule function

Malformations of cortical development (MCDs) are a group of severe brain malformations associated with intellectual disability and refractory childhood epilepsy. Human missense heterozygous mutations in the 9 α-tubulin and 10 β-tubulin isoforms forming the heterodimers that assemble into microtubules (MTs) were found to cause MCDs. However, how a single mutated residue in a given tubulin isoform can perturb the entire microtubule population in a neuronal cell remains a crucial question. Here, we examined 85 MCD-associated tubulin mutations occurring in TUBA1A, TUBB2, and TUBB3 and their location in a three-dimensional (3D) microtubule cylinder. Mutations hitting residues exposed on the outer microtubule surface are likely to alter microtubule association with partners, while alteration of intradimer contacts may impair dimer stability and straightness. Other types of mutations are predicted to alter interdimer and lateral contacts, which are responsible for microtubule cohesion, rigidity, and dynamics. MCD-associated tubulin mutations surprisingly fall into all categories, thus providing unexpected insights into how a single mutation may impair microtubule function and elicit dominant effects in neurons.

Arsenic trioxide disturbs the LIS1/NDEL1/dynein microtubule dynamic complex by disrupting the CLIP170 zinc finger in head and neck cancer

Cancer mortality is mainly caused by metastasis, which requires dynamic remodeling of cytoskeletal components such as microtubules. Targeting microtubules presents a promising antimetastatic strategy that could prevent cancer spreading and recurrence. It is known that arsenic trioxide (ATO) is able to inhibit the migration and invasion of solid malignant tumors, but its exact molecular mechanism remains unclear. Here, we report a novel molecular target and antimetastatic mechanism of ATO in head and neck squamous cell carcinoma (HNSCC). We found that cytoplasmic linker protein 170 (CLIP170) was overexpressed in HNSCC tissues and cells compared to normal controls. ATO at non-cytotoxic level (1 μM) inhibited the migration and invasion of HNSCC cells by displacing zinc in the zinc finger motif of CLIP170, which is a key protein that controls microtubule dynamics. The antimetastatic effects of ATO were equivalent to those of siRNA-mediated CLIP170 knockdown. Furthermore, ATO dysregulated microtubule polymerization via the CLIP170/LIS1/NDEL1/dynein signaling pathway in Cal27 cells as a functional consequence of CLIP170 zinc finger disruption. The effect was partially reversed by zinc supplementation. Taken together, these findings reveal that CLIP170 is a novel molecular target of ATO and demonstrate the capability and underlying mechanisms of ATO as a potential antimetastatic agent for HNSCC treatment.

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 IQ67 DOMAIN5 Modulates Microtubule Dynamics and Pavement Cell Shape

The dynamic arrangement of cortical microtubules (MTs) plays a pivotal role in controlling cell growth and shape formation in plants, but the mechanisms by which cortical MTs are organized to regulate these processes are not well characterized. In particular, the dynamic behavior of cortical MTs is critical for their spatial organization, yet the molecular mechanisms controlling MT dynamics remain poorly understood. In this study, we used the puzzle piece-shaped pavement cells of Arabidopsis (Arabidopsis thaliana) leaves as a model system in which to study cortical MT organization. We isolated an ethyl methanesulfonate mutant with reduced interdigitation of pavement cells in cotyledons. This line carried a mutation in IQ67 DOMAIN5 (IQD5), which encodes a member of the plant-specific IQ motif protein family. Live-cell imaging and biochemical analyses demonstrated that IQD5 binds to MTs and promotes MT assembly. MT-depolymerizing drug treatment and in vivo MT dynamics assays suggested that IQD5 functions to stabilize MTs. Hence, our findings provide genetic, cell biological, and biochemical evidence that IQD5 regulates MT dynamics that affect MT organization and subsequent cell shape formation.

Structure-Function Relationship of the Bik1-Bim1 Complex

In budding yeast, the microtubule plus-end tracking proteins Bik1 (CLIP-170) and Bim1 (EB1) form a complex that interacts with partners involved in spindle positioning, including Stu2 and Kar9. Here, we show that the CAP-Gly and coiled-coil domains of Bik1 interact with the C-terminal ETF peptide of Bim1 and the C-terminal tail region of Stu2, respectively. The crystal structures of the CAP-Gly domain of Bik1 (Bik1CG) alone and in complex with an ETF peptide revealed unique, functionally relevant CAP-Gly elements, establishing Bik1CG as a specific C-terminal phenylalanine recognition domain. Unlike the mammalian CLIP-170-EB1 complex, Bik1-Bim1 forms ternary complexes with the EB1-binding motifs SxIP and LxxPTPh, which are present in diverse proteins, including Kar9. Perturbation of the Bik1-Bim1 interaction in vivo affected Bik1 localization and astral microtubule length. Our results provide insight into the role of the Bik1-Bim1 interaction for cell division, and demonstrate that the CLIP-170-EB1 module is evolutionarily flexible.

Motor axon navigation relies on Fidgetin-like 1-driven microtubule plus end dynamics

Fassier et al. identify Fidgetin-like 1 (Fignl1) as a key growth cone (GC)-enriched microtubule (MT)-associated protein in motor circuit wiring. They show that Fignl1 modulates motor GC morphology and steering behavior by down-regulating EB binding at MT plus ends and promoting MT depolymerization beneath the cell cortex.

