Identification of a novel small Arabidopsis protein interacting with gamma-tubulin complex protein 3

Structural insights into the interplay between microtubule polymerases, γ-tubulin complexes and their receptors

The γ-tubulin ring complex (γ-TuRC) is a structural template for controlled nucleation of microtubules from α/β-tubulin heterodimers. At the cytoplasmic side of the yeast spindle pole body, the CM1-containing receptor protein Spc72 promotes γ-TuRC assembly from seven γ-tubulin small complexes (γ-TuSCs) and recruits the microtubule polymerase Stu2, yet their molecular interplay remains unclear. Here, we determine the cryo-EM structure of the Candida albicans cytoplasmic nucleation unit at 3.6 Å resolution, revealing how the γ-TuRC is assembled and conformationally primed for microtubule nucleation by the dimerised Spc72 CM1 motif. Two coiled-coil regions of Spc72 interact with the conserved C-terminal α-helix of Stu2 and thereby position the α/β-tubulin-binding TOG domains of Stu2 in the vicinity of the microtubule assembly site. Collectively, we reveal the function of CM1 motifs in γ-TuSC oligomerisation and the recruitment of microtubule polymerases to the γ-TuRC.

Regulation of cytoskeleton dynamics and its interplay with force in plant cells

The plant cytoskeleton is an intricate network composed of actin filaments and microtubules. The cytoskeleton undergoes continuous dynamic changes that provide the basis for rapidly responding to intrinsic and extrinsic stimuli, including mechanical stress. Microtubules can respond to alterations of mechanical stress and reorient along the direction of maximal tensile stress in plant cells. The cytoskeleton can also generate driving force for cytoplasmic streaming, organelle movement, and vesicle transportation. In this review, we discuss the progress of how the plant cytoskeleton responds to mechanical stress. We also summarize the roles of the cytoskeleton in generating force that drive organelles and nuclear transportation in plant cells. Finally, some hypotheses concerning the link between the roles of the cytoskeleton in force response and organelle movement, as well as several key questions that remain to be addressed in the field, are highlighted.

Microtubule nucleation for spindle assembly: one molecule at a time

The cell orchestrates the dance of chromosome segregation with remarkable speed and fidelity. The mitotic spindle is built from scratch after interphase through microtubule (MT) nucleation, which is dependent on the γ-tubulin ring complex (γ-TuRC), the universal MT template. Although several MT nucleation pathways build the spindle framework, the question of when and how γ-TuRC is targeted to these nucleation sites in the spindle and subsequently activated remains an active area of investigation. Recent advances facilitated the discovery of new MT nucleation effectors and their mechanisms of action. In this review, we illuminate each spindle assembly pathway and subsequently consider how the pathways are merged to build a spindle.

Identification of a putative kinase interacting domain in the durum wheat catalase 1 (TdCAT1) protein

Catalases are crucial antioxidant enzymes that regulate plants responses to different biotic and abiotic stresses. It has been previously shown that the activities of durum wheat catalase proteins (TdCAT1) were stimulated in the presence of divalent cations Mn2+, Mg2+, Fe2+, Zn2+, and Ca2+. In addition, TdCAT1s can interact with calmodulins in calcium-independent manner, and this interaction stimulates its catalytic activity in a calcium-dependent manner. Moreover, this activity is further enhanced by Mn2+ cations. The current study showed that wheat catalase presents different phosphorylation targets. Besides, we demonstrated that catalase is able to interact with Mitogen Activated Proteins kinases via a conserved domain. This interaction activates wheat catalase independently of its phosphorylation status but is more promoted by Mn2+, Fe2+ and Ca2+ divalent cations. Interestingly, we have demonstrated that durum wheat catalase activity is differentially regulated by Mitogen Activated Proteins kinases and Calmodulins in the presence of calcium. Moreover, the V0 of the reaction increase gradually following the increasing quantities of Mn2+ divalent cations. Such results have never been described before and suggest i) complex regulatory mechanisms exerted on wheat catalase, ii) divalent cations (Mn2+; Mg2+; Ca2+ and Fe2+) act as key cofactors in these regulatory mechanisms.

