KATANIN-mediated microtubule severing is required for MTOC organisation and function in Marchantia polymorpha

Microtubule organising centres (MTOCs) are sites of localised microtubule nucleation in eukaryotic cells. Regulation of microtubule dynamics often involves KATANIN (KTN): a microtubule severing enzyme that cuts microtubules to generate new negative ends, leading to catastrophic depolymerisation. In Arabidopsis thaliana, KTN is required for the organisation of microtubules in the cell cortex, preprophase band, mitotic spindle and phragmoplast. However, as angiosperms lack MTOCs, the role of KTN in MTOC formation has yet to be studied in plants. Two unique MTOCs - the polar organisers - form on opposing sides of the preprophase nucleus in liverworts. Here, we show that KTN-mediated microtubule depolymerisation regulates the number and organisation of polar organisers formed in Marchantia polymorpha. Mpktn mutants that lacked KTN function had supernumerary disorganised polar organisers compared with wild type. This was in addition to defects in the microtubule organisation in the cell cortex, preprophase band, mitotic spindle and phragmoplast. These data are consistent with the hypothesis that KTN-mediated microtubule dynamics are required for the de novo formation of MTOCs, a previously unreported function in plants.

Microtubule-binding domains in Katanin p80 subunit are essential for severing activity in C. elegans

Microtubule-severing enzymes (MSEs), such as Katanin, Spastin, and Fidgetin play essential roles in cell division and neurogenesis. They damage the microtubule (MT) lattice, which can either destroy or amplify the MT cytoskeleton, depending on the cellular context. However, little is known about how they interact with their substrates. We have identified the microtubule-binding domains (MTBD) required for Katanin function in C. elegans. Katanin is a heterohexamer of dimers containing a catalytic subunit p60 and a regulatory subunit p80, both of which are essential for female meiotic spindle assembly. Here, we report that p80-like(MEI-2) dictates Katanin binding to MTs via two MTBDs composed of basic patches. Substituting these patches reduces Katanin binding to MTs, compromising its function in female meiotic-spindle assembly. Structural alignments of p80-like(MEI-2) with p80s from different species revealed that the MTBDs are evolutionarily conserved, even if the specific amino acids involved vary. Our findings highlight the critical importance of the regulatory subunit (p80) in providing MT binding to the Katanin complex.

New insights into the functions and regulations of MAP215/MOR1 and katanin, two conserved microtubule-associated proteins in Arabidopsis

Plant microtubules (MTs) form highly dynamic and distinct arrays throughout the cell cycle and are essential for cell and organ morphogenesis. A plethora of microtubule associated-proteins (MAPs), both conserved and plant-specific, ensure the dynamic response of MTs to internal and external cues. The MAP215 family MT polymerase/nucleation factor and the MT severing enzyme katanin are among the most conserved MAPs in eukaryotes. Recent studies have revealed unexpected functional and physical interactions between MICROTUBULE ORGANIZATION 1 (MOR1), the Arabidopsis homolog of MAP215, and KATANIN 1 (KTN1), the catalytic subunit of katanin. In this minireview, we provide a short overview on current understanding of the functions and regulations of MOR1 and katanin in cell morphogenesis and plant growth and development.

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.

Using Drosophila Larval Neuromuscular Junction and Muscle Cells to Visualize Microtubule Network

The microtubule network is an essential component of the nervous system. Mutations in many microtubules regulatory proteins are associated with neurodevelopmental disorders and neurological diseases, such as microtubule-associated protein Tau to neurodegenerative diseases, microtubule severing protein Spastin and Katanin 60 cause hereditary spastic paraplegia and neurodevelopmental abnormalities, respectively. Detection of microtubule networks in neurons is advantageous for elucidating the pathogenesis of neurological disorders. However, the small size of neurons and the dense arrangement of axonal microtubule bundles make visualizing the microtubule networks challenging. In this study, we describe a method for dissection of the larval neuromuscular junction and muscle cells, as well as immunostaining of α-tubulin and microtubule-associated protein Futsch to visualize microtubule networks in Drosophila melanogaster. The neuromuscular junction permits us to observe both pre-and post-synaptic microtubules, and the large size of muscle cells in Drosophila larva allows for clear visualization of the microtubule network. Here, by mutating and overexpressing Katanin 60 in Drosophila melanogaster, and then examining the microtubule networks in the neuromuscular junction and muscle cells, we accurately reveal the regulatory role of Katanin 60 in neurodevelopment. Therefore, combined with the powerful genetic tools of Drosophila melanogaster, this protocol greatly facilitates genetic screening and microtubule dynamics analysis for the role of microtubule network regulatory proteins in the nervous system.