During neural circuit assembly, extrinsic signals are integrated into changes in growth cone (GC) cytoskeleton underlying axon guidance decisions. Microtubules (MTs) were shown to play an instructive role in GC steering. However, the numerous actors required for MT remodeling during axon navigation and their precise mode of action are far from being deciphered. Using loss- and gain-of-function analyses during zebrafish development, we identify in this study the meiotic clade adenosine triphosphatase Fidgetin-like 1 (Fignl1) as a key GC-enriched MT-interacting protein in motor circuit wiring and larval locomotion. We show that Fignl1 controls GC morphology and behavior at intermediate targets by regulating MT plus end dynamics and growth directionality. We further reveal that alternative translation of Fignl1 transcript is a sophisticated mechanism modulating MT dynamics: a full-length isoform regulates MT plus end–tracking protein binding at plus ends, whereas shorter isoforms promote their depolymerization beneath the cell cortex. Our study thus pinpoints Fignl1 as a multifaceted key player in MT remodeling underlying motor circuit connectivity.

Poly(ADP-ribose) mediates asymmetric division of mouse oocyte

Before fertilization, mammalian oocyte undergoes an asymmetric division which depends on eccentric positioning of the spindle at the oocyte cortex to form a polar body and an egg. Since the centriole is absent and, as a result, the polar array microtubules are not fully developed in oocytes, microtubules have seldom been considered as required for eccentric positioning of the spindle, while actin-related forces have instead been proposed to be primarily responsible for this process. However, the existing models are largely conflicting and the underlying mechanism of asymmetric division is still elusive. Here we show that poly(ADP-ribose) (PAR) is enriched at mouse oocyte cortical area throughout meiosis. Specific removal of cortical PAR results in an ectopic spindle and a failure of asymmetric division. During spindle migration, when the spindle deviates from the center of oocyte by a pushing force of cytoplasmic actin, the short polar array microtubules emanating from the juxtacortical spindle pole extend to the cortex and penetrate into cortical PAR, docking and stabilizing the spindle at the cortex which facilitates the asymmetric division. This process depends on the affinity between PAR and microtubule-associated proteins such as Spindly, which contributes to a physical link for cortical PAR and the spindle. Notably, fusing a PAR-binding domain to end-binding protein 3, a plus-end tracking protein at the polar array microtubules, restores the asymmetric division of oocytes with Spindly knockdown. Thus, our work demonstrates a comprehensive mechanism for oocyte spindle positioning and asymmetric division.

Tubulin Post-Translational Modifications and Microtubule Dynamics

Microtubules are hollow tube-like polymeric structures composed of α,β-tubulin heterodimers. They play an important role in numerous cellular processes, including intracellular transport, cell motility and segregation of the chromosomes during cell division. Moreover, microtubule doublets or triplets form a scaffold of a cilium, centriole and basal body, respectively. To perform such diverse functions microtubules have to differ in their properties. Post-translational modifications are one of the factors that affect the properties of the tubulin polymer. Here we focus on the direct and indirect effects of post-translational modifications of tubulin on microtubule dynamics.

Understanding force-generating microtubule systems through in vitro reconstitution

Microtubules switch between growing and shrinking states, a feature known as dynamic instability. The biochemical parameters underlying dynamic instability are modulated by a wide variety of microtubule-associated proteins that enable the strict control of microtubule dynamics in cells. The forces generated by controlled growth and shrinkage of microtubules drive a large range of processes, including organelle positioning, mitotic spindle assembly, and chromosome segregation. In the past decade, our understanding of microtubule dynamics and microtubule force generation has progressed significantly. Here, we review the microtubule-intrinsic process of dynamic instability, the effect of external factors on this process, and how the resulting forces act on various biological systems. Recently, reconstitution-based approaches have strongly benefited from extensive biochemical and biophysical characterization of individual components that are involved in regulating or transmitting microtubule-driven forces. We will focus on the current state of reconstituting increasingly complex biological systems and provide new directions for future developments.

Dynactin binding to tyrosinated microtubules promotes centrosome centration in C. elegans by enhancing dynein-mediated organelle transport

The microtubule-based motor dynein generates pulling forces for centrosome centration and mitotic spindle positioning in animal cells. How the essential dynein activator dynactin regulates these functions of the motor is incompletely understood. Here, we dissect the role of dynactin's microtubule binding activity, located in the p150 CAP-Gly domain and an adjacent basic patch, in the C. elegans zygote. Analysis of p150 mutants engineered by genome editing suggests that microtubule tip tracking of dynein-dynactin is dispensable for targeting the motor to the cell cortex and for generating robust cortical pulling forces. Instead, mutations in p150's CAP-Gly domain inhibit cytoplasmic pulling forces responsible for centration of centrosomes and attached pronuclei. The centration defects are mimicked by mutations of α-tubulin's C-terminal tyrosine, and both p150 CAP-Gly and tubulin tyrosine mutants decrease the frequency of early endosome transport from the cell periphery towards centrosomes during centration. Our results suggest that p150 GAP-Gly domain binding to tyrosinated microtubules promotes initiation of dynein-mediated organelle transport in the dividing one-cell embryo, and that this function of p150 is critical for generating cytoplasmic pulling forces for centrosome centration.

