Cellular Samurai: katanin and the severing of microtubules

A potential posttranscriptional regulator for p60-katanin: miR-124-3p

Katanin is a microtubule severing protein belonging to the ATPase family and consists of two subunits; p60-katanin synthesized by the KATNA1 gene and p80-katanin synthesized by the KATNB1 gene. Microtubule severing is one of the mechanisms that allow the reorganization of microtubules depending on cellular needs. While this reorganization of microtubules is associated with mitosis in dividing cells, it primarily takes part in the formation of structures such as axons and dendrites in nondividing mature neurons. Therefore, it is extremely important in neuronal branching. p60 and p80 katanin subunits coexist in the cell. While p60-katanin is responsible for cutting microtubules with its ATPase function, p80-katanin is responsible for the regulation of p60-katanin and its localization in the centrosome. Although katanin has vital functions in the cell, there are no known posttranscriptional regulators of it. MicroRNAs (miRNAs) are a group of small noncoding ribonucleotides that have been found to have important roles in regulating gene expression posttranscriptionally. Despite being important in gene regulation, so far no microRNA has been experimentally associated with katanin regulation. In this study, the effects of miR-124-3p, which we detected as a result of bioinformatics analysis to have the potential to bind to the p60 katanin mRNA, were investigated. For this aim, in this study, SH-SY5Y neuroblastoma cells were transfected with pre-miR-124-3p mimics and pre-mir miRNA precursor as a negative control, and the effect of this transfection on p60-katanin expression was measured at both RNA and protein levels by quantitative real-time PCR (qRT-PCR) and western blotting, respectively. The results of this study showed for the first time that miR-124-3p, which was predicted to bind p60-katanin mRNA by bioinformatic analysis, may regulate the expression of the KATNA1 gene. The data obtained within the scope of this study will make important contributions in order to better understand the regulation of the expression of p60-katanin which as well will have an incontrovertible impact on the understanding of the importance of cytoskeletal reorganization in both mitotic and postmitotic cells.

UNC-45A breaks the microtubule lattice independently of its effects on non-muscle myosin II

In invertebrates, UNC-45 regulates myosin stability and functions. Vertebrates have two distinct isoforms of the protein: UNC-45B, expressed in muscle cells only, and UNC-45A, expressed in all cells and implicated in regulating both non-muscle myosin II (NMII)- and microtubule (MT)-associated functions. Here, we show that, in vitro and in human and rat cells, UNC-45A binds to the MT lattice, leading to MT bending, breakage and depolymerization. Furthermore, we show that UNC-45A destabilizes MTs independent of its C-terminal NMII-binding domain and even in the presence of the NMII inhibitor blebbistatin. These findings identified UNC-45A as a novel type of MT-severing protein with a dual non-mutually exclusive role in regulating NMII activity and MT stability. Because many human diseases, from cancer to neurodegenerative diseases, are caused by or associated with deregulation of MT stability, our findings have profound implications in the biology of MTs, as well as the biology of human diseases and possible therapeutic implications for their treatment.This article has an associated First Person interview with the joint first authors of the paper.

p60-katanin: a novel interacting partner for p53

Katanin, one of the best-characterized microtubule (MT) severing proteins, is composed of two subunits: catalytic p60-katanin, and regulatory p80-katanin. p60-katanin triggers MT reorganization by severing them. MT reorganization is essential for both mitotic cells and post-mitotic neurons in numerous vital processes such as intracellular transport, mitosis, cellular differentiation and apoptosis. Due to the deleterious effect of continuous severing for cells, p60-katanin requires a strategic regulation. However, there are only a few known regulators of p60-katanin. p53 functions in similar cellular processes as katanin such as cell cycle, differentiation, and apoptosis depending on its interacting partners. Considering this similarity, in this study we investigated p53 as a potential regulatory candidate of p60-katanin, and examined their interaction. Co-immunoprecipitation analyses revealed that p60-katanin interacts with p53. We were able to locate a potential interaction site for the two proteins by deleting different candidate regions We showed for the first time that p53 and p60-katanin interact. This interaction appears to occur via p53's DNA binding domain and p60-katanin's C-terminal. This study will pave the way for future studies regarding the functional outcomes of this interaction which is vital for understanding the regulation of cellular events such as cell cycle, differentiation, and apoptosis in disease and in health.

p53 regulates katanin-p60 promoter in HCT 116 cells

Tumor suppressor protein p53, which functions in the cell cycle, apoptosis and neuronal differentiation via transcriptional regulations of target genes or interactions with several proteins, has been associated with neurite outgrowth through microtubule re-organization. We previously demonstrated in neurons that upon p53 induction, the level of microtubule severing protein Katanin-p60 increases, indicating that p53 might be a transcriptional regulator of the KATNA1 gene encoding Katanin-p60. In this context, we firstly elucidated the activity of KATNA1 regulatory regions and endogenous KATNA1 mRNA levels in the presence or absence of p53 using HCT 116 WT and HCT 116 p53 (-/-) cells. Next, we demonstrated the binding of p53 to the KATNA1 promoter and then investigated the role of p53 on KATNA1 gene expression by ascertaining KATNA1 mRNA and Katanin-p60 protein levels upon p53 overexpression and activation in both cells. Moreover, we showed changes in microtubule network upon increased Katanin-p60 level due to p53 overexpression. Also, the changes in KATNA1 mRNA and Katanin-p60 protein levels upon p53 knockdown were investigated. Our results indicate that p53 is an activator of KATNA1 gene expression and we show that both p53 and Katanin-p60 expression have strict regulations and are maintained at balanced levels as they are vital proteins to orchestrate either survival and apoptosis or differentiation.

Is toxicant-induced Sertoli cell injury in vitro a useful model to study molecular mechanisms in spermatogenesis?

Sertoli cells isolated from rodents or humans and cultured in vitro are known to establish a functional tight junction (TJ)-permeability barrier that mimics the blood-testis barrier (BTB) in vivo. This model has been widely used by investigators to study the biology of the TJ and the BTB. Studies have shown that environmental toxicants (e.g., perfluorooctanesulfonate (PFOS), bisphenol A (BPA) and cadmium) that exert their disruptive effects to induce Sertoli cell injury using this in vitro model are reproducible in studies in vivo. Thus, this in vitro system provides a convenient approach to probe the molecular mechanism(s) underlying toxicant-induced testis injury but also to provide new insights in understanding spermatogenesis, such as the biology of cell adhesion, BTB restructuring that supports preleptotene spermatocyte transport, and others. Herein, we provide a brief and critical review based on studies using this in vitro model of Sertoli cell cultures using primary cells isolated from rodent testes vs. humans to monitor environmental toxicant-mediated Sertoli cell injury. In short, recent findings have shown that environmental toxicants exert their effects on Sertoli cells to induce testis injury through their action on Sertoli cell actin- and/or microtubule-based cytoskeleton. These effects are mediated via their disruptive effects on actin- and/or microtubule-binding proteins. Sertoli cells also utilize differential spatiotemporal expression of these actin binding proteins to confer plasticity to the BTB to regulate germ cell transport across the BTB.