IQ67 DOMAIN protein 21 is critical for indentation formation in pavement cell morphogenesis

In plants, cortical microtubules anchor to the plasma membrane in arrays and play important roles in cell shape. However, the molecular mechanism of microtubule binding proteins, which connect the plasma membrane and cortical microtubules in cell morphology remains largely unknown. Here, we report that a plasma membrane and microtubule dual-localized IQ67 domain protein, IQD21, is critical for cotyledon pavement cell (PC) morphogenesis in Arabidopsis. iqd21 mutation caused increased indentation width, decreased lobe length, and similar lobe number of PCs, whereas IQD21 overexpression had a different effect on cotyledon PC shape. Weak overexpression led to increased lobe number, decreased indentation width, and similar lobe length, while moderate or great overexpression resulted in decreased lobe number, indentation width, and lobe length of PCs. Live-cell observations revealed that IQD21 accumulation at indentation regions correlates with lobe initiation and outgrowth during PC development. Cell biological and genetic approaches revealed that IQD21 promotes transfacial microtubules anchoring to the plasma membrane via its polybasic sites and bundling at the indentation regions in both periclinal and anticlinal walls. IQD21 controls cortical microtubule organization mainly through promoting Katanin 1-mediated microtubule severing during PC interdigitation. These findings provide the genetic evidence that transfacial microtubule arrays play a determinant role in lobe formation, and the insight into the molecular mechanism of IQD21 in transfacial microtubule organization at indentations and puzzle-shaped PC development.

Brassinosteroid signals cooperate with katanin-mediated microtubule severing to control stamen filament elongation

Proper stamen filament elongation is essential for pollination and plant reproduction. Plant hormones are extensively involved in every stage of stamen development; however, the cellular mechanisms by which phytohormone signals couple with microtubule dynamics to control filament elongation remain unclear. Here, we screened a series of Arabidopsis thaliana mutants showing different microtubule defects and revealed that only those unable to sever microtubules, lue1 and ktn80.1234, displayed differential floral organ elongation with less elongated stamen filaments. Prompted by short stamen filaments and severe decrease in KTN1 and KTN80s expression in qui-2 lacking five BZR1-family transcription factors (BFTFs), we investigated the crosstalk between microtubule severing and brassinosteroid (BR) signaling. The BFTFs transcriptionally activate katanin-encoding genes, and the microtubule-severing frequency was severely reduced in qui-2. Taken together, our findings reveal how BRs can regulate cytoskeletal dynamics to coordinate the proper development of reproductive organs.

Crystal structure of the C-terminal domain of the plant-specific microtubule-associated protein Spiral2

Plant cells form microtubule arrays, called `cortical microtubules', beneath the plasma membrane which are critical for cell-wall organization and directional cell growth. Cortical microtubules are nucleated independently of centrosomes. Spiral2 is a land-plant-specific microtubule minus-end-targeting protein that stabilizes the minus ends by inhibiting depolymerization of the filament. Spiral2 possesses an N-terminal microtubule-binding domain and a conserved C-terminal domain whose function is unknown. In this study, the crystal structure of the conserved C-terminal domain of Spiral2 was determined using the single-wavelength anomalous dispersion method. Refinement of the model to a resolution of 2.2 Å revealed a helix-turn-helix fold with seven α-helices. The protein crystallized as a dimer, but SEC-MALS analysis showed the protein to be monomeric. A structural homology search revealed that the protein has similarity to the C-terminal domain of the katanin regulatory subunit p80. The structure presented here suggests that the C-terminal domain of Spiral2 represents a new class of microtubule dynamics modulator across the kingdom.