The IQD Family of Calmodulin-Binding Proteins Links Calcium Signaling to Microtubules, Membrane Subdomains, and the Nucleus

Calcium (Ca2+) signaling and dynamic reorganization of the cytoskeleton are essential processes for the coordination and control of plant cell shape and cell growth. Calmodulin (CaM) and closely related calmodulin-like (CML) polypeptides are principal sensors of Ca2+ signals. CaM/CMLs decode and relay information encrypted by the second messenger via differential interactions with a wide spectrum of targets to modulate their diverse biochemical activities. The plant-specific IQ67 DOMAIN (IQD) family emerged as possibly the largest class of CaM-interacting proteins with undefined molecular functions and biological roles. Here, we show that the 33 members of the IQD family in Arabidopsis (Arabidopsis thaliana) differentially localize, using green fluorescent protein (GFP)-tagged proteins, to multiple and distinct subcellular sites, including microtubule (MT) arrays, plasma membrane subdomains, and nuclear compartments. Intriguingly, the various IQD-specific localization patterns coincide with the subcellular patterns of IQD-dependent recruitment of CaM, suggesting that the diverse IQD members sequester Ca2+-CaM signaling modules to specific subcellular sites for precise regulation of Ca2+-dependent processes. Because MT localization is a hallmark of most IQD family members, we quantitatively analyzed GFP-labeled MT arrays in Nicotiana benthamiana cells transiently expressing GFP-IQD fusions and observed IQD-specific MT patterns, which point to a role of IQDs in MT organization and dynamics. Indeed, stable overexpression of select IQD proteins in Arabidopsis altered cellular MT orientation, cell shape, and organ morphology. Because IQDs share biochemical properties with scaffold proteins, we propose that IQD families provide an assortment of platform proteins for integrating CaM-dependent Ca2+ signaling at multiple cellular sites to regulate cell function, shape, and growth.

Towards understanding cellular structure biology: In-cell NMR

To watch biological macromolecules perform their functions inside the living cells is the dream of any biologists. In-cell nuclear magnetic resonance is a branch of biomolecular NMR spectroscopy that can be used to observe the structures, interactions and dynamics of these molecules in the living cells at atomic level. In principle, in-cell NMR can be applied to different cellular systems to achieve biologically relevant structural and functional information. In this review, we summarize the existing approaches in this field and discuss its applications in protein interactions, folding, stability and post-translational modifications. We hope this review will emphasize the effectiveness of in-cell NMR for studies of intricate biological processes and for structural analysis in cellular environments.

Molecular Determinants of Tubulin's C-Terminal Tail Conformational Ensemble

Tubulin is important for a wide variety of cellular processes including cell division, ciliogenesis, and intracellular trafficking. To perform these diverse functions, tubulin is regulated by post-translational modifications (PTM), primarily at the C-terminal tails of both the α- and β-tubulin heterodimer subunits. The tubulin C-terminal tails are disordered segments that are predicted to extend from the ordered tubulin body and may regulate both intrinsic properties of microtubules and the binding of microtubule associated proteins (MAP). It is not understood how either interactions with the ordered tubulin body or PTM affect tubulin's C-terminal tails. To probe these questions, we developed a method to isotopically label tubulin for C-terminal tail structural studies by NMR. The conformational changes of the tubulin tails as a result of both proximity to the ordered tubulin body and modification by mono- and polyglycine PTM were determined. The C-terminal tails of the tubulin dimer are fully disordered and, in contrast with prior simulation predictions, exhibit a propensity for β-sheet conformations. The C-terminal tails display significant chemical shift differences as compared to isolated peptides of the same sequence, indicating that the tubulin C-terminal tails interact with the ordered tubulin body. Although mono- and polyglycylation affect the chemical shift of adjacent residues, the conformation of the C-terminal tail appears insensitive to the length of polyglycine chains. Our studies provide important insights into how the essential disordered domains of tubulin function.

AMP-activated protein kinase fortifies epithelial tight junctions during energetic stress via its effector GIV/Girdin

Loss of epithelial polarity impacts organ development and function; it is also oncogenic. AMPK, a key sensor of metabolic stress stabilizes cell-cell junctions and maintains epithelial polarity; its activation by Metformin protects the epithelial barrier against stress and suppresses tumorigenesis. How AMPK protects the epithelium remains unknown. Here, we identify GIV/Girdin as a novel effector of AMPK, whose phosphorylation at a single site is both necessary and sufficient for strengthening mammalian epithelial tight junctions and preserving cell polarity and barrier function in the face of energetic stress. Expression of an oncogenic mutant of GIV (cataloged in TCGA) that cannot be phosphorylated by AMPK increased anchorage-independent growth of tumor cells and helped these cells to evade the tumor-suppressive action of Metformin. This work defines a fundamental homeostatic mechanism by which the AMPK-GIV axis reinforces cell junctions against stress-induced collapse and also provides mechanistic insight into the tumor-suppressive action of Metformin.

Activity-Dependent Regulation of Distinct Transport and Cytoskeletal Remodeling Functions of the Dendritic Kinesin KIF21B

The dendritic arbor is subject to continual activity-dependent remodeling, requiring a balance between directed cargo trafficking and dynamic restructuring of the underlying microtubule tracks. How cytoskeletal components are able to dynamically regulate these processes to maintain this balance remains largely unknown. By combining single-molecule assays and live imaging in rat hippocampal neurons, we have identified the kinesin-4 KIF21B as a molecular regulator of activity-dependent trafficking and microtubule dynamicity in dendrites. We find that KIF21B contributes to the retrograde trafficking of brain-derived neurotrophic factor (BDNF)-TrkB complexes and also regulates microtubule dynamics through a separable, non-motor microtubule-binding domain. Neuronal activity enhances the motility of KIF21B at the expense of its role in cytoskeletal remodeling, the first example of a kinesin whose function is directly tuned to neuronal activity state. These studies suggest a model in which KIF21B navigates the complex cytoskeletal environment of dendrites by compartmentalizing functions in an activity-dependent manner.