Molecular insight into how γ-TuRC makes microtubules

As one of four filament types, microtubules are a core component of the cytoskeleton and are essential for cell function. Yet how microtubules are nucleated from their building blocks, the αβ-tubulin heterodimer, has remained a fundamental open question since the discovery of tubulin 50 years ago. Recent structural studies have shed light on how γ-tubulin and the γ-tubulin complex proteins (GCPs) GCP2 to GCP6 form the γ-tubulin ring complex (γ-TuRC). In parallel, functional and single-molecule studies have informed on how the γ-TuRC nucleates microtubules in real time, how this process is regulated in the cell and how it compares to other modes of nucleation. Another recent surprise has been the identification of a second essential nucleation factor, which turns out to be the well-characterized microtubule polymerase XMAP215 (also known as CKAP5, a homolog of chTOG, Stu2 and Alp14). This discovery helps to explain why the observed nucleation activity of the γ-TuRC in vitro is relatively low. Taken together, research in recent years has afforded important insight into how microtubules are made in the cell and provides a basis for an exciting era in the cytoskeleton field.

MZT Proteins Form Multi-Faceted Structural Modules in the γ-Tubulin Ring Complex

Microtubule organization depends on the γ-Tubulin ring complex (γ-TuRC), a ~2.3-MDa nucleation factor comprising an asymmetric assembly of γ-Tubulin and GCP2-GCP6. However, it is currently unclear how the γ-TuRC-associated microproteins MZT1 and MZT2 contribute to the structure and regulation of the holocomplex. Here, we report cryo-EM structures of MZT1 and MZT2 in the context of the native human γ-TuRC. MZT1 forms two subcomplexes with the N-terminal α-helical domains of GCP3 or GCP6 (GCP-NHDs) within the γ-TuRC “lumenal bridge.” We determine the X-ray structure of recombinant MZT1/GCP6-NHD and find it is similar to that within the native γ-TuRC. We identify two additional MZT/GCP-NHD-like subcomplexes, one of which is located on the outer face of the γ-TuRC and comprises MZT2 and GCP2-NHD in complex with a centrosomin motif 1 (CM1)-containing peptide. Our data reveal how MZT1 and MZT2 establish multi-faceted, structurally mimetic “modules” that can expand structural and regulatory interfaces in the γ-TuRC.

Wieczorek et al. show how the microproteins MZT1 and MZT2 expand binding interfaces across the γ-TuRC—the cell’s microtubule nucleating machinery—by forming similarly shaped, “modular” subcomplexes with the α-helical N-terminal domains of different γ-Tubulin complex proteins (GCPs).

Regulation and Physiological Significance of the Nuclear Shape in Plants

The shape of plant nuclei varies among different species, tissues, and cell types. In Arabidopsis thaliana seedlings, nuclei in meristems and guard cells are nearly spherical, whereas those of epidermal cells in differentiated tissues are elongated spindle-shaped. The vegetative nuclei in pollen grains are irregularly shaped in angiosperms. In the past few decades, it has been revealed that several nuclear envelope (NE) proteins play the main role in the regulation of the nuclear shape in plants. Some plant NE proteins that regulate nuclear shape are also involved in nuclear or cellular functions, such as nuclear migration, maintenance of chromatin structure, gene expression, calcium and reactive oxygen species signaling, plant growth, reproduction, and plant immunity. The shape of the nucleus has been assessed both by labeling internal components (for instance chromatin) and by labeling membranes, including the NE or endoplasmic reticulum in interphase cells and viral-infected cells of plants. Changes in NE are correlated with the formation of invaginations of the NE, collectively called the nucleoplasmic reticulum. In this review, what is known and what is unknown about nuclear shape determination are presented, and the physiological significance of the control of the nuclear shape in plants is discussed.

Is the plant nucleus a mechanical rheostat?

Beyond its biochemical nature, the nucleus is also a physical object. There is accumulating evidence that its mechanics plays a key role in gene expression, cytoskeleton organization, and more generally in cell and developmental biology. Building on data mainly obtained from the animal literature, we show how nuclear mechanics may orchestrate development and gene expression. In other words, the nucleus may play the additional role of a mechanical rheostat. Although data from plant systems are still scarce, we pinpoint recent advances and highlight some differences with animal systems. Building on this survey, we propose a list of prospects for future research in plant nuclear mechanotransduction and development.