Microtubule Severing Enzymes Oligomerization and Allostery: A Tale of Two Domains

Severing proteins are nanomachines from the AAA+ (ATPases associated with various cellular activities) superfamily whose function is to remodel the largest cellular filaments, microtubules. The standard AAA+ machines adopt hexameric ring structures for functional reasons, while being primarily monomeric in the absence of the nucleotide. Both major severing proteins, katanin and spastin, are believed to follow this trend. However, studies proposed that they populate lower-order oligomers in the presence of cofactors, which are functionally relevant. Our simulations show that the preferred oligomeric assembly is dependent on the binding partners and on the type of severing protein. Essential dynamics analysis predicts that the stability of an oligomer is dependent on the strength of the interface between the helical bundle domain (HBD) of a monomer and the convex face of the nucleotide binding domain (NBD) of a neighboring monomer. Hot spots analysis found that the region consisting of the HBD tip and the C-terminal (CT) helix is the only common element between the allosteric networks responding to nucleotide, substrate, and intermonomer binding. Clustering analysis indicates the existence of multiple pathways for the transition between the secondary structure of the HBD tip in monomers and the structure(s) it adopts in oligomers.

The IPGA1-ANGUSTIFOLIA module regulates microtubule organisation and pavement cell shape in Arabidopsis

Plant cells continuously experience mechanical stress resulting from the cell wall that bears internal turgor pressure. Cortical microtubules align with the predicted maximal tensile stress direction to guide cellulose biosynthesis and therefore results in cell wall reinforcement. We have previously identified Increased Petal Growth Anisotropy (IPGA1) as a putative microtubule-associated protein in Arabidopsis, but the function of IPGA1 remains unclear. Here, using the Arabidopsis cotyledon pavement cell as a model, we demonstrated that IPGA1 forms protein granules and interacts with ANGUSTIFOLIA (AN) to cooperatively regulate microtubule organisation in response to stress. Application of mechanical perturbations, such as cell ablation, led to microtubule reorganisation into aligned arrays in wild-type cells. This microtubule response to stress was enhanced in the IPGA1 loss-of-function mutant. Mechanical perturbations promoted the formation of IPGA1 granules on microtubules. We further showed that IPGA1 physically interacted with AN both in vitro and on microtubules. The ipga1 mutant alleles exhibited reduced interdigitated growth of pavement cells, with smooth shape. IPGA1 and AN had a genetic interaction in regulating pavement cell shape. Furthermore, IPGA1 genetically and physically interacted with the microtubule-severing enzyme KATANIN. We propose that the IPGA1-AN module regulates microtubule organisation and pavement cell shape.

PP2A interacts with KATANIN to promote microtubule organization and conical cell morphogenesis

The organization of the microtubule cytoskeleton is critical for cell and organ morphogenesis. The evolutionarily conserved microtubule-severing enzyme KATANIN plays critical roles in microtubule organization in the plant and animal kingdoms. We previously used conical cell of Arabidopsis thaliana petals as a model system to investigate cortical microtubule organization and cell morphogenesis and determined that KATANIN promotes the formation of circumferential cortical microtubule arrays in conical cells. Here, we demonstrate that the conserved protein phosphatase PP2A interacts with and dephosphorylates KATANIN to promote the formation of circumferential cortical microtubule arrays in conical cells. KATANIN undergoes cycles of phosphorylation and dephosphorylation. Using co-immunoprecipitation coupled with mass spectrometry, we identified PP2A subunits as KATANIN-interacting proteins. Further biochemical studies showed that PP2A interacts with and dephosphorylates KATANIN to stabilize its cellular abundance. Similar to the katanin mutant, mutants for genes encoding PP2A subunits showed disordered cortical microtubule arrays and defective conical cell shape. Taken together, these findings identify PP2A as a regulator of conical cell shape and suggest that PP2A mediates KATANIN phospho-regulation during plant cell morphogenesis.

MOR1/MAP215 acts synergistically with katanin to control cell division and anisotropic cell elongation in Arabidopsis

The MAP215 family of microtubule (MT) polymerase/nucleation factors and the MT severing enzyme katanin are widely conserved MT-associated proteins (MAPs) across the plant and animal kingdoms. However, how these two essential MAPs coordinate to regulate plant MT dynamics and development remains unknown. Here, we identified novel hypomorphic alleles of MICROTUBULE ORGANIZATION 1 (MOR1), encoding the Arabidopsis thaliana homolog of MAP215, in genetic screens for mutants oversensitive to the MT-destabilizing drug propyzamide. Live imaging in planta revealed that MOR1-green fluorescent protein predominantly tracks the plus-ends of cortical MTs (cMTs) in interphase cells and labels preprophase band, spindle and phragmoplast MT arrays in dividing cells. Remarkably, MOR1 and KATANIN 1 (KTN1), the p60 subunit of Arabidopsis katanin, act synergistically to control the proper formation of plant-specific MT arrays, and consequently, cell division and anisotropic cell expansion. Moreover, MOR1 physically interacts with KTN1 and promotes KTN1-mediated severing of cMTs. Our work establishes the Arabidopsis MOR1-KTN1 interaction as a central functional node dictating MT dynamics and plant growth and development.