A role for the yeast CLIP170 ortholog, the plus-end-tracking protein Bik1, and the Rho1 GTPase in Snc1 trafficking

The diversity of microtubule functions is dependent on the status of tubulin C-termini. To address the physiological role of the C-terminal aromatic residue of α-tubulin, a tub1-Glu yeast strain expressing an α-tubulin devoid of its C-terminal amino acid was used to perform a genome-wide-lethality screen. The identified synthetic lethal genes suggested links with endocytosis and related processes. In the tub1-Glu strain, the routing of the v-SNARE Snc1 was strongly impaired, with a loss of its polarized distribution in the bud, and Abp1, an actin patch or endocytic marker, developed comet-tail structures. Snc1 trafficking required dynamic microtubules but not dynein and kinesin motors. Interestingly, deletion of the microtubule plus-end-tracking protein Bik1 (a CLIP170 ortholog), which is preferentially recruited to the C-terminal residue of α-tubulin, similarly resulted in Snc1 trafficking defects. Finally, constitutively active Rho1 rescued both Bik1 localization at the microtubule plus-ends in tub1-Glu strain and a correct Snc1 trafficking in a Bik1-dependent manner. Our results provide the first evidence for a role of microtubule plus-ends in membrane cargo trafficking in yeast, through Rho1- and Bik1-dependent mechanisms, and highlight the importance of the C-terminal α-tubulin amino acid in this process.

Microtubule plus-end tracking proteins in neuronal development

Regulation of the microtubule cytoskeleton is of pivotal importance for neuronal development and function. One such regulatory mechanism centers on microtubule plus-end tracking proteins (+TIPs): structurally and functionally diverse regulatory factors, which can form complex macromolecular assemblies at the growing microtubule plus-ends. +TIPs modulate important properties of microtubules including their dynamics and their ability to control cell polarity, membrane transport and signaling. Several neurodevelopmental and neurodegenerative diseases are associated with mutations in +TIPs or with misregulation of these proteins. In this review, we focus on the role and regulation of +TIPs in neuronal development and associated disorders.

Microtubule plus end-associated CLIP-170 initiates HSV-1 retrograde transport in primary human cells

Herpes simplex virus particles that enter the cell do not randomly associate with microtubule filaments, but require plus end–binding proteins EB1, CLIP-170, and dynactin to initiate retrograde transport to the nucleus.

Dynamic microtubules (MTs) continuously explore the intracellular environment and, through specialized plus end–tracking proteins (+TIPs), engage a variety of targets. However, the nature of cargoes that require +TIP-mediated capture for their movement on MTs remains poorly understood. Using RNA interference and dominant-negative approaches, combined with live cell imaging, we show that herpes simplex virus particles that have entered primary human cells exploit a +TIP complex comprising end-binding protein 1 (EB1), cytoplasmic linker protein 170 (CLIP-170), and dynactin-1 (DCTN1) to initiate retrograde transport. Depletion of these +TIPs completely blocked post-entry long-range transport of virus particles and suppressed infection ∼5,000-fold, whereas transferrin uptake, early endosome organization, and dynein-dependent movement of lysosomes and mitochondria remained unaffected. These findings provide the first insights into the earliest stages of viral engagement of MTs through specific +TIPs, akin to receptors, with therapeutic implications, and identify herpesvirus particles as one of a very limited number of cargoes absolutely dependent on CLIP-170–mediated capture to initiate transport in primary human cells.

TTBK2 with EB1/3 regulates microtubule dynamics in migrating cells through KIF2A phosphorylation

The microtubule (MT) plus end–tracking protein TTBK2 phosphorylates kinesin-13 family MT depolymerase KIF2A and removes it from MTs, thereby antagonizing KIF2A-induced depolymerization at MT plus ends during cell migration.

Microtubules (MTs) play critical roles in various cellular events, including cell migration. End-binding proteins (EBs) accumulate at the ends of growing MTs and regulate MT end dynamics by recruiting other plus end–tracking proteins (+TIPs). However, how EBs contribute to MT dynamics through +TIPs remains elusive. We focused on tau-tubulin kinase 2 (TTBK2) as an EB1/3-binding kinase and confirmed that TTBK2 acted as a +TIP. We identified MT-depolymerizing kinesin KIF2A as a novel substrate of TTBK2. TTBK2 phosphorylated KIF2A at S135 in intact cells in an EB1/3-dependent fashion and inactivated its MT-depolymerizing activity in vitro. TTBK2 depletion reduced MT lifetime (facilitated shrinkage and suppressed rescue) and impaired HeLa cell migration, and these phenotypes were partially restored by KIF2A co-depletion. Expression of nonphosphorylatable KIF2A, but not wild-type KIF2A, reduced MT lifetime and slowed down the cell migration. These findings indicate that TTBK2 with EB1/3 phosphorylates KIF2A and antagonizes KIF2A-induced depolymerization at MT plus ends for cell migration.