Mathematical modeling of bacterial track-altering motors: Track cleaving through burnt-bridge ratchets

The generation of directed movement of cellular components frequently requires the rectification of Brownian motion. Molecular motor enzymes that use ATP to walk on filamentous tracks are typically involved in cell transport, however, a track-altering motor can arise when an enzyme interacts with and alters its track. In Caulobacter crescentus and other bacteria, an active DNA partitioning (Par) apparatus is employed to segregate replicated chromosome regions to specific locations in dividing cells. The Par apparatus is composed of two proteins: ParA, an ATPase that can form polymeric structures on the nucleoid, and ParB, a protein that can bind and destabilize ParA structures. It has been proposed that the ParB-mediated alteration of ParA structures could be responsible for generating the directed movement of DNA during bacterial division. How precisely these actions are coordinated and translated into directed movement is not clear. In this paper we consider the C. crescentus segregation apparatus as an example of a track altering motor that operates using a so-called burnt-bridge mechanism. We develop and analyze mathematical models that examine how diffusion and ATP-hydrolysis-mediated monomer removal (or cleaving) can be combined to generate directed movement. Using a mean first passage approach, we analytically calculate the effective ParA track-cleaving velocities, effective diffusion coefficient, and other higher moments for the movement a ParB protein cluster that breaks monomers away at random locations on a single ParA track. Our model results indicate that cleaving velocities and effective diffusion constants are sensitive to ParB-induced ATP hydrolysis rates. Our analytical results are in excellent agreement with stochastic simulation results.

Protective effects of transforming growth factor β2 in intestinal epithelial cells by regulation of proteins associated with stress and endotoxin responses

Transforming growth factor (TGF)-β2 is an important anti-inflammatory protein in milk and colostrum. TGF-β2 supplementation appears to reduce gut inflammatory diseases in early life, such as necrotizing enterocolitis (NEC) in young mice. However, the molecular mechanisms by which TGF-β2 protects immature intestinal epithelial cells (IECs) remain to be more clearly elucidated before interventions in infants can be considered. Porcine IECs PsIc1 were treated with TGF-β2 and/or lipopolysaccharide (LPS), and changes in the cellular proteome were subsequently analyzed using two-dimensional gel electrophoresis-MS and LC-MS-based proteomics. TGF-β2 alone induced the differential expression of 13 proteins and the majority of the identified proteins were associated with stress responses, TGF-β and Toll-like receptor 4 signaling cascades. In particular, a series of heat shock proteins had similar differential trends as previously shown in the intestine of NEC-resistant preterm pigs and young mice. Furthermore, LC-MS-based proteomics and Western blot analyses revealed 20 differentially expressed proteins following treatment with TGF-β2 in LPS-challenged IECs. Thirteen of these proteins were associated with stress response pathways, among which five proteins were altered by LPS and restored by TGF-β2, whereas six were differentially expressed only by TGF-β2 in LPS-challenged IECs. Based on previously reported biological functions, these patterns indicate the anti-stress and anti-inflammatory effects of TGF-β2 in IECs. We conclude that TGF-β2 of dietary or endogenous origin may regulate the IEC responses against LPS stimuli, thereby supporting cellular homeostasis and innate immunity in response to bacterial colonization, and the first enteral feeding in early life.

Katanin p60 contributes to microtubule instability around the midbody and facilitates cytokinesis in rat cells

The completion of cytokinesis is crucial for mitotic cell division. Cleavage furrow ingression is followed by the breaking and resealing of the intercellular bridge, but the detailed mechanism underlying this phenomenon remains unknown. Katanin is a microtubule-severing protein comprised of an AAA ATPase subunit and an accessory subunit designated as p60 and p80, respectively. Localization of katanin p60 was observed at the midzone to midbody from anaphase to cytokinesis in rat cells, and showed a ring-shaped distribution in the gap between the inside of the contractile ring and the central spindle bundle in telophase. Katanin p60 did not bind with p80 at the midzone or midbody, and localization was shown to be dependent on microtubules. At the central spindle and the midbody, no microtubule growth plus termini were seen with katanin p60, and microtubule density was inversely correlated with katanin p60 density in the region of katanin p60 localization that seemed to lead to microtubule destabilization at the midbody. Inhibition of katanin p60 resulted in incomplete cytokinesis by regression and thus caused the appearance of binucleate cells. These results suggest that katanin p60 contributes to microtubule instability at the midzone and midbody and facilitates cytokinesis in rat cells.

Microtubules: greater than the sum of the parts

The post-genomic era has produced a variety of new investigation technologies, techniques and approaches that may offer exciting insights into many long-standing questions of scientific research. The microtubule cytoskeleton is a highly conserved system that shows a high degree of internal complexity, is known to be integral to many cell systems and functions on a fundamental level. After decades of study, much is still unknown about microtubules in vivo from the control of dynamics in living cells to their responses to environmental changes and responses to other cellular processes. In the present article, we examine some outstanding questions in the microtubule field and propose a combination of emerging interdisciplinary approaches, i.e. high-throughput functional genomics techniques, quantitative and super-resolution microscopy, and in silico modelling, that could shed light on the systemic regulation of microtubules in cells by networks of regulatory factors. We propose that such an integrative approach is key to elucidate the function of the microtubule cytoskeleton as a complete responsive integral biological system.