Katanin subunits p60 and p80, potential biomarkers for papillary thyroid carcinoma to distinguish nodular goiter: STROBE

Katanin subunits p60 and p80 are involved in microtubule-mediated cytoskeletal organization during cell division. Their aberrant expression has been found in prostate, breast, and non-small cell lung (NSCLC) cancers. It has recently been reported that compared with adjacent papillary thyroid carcinoma (PTC) tissues, both are highly expressed in tumor tissues. Here, we investigated whether katanin subunits p60 and p80 can be used as potential biomarkers for PTC to distinguish nodular goiter (NG).Immunohistochemistry was performed to investigate the expression of katanin subunits p60 and p80 in the tissues of 97 cases of PTC and NG. This cohort included 87 cases with PTC (74 classical or conventional (CPTC) and 13 follicular (FVPTC) variants) and 10 cases with NG.We found that katanin subunits p60 and p80 were expressed in PTC, but not in NG. The cutoff values of katanin p60 and p80 for PTC were 22.43% and 0.83%, respectively. The katanin subunit p60 was significantly associated with lymph node metastasis. Katanin subunit p80 was more highly expressed in CPTC than in FVPTC. The expression of the katanin subunit p60 was positively correlated with the expression of katanin p80 in PTC. Importantly, we found that overexpression of katanin p60 increased the expression of katanin p80 in a human papillary thyroid carcinoma KTC-1 cell line, which further supports the existence of katanin p60 and p80 feedback loops.Our results indicate that katanin subunits p60 and p80 may be used as potential PTC biomarkers to distinguish NG and may be novel therapeutic targets for PTC.

STRIPAK regulation of katanin microtubule severing in the Caenorhabditis elegans embryo

Microtubule severing plays important role in cell structure and cell division. The microtubule severing protein katanin, composed of the MEI-1/MEI-2 subunits in Caenorhabditis elegans, is required for oocyte meiotic spindle formation; however, it must be inactivated for mitosis to proceed as continued katanin expression is lethal. Katanin activity is regulated by 2 ubiquitin-based protein degradation pathways. Another ubiquitin ligase, HECD-1, the homolog of human HECTD1/HECT domain E3 ubiquitin protein ligase 1, regulates katanin activity without affecting katanin levels. In other organisms, HECD-1 is a component of the striatin-interacting kinase phosphatase complex, which affects cell proliferation and a variety of signaling pathways. Here we conducted a systematic screen of how mutations in striatin-interacting kinase phosphatase components affect katanin function in C. elegans. Striatin-interacting kinase phosphatase core components (FARL-11, CASH-1, LET-92, and GCK-1) were katanin inhibitors in mitosis and activators in meiosis, much like HECD-1. By contrast, variable components (SLMP-1, OTUB-2) functioned as activators of katanin activity in mitosis, indicating they may function to alter striatin-interacting kinase phosphatase core function. The core component CCM-3 acted as an inhibitor at both divisions, while other components (MOB-4, C49H3.6) showed weak interactions with katanin mutants. Additional experiments indicate that katanin may be involved with the centralspindlin complex and a tubulin chaperone. HECD-1 shows ubiquitous expression in the cytoplasm throughout meiosis and early development. The differing functions of the different subunits could contribute to the diverse functions of the striatin-interacting kinase phosphatase complex in C. elegans and other organisms.