Control of microtubule organization and dynamics: two ends in the limelight

Microtubules have fundamental roles in many essential biological processes, including cell division and intracellular transport. They assemble and disassemble from their two ends, denoted the plus end and the minus end. Significant advances have been made in our understanding of microtubule plus-end-tracking proteins (+TIPs) such as end-binding protein 1 (EB1), XMAP215, selected kinesins and dynein. By contrast, information on microtubule minus-end-targeting proteins (-TIPs), such as the calmodulin-regulated spectrin-associated proteins (CAMSAPs) and Patronin, has only recently started to emerge. Here, we review our current knowledge of factors, including microtubule-targeting agents, that associate with microtubule ends to control the dynamics and function of microtubules during the cell cycle and development.

CLIP-170 tethers kinetochores to microtubule plus ends against poleward force by dynein for stable kinetochore-microtubule attachment

The cytoplasmic linker protein (CLIP)-170 localizes to kinetochores and is suggested to function in stable attachment of kinetochores to microtubule ends. Here we show that defects in kinetochore-microtubule attachment and chromosome alignment in CLIP-170-depleted cells were rescued by co-depletion of p150glued, a dynactin subunit required for kinetochore localization of CLIP-170. CLIP-170 recruited p150glued to microtubule ends. Kinetochore localization at microtubule ends was perturbed by CLIP-170 depletion, which was rescued by co-depleting p150glued. Our results imply that CLIP-170 tethers kinetochores to microtubule ends against the dynein-mediated poleward force to slide kinetochores along microtubules, facilitating the stable kinetochore attachment to microtubules.

Drosophila CLIP-190 and mammalian CLIP-170 display reduced microtubule plus end association in the nervous system

Usually, CLIPs promote microtubule growth by binding their plus ends. However, in neurons, mouse CLIP-170 and fly CLIP-190 are weak end binders, instead forming actin/myosin VI–dependent patches in the center of growth cones. Total CLIP-190 loss, even together with four other plus end binders, reveals no role in neuronal MT regulation.

Axons act like cables, electrically wiring the nervous system. Polar bundles of microtubules (MTs) form their backbones and drive their growth. Plus end–tracking proteins (+TIPs) regulate MT growth dynamics and directionality at their plus ends. However, current knowledge about +TIP functions, mostly derived from work in vitro and in nonneuronal cells, may not necessarily apply to the very different context of axonal MTs. For example, the CLIP family of +TIPs are known MT polymerization promoters in nonneuronal cells. However, we show here that neither Drosophila CLIP-190 nor mammalian CLIP-170 is a prominent MT plus end tracker in neurons, which we propose is due to low plus end affinity of the CAP-Gly domain–containing N-terminus and intramolecular inhibition through the C-terminus. Instead, both CLIP-190 and CLIP-170 form F-actin–dependent patches in growth cones, mediated by binding of the coiled-coil domain to myosin-VI. Because our loss-of-function analyses in vivo and in culture failed to reveal axonal roles for CLIP-190, even in double-mutant combinations with four other +TIPs, we propose that CLIP-190 and -170 are not essential axon extension regulators. Our findings demonstrate that +TIP functions known from nonneuronal cells do not necessarily apply to the regulation of the very distinct MT networks in axons.

Regulation of α2B-Adrenerigc Receptor Export Trafficking by Specific Motifs

Intracellular trafficking and precise targeting to specific locations of G protein-coupled receptors (GPCRs) control the physiological functions of the receptors. Compared to the extensive efforts dedicated to understanding the events involved in the endocytic and recycling pathways, the molecular mechanisms underlying the transport of the GPCR superfamily from the endoplasmic reticulum (ER) through the Golgi to the plasma membrane are relatively less well defined. Over the past years, we have used α(2B)-adrenergic receptor (α(2B)-AR) as a model to define the factors that control GPCR export trafficking. In this chapter, we will review specific motifs identified to mediate the export of nascent α(2B)-AR from the ER and the Golgi and discuss the possible underlying mechanisms. As these motifs are highly conserved among GPCRs, they may provide common mechanisms for export trafficking of these receptors.

Microtubule plus-end tracking proteins and their roles in cell division

Microtubules are cellular components that are required for a variety of essential processes such as cell motility, mitosis, and intracellular transport. This is possible because of the inherent dynamic properties of microtubules. Many of these properties are tightly regulated by a number of microtubule plus-end-binding proteins or +TIPs. These proteins recognize the distal end of microtubules and are thus in the right context to control microtubule dynamics. In this review, we address how microtubule dynamics are regulated by different +TIP families, focusing on how functionally diverse +TIPs spatially and temporally regulate microtubule dynamics during animal cell division.

Reconstitution of a hierarchical +TIP interaction network controlling microtubule end tracking of dynein

Growing microtubule end regions recruit a variety of proteins collectively termed +TIPs, which confer local functions to the microtubule cytoskeleton. +TIPs form dynamic interaction networks whose behaviour depends on a number of potentially competitive and hierarchical interaction modes. The rules that determine which of the various +TIPs are recruited to the limited number of available binding sites at microtubule ends remain poorly understood. Here we examined how the human dynein complex, the main minus-end-directed motor and an important +TIP (refs , , ), is targeted to growing microtubule ends in the presence of different +TIP competitors. Using a total internal reflection fluorescence microscopy-based reconstitution assay, we found that a hierarchical recruitment mode targets the large dynactin subunit p150Glued to growing microtubule ends via EB1 and CLIP-170 in the presence of competing SxIP-motif-containing peptides. We further show that the human dynein complex is targeted to growing microtubule ends through an interaction of the tail domain of dynein with p150Glued. Our results highlight how the connectivity and hierarchy within dynamic +TIP networks are orchestrated.