Mechanical Shielding in Plant Nuclei

In animal single cells in culture, nuclear geometry and stiffness can be affected by mechanical cues, with important consequences for chromatin status and gene expression. This calls for additional investigation into the corresponding physiological relevance in a multicellular context and in different mechanical environments. Using the Arabidopsis root as a model system, and combining morphometry and micro-rheometry, we found that hyperosmotic stress decreases nuclear circularity and size and increases nuclear stiffness in meristematic cells. These changes were accompanied by enhanced expression of touch response genes. The nuclear response to hyperosmotic stress was rescued upon return to iso-osmotic conditions and could even lead to opposite trends upon hypo-osmotic stress. Interestingly, nuclei in a mutant impaired in the functions of the gamma-tubulin complex protein 3 (GCP3) interacting protein (GIP)/MZT1 proteins at the nuclear envelope were almost insensitive to such osmotic changes. The gip1gip2 mutant exhibited constitutive hyperosmotic stress response with stiffer and deformed nuclei, as well as touch response gene induction. The mutant was also resistant to lethal hyperosmotic conditions. Altogether, we unravel a stereotypical geometric, mechanical, and genetic nuclear response to hyperosmotic stress in plants. Our data also suggest that chromatin acts as a gel that stiffens in hyperosmotic conditions and that the nuclear-envelope-associated protein GIPs act as negative regulators of this response.

Microtubule nucleation by γ-tubulin complexes and beyond

In this short review, we give an overview of microtubule nucleation within cells. It is nearly 30 years since the discovery of γ-tubulin, a member of the tubulin superfamily essential for proper microtubule nucleation in all eukaryotes. γ-tubulin associates with other proteins to form multiprotein γ-tubulin ring complexes (γ-TuRCs) that template and catalyse the otherwise kinetically unfavourable assembly of microtubule filaments. These filaments can be dynamic or stable and they perform diverse functions, such as chromosome separation during mitosis and intracellular transport in neurons. The field has come a long way in understanding γ-TuRC biology but several important and unanswered questions remain, and we are still far from understanding the regulation of microtubule nucleation in a multicellular context. Here, we review the current literature on γ-TuRC assembly, recruitment, and activation and discuss the potential importance of γ-TuRC heterogeneity, the role of non-γ-TuRC proteins in microtubule nucleation, and whether γ-TuRCs could serve as good drug targets for cancer therapy.

γ-TuRC Heterogeneity Revealed by Analysis of Mozart1

Microtubules are essential for various cell processes [1] and are nucleated by multi-protein γ-tubulin ring complexes (γ-TuRCs) at various microtubule organizing centers (MTOCs), including centrosomes [2-6]. Recruitment of γ-TuRCs to different MTOCs at different times influences microtubule array formation, but how this is regulated remains an open question. It also remains unclear whether all γ-TuRCs within the same organism have the same composition and how any potential heterogeneity might influence γ-TuRC recruitment. MOZART1 (Mzt1) was recently identified as a γ-TuRC component [7, 8] and is conserved in nearly all eukaryotes [6, 9]. Mzt1 has so far been studied in cultured human cells, yeast, and plants; its absence leads to failures in γ-TuRC recruitment and cell division, resulting in cell death [7, 9-15]. Mzt1 is small (∼8.5 kDa), binds directly to core γ-TuRC components [9, 10, 14, 15], and appears to mediate the interaction between γ-TuRCs and proteins that tether γ-TuRCs to MTOCs [9, 15]. Here, we use Drosophila to investigate the function of Mzt1 in a multicellular animal for the first time. Surprisingly, we find that Drosophila Mzt1 is expressed only in the testes and is present in γ-TuRCs recruited to basal bodies, but not to mitochondria, in developing sperm cells. mzt1 mutants are viable but have defects in basal body positioning and γ-TuRC recruitment to centriole adjuncts; sperm formation is affected and mutants display a rapid age-dependent decline in sperm motility and male fertility. Our results reveal that tissue-specific and MTOC-specific γ-TuRC heterogeneity exist in Drosophila and highlight the complexity of γ-TuRC recruitment in a multicellular animal.