KATNAL1 regulation of sertoli cell microtubule dynamics is essential for spermiogenesis and male fertility

Spermatogenesis is a complex process reliant upon interactions between germ cells (GC) and supporting somatic cells. Testicular Sertoli cells (SC) support GCs during maturation through physical attachment, the provision of nutrients, and protection from immunological attack. This role is facilitated by an active cytoskeleton of parallel microtubule arrays that permit transport of nutrients to GCs, as well as translocation of spermatids through the seminiferous epithelium during maturation. It is well established that chemical perturbation of SC microtubule remodelling leads to premature GC exfoliation demonstrating that microtubule remodelling is an essential component of male fertility, yet the genes responsible for this process remain unknown. Using a random ENU mutagenesis approach, we have identified a novel mouse line displaying male-specific infertility, due to a point mutation in the highly conserved ATPase domain of the novel KATANIN p60-related microtubule severing protein Katanin p60 subunit A-like1 (KATNAL1). We demonstrate that Katnal1 is expressed in testicular Sertoli cells (SC) from 15.5 days post-coitum (dpc) and that, consistent with chemical disruption models, loss of function of KATNAL1 leads to male-specific infertility through disruption of SC microtubule dynamics and premature exfoliation of spermatids from the seminiferous epithelium. The identification of KATNAL1 as an essential regulator of male fertility provides a significant novel entry point into advancing our understanding of how SC microtubule dynamics promotes male fertility. Such information will have resonance both for future treatment of male fertility and the development of non-hormonal male contraceptives.

An essential role for katanin p80 and microtubule severing in male gamete production

Katanin is an evolutionarily conserved microtubule-severing complex implicated in multiple aspects of microtubule dynamics. Katanin consists of a p60 severing enzyme and a p80 regulatory subunit. The p80 subunit is thought to regulate complex targeting and severing activity, but its precise role remains elusive. In lower-order species, the katanin complex has been shown to modulate mitotic and female meiotic spindle dynamics and flagella development. The in vivo function of katanin p80 in mammals is unknown. Here we show that katanin p80 is essential for male fertility. Specifically, through an analysis of a mouse loss-of-function allele (the Taily line), we demonstrate that katanin p80, most likely in association with p60, has an essential role in male meiotic spindle assembly and dissolution and the removal of midbody microtubules and, thus, cytokinesis. Katanin p80 also controls the formation, function, and dissolution of a microtubule structure intimately involved in defining sperm head shaping and sperm tail formation, the manchette, and plays a role in the formation of axoneme microtubules. Perturbed katanin p80 function, as evidenced in the Taily mouse, results in male sterility characterized by decreased sperm production, sperm with abnormal head shape, and a virtual absence of progressive motility. Collectively these data demonstrate that katanin p80 serves an essential and evolutionarily conserved role in several aspects of male germ cell development.

Microtubules are critical components of cells, acting as a “scaffold” for the movement of organelles and proteins within the cytoplasm. The control of microtubule length, number, and movement is essential for many cellular processes, including division, architecture, and migration. We have defined the role of the microtubule severing protein katanin p80 in male germ cell development. Male mice carrying a point mutation in the p80 gene are sterile as a consequence of low numbers of sperm, abnormal sperm morphology, and poor motility (ability to “swim”). We show that this mutation is associated with defects in microtubule structures involved in the division of immature sperm cells, in structures that shape the sperm head, and in the sperm tail, which is essential for sperm movement in the female reproductive tract. This study is the first to show that katanin p80, via its effects on microtubule dynamics within the testis, is required for male fertility.

A role for katanin in plant cell division: microtubule organization in dividing root cells of fra2 and lue1Arabidopsis thaliana mutants

Severing of microtubules by katanin has proven to be crucial for cortical microtubule organization in elongating and differentiating plant cells. On the contrary, katanin is currently not considered essential during cell division in plants as it is in animals. However, defects in cell patterning have been observed in katanin mutants, implying a role for it in dividing plant cells. Therefore, microtubule organization was studied in detail by immunofluorescence in dividing root cells of fra2 and lue1 katanin mutants of Arabidopsis thaliana. In both, early preprophase bands consisted of poorly aligned microtubules, prophase spindles were multipolar, and the microtubules of expanding phragmoplasts were elongated, bended toward and connected to the surface of daughter nuclei. Accordingly, severing by katanin seems to be necessary for the proper organization of these microtubule arrays. In both fra2 and lue1, metaphase/anaphase spindles and initiating phragmoplasts exhibited typical organization. However, they were obliquely oriented more frequently than in the wild type. It is proposed that this oblique orientation may be due to prophase spindle multipolarity and results in a failure of the cell plate to follow the predetermined division plane, during cytokinesis, producing oblique cell walls in the roots of both mutants. It is therefore concluded that, like in animal cells, katanin is important for plant cell division, influencing the organization of several microtubule arrays. Moreover, failure in microtubule severing indirectly affects the orientation of the division plane.

Somatotropin-mediated gene expression profiling of differentially displayed ESTs during lactation in Indian buffalo (Bubalus bubalis)

The study of bovine mammary gland functional genomics requires appropriate cDNA library collections to access gene expression patterns from different developmental and physiological stages. The present study was undertaken with the objective to identify candidate genes involved in the process of increased milk synthesis following 0, 48 and 96 h of recombinant bovine somatotropin (rbST) treatment to Surti buffalo (Bubalus bubalis) through differential display reverse transcriptase PCR (DDRT-PCR). Of a total 50 sequenced DD bands, 64% of ESTs were differentially expressed (appeared only in post-treatment samples, i.e. 48 h and 96 h) and 36% were up-regulated after rbST treatment. Of the ESTs 32%were found to be located on Bos taurus chromosome 24 (equivalent to buffalo chromosome 22), whereas 16% of ESTs could not be mapped, indicating that they are specific to buffalo. Quantitative real time PCR assay of 15 ESTs revealed transcript level surge in 13 ESTs, and decline in one EST, while one showed up-regulation in expression level at 48 h while down-regulation at 96 h. This study indicates more than 30 novel transcripts, with unknown function, involved in increased milk synthesis and also the involvement of many more genes in the physiology of milk production than once thought.

Drosophila katanin is a microtubule depolymerase that regulates cortical-microtubule plus-end interactions and cell migration

Regulation of microtubule dynamics at the cell cortex is important for cell motility, morphogenesis and division. Here we show that the Drosophila katanin Dm-Kat60 functions to generate a dynamic cortical-microtubule interface in interphase cells. Dm-Kat60 concentrates at the cell cortex of S2 Drosophila cells during interphase, where it suppresses the polymerization of microtubule plus-ends, thereby preventing the formation of aberrantly dense cortical arrays. Dm-Kat60 also localizes at the leading edge of migratory D17 Drosophila cells and negatively regulates multiple parameters of their motility. Finally, in vitro, Dm-Kat60 severs and depolymerizes microtubules from their ends. On the basis of these data, we propose that Dm-Kat60 removes tubulin from microtubule lattice or microtubule ends that contact specific cortical sites to prevent stable and/or lateral attachments. The asymmetric distribution of such an activity could help generate regional variations in microtubule behaviours involved in cell migration.