Structural basis for the extended CAP-Gly domains of p150(glued) binding to microtubules and the implication for tubulin dynamics

p150(glued) belongs to a group of proteins accumulating at microtubule plus ends (+TIPs). It plays a key role in initiating retrograde transport by recruiting and tethering endosomes and dynein to microtubules. p150(glued) contains an N-terminal microtubule-binding cytoskeleton-associated protein glycine-rich (CAP-Gly) domain that accelerates tubulin polymerization. Although this copolymerization is well-studied using light microscopic techniques, structural consequences of this interaction are elusive. Here, using electron-microscopic and spectroscopic approaches, we provide a detailed structural view of p150(glued) CAP-Gly binding to microtubules and tubulin. Cryo-EM 3D reconstructions of p150(glued)-CAP-Gly complexed with microtubules revealed the recognition of the microtubule surface, including tubulin C-terminal tails by CAP-Gly. These binding surfaces differ from other retrograde initiation proteins like EB1 or dynein, which could facilitate the simultaneous attachment of all accessory components. Furthermore, the CAP-Gly domain, with its basic extensions, facilitates lateral and longitudinal interactions of tubulin molecules by covering the tubulin acidic tails. This shielding effect of CAP-Gly and its basic extensions may provide a molecular basis of the roles of p150(glued) in microtubule dynamics.

Promoting microtubule assembly: A hypothesis for the functional significance of the +TIP network

Regulation of microtubule (MT) dynamics is essential for many cellular processes, but the machinery that controls MT dynamics remains poorly understood. MT plus-end tracking proteins (+TIPs) are a set of MT-associated proteins that dynamically track growing MT ends and are uniquely positioned to govern MT dynamics. +TIPs associate with each other in a complex array of inter- and intra-molecular interactions known as the "+TIP network." Why do so many +TIPs bind to other +TIPs? Typical answers include the ideas that these interactions localize proteins where they are needed, deliver proteins to the cortex, and/or create regulatory pathways. We propose an additional and more mechanistic hypothesis: that +TIPs bind each other to create a superstructure that promotes MT assembly by constraining the structural fluctuations of the MT tip, thus acting as a polymerization chaperone.

Characterization of microtubule-binding and dimerization activity of Giardia lamblia end-binding 1 protein

End-binding 1 (EB1) proteins are evolutionarily conserved components of microtubule (MT) plus-end tracking protein that regulate MT dynamics. Giardia lamblia, with two nuclei and cytoskeletal structures, requires accurate MT distribution for division. In this study, we show that a single EB1 homolog gene of G. lamblia regulates MT dynamics in mitosis. The haemagglutinin-tagged G. lamblia EB1 (GlEB1) localizes to the nuclear envelopes and median bodies, and is transiently present in mitotic spindles of dividing cells. Knockdown of GlEB1 expression using the morpholinos-based anti-EB1 oligonucleotides, resulted in a significant defect in mitosis of Giardia trophozoites. The MT-binding assays using recombinant GlEB1 (rGlEB1) proteins demonstrated that rGlEB1_102–238, but not rGlEB1_1–184, maintains an MT-binding ability comparable with that of the full length protein, rGlEB1_1–238. Size exclusion chromatography showed that rGlEB1 is present as a dimer formed by its C-terminal domain and a disulfide bond. In vitro-mutagenesis of GlEB1 indicated that an intermolecular disulfide bond is made between cysteine #13 of the two monomers. Complementation assay using the BIM1 knockout mutant yeast, the yeast homolog of mammalian EB1, indicated that expression of the C13S mutant GlEB1 protein cannot rescue the mitotic defect of the BIM1 mutant yeast. These results suggest that dimerization of GlEB1 via the 13th cysteine residues plays a role during mitosis in Giardia.

Cross-saturation and transferred cross-saturation experiments

Structural analyses of protein-protein interactions are required to reveal their functional mechanisms, and accurate protein-protein complex models, based on experimental results, are the starting points for drug development. In addition, structural information about proteins under physiologically relevant conditions is crucially important for understanding biological events. However, for proteins such as those embedded in lipid bilayers and transiently complexed with their effectors under physiological conditions, structural analyses by conventional methods are generally difficult, due to their large molecular weights and inhomogeneity. We have developed the cross-saturation (CS) method, which is an nuclear magnetic resonance measurement technique for the precise identification of the interfaces of protein-protein complexes. In addition, we have developed an extended version of the CS method, termed transferred cross-saturation (TCS), which enables the identification of the residues of protein ligands in close proximity to huge (>150 kDa) and heterogeneous complexes under fast exchange conditions (>0.1 s(-1)). Here, we discuss the outline, basic theory, and practical considerations of the CS and TCS methods. In addition, we will review the recent progress in the construction of models of protein-protein complexes, based on CS and TCS experiments, and applications of TCS to in situ analyses of biologically and medically important proteins in physiologically relevant states.