NMR secondary structure and interactions of recombinant human MOZART1 protein, a component of the gamma-tubulin complex

Mitotic‐spindle organizing protein associated with a ring of γ‐tubulin 1 (MOZART1) is an 8.5 kDa protein linked to regulation of γ‐tubulin ring complexes (γTuRCs), which are involved in nucleation of microtubules. Despite its small size, MOZART1 represents a challenging target for detailed characterization in vitro. We described herein a protocol for efficient production of recombinant human MOZART1 in Escherichia coli and assessed the properties of the purified protein using a combination of size exclusion chromatography coupled with multiangle light scattering (SEC‐MALS), dynamic light scattering (DLS), and nuclear magnetic resonance (NMR) experiments. MOZART1 forms heterogeneous oligomers in solution. We identified optimal detergent and buffer conditions for recording well resolved NMR experiments allowing nearly full protein assignment and identification of three distinct alpha‐helical structured regions. Finally, using NMR, we showed that MOZART1 interacts with the N‐terminus (residues 1–250) of GCP3 (γ‐tubulin complex protein 3). Our data illustrate the capacity of MOZART1 to form oligomers, promoting multiple contacts with a subset of protein partners in the context of microtubule nucleation.

Nuclear envelope: a new frontier in plant mechanosensing?

In animals, it is now well established that forces applied at the cell surface are propagated through the cytoskeleton to the nucleus, leading to deformations of the nuclear structure and, potentially, to modification of gene expression. Consistently, altered nuclear mechanics has been related to many genetic disorders, such as muscular dystrophy, cardiomyopathy and progeria. In plants, the integration of mechanical signals in cell and developmental biology has also made great progress. Yet, while the link between cell wall stresses and cytoskeleton is consolidated, such cortical mechanical cues have not been integrated with the nucleoskeleton. Here, we propose to take inspiration from studies on animal nuclei to identify relevant methods amenable to probing nucleus mechanics and deformation in plant cells, with a focus on microrheology. To identify potential molecular targets, we also compare the players at the nuclear envelope, namely lamina and LINC complex, in both plant and animal nuclei. Understanding how mechanical signals are transduced to the nucleus across kingdoms will likely have essential implications in development (e.g. how mechanical cues add robustness to gene expression patterns), in the nucleoskeleton-cytoskeleton nexus (e.g. how stress is propagated in turgid/walled cells), as well as in transcriptional control, chromatin biology and epigenetics.

Cell Biology of the Plant Nucleus

The eukaryotic nucleus is enclosed by the nuclear envelope, which is perforated by the nuclear pores, the gateways of macromolecular exchange between the nucleoplasm and cytoplasm. The nucleoplasm is organized in a complex three-dimensional fashion that changes over time and in response to stimuli. Within the cell, the nucleus must be viewed as an organelle (albeit a gigantic one) that is a recipient of cytoplasmic forces and capable of morphological and positional dynamics. The most dramatic reorganization of this organelle occurs during mitosis and meiosis. Although many of these aspects are less well understood for the nuclei of plants than for those of animals or fungi, several recent discoveries have begun to place our understanding of plant nuclei firmly into this broader cell-biological context.

Regulation of the wheat MAP kinase phosphatase 1 by 14-3-3 proteins

Plant MAP kinase phosphatases (MKPs) are major regulators of MAPK signaling pathways and play crucial roles in controlling growth, development and stress responses. The presence of several functional domains in plant MKPs such as a dual specificity phosphatase catalytic domain, gelsolin, calmodulin-binding and serine-rich domains, suggests that MKPs can interact with distinct cellular partners, others than MAPKs. In this report, we identified a canonical mode I 14-3-3-binding motif (₅₇₄KLPSLP₅₇₉) located at the carboxy-terminal region of the wheat MKP, TMKP1. We found that this motif is well-conserved among other MKPs from monocots including Hordeum vulgare, Brachypodium distachyon and Aegilops taushii. Using co-immunoprecipitation assays, we provide evidence for interaction between TMKP1 and 14-3-3 proteins in wheat. Moreover, the phosphatase activity of TMKP1 is increased in a phospho-dependent manner by either Arabidopsis or yeast 14-3-3 isoforms. TMKP1 activation by 14-3-3 proteins is enhanced by Mn²⁺, whereas in the presence of Ca²⁺ ions, TMKP1 activation was limited to Arabidopsis 14-3-3φ (phi), an isoform harboring an EF-hand motif. Such findings strongly suggest that 14-3-3 proteins, in conjunction with specific divalent cations, may stimulate TMKP1 activity and point-out that 14-3-3 proteins bind and regulate the activity of a MKP in eukaryotes.