Role of a novel coiled-coil domain-containing protein CCDC69 in regulating central spindle assembly

The formation of the central spindle (or the spindle midzone) is essential for cytokinesis in animal cells. In this study, we report that coiled-coil domain-containing protein 69 (CCDC69) is implicated in controlling the assembly of central spindles and the recruitment of midzone components. Exogenous expression of CCDC69 in HeLa cells interfered with microtubule polymerization and disrupted the formation of bipolar mitotic spindles. Endogenous CCDC69 proteins were localized to the central spindle during anaphase. RNA interference (RNAi)-mediated knockdown of CCDC69 led to the formation of aberrant central spindles and disrupted the localization of midzone components such as aurora B kinase, protein regulator of cytokinesis 1 (PRC1), MgcRacGAP/HsCYK-4, and polo-like kinase 1 (Plk1) at the central spindle. Aurora B kinase was found to bind to CCDC69 and this binding depended on the coiled-coil domains at the C-terminus of CCDC69. Further, disruption of aurora B function in HeLa cells by treatment with a small chemical inhibitor led to the mislocalization of CCDC69 at the central spindle. Our results indicate that CCDC69 acts as a scaffold to regulate the recruitment of midzone components and the assembly of central spindles.

Microtubule length distributions in the presence of protein-induced severing

Microtubules are highly regulated dynamic elements of the cytoskeleton of eukaryotic cells. One of the regulation mechanisms observed in living cells is the severing by the proteins katanin and spastin. We introduce a model for the dynamics of microtubules in the presence of randomly occurring severing events. Under the biologically motivated assumption that the newly created plus end undergoes a catastrophe, we investigate the steady-state length distribution. We show that the presence of severing does not affect the number of microtubules, regardless of the distribution of severing events. In the special case in which the microtubules cannot recover from the depolymerizing state (no rescue events) we derive an analytical expression for the length distribution. In the general case we transform the problem into a single ordinary differential equation that is solved numerically.

From birth till death: neurogenesis, cell cycle, and neurodegeneration

Neurogenesis in the embryo involves many signaling pathways and transcriptional programs and an elaborate orchestration of cell cycle exit in differentiating precursors. However, while the neurons differentiate into a plethora of different subtypes and different identities, they also presume a highly polar structure with a particular morphology of the cytoskeleton, thereby making it almost impossible for any differentiated cell to re-enter the cell cycle. It has been observed that dysregulated or forced cell cycle reentry is closely linked to neurodegeneration and apoptosis in neurons, most likely through changes in the neurocytoskeleton. However, proliferative cells still exist within the nervous system, and adult neural stem cells (NSCs) have been identified in the Central Nervous System (CNS) in the past decade, raising a great stir in the neuroscience community. NSCs present a new therapeutic potential, and much effort has since gone into understanding the molecular mechanisms driving differentiation of specific neuronal lineages, such as dopaminergic neurons, for use in regenerative medicine, either through transplanted NSCs or manipulation of existing ones. Nevertheless, differentiation and proliferation are two sides of the same coin, just like tumorigenesis and degeneration. Tumor formation may be regarded as a de-differentiation of tissues, where cell cycle mechanisms are reactivated in differentiated cell types. It is thus important to understand the molecular mechanisms underlying various brain tumors in this perspective. The recent Cancer Stem Cell (CSC) hypothesis also suggests the presence of Brain Tumor Initiating Cells (BTICs) within a tumor population, although the exact origin of these rare and mostly elusive BTICs are yet to be identified. This review attempts to investigate the correlation of neural stem cells/precursors, mature neurons, BTICs and brain tumors with respect to cell cycle regulation and the impact of cell cycle in neurodegeneration.

Predominant regulators of tubulin monomer-polymer partitioning and their implication for cell polarization

The microtubule-system organizes the cytoplasm during interphase and segregates condensed chromosomes during mitosis. Four unrelated conserved proteins, XMAP215/Dis1/TOGp, MCAK, MAP4 and Op18/stathmin, have all been implicated as predominant regulators of tubulin monomer-polymer partitioning in animal cells. However, while studies employing the Xenopus egg extract model system indicate that the partitioning is largely governed by the counteractive activities of XMAP215 and MCAK, studies of human cell lines indicate that MAP4 and Op18 are the predominant regulators of the interphase microtubule-array. Here, we review functional interplay of these proteins during interphase and mitosis in various cell model systems. We also review the evidence that MAP4 and Op18 have interphase-specific, counteractive and phosphorylation-inactivated activities that govern tubulin subunit partitioning in many mammalian cell types. Finally, we discuss evidence indicating that partitioning regulation by MAP4 and Op18 may be of significance to establish cell polarity.

Initiation of hepatitis C virus infection requires the dynamic microtubule network: role of the viral nucleocapsid protein

Early events leading to the establishment of hepatitis C virus (HCV) infection are not completely understood. We show that intact and dynamic microtubules play a key role in the initiation of productive HCV infection. Microtubules were required for virus entry into cells, as evidenced using virus pseudotypes presenting HCV envelope proteins on their surface. Studies carried out using the recent infectious HCV model revealed that microtubules also play an essential role in early, postfusion steps of the virus cycle. Moreover, low concentrations of vinblastin and nocodazol, microtubule-affecting drugs, and paclitaxel, which stabilizes microtubules, inhibited infection, suggesting that microtubule dynamic instability and/or treadmilling mechanisms are involved in HCV internalization and early transport. By protein chip and direct core-dependent pull-down assays, followed by mass spectrometry, we identified beta- and alpha-tubulin as cellular partners of the HCV core protein. Surface plasmon resonance analyses confirmed that core directly binds to tubulin with high affinity via amino acids 2-117. The interaction of core with tubulin in vitro promoted its polymerization and enhanced the formation of microtubules. Immune electron microscopy showed that HCV core associates, at least temporarily, with microtubules polymerized in its presence. Studies by confocal microscopy showed a juxtaposition of core with microtubules in HCV-infected cells. In summary, we report that intact and dynamic microtubules are required for virus entry into cells and for early postfusion steps of infection. HCV may exploit a direct interaction of core with tubulin, enhancing microtubule polymerization, to establish efficient infection and promote virus transport and/or assembly in infected cells.