Katanin-Dependent Microtubule Ordering in Association with ABA Is Important for Root Hydrotropism

Root hydrotropism refers to root directional growth toward soil moisture. Cortical microtubule arrays are essential for determining the growth axis of the elongating cells in plants. However, the role of microtubule reorganization in root hydrotropism remains elusive. Here, we demonstrate that the well-ordered microtubule arrays and the microtubule-severing protein KATANIN (KTN) play important roles in regulating root hydrotropism in Arabidopsis. We found that the root hydrotropic bending of the ktn1 mutant was severely attenuated but not root gravitropism. After hydrostimulation, cortical microtubule arrays in cells of the elongation zone of wild-type (WT) Col-0 roots were reoriented from transverse into an oblique array along the axis of cell elongation, whereas the microtubule arrays in the ktn1 mutant remained in disorder. Moreover, we revealed that abscisic acid (ABA) signaling enhanced the root hydrotropism of WT and partially rescued the oryzalin (a microtubule destabilizer) alterative root hydrotropism of WT but not ktn1 mutants. These results suggest that katanin-dependent microtubule ordering is required for root hydrotropism, which might work downstream of ABA signaling pathways for plant roots to search for water.

KATANIN and cortical microtubule organization have a pivotal role in early pollen tube guidance

Following pollen deposition on the receptive surface of the stigma, pollen germinates a tube that carries male gametes toward the ovule where fertilization occurs. As soon as it emerges from the pollen grain, the pollen tube has to be properly guided through the pistil tissues so as to reach the ovule and ensure double fertilization. Chemical attractants, nutrients as well as receptor kinase-dependent signaling pathways have been implicated in this guidance. Recently, we showed in Arabidopsis that the microtubule severing enzyme KATANIN, by acting both on cortical microtubule (CMT) dynamics and cellulose microfibril (CMF) deposition, conferred particular mechanical properties to the papilla cell wall that act as active guidance factors. Here we confirm the importance of KATANIN and CMT orientation in pollen tube directionality by examining another katanin mutant.

CsKTN1 for a katanin p60 subunit is associated with the regulation of fruit elongation in cucumber (Cucumis sativus L.)

We identified a short fruit3 (sf3) mutant in cucumber. Map-based cloning revealed that CsKTN1 gene encodes a katanin p60 subunit, which is associated with the regulation of fruit elongation. Fruit length is an important horticultural trait for both fruit yield and quality of cucumber (Cucumis sativus L.). Knowledge on the molecular regulation of fruit elongation in cucumber is very limited. In this study, we identified and characterized a cucumber short fruit3 (sf3) mutant. Histological examination indicated that the shorter fruit in the mutant was due to reduced cell numbers. Genetic analysis revealed that the phenotype of the sf3 mutant was controlled by a single gene with semi-dominant inheritance. By map-based cloning and Arabidopsis genetic transformation, we showed that Sf3 was a homolog of KTN1 (CsKTN1) encoding a katanin p60 subunit. A non-synonymous mutation in the fifth exon of CsKTN1 resulted in an amino acid substitution from Serine in the wild type to Phenylalanine in the sf3 mutant. CsKTN1 expressed in all tissues of both the wild type and the sf3 mutant. However, there was no significant difference in CsKTN1 expression levels between the wild type and the sf3 mutant. The hormone quantitation and RNA-seq analysis suggested that auxin and gibberellin contents are decreased in sf3 by changing the expression levels of genes related with auxin and gibberellin metabolism and signaling. This work helps understand the function of the katanin and the molecular mechanisms of fruit growth regulation in cucumber.

WDR47 protects neuronal microtubule minus ends from katanin-mediated severing

Axons and dendrites are long extensions of neurons that contain arrays of noncentrosomal microtubules. Calmodulin-regulated spectrin-associated proteins (CAMSAPs) bind to and stabilize free microtubule minus ends and are critical for proper neuronal development and function. Previous studies have shown that the microtubule-severing ATPase katanin interacts with CAMSAPs and limits the length of CAMSAP-decorated microtubule stretches. However, how CAMSAP and microtubule minus end dynamics are regulated in neurons is poorly understood. Here, we show that the neuron-enriched protein WDR47 interacts with CAMSAPs and is critical for axon and dendrite development. We find that WDR47 accumulates at CAMSAP2-decorated microtubules, is essential for maintaining CAMSAP2 stretches, and protects minus ends from katanin-mediated severing. We propose a model where WDR47 protects CAMSAP2 at microtubule minus ends from katanin activity to ensure proper stabilization of the neuronal microtubule network.