Synergistic role of fission yeast Alp16GCP6 and Mzt1MOZART1 in γ-tubulin complex recruitment to mitotic spindle pole bodies and spindle assembly

In fission yeast, γ-tubulin ring complex (γTuRC)-specific components Gfh1(GCP4), Mod21(GCP5), and Alp16(GCP6) are nonessential for cell growth. Of these deletion mutants, only alp16Δ shows synthetic lethality with temperature-sensitive mutants of Mzt1(MOZART1), a component of the γTuRC required for recruitment of the complex to microtubule-organizing centers. γ-Tubulin small complex levels at mitotic spindle pole bodies (SPBs, the centrosome equivalent in fungi) and microtubule levels for preanaphase spindles are significantly reduced in alp16Δ cells but not in gfh1Δ or mod21Δ cells. Furthermore, alp16Δ cells often form monopolar spindles and frequently lose a minichromosome when the spindle assembly checkpoint is inactivated. Alp16(GCP6) promotes Mzt1-dependent γTuRC recruitment to mitotic SPBs and enhances spindle microtubule assembly in a manner dependent on its expression levels. Gfh1(GCP4) and Mod21(GCP5) are not required for Alp16(GCP6)-dependent γTuRC recruitment. Mzt1 has an additional role in the activation of the γTuRC for spindle microtubule assembly. The ratio of Mzt1 to γTuRC levels for preanaphase spindles is higher than at other stages of the cell cycle. Mzt1 overproduction enhances spindle microtubule assembly without affecting γTuRC levels at mitotic SPBs. We propose that Alp16(GCP6) and Mzt1 act synergistically for efficient bipolar spindle assembly to ensure faithful chromosome segregation.

Regulation of nuclear shape and size in plants

Nuclear shape and size changes have long been used by cytopathologists to diagnose, stage, and prognose cancer. However, the underlying causalities and molecular mechanisms are largely unknown. The current eukaryotic tree of life groups eukaryotes into five supergroups, with all organisms between humans and yeast falling into the supergroup Opisthokonta. The emergence of model organisms with strong molecular genetic methodology in the other supergroups has recently facilitated a broader evolutionary approach to pressing biological questions. Here, we review what is known about the control of nuclear shape and size in the Archaeplastidae, the supergroup containing the higher plants. We discuss common themes as well as differences toward a more generalized model of how eukaryotic organisms regulate nuclear morphology.

GIP Contributions to the Regulation of Centromere at the Interface Between the Nuclear Envelope and the Nucleoplasm

Centromeres are known as specific chromatin domains without which eukaryotic cells cannot divide properly during mitosis. Despite the considerable efforts to understand the centromere/kinetochore assembly during mitosis, until recently, comparatively few studies have dealt with the regulation of centromere during interphase. Here, we briefly review and discuss past and recent advances about the architecture of centromeres and their regulation during the cell cycle. Furthermore, we highlight and discuss new findings and hypotheses regarding the specific regulation of centromeres in both plant and animal nuclei, especially with GIP proteins at the interface between the nuclear envelope and the nucleoplasm.

Centromeric chromatin and its dynamics in plants

Centromeres are chromatin structures that are required for proper separation of chromosomes during mitosis and meiosis. The centromere is composed of centromeric DNA, often enriched in satellite repeats, and kinetochore complex proteins. To date, over 100 kinetochore components have been identified in various eukaryotes. Kinetochore assembly begins with incorporation of centromeric histone H3 variant CENH3 into centromeric nucleosomes. Protein components of the kinetochore are either present at centromeres throughout the cell cycle or localize to centromeres transiently, prior to attachment of microtubules to each kinetochore in prometaphase of mitotic cells. This is the case for the spindle assembly checkpoint (SAC) proteins in animal cells. The SAC complex ensures equal separation of chromosomes between daughter nuclei by preventing anaphase onset before metaphase is complete, i.e. the sister kinetochores of all chromosomes are attached to spindle fibers from opposite poles. In this review, we focus on the organization of centromeric DNA and the kinetochore assembly in plants. We summarize recent advances regarding loading of CENH3 into the centromere, and the subcellular localization and protein-protein interactions of Arabidopsis thaliana proteins involved in kinetochore assembly and function. We describe the transcriptional activity of corresponding genes based on in silico analysis of their promoters and cell cycle-dependent expression. Additionally, barley homologs of all selected A. thaliana proteins have been identified in silico, and their sequences and domain structures are presented.