Visualization of microtubule growth in living platelets reveals a dynamic marginal band with multiple microtubules

The marginal band of microtubules maintains the discoid shape of resting blood platelets. Although studies of platelet microtubule coil structure conclude that it is composed of a single microtubule, no investigations of its dynamics exist. In contrast to previous studies, permeabilized platelets incubated with GTP-rhodamine-tubulin revealed tubulin incorporation at 7.9 (+/- 1.9) points throughout the coil, and anti-EB1 antibodies stained 8.7 (+/- 2.0) sites, indicative of multiple free microtubules. To pursue this result, we expressed the microtubule plus-end marker EB3-GFP in megakaryocytes and examined its behavior in living platelets released from these cells. Time-lapse microscopy of EB3-GFP in resting platelets revealed multiple assembly sites within the coil and a bidirectional pattern of assembly. Consistent with these findings, tyrosinated tubulin, a marker of newly assembled microtubules, localized to resting platelet microtubule coils. These results suggest that the resting platelet marginal band contains multiple highly dynamic microtubules of mixed polarity. Analysis of microtubule coil diameters in newly formed resting platelets indicates that microtubule coil shrinkage occurs with aging. In addition, activated EB3-GFP-expressing platelets exhibited a dramatic increase in polymerizing microtubules, which travel outward and into filopodia. Thus, the dynamic microtubules associated with the marginal band likely function during both resting and activated platelet states.

Rapid spatiotemporal patterning of cytosolic Ca2+ underlies flagellar excision in Chlamydomonas reinhardtii

Ca(2+)-dependent signalling processes are implicated in many aspects of flagella function in the green alga, Chlamydomonas. In this study, we examine the spatiotemporal dynamics of cytosolic Ca2+ ([Ca2+](cyt)) in single Chlamydomonas cells during the process of flagellar excision, using biolistically loaded calcium-responsive dyes. Acid-induced deflagellation occurred in parallel with a single transient elevation in whole-cell [Ca2+](cyt), which was absent in the acid deflagellation-deficient adf1 mutant. Deflagellation could also be induced by elevated external Ca2+ ([Ca2+](ext)), which promoted very rapid spiking of [Ca2+](cyt) across the whole cell and in the flagella. We also detected very rapid apically localised Ca2+ signalling events with an approximate duration of 500 msec. Ninety-seven per cent of deflagellation events coincided with a rapid elevation in [Ca2+](cyt) in the apical region of the cell, either in the form of a whole cell or an apically localised increase, indicating that [Ca2+](cyt) elevations in the apical region play an underlying role in deflagellation. Our data indicate that elevated [Ca2+](ext) acts to disrupt Ca2+ homeostasis which induces deflagellation by both Adf1-dependent and Adf1-independent mechanisms. Elevated [Ca2+](ext) also results in further [Ca2+](cyt) elevations after the main period of whole cell spiking which are very strongly associated with deflagellation, exhibit a high degree of apical localisation and are largely absent in the adf1 mutant. We propose that these later elevations may act as specific signals for deflagellation.

Mitotic spindle dynamics in Drosophila

Mitosis, the process by which the replicated chromosomes are segregated equally into daughter cells, has been studied for over a century. Drosophila melanogaster is an ideal organism for this research. Drosophila embryos are well suited to image mitosis, because during cycles 10-13 nuclei divide rapidly at the surface of the embryo, but mitotic cells during larval stages and spermatocytes are also used for the study of mitosis. Drosophila can be easily maintained, many mutant stocks exist, and transgenic flies expressing mutated or fluorescently labeled proteins can be made. In addition, the genome has been completed and RNA interference can be used in Drosophila tissue culture cells. Here, we review our current understanding of spindle dynamics, looking at the experiments and quantitative modeling on which it is based. Many molecular players in the Drosophila mitotic spindle are similar to those in mammalian spindles, so findings in Drosophila can be extended to other organisms.

Role of cofactors B (TBCB) and E (TBCE) in tubulin heterodimer dissociation

Tubulin folding cofactors B (TBCB) and E (TBCE) are alpha-tubulin binding proteins that, together with Arl2 and cofactors D (TBCD), A (TBCA or p14) and C (TBCC), participate in tubulin biogenesis. TBCD and TBCE have also been implicated in microtubule dynamics through regulation of tubulin heterodimer dissociation. Understanding the in vivo function of these proteins will shed light on the Kenny-Caffey/Sanjad-Sakati syndrome, an important human disorder associated with TBCE. Here we show that, when overexpressed, TBCB depolymerizes microtubules. We found that this function is based on the ability of TBCB to form a binary complex with TBCE that greatly enhances the efficiency of this cofactor to dissociate tubulin in vivo and in vitro. We also show that TBCE, TBCB and alpha-tubulin form a ternary complex after heterodimer dissociation, whereas the free beta-tubulin subunit is recovered by TBCA. These complexes might serve to escort alpha-tubulin towards degradation or recycling, depending on the cell requirements.

Purification of MINUS: A negative regulator of microtubule nucleation in a variety of organisms

Microtubules (MT) are important for cell behavior and maintenance, yet the factors regulating MT assembly in vivo remain obscure. In a biochemical search, we have isolated a small (4.7 kDa) acidic, phosphorylated polypeptide, which we named MINUS (microtubule nucleation suppressor) for its activity to inhibit MT nucleation [P. Fanara, B. Oback, K. Ashman, A. Podtelejnikov, R. Brandt, EMBO J. 18 (1999) 565]. Here, the purification strategy was optimized and the polypeptide purified to homogeneity from bovine brain, Drosophila, Caenorhabditis elegans and yeast. Amino acid analysis showed similar composition of MINUS from different species. In particular, MINUS was rich in glycine, threonine, isoleucine, leucine and acidic amino acids. Inductively coupled plasma mass spectrometry revealed a large peak for phosphorus confirming its identity as a phosphopeptide. For further purification, MINUS was separated as a single peak on reverse phase-HPLC (RP-HPLC). Preliminary sequence analysis suggested MINUS to be N-terminally blocked. However, conventional enzymatic digestions did not reveal differences in the peak profile compared to undigested MINUS. Hence, partial acid hydrolysis and proteinase K digestion was performed followed by RP-HPLC. The proteinase K digested peaks were subjected to Edman degradation (first peak, ser-pro-ser/gly-ser; second peak, tyr/arg-leu), mass spectrometry (no result) and MALDI analysis (no result). Collectively, the data suggest that MINUS belongs to a new class of MT assembly regulators. Sequence information and antibody development will be useful to examine its biological role in a definitive manner.