Plk1 phosphorylates CLIP-170 and regulates its binding to microtubules for chromosome alignment

The microtubule (MT) cytoskeleton is essential for cellular morphogenesis, cell migration, and cell division. MT organization is primarily mediated by a variety of MT-associated proteins. Among these proteins, plus-end-tracking proteins (+TIPs) are evolutionarily conserved factors that selectively accumulate at growing MT plus ends. Cytoplasmic linker protein (CLIP)-170 is a +TIP that associates with diverse proteins to determine the behavior of MT ends and their linkage to intracellular structures, including mitotic chromosomes. However, how CLIP-170 activity is spatially and temporally controlled is largely unknown. Here, we show that phosphorylation at Ser312 in the third serine-rich region of CLIP-170 is increased during mitosis. Polo-like kinase 1 (Plk1) is responsible for this phosphorylation during the mitotic phase of dividing cells. In vitro analysis using a purified CLIP-170 N-terminal fragment showed that phosphorylation by Plk1 diminishes CLIP-170 binding to the MT ends and lattice without affecting binding to EB3. Furthermore, we demonstrate that during mitosis, stable kinetochore/MT attachment and subsequent chromosome alignment require CLIP-170 and a proper phosphorylation/dephosphorylation cycle at Ser312. We propose that CLIP-170 phosphorylation by Plk1 regulates proper chromosome alignment by modulating the interaction between CLIP-170 and MTs in mitotic cells and that CLIP-170 activity is stringently controlled by its phosphorylation state, which depends on the cellular context.

Ordered recruitment of dynactin to the microtubule plus-end is required for efficient initiation of retrograde axonal transport

Long-range retrograde axonal transport in neurons is driven exclusively by the microtubule motor cytoplasmic dynein. The efficient initiation of dynein-mediated transport from the distal axon is critical for normal neuronal function, and neurodegenerative disease-associated mutations have been shown to specifically disrupt this process. Here, we examine the role of dynamic microtubules and microtubule plus-end binding proteins (+TIPs) in the initiation of dynein-mediated retrograde axonal transport using live-cell imaging of cargo motility in primary mouse dorsal root ganglion neurons. We show that end-binding (EB)-positive dynamic microtubules are enriched in the distal axon. The +TIPs EB1, EB3, and cytoplasmic linker protein-170 (CLIP-170) interact with these dynamic microtubules, recruiting the dynein activator dynactin in an ordered pathway, leading to the initiation of retrograde transport by the motor dynein. Once transport has initiated, however, neither the EBs nor CLIP-170 are required to maintain transport flux along the mid-axon. In contrast, the +TIP Lis1 activates transport through a distinct mechanism and is required to maintain processive organelle transport along both the distal and mid-axon. Further, we show that the EB/CLIP-170/dynactin-dependent mechanism is required for the efficient initiation of transport from the distal axon for multiple distinct cargos, including mitochondria, Rab5-positive early endosomes, late endosomes/lysosomes, and TrkA-, TrkB-, and APP-positive organelles. Our observations indicate that there is an essential role for +TIPs in the regulation of retrograde transport initiation in the neuron.

The architecture of Trypanosoma brucei tubulin-binding cofactor B and implications for function

Tubulin-binding cofactor (TBC)-B is implicated in the presentation of α-tubulin ready to polymerize, and at the correct levels to form microtubules. Bioinformatics analyses, including secondary structure prediction, CD, and crystallography, were combined to characterize the molecular architecture of Trypanosoma brucei TBC-B. An efficient recombinant expression system was prepared, material-purified, and characterized by CD. Extensive crystallization screening, allied with the use of limited proteolysis, led to structures of the N-terminal ubiquitin-like and C-terminal cytoskeleton-associated protein with glycine-rich segment domains at 2.35-Å and 1.6-Å resolution, respectively. These are compact globular domains that appear to be linked by a flexible segment. The ubiquitin-like domain contains two lysines that are spatially conserved with residues known to participate in ubiquitinylation, and so may represent a module that, through covalent attachment, regulates the signalling and/or protein degradation associated with the control of microtubule assembly, catastrophe, or function. The TBC-B C-terminal cytoskeleton-associated protein with glycine-rich segment domain, a known tubulin-binding structure, is the only such domain encoded by the T. brucei genome. Interestingly, in the crystal structure, the peptide-binding groove of this domain forms intermolecular contacts with the C-terminus of a symmetry-related molecule, an association that may mimic interactions with the C-terminus of α-tubulin or other physiologically relevant partners. The interaction of TBC-B with the α-tubulin C-terminus may, in particular, protect from post-translational modifications, or simply assist in the shepherding of the protein into polymerization.

Cytoskeletal roles in cardiac ion channel expression

The cytoskeleton and cardiac ion channel expression are closely linked. From the time that newly synthesized channels exit the endoplasmic reticulum, they are either traveling along the microtubule or actin cytoskeletons or likely anchored in the plasma membrane or in internal vesicular pools by those scaffolds. Molecular motors, small GTPases and even the dynamics of the cytoskeletons themselves influence the trafficking and expression of the channels. In some cases, the functioning of the channels themselves has profound influences on the cytoskeleton. Here we provide an overview of the current state of knowledge on the involvement of the actin and microtubule cytoskeletons in the trafficking, targeting and expression of cardiac ion channels and a few channels expressed elsewhere. We highlight, also, some of the many questions that remain about these processes. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

The angiotensin II type 1 receptor C-terminal Lys residues interact with tubulin and modulate receptor export trafficking