KATANIN and CLASP function at different spatial scales to mediate microtubule response to mechanical stress in Arabidopsis cotyledons

Mechanical stress influences cell- and tissue-scale processes across all kingdoms. It remains challenging to delineate how mechanical stress, originating at these different length scales, impacts cell and tissue form. We combine growth tracking of cells, quantitative image analysis, as well as molecular and mechanical perturbations to address this problem in pavement cells of Arabidopsis thaliana cotyledon tissue. We show that microtubule organization based on chemical signals and cell-shape-derived mechanical stress varies during early stages of pavement cell development and is mediated by the evolutionary conserved proteins, KATANIN and CLASP. However, we find that these proteins regulate microtubule organization in response to tissue-scale mechanical stress to different extents in the cotyledon epidermis. Our results further demonstrate that regulation of cotyledon form is uncoupled from the mechanical-stress-dependent control of pavement cell shape that relies on microtubule organization governed by subcellular mechanical stress.

ABNORMAL SHOOT 6 interacts with KATANIN 1 and SHADE AVOIDANCE 4 to promote cortical microtubule severing and ordering in Arabidopsis

Plant interphase cortical microtubules (cMTs) mediate anisotropic cell expansion in response to environmental and developmental cues. In Arabidopsis thaliana, KATANIN 1 (KTN1), the p60 catalytic subunit of the conserved MT-severing enzyme katanin, is essential for cMT ordering and anisotropic cell expansion. However, the regulation of KTN1-mediated cMT severing and ordering remains unclear. In this work, we report that the Arabidopsis IQ67 DOMAIN (IQD) family gene ABNORMAL SHOOT 6 (ABS6) encodes a MT-associated protein. Overexpression of ABS6 leads to elongated cotyledons, directional pavement cell expansion, and highly ordered transverse cMT arrays. Genetic suppressor analysis revealed that ABS6-mediated cMT ordering is dependent on KTN1 and SHADE AVOIDANCE 4 (SAV4). Live imaging of cMT dynamics showed that both ABS6 and SAV4 function as positive regulators of cMT severing. Furthermore, ABS6 directly interacts with KTN1 and SAV4 and promotes their recruitment to the cMTs. Finally, analysis of loss-of-function mutant combinations showed that ABS6, SAV4, and KTN1 work together to ensure the robust ethylene response in the apical hook of dark-grown seedlings. Together, our findings establish ABS6 and SAV4 as positive regulators of cMT severing and ordering, and highlight the role of cMT dynamics in fine-tuning differential growth in plants.

Phosphorylation of the microtubule-severing AAA+ enzyme Katanin regulates C. elegans embryo development

The evolutionarily conserved microtubule (MT)-severing AAA-ATPase enzyme Katanin is emerging as a critical regulator of MT dynamics. In Caenorhabditis elegans, Katanin MT-severing activity is essential for meiotic spindle assembly but is toxic for the mitotic spindle. Here we analyzed Katanin dynamics in C. elegans and deciphered the role of Katanin phosphorylation in the regulation of its activity and stability. Katanin is abundant in oocytes, and its levels drop after meiosis, but unexpectedly, a significant fraction is present throughout embryogenesis, where it is dynamically recruited to the centrosomes and chromosomes during mitosis. We show that the minibrain kinase MBK-2, which is activated during meiosis, phosphorylates Katanin at multiple serines. We demonstrate unequivocally that Katanin phosphorylation at a single residue is necessary and sufficient to target Katanin for proteasomal degradation after meiosis, whereas phosphorylation at the other sites only inhibits Katanin ATPase activity stimulated by MTs. Our findings suggest that cycles of phosphorylation and dephosphorylation fine-tune Katanin level and activity to deliver the appropriate MT-severing activity during development.

The LisH Domain-Containing N-Terminal Fragment is Important for the Localization, Dimerization, and Stability of Katnal2 in Tetrahymena

Katanin-like 2 protein (Katnal2) orthologs have a tripartite domain organization. Two highly conserved regions, an N-terminal LisH (Lis-homology) domain and a C-terminal AAA catalytic domain, are separated by a less conserved linker. The AAA domain of Katnal2 shares the highest amino acid sequence homology with the AAA domain of the canonical katanin p60. Katnal2 orthologs are present in a wide range of eukaryotes, from protists to humans. In the ciliate Tetrahymena thermophila, a Katnal2 ortholog, Kat2, co-localizes with the microtubular structures, including basal bodies and ciliary outer doublets, and this co-localization is sensitive to levels of microtubule glutamylation. The functional analysis of Kat2 domains suggests that an N-terminal fragment containing a LisH domain plays a role in the subcellular localization, dimerization, and stability of Kat2.