Arabidopsis MZT1 homologs GIP1 and GIP2 are essential for centromere architecture

Centromeres play a pivotal role in maintaining genome integrity by facilitating the recruitment of kinetochore and sister-chromatid cohesion proteins, both required for correct chromosome segregation. Centromeres are epigenetically specified by the presence of the histone H3 variant (CENH3). In this study, we investigate the role of the highly conserved γ-tubulin complex protein 3-interacting proteins (GIPs) in Arabidopsis centromere regulation. We show that GIPs form a complex with CENH3 in cycling cells. GIP depletion in the gip1gip2 knockdown mutant leads to a decreased CENH3 level at centromeres, despite a higher level of Mis18BP1/KNL2 present at both centromeric and ectopic sites. We thus postulate that GIPs are required to ensure CENH3 deposition and/or maintenance at centromeres. In addition, the recruitment at the centromere of other proteins such as the CENP-C kinetochore component and the cohesin subunit SMC3 is impaired in gip1gip2. These defects in centromere architecture result in aneuploidy due to severely altered centromeric cohesion. Altogether, we ascribe a central function to GIPs for the proper recruitment and/or stabilization of centromeric proteins essential in the specification of the centromere identity, as well as for centromeric cohesion in somatic cells.

Microtubule nucleating and severing enzymes for modifying microtubule array organization and cell morphogenesis in response to environmental cues

In higher plants, reorientation of cortical microtubule arrays has been postulated to be of importance for modifying cell growth to adapt to environmental conditions. However, the process of microtubule reorientation is largely unknown. Recent genetic and live cell imaging studies of microtubule dynamics shed light on the regulatory mechanisms of microtubule molecular nucleation and severing apparatuses, which are required for array reorientation in response to blue light signaling. Branching nucleation from γ-tubulin complexes creates a small population of discordant microtubules that are acted on by KATANIN-mediated severing in two ways. KATANIN releases microtubules from nucleation sites and rapidly amplifies discordant microtubules by severing at microtubule crossovers. In this review, I focus on the molecular details of these two enzymes, which enable microtubule array transition.

The activity of the wheat MAP kinase phosphatase 1 is regulated by manganese and by calmodulin

MAPK phosphatases (MKPs) are negative regulators of MAPKs in eukaryotes and play key roles in the regulation of different cellular processes. However in plants, little is known about the regulation of these Dual Specific Phosphatases (DSPs) by Ca(2+) and calmodulin (CaM). Here, we showed that the wheat MKP (TMKP1) harboring a calmodulin (CaM) binding domain, binds to CaM in a Ca(2+)-dependent manner. In addition, TMKP1 exhibited a phosphatase activity in vitro that is specifically enhanced by Mn(2+) and to a lesser extent by Mg(2+), but without any synergistic effect between the two bivalent cations. Most interestingly, CaM/Ca(2+) complex inhibits the catalytic activity of TMKP1 in a CaM-dose dependent manner. However, in the presence of Mn(2+) this activity is enhanced by CaM/Ca(2+) complex. These dual regulatory effects seem to be mediated via interaction of CaM/Ca(2+) to the CaM binding domain in the C-terminal part of TMKP1. Such effects were not reported so far, and raise a possible role for CaM and Mn(2+) in the regulation of plant MKPs during cellular response to external signals.

GIP/MZT1 proteins orchestrate nuclear shaping

The functional organization of the nuclear envelope (NE) is only just emerging in plants with the recent characterization of NE protein complexes and their molecular links to the actin cytoskeleton. The NE also plays a role in microtubule nucleation by recruiting γ-Tubulin Complexes (γ-TuCs) which contribute to the establishment of a robust mitotic spindle. γ-tubulin Complex Protein 3 (GCP3)-interacting proteins (GIPs) have been identified recently as integral components of γ-TuCs. GIPs have been conserved throughout evolution and are also named MZT1 (mitotic-spindle organizing protein 1). This review focuses on recent data investigating the role of GIP/MZT1 at the NE, including insights from the study of GIP partners. It also uncovers new functions for GIP/MZT1 during interphase and highlights a current view of NE-associated components which are critical for nuclear shaping during both cell division and differentiation.

A ring for all: γ-tubulin-containing nucleation complexes in acentrosomal plant microtubule arrays

The construction of dynamic polar microtubules from 13 protofilaments consisting of α-tubulin and β-tubulin heterodimers requires a preformed nucleation seed that specifies subcellular localization and timing of microtubule polymerization in vivo. An evolutionarily conserved γ-tubulin-containing ring complex is recruited to the lateral wall of preexisting microtubules or outer nuclear membranes in plant cells, and is then activated as a template for new microtubules of defined geometry. Specific regulators are thought to target/activate the ring complex to nucleate nascent microtubules in distinct polymerization patterns, as seen in interphase and mitotic arrays. The augmin complex, which was initially identified in metazoan cells, recruits the ring complex to plant mitotic microtubules, where new polymers are abundantly generated at shallow angles.