The physiological and pathological functions of angiotensin II are largely mediated through activating the cell surface angiotensin II type 1 receptor (AT1R). However, the molecular mechanisms underlying the transport of newly synthesized AT1R from the endoplasmic reticulum (ER) to the cell surface remain poorly defined. Here we demonstrated that the C-terminus (CT) of AT1R directly and strongly bound to tubulin and the binding domains were mapped to two consecutive Lys residues at positions 310 and 311 in the CT membrane-proximal region of AT1R and the acidic CT of tubulin, suggestive of essentially ionic interactions between AT1R and tubulin. Furthermore, mutation to disrupt tubulin binding dramatically inhibited the cell surface expression of AT1R, arrested AT1R in the ER, and attenuated AT1R-mediated signaling measured as ERK1/2 activation. These data demonstrate for the first time that specific Lys residues in the CT juxtamembrane region regulate the processing of AT1R through interacting with tubulin. These data also suggest an important role of the microtubule network in the cell surface transport of AT1R.

Autoinhibition of TBCB regulates EB1-mediated microtubule dynamics

Tubulin cofactors (TBCs) participate in the folding, dimerization, and dissociation pathways of the tubulin dimer. Among them, TBCB and TBCE are two CAP-Gly domain-containing proteins that together efficiently interact with and dissociate the tubulin dimer. In the study reported here we showed that TBCB localizes at spindle and midzone microtubules during mitosis. Furthermore, the motif DEI/M-COO(-) present in TBCB, which is similar to the EEY/F-COO(-) element characteristic of EB proteins, CLIP-170, and α-tubulin, is required for TBCE-TBCB heterodimer formation and thus for tubulin dimer dissociation. This motif is responsible for TBCB autoinhibition, and our analysis suggests that TBCB is a monomer in solution. Mutants of TBCB lacking this motif are derepressed and induce microtubule depolymerization through an interaction with EB1 associated with microtubule tips. TBCB is also able to bind to the chaperonin complex CCT containing α-tubulin, suggesting that it could escort tubulin to facilitate its folding and dimerization, recycling or degradation.

Microtubule plus-ends within a mitotic cell are 'moving platforms' with anchoring, signalling and force-coupling roles

The microtubule polymer grows and shrinks predominantly from one of its ends called the 'plus-end'. Plus-end regulation during interphase is well understood. However, mitotic regulation of plus-ends is only beginning to be understood in mammalian cells. During mitosis, the plus-ends are tethered to specialized microtubule capture sites. At these sites, plus-end-binding proteins are loaded and unloaded in a regulated fashion. Proper tethering of plus-ends to specialized sites is important so that the microtubule is able to translate its growth and shrinkage into pushing and pulling forces that move bulky subcellular structures. We discuss recent advances on how mitotic plus-ends are tethered to distinct subcellular sites and how plus-end-bound proteins can modulate the forces that move subcellular structures. Using end binding 1 (EB1) as a prototype plus-end-binding protein, we highlight the complex network of plus-end-binding proteins and their regulation through phosphorylation. Finally, we develop a speculative 'moving platform' model that illustrates the plus-end's role in distinguishing correct versus incorrect microtubule interactions.

+TIPs: SxIPping along microtubule ends

+TIPs are a heterogeneous class of proteins that specifically bind to growing microtubule ends. Because dynamic microtubules are essential for many intracellular processes, +TIPs play important roles in regulating microtubule dynamics and microtubule interactions with other intracellular structures. End-binding proteins (EBs) recognize a structural cap at growing microtubule ends, and have emerged as central adaptors that mediate microtubule plus-end tracking of potentially all other +TIPs. The majority of these +TIPs bind to EBs through a short hydrophobic (S/T)x(I/L)P sequence motif (SxIP) and surrounding electrostatic interactions. These recent discoveries have resulted in a rapid expansion of the number of possible +TIPs. In this review, we outline our current understanding of the molecular mechanism of plus-end tracking and provide an overview of SxIP-recruited +TIPs.

Tyrosine-dependent capture of CAP-Gly domain-containing proteins in complex mixture by EB1 C-terminal peptidic probes

Microtubule dynamics is regulated by an array of microtubule associated proteins of which the microtubule plus-end tracking proteins (+TIPs) are prominent examples. +TIPs form dynamic interaction networks at growing microtubule ends in an EB1-dependent manner. The interaction between the C-terminal domain of EB1 and the CAP-Gly domains of the +TIP CLIP-170 depends on the last tyrosine residue of EB1. In the present study, we generated peptidic probes corresponding to the C-terminal tail of EB1 to affinity-capture binding partners from cell lysates. Using an MS-based approach, we showed that the last 15 amino-acid residues of EB1, either free or immobilized on beads, bound recombinant CAP-Gly domains of CLIP-170. We further demonstrate that this binding was prevented when the C-terminal tyrosine of EB1 was absent in the peptidic probes. Western blotting in combination with a label-free quantitative proteomic analysis revealed that the peptidic probe harboring the C-terminal tyrosine of EB1 effectively pulled-down proteins with CAP-Gly domains from endothelial cell extracts. Additional proteins known to interact directly or indirectly with EB1 and the microtubule cytoskeleton were also identified. Our peptidic probes represent valuable tools to detect changes induced in EB1-dependent +TIP networks by external cues such as growth factors and small molecules.