Katanin Grips the β-Tubulin Tail through an Electropositive Double Spiral to Sever Microtubules

The AAA ATPase katanin severs microtubules. It is critical in cell division, centriole biogenesis, and neuronal morphogenesis. Its mutation causes microcephaly. The microtubule templates katanin hexamerization and activates its ATPase. The structural basis for these activities and how they lead to severing is unknown. Here, we show that β-tubulin tails are necessary and sufficient for severing. Cryoelectron microscopy (cryo-EM) structures reveal the essential tubulin tail glutamates gripped by a double spiral of electropositive loops lining the katanin central pore. Each spiral couples allosterically to the ATPase and binds alternating, successive substrate residues, with consecutive residues coordinated by adjacent protomers. This tightly couples tail binding, hexamerization, and ATPase activation. Hexamer structures in different states suggest an ATPase-driven, ratchet-like translocation of the tubulin tail through the pore. A disordered region outside the AAA core anchors katanin to the microtubule while the AAA motor exerts the forces that extract tubulin dimers and sever the microtubule.

Cell Wall Modifications in Giant Cells Induced by the Plant Parasitic Nematode Meloidogyne incognita in Wild-Type (Col-0) and the fra2 Arabidopsis thaliana Katanin Mutant

Meloidogyne incognita is a root knot nematode (RKN) species which is among the most notoriously unmanageable crop pests with a wide host range. It inhabits plants and induces unique feeding site structures within host roots, known as giant cells (GCs). The cell walls of the GCs undergo the process of both thickening and loosening to allow expansion and finally support nutrient uptake by the nematode. In this study, a comparative in situ analysis of cell wall polysaccharides in the GCs of wild-type Col-0 and the microtubule-defective fra2 katanin mutant, both infected with M. incognita has been carried out. The fra2 mutant had an increased infection rate. Moreover, fra2 roots exhibited a differential pectin and hemicellulose distribution when compared to Col-0 probably mirroring the fra2 root developmental defects. Features of fra2 GC walls include the presence of high-esterified pectic homogalacturonan and pectic arabinan, possibly to compensate for the reduced levels of callose, which was omnipresent in GCs of Col-0. Katanin severing of microtubules seems important in plant defense against M. incognita, with the nematode, however, to be nonchalant about this "katanin deficiency" and eventually induce the necessary GC cell wall modifications to establish a feeding site.

Xyloglucans and Microtubules Synergistically Maintain Meristem Geometry and Phyllotaxis

The shoot apical meristem (SAM) gives rise to all aerial plant organs. Cell walls are thought to play a central role in this process, translating molecular regulation into dynamic changes in growth rate and direction, although their precise role in morphogenesis during organ formation is poorly understood. Here, we investigated the role of xyloglucans (XyGs), a major, yet functionally poorly characterized, wall component in the SAM of Arabidopsis (Arabidopsis thaliana). Using immunolabeling, biochemical analysis, genetic approaches, microindentation, laser ablation, and live imaging, we showed that XyGs are important for meristem shape and phyllotaxis. No difference in the Young's modulus (i.e. an indicator of wall stiffness) of the cell walls was observed when XyGs were perturbed. Mutations in enzymes required for XyG synthesis also affect other cell wall components such as cellulose content and pectin methylation status. Interestingly, control of cortical microtubule dynamics by the severing enzyme KATANIN became vital when XyGs were perturbed or absent. This suggests that the cytoskeleton plays an active role in compensating for altered cell wall composition.

Primary cilium loss in mammalian cells occurs predominantly by whole-cilium shedding

The primary cilium is a central signaling hub in cell proliferation and differentiation and is built and disassembled every cell cycle in many animal cells. Disassembly is critically important, as misregulation or delay of cilia loss leads to cell cycle defects. The physical means by which cilia are lost are poorly understood but are thought to involve resorption of ciliary components into the cell body. To investigate cilium loss in mammalian cells, we used live-cell imaging to comprehensively characterize individual events. The predominant mode of cilium loss was rapid deciliation, in which the membrane and axoneme of the cilium was shed from the cell. Gradual resorption was also observed, as well as events in which a period of gradual resorption was followed by rapid deciliation. Deciliation resulted in intact shed cilia that could be recovered from culture medium and contained both membrane and axoneme proteins. We modulated levels of katanin and intracellular calcium, two putative regulators of deciliation, and found that excess katanin promotes cilia loss by deciliation, independently of calcium. Together, these results suggest that mammalian ciliary loss involves a tunable decision between deciliation and resorption.