Mzt1/Tam4, a fission yeast MOZART1 homologue, is an essential component of the γ-tubulin complex and directly interacts with GCP3(Alp6)

In humans, MOZART1 plays an essential role in mitotic spindle formation as a component of the γ-tubulin ring complex. We report that the fission yeast homologue of MOZART1, Mzt1/Tam4, is located at microtubule-organizing centers (MTOCs) and coimmunoprecipitates with γ-tubulin Gtb1 from cell extracts. We show that mzt1/tam4 is an essential gene in fission yeast, encoding a 64-amino acid peptide, depletion of which leads to aberrant microtubule structure, including malformed mitotic spindles and impaired interphase microtubule array. Mzt1/Tam4 depletion also causes cytokinesis defects, suggesting a role of the γ-tubulin complex in the regulation of cytokinesis. Yeast two-hybrid analysis shows that Mzt1/Tam4 forms a complex with Alp6, a fission yeast homologue of γ-tubulin complex protein 3 (GCP3). Biophysical methods demonstrate that there is a direct interaction between recombinant Mzt1/Tam4 and the N-terminal region of GCP3(Alp6). Together our results suggest that Mzt1/Tam4 contributes to the MTOC function through regulation of GCP3(Alp6).

Fission yeast MOZART1/Mzt1 is an essential γ-tubulin complex component required for complex recruitment to the microtubule organizing center, but not its assembly

γ-Tubulin plays a universal role in microtubule nucleation from microtubule organizing centers (MTOCs) such as the animal centrosome and fungal spindle pole body (SPB). γ-Tubulin functions as a multiprotein complex called the γ-tubulin complex (γ-TuC), consisting of GCP1-6 (GCP1 is γ-tubulin). In fungi and flies, it has been shown that GCP1-3 are core components, as they are indispensable for γ-TuC complex assembly and cell division, whereas the other three GCPs are not. Recently a novel conserved component, MOZART1, was identified in humans and plants, but its precise functions remain to be determined. In this paper, we characterize the fission yeast homologue Mzt1, showing that it is essential for cell viability. Mzt1 is present in approximately equal stoichiometry with Alp4/GCP2 and localizes to all the MTOCs, including the SPB and interphase and equatorial MTOCs. Temperature-sensitive mzt1 mutants display varying degrees of compromised microtubule organization, exhibiting multiple defects during both interphase and mitosis. Mzt1 is required for γ-TuC recruitment, but not sufficient to localize to the SPB, which depends on γ-TuC integrity. Intriguingly, the core γ-TuC assembles in the absence of Mzt1. Mzt1 therefore plays a unique role within the γ-TuC components in attachment of this complex to the major MTOC site.

Microtubule nucleation and establishment of the mitotic spindle in vascular plant cells

The microtubular cytoskeleton plays a major role in cellular organization and proliferation. The first step in construction of a microtubule is microtubule nucleation. Individual microtubules then participate in organization of more complex microtubule arrays. A strong body of evidence suggests that the underlying molecular mechanisms involve protein complexes that are conserved among eukaryotes. However, plant cell specificities, mainly characterized by the presence of a cell wall and the absence of centrosomes, must be taken into account to understand their mitotic processes. The goal of this review is to summarize and discuss current knowledge regarding the mechanisms involved in plant spindle assembly during early mitotic events. The functions of the proteins currently characterized at microtubule nucleation sites and involved in spindle assembly are considered during cell-cycle progression from G2 phase to metaphase.