Microtubules and maps

Microtubules are very dynamic polymers whose assembly and disassembly is determined by whether their heterodimeric tubulin subunits are in a straight or curved conformation. Curvature is introduced by bending at the interfaces between monomers. Assembly and disassembly are primarily controlled by the hydrolysis of guanosine triphosphate (GTP) in a site that is completed by the association of two heterodimers. However, a multitude of associated proteins are able to fine-tune these dynamics so that microtubules are assembled and disassembled where and when they are required by the cell. We review the recent progress that has been made in obtaining a glimpse of the structural interactions involved.

Possible regulation of microtubules through destabilization of tubulin

Chaperones and folding cofactors are known to mediate posttranslational folding of nascent tubulin, thus forming functional dimers. Based on sequence likeness, a novel protein similar to cofactor E, E-like protein (El), was identified. In overexpression experiments El, similar to a subset of folding factors (i.e. cofactors D and E), appears to disrupt functional dimers and target them for destruction by the proteasome. El apparently does not interact with microtubules directly and has no function in the tubulin folding pathway. Suppression of El expression seems to increase the cellular content of stable, posttranslationally modified microtubules by an unknown mechanism. Degradation of functional tubulin dimers as well as the alteration of the cellular content of stable microtubules through El might regulate the distribution and organization of organelles in vivo.

Identification and Characterization of Factors Required for Microtubule Growth and Nucleation in the Early C. elegans Embryo

Microtubules (MTs) are dynamic polymers that undergo cell cycle and position-sensitive regulation of polymerization and depolymerization. Although many different factors that regulate MT dynamics have been described, to date there has been no systematic analysis of genes required for MT dynamics in a single system. Here, we use a transgenic EB1::GFP strain, which labels the growing plus ends of MTs, to analyze the growth rate, nucleation rate, and distribution of growing MTs in the Caenorhabditis elegans embryo. We also present the results from an RNAi screen of 40 genes previously implicated in MT-based processes. Our findings suggest that fast microtubule growth is dependent on the amount of free tubulin and the ZYG-9-TAC-1 complex. Robust MT nucleation by centrosomes requires AIR-1, SPD-2, SPD-5, and gamma-tubulin. However, we found that centrosomes do not nucleate MTs to saturation; rather, the depolymerizing kinesin-13 subfamily member KLP-7 is required to limit microtubule outgrowth from centrosomes.

Identification of a novel tubulin-destabilizing protein related to the chaperone cofactor E

Factors that regulate the microtubule cytoskeleton are critical in determining cell behavior. Here we describe the function of a novel protein that we term E-like based on its sequence similarity to the tubulin-specific chaperone cofactor E. We find that upon overexpression, E-like depolymerizes microtubules by committing tubulin to proteosomal degradation. Our data suggest that this function is direct and is based on the ability of E-like to disrupt the tubulin heterodimer in vitro. Suppression of E-like expression results in an increase in the number of stable microtubules and a tight clustering of endocellular membranes around the microtubule-organizing center, while the properties of dynamic microtubules are unaffected. These observations define E-like as a novel regulator of tubulin stability, and provide a link between tubulin turnover and vesicle transport.

Microtubule-associated proteins and their essential roles during mitosis

Microtubules play essential roles during mitosis, including chromosome capture, congression, and segregation. In addition, microtubules are also required for successful cytokinesis. At the heart of these processes is the ability of microtubules to do work, a property that derives from their intrinsic dynamic behavior. However, if microtubule dynamics were not properly regulated, it is certain that microtubules alone could not accomplish any of these tasks. In vivo, the regulation of microtubule dynamics is the responsibility of microtubule-associated proteins. Among these, we can distinguish several classes according to their function: (1) promotion and stabilization of microtubule polymerization, (2) destabilization or severance of microtubules, (3) functioning as linkers between various structures, or (4) motility-related functions. Here we discuss how the various properties of microtubule-associated proteins can be used to assemble an efficient mitotic apparatus capable of ensuring the bona fide transmission of the genetic information in animal cells.

An aurora kinase is essential for flagellar disassembly in Chlamydomonas

Cilia and flagella play key roles in development and sensory transduction, and several human disorders, including polycystic kidney disease, are associated with the failure to assemble cilia. Here, we show that the aurora protein kinase CALK in the biflagellated alga Chlamydomonas has a central role in two pathways for eliminating flagella. Cells rendered deficient in CALK were defective in regulated flagellar excision and regulated flagellar disassembly. Exposure of cells to altered ionic conditions, the absence of a centriole/basal body for nucleating flagellar assembly, cessation of delivery of flagellar components to their tip assembly site, and formation of zygotes all led to activation of the regulated disassembly pathway as indicated by phosphorylation of CALK and the absence of flagella. We propose that cells have a sensory pathway that detects conditions that are inappropriate for possession of a flagellum, and that CALK is a key effector of flagellar disassembly in that pathway.

The role of the cytoskeleton in the morphogenesis and function of stomatal complexes

Microtubules (MTs) and actin filaments (AFs) form highly organized arrays in stomatal cells that play key roles in the morphogenesis of stomatal complexes. The cortical MTs controlling the orientation of the depositing cellulose microfibrils (CMs) and affecting the pattern of local wall thickenings define the mechanical properties of the walls of stomatal cells, thus regulating accurately their shape. Besides, they are involved in determination of the cell division plane. Substomatal cavity and stomatal pore formation are also MT-dependent processes. Among the cortical MT arrays, the radial ones lining the periclinal walls are of particular morphogenetic importance. Putative MT organizing centers (MTOCs) function at their focal regions, at least in guard cells (GCs), or alternatively, these regions either organize or nucleate cortical MTs. AFs are involved in cell polarization preceding asymmetrical divisions, in determination of the cell division plane and final cell plate alignment and probably in transduction of stimuli implicated in stomatal complex morphogenesis. Mature kidney-shaped GCs display radial AF arrays, undergoing definite organization cycles during stomatal movement. They are involved in stomatal movement, probably by controlling plasmalemma ion-channel activities. Radial MT arrays also persist in mature GCs, but a role in stomatal function cannot yet be attributed to them. Contents Summary 613 I. Introduction 614 II. Cytoskeleton and development of the stomatal complexes 614 III. Cytoskeleton and stomatal cell shaping 620 IV. Stomatal pore formation 624 V. Substomatal cavity formation 625 VI. Stomatal complex morphogenesis in mutants 626 VII. Cytoskeleton dynamics in functioning stomata 628 VIII. Mechanisms of microtubule organization in stomatal cells 631 IX. Conclusions-perspectives 634 References 635.