The GIP gamma-tubulin complex-associated proteins are involved in nuclear architecture in Arabidopsis thaliana

During interphase, the microtubular cytoskeleton of cycling plant cells is organized in both cortical and perinuclear arrays. Perinuclear microtubules (MTs) are nucleated from γ-Tubulin Complexes (γ-TuCs) located at the surface of the nucleus. The molecular mechanisms of γ-TuC association to the nuclear envelope (NE) are currently unknown. The γ-TuC Protein 3 (GCP3)-Interacting Protein 1 (GIP1) is the smallest γ-TuC component identified so far. AtGIP1 and its homologous protein AtGIP2 participate in the localization of active γ-TuCs at interphasic and mitotic MT nucleation sites. Arabidopsis gip1gip2 mutants are impaired in establishing a fully functional mitotic spindle and exhibit severe developmental defects. In this study, gip1gip2 knock down mutants were further characterized at the cellular level. In addition to defects in both the localization of γ-TuC core proteins and MT fiber robustness, gip1gip2 mutants exhibited a severe alteration of the nuclear shape associated with an abnormal distribution of the nuclear pore complexes. Simultaneously, they showed a misorganization of the inner nuclear membrane protein AtSUN1. Furthermore, AtGIP1 was identified as an interacting partner of AtTSA1 which was detected, like the AtGIP proteins, at the NE. These results provide the first evidence for the involvement of a γ-TuC component in both nuclear shaping and NE organization. Functional hypotheses are discussed in order to propose a model for a GIP-dependent nucleo-cytoplasmic continuum.

Arabidopsis GCP3-interacting protein 1/MOZART 1 is an integral component of the γ-tubulin-containing microtubule nucleating complex

Microtubules in eukaryotic cells are nucleated from ring-shaped complexes that contain γ-tubulin and a family of homologous γ-tubulin complex proteins (GCPs), but the subunit composition of the complexes can vary among fungi, animals and plants. Arabidopsis GCP3-interacting protein 1 (GIP1), a small protein with no homology to the GCP family, interacts with GCP3 in vitro, and is a plant homolog of vertebrate mitotic-spindle organizing protein associated with a ring of γ-tubulin 1 (MOZART1), a recently identified component of the γ-tubulin complex in human cell lines. In this study, we characterized two closely related Arabidopsis GIP1s: GIP1a and GIP1b. Single mutants of gip1a and gip1b were indistinguishable from wild-type plants, but their double mutant was embryonic lethal, and showed impaired development of male gametophytes. Functional fusions of GIP1a with green fluorescent protein (GFP) were used to purify GIP1a-containing complexes from Arabidopsis plants, which contained all the subunits (except NEDD1) previously identified in the Arabidopsis γ-tubulin complexes. GIP1a and GIP1b interacted specifically with Arabidopsis GCP3 in yeast. GFP-GIP1a labeled mitotic microtubule arrays in a pattern largely consistent with, but partly distinct from, the localization of the γ-tubulin complex containing GCP2 or GCP3 in planta. In interphase cortical arrays, the labeled complexes were preferentially recruited to existing microtubules, from which new microtubules were efficiently nucleated. However, in contrast to complexes labeled with tagged GCP2 or GCP3, their recruitment to cortical areas with no microtubules was rarely observed. These results indicate that GIP1/MOZART1 is an integral component of a subset of the Arabidopsis γ-tubulin complexes.

TMKP1 is a novel wheat stress responsive MAP Kinase phosphatase localized in the nucleus

The regulation of plant signalling responses by Mitogen-Activated Protein Kinases (MAPKs)-mediated protein phosphorylation is well recognized. MAP kinase phosphatases (MKPs) are negative regulators of MAPKs in eukaryotes. We report here the identification and the characterization of TMKP1, the first wheat MKP (Triticum turgidum L. subsp. Durum). Expression profile analyses performed in two durum wheat cultivars showing a marked difference in salt and drought stress tolerance, revealed a differential regulation of TMKP1. Under salt and osmotic stress, TMKP1 is induced in the sensitive wheat variety and repressed in the tolerant one. A recombinant TMKP1 was shown to be an active phosphatase and capable to interact specifically with two wheat MAPKs (TMPK3 and TMPK6). In BY2 tobacco cells transiently expressing GFP::TMKP1, the fusion protein was localized into the nucleus. Interestingly, the deletion of the N-terminal non catalytic domain results in a strong accumulation of the truncated fusion protein in the cytoplasm. In addition, when expressed in BY2 cells, TMPK3 and TMPK6 fused to red fluorescent protein (RFP) were shown to be present predominantly in the nucleus. Surprisingly, when co-expressed with the N-terminal truncated TMKP1 fusion protein; both kinases are excluded from the nuclear compartment and accumulate in the cytoplasm. This strongly suggests that TMKP1 interacts in vivo with TMPK3 and TMPK6 and controls their subcellular localization. Taken together, our results show that the newly isolated wheat MKP might play an active role in modulating the plant cell responses to salt and osmotic stress responses.