Cellular Deflagellation

Deciliation, also known as deflagellation, flagellar autotomy, flagellar excision, or flagellar shedding, refers to the process whereby eukaryotic cells shed their cilia or flagella, often in response to stress. Used for many decades as a tool for scientists interested in the structure, function, and genesis of cilia, deciliation itself is a process worthy of scientific investigation. Deciliation has numerous direct medical implications, but more profoundly, intriguing relationships between deciliation, ciliogenesis, and the cell cycle indicate that understanding the mechanism of deciliation will contribute to a deeper understanding of broad aspects of cell biology. This review provides a critical examination of diverse data bearing on this problem. It also highlights current deficiencies in our understanding of the mechanism of deciliation.

MAPping the Eukaryotic Tree of Life: Structure, Function, and Evolution of the MAP215⧸Dis1 Family of Microtubule-Associated Proteins

The MAP215/Dis1 family of proteins is an evolutionarily ancient family of microtubule-associated proteins, with characterized members in all major kingdoms of eukaryotes, including fungi (Stu2 in S. cerevisiae, Dis1 and Alp14 in S. pombe), Dictyostelium (DdCP224), plants (Mor1 in A. thaliana and TMBP200 in N. tabaccum), and animals (Zyg9 in C. elegans, Msps in Drosophila, XMAP215 in Xenopus, and ch-TOG in humans). All MAP215/Dis1 proteins (with the exception of those in plants) localize to microtubule-organizing centers (MTOCs), including spindle pole bodies in yeast and centrosomes in animals, and all bind to microtubules in vitro and?or in vivo. Diverse roles in regulating microtubule assembly and organization have been proposed for individual family members, and a substantial body of evidence suggests that MAP215/Dis1-related proteins play critical roles in the assembly and function of the meiotic/mitotic spindles and/or cell division. An extensive search of public databases (including both EST and genome databases) identified partial sequences predicted to encode more than three dozen new members of the MAP215/Dis1 family, including putative MAP215/Dis1-related proteins in Giardia lamblia and four other protists, sixteen additional species of fungi, six plants, and twelve animals. The structure and function of MAP215/Dis1 proteins are discussed in relation to the evolution of this ancient family of microtubule-associated proteins.

Microtubule Dynamics may Embody a Stationary Bipolarity Forming Mechanism Related to the Prokaryotic Division Site Mechanism (Pole-to-Pole Oscillations)

Cell division mechanisms in eukaryotes and prokaryotes have until recently been seen as being widely different. However, pole-to-pole oscillations of proteins like MinE in prokaryotes are now known to determine the division plane. These protein waves arise through spontaneous pattern forming reaction-diffusion mechanisms, based on cooperative binding of the proteins to a quasistationary matrix (like the cell membrane or DNA). Rather than waves, stationary bipolar pattern formation may arise as well. Some of the involved proteins have eukaryotic homologs (e.g. FtsZ and tubulin), pointing to a possible ancient shared mechanism. Tubulin polymerizes to microtubules in the spindle. Mitotic microtubules are in a highly dynamical state, frequently undergoing rapid shortening (catastrophe), and fragments formed from the microtubule ends are inferred to enhance the destabilization. Here, we show that cooperative binding of such fragments to microtubules may set up a similar pattern forming mechanism as seen in prokaryotes. The result is a spontaneously formed, well controllable, bipolar state of microtubule dynamics in the cell, which may contribute to defining the bipolar spindle.

Cell and molecular biology of spindle poles and NuMA

Mitotic and meiotic cells contain a bipolar spindle apparatus of microtubules and associated proteins. To arrange microtubules into focused spindle poles, different mechanisms are used by various organisms. Principally, two major pathways have been characterized: nucleation and anchorage of microtubules at preexisting centers such as centrosomes or spindle pole bodies, or microtubule growth off the surface of chromosomes, followed by sorting and focusing into spindle poles. These two mechanisms can even be found in cells of the same organism: whereas most somatic animal cells utilize the centrosome as an organizing center for spindle microtubules, female meiotic cells build an acentriolar spindle apparatus. Most interestingly, the molecular components that drive acentriolar spindle pole formation are also present in cells containing centrosomes. They include microtubule-dependent motor proteins and a variety of structural proteins that regulate microtubule orientation, anchoring, and stability. The first of these spindle pole proteins, NuMA, had already been identified more than 20 years ago. In addition, several new proteins have been characterized more recently. This review discusses their role during spindle formation and their regulation in the cell cycle.

Dynamics and regulation of plant interphase microtubules: a comparative view

Microtubule and actin cytoskeletons are fundamental to a variety of cellular activities within eukaryotic organisms. Extensive information on the dynamics and functions of microtubules, as well as on their regulatory proteins, have been revealed in fungi and animals, and corresponding pictures are now slowly emerging in plants. During interphase, plant cells contain highly dynamic cortical microtubules that organize into ordered arrays, which are apparently regulated by distinct groups of microtubule regulators. Comparison with fungal and animal microtubules highlights both conserved and unique mechanisms for the regulation of the microtubule cytoskeleton in plants.

Kinesin superfamily protein 2A (KIF2A) functions in suppression of collateral branch extension

Through interactions with microtubules, the kinesin superfamily of proteins (KIFs) could have multiple roles in neuronal function and development. During neuronal development, postmitotic neurons develop primary axons extending toward targets, while other collateral branches remain short. Although the process of collateral branching is important for correct wiring of the brain, the mechanisms involved are not well understood. In this study, we analyzed kif2a(-/-) mice, whose brains showed multiple phenotypes, including aberrant axonal branching due to overextension of collateral branches. In kif2a(-/-) growth cones, microtubule-depolymerizing activity decreased. Moreover, many individual microtubules showed abnormal behavior at the kif2a(-/-) cell edge. Based on these results, we propose that KIF2A regulates microtubule dynamics at the growth cone edge by depolymerizing microtubules and that it plays an important role in the suppression of collateral branch extension.

Alteration of microtubule dynamic instability during preprophase band formation revealed by yellow fluorescent protein-CLIP170 microtubule plus-end labeling

At the onset of mitosis, plant cells form a microtubular preprophase band that defines the plane of cell division, but the mechanism of its formation remains a mystery. Here, we describe the use of mammalian yellow fluorescent protein-tagged CLIP170 to visualize the dynamic plus ends of plant microtubules in transfected cowpea protoplasts and in stably transformed and dividing tobacco Bright Yellow 2 cells. Using plus-end labeling, we observed dynamic instability in different microtubular conformations in live plant cells. The interphase plant microtubules grow at 5 micro m/min, shrink at 20 micro m/min, and display catastrophe and rescue frequencies of 0.02 and 0.08 events/s, respectively, exhibiting faster turnover than their mammalian counterparts. Strikingly, during preprophase band formation, the growth rate and catastrophe frequency of plant microtubules double, whereas the shrinkage rate and rescue frequency remain unchanged, making microtubules shorter and more dynamic. Using these novel insights and four-dimensional time-lapse imaging data, we propose a model that can explain the mechanism by which changes in microtubule dynamic instability drive the dramatic rearrangements of microtubules during preprophase band and spindle formation in plant cells.

Converging populations of f-actin promote breakage of associated microtubules to spatially regulate microtubule turnover in migrating cells

BACKGROUND

In migrating cells, the retrograde flow of filamentous actin (f-actin) from the leading edge toward the cell body is accompanied by the synchronous motion of microtubules (MTs, ), whose plus ends undergo net growth. Thus, MTs must depolymerize elsewhere in the cell to maintain polymer mass over time. The source and location of depolymerized MTs is unknown. Here, we test the hypothesis that MT polymer loss occurs in central cell regions and is induced by the convergence of actin retrograde and anterograde flow, which buckles and breaks associated MTs and promotes minus-end depolymerization.

RESULTS

We characterized the effects of calyculin A and ML-7 on the movement of f-actin and MTs by multi-spectral fluorescence recovery after photobleaching (FRAP) and fluorescent speckle microscopy (FSM). Our studies show that these drugs affect the rate of f-actin and MT convergence and MT buckling in a central cell region we call the "convergence zone." Increases in f-actin convergence are associated with faster MT turnover and an increase in both MT breakage and minus-end depolymerization, but they have no effect on MT plus end dynamic instability.

CONCLUSIONS

We propose that f-actin movement into the convergence zone plays a major role in spatially modulating MT turnover during cell migration by regulating MT breakage, and thus minus-end dynamics, in central cell regions.

Centrosomal TACCtics

Although the centrosome was first described over 100 years ago, we still know relatively little of the molecular mechanisms responsible for its functions. Recently, members of a novel family of centrosomal proteins have been identified in a wide variety of organisms. The transforming acidic coiled-coil-containing (TACC) proteins all appear to play important roles in cell division and cellular organisation in both embryonic and somatic systems. These closely related molecules have been implicated in microtubule stabilisation, acentrosomal spindle assembly, translational regulation, haematopoietic development and cancer progression. In this review, I summarise what we already know of this protein family and will use the TACC proteins to illustrate the many facets that centrosomes have developed during the course of evolution.

Microtubules and microfilaments in cell morphogenesis in higher plants

Microtubules and microfilaments play important roles in cell morphogenesis. The picture emerging from drug studies and molecular-genetic analyses of mutant higher plants defective in cell morphogenesis shows that the roles played by them remain the same in both tip-growing and diffuse-growing cells. Microtubules are important for establishing and maintaining growth polarity whereas actin microfilaments deliver the materials required for growth to specified sites. The recent cloning of several cell morphogenesis genes has revealed that conserved mechanisms as well as novel signal transduction pathways spatially organize the plant cytoskeleton.

Structural microtubule cap: stability, catastrophe, rescue, and third state

Microtubules polymerize from GTP-liganded tubulin dimers, but are essentially made of GDP-liganded tubulin. We investigate the tug-of-war resulting from the fact that GDP-liganded tubulin favors a curved configuration, but is forced to remain in a straight one when part of a microtubule. We point out that near the end of a microtubule, the proximity of the end shifts the balance in this tug-of-war, with some protofilament bending as result. This somewhat relaxes the microtubule lattice near its end, resulting in a structural cap. This structural cap thus is a simple mechanical consequence of two well-established facts: protofilaments made of GDP-liganded tubulin have intrinsic curvature, and microtubules are elastic, made from material that can yield to forces, in casu its own intrinsic forces. We explore possible properties of this structural cap, and demonstrate 1) how it allows both polymerization from GTP-liganded tubulin and rapid depolymerization in its absence; 2) how rescue can occur; 3) how a third, meta-stable intermediate state is possible and can explain some experimental results; and 4) how the tapered tips observed at polymerizing microtubule ends are stabilized during growth, though unable to accommodate a lateral cap. This scenario thus supports the widely accepted GTP-cap model by suggesting a stabilizing mechanism that explains the many aspects of dynamic instability.

Regulation of microtubule-associated proteins

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

Microtubule organization in the green kingdom: chaos or self-order?

Plant microtubule arrays differ fundamentally from their animal, fungal and protistan counterparts. These differences largely reflect the requirements of plant composite polymer cell walls and probably also relate to the acquisition of chloroplasts. Plant microtubules are usually dispersed and lack conspicuous organizing centres. The key to understanding this dispersed nature is the identification of proteins that interact with and regulate the spatial and dynamic properties of microtubules. Over the past decade, a number of these proteins have been uncovered, including numerous kinesin-related proteins and a 65 kDa class of structural microtubule-associated proteins that appear to be unique to plants. Mutational analysis has identified MOR1, a probable stabilizer of microtubules that is a homologue of the TOGp-XMAP215 class of high-molecular-weight microtubule-associated proteins, and a katanin p60 subunit homologue implicated in the severing of microtubules. The identification of these two proteins provides new insights into the mechanisms controlling microtubule assembly and dynamics, particularly in the dispersed cortical array found in highly polarized plant cells.

Polyglycylation domain of beta-tubulin maintains axonemal architecture and affects cytokinesis in Tetrahymena

Polyglycylation occurs through the post-translational addition of a polyglycine peptide to the gamma-carboxyl group of glutamic acids near the C terminus of alpha- and beta-tubulin, and has been found only in cells with axonemes, from protists to humans. In Tetrahymena thermophila, multiple sites of polyglycylation on alpha-tubulin are dispensable. By contrast, mutating similar sites on beta-tubulin has site-specific effects, affecting cell motility and cytokinesis, or resulting in cell death. Here, we address the lethality of a polyglycylation deficiency in T. thermophila using heterokaryons. Cells with a lethal mutation in the polyglycylation domain of beta-tubulin assembled axonemes that lack the central pair, B-subfibres and the transitional zone of outer microtubules (MTs). Furthermore, an arrest in cytokinesis occurred, and was associated with incomplete severing of cortical MTs positioned near the cleavage furrow. Thus, tubulin polyglycylation is required for the maintenance of some stable microtubular organelles that are all known to be polyglycylated in vivo, but its effects on MTs appear to be organelle-specific.