Microtubule bundling by MAP65-1 protects against severing by inhibiting the binding of katanin

The microtubule-severing enzyme katanin (KTN1) regulates the organization and turnover of microtubule arrays by the localized breakdown of microtubule polymers. In land plants, KTN1 activity is essential for the formation of linearly organized cortical microtubule arrays that determine the axis of cell expansion. Cell biological studies have shown that even though KTN1 binds to the sidewalls of single and bundled microtubules, severing activity is restricted to microtubule cross-over and nucleation sites, indicating that cells contain protective mechanisms to prevent indiscriminate microtubule severing. Here, we show that the microtubule-bundling protein MAP65-1 inhibits KTN1-mediated microtubule severing in vitro. Severing is inhibited at bundled microtubule segments and the severing rate of nonbundled microtubules is reduced by MAP65-1 in a concentration-dependent manner. Using various MAP65-1 mutant proteins, we demonstrate that efficient cross-linking of microtubules is crucial for this protective effect and that microtubule binding alone is not sufficient. Reduced severing due to microtubule bundling by MAP65-1 correlated to decreased binding of KTN1 to these microtubules. Taken together, our work reveals that cross-linking of microtubules by MAP65-1 confers resistance to severing by inhibiting the binding of KTN1 and identifies the structural features of MAP65-1 that are important for this activity.

Elk1 affects katanin and spastin proteins via differential transcriptional and post-transcriptional regulations

Microtubule severing, which is highly critical for the survival of both mitotic and post-mitotic cells, has to be precisely adjusted by regulating the expression levels of severing proteins, katanin and spastin. Even though severing mechanism is relatively well-studied, there are limited studies for the transcriptional regulation of microtubule severing proteins. In this study, we identified the main regulatory region of KATNA1 gene encoding katanin-p60 as 5’ UTR, which has a key role for its expression, and showed Elk1 binding to KATNA1. Furthermore, we identified that Elk1 decreased katanin-p60 and spastin protein expressions, while mRNA levels were increased upon Elk1 overexpression. In addition, SUMOylation is a known post-translational modification regulating Elk1 activity. A previous study suggested that K230, K249, K254 amino acids in the R domain are the main SUMOylation sites; however, we identified that these amino acids are neither essential nor substantial for Elk1 SUMOylation. Also, we determined that KATNA1 methylation results in the reduction of Elk1 binding whereas SPG4 methylation does not. Together, our findings emphasizing the impacts of both transcriptional and post-transcriptional regulations of katanin-p60 and spastin suggest that Elk1 has a key role for differential expression patterns of microtubule severing proteins, thereby regulating cellular functions through alterations of microtubule organization.

CLASP stabilization of plus ends created by severing promotes microtubule creation and reorientation

Central to the building and reorganizing cytoskeletal arrays is creation of new polymers. Although nucleation has been the major focus of study for microtubule generation, severing has been proposed as an alternative mechanism to create new polymers, a mechanism recently shown to drive the reorientation of cortical arrays of higher plants in response to blue light perception. Severing produces new plus ends behind the stabilizing GTP-cap. An important and unanswered question is how these ends are stabilized in vivo to promote net microtubule generation. Here we identify the conserved protein CLASP as a potent stabilizer of new plus ends created by katanin severing in plant cells. Clasp mutants are defective in cortical array reorientation. In these mutants, both rescue of shrinking plus ends and the stabilization of plus ends immediately after severing are reduced. Computational modeling reveals that it is the specific stabilization of severed ends that best explains CLASP's function in promoting microtubule amplification by severing and array reorientation.

Severing enzymes amplify microtubule arrays through lattice GTP-tubulin incorporation

Spastin and katanin sever and destabilize microtubules. Paradoxically, despite their destructive activity they increase microtubule mass in vivo. We combined single-molecule total internal reflection fluorescence microscopy and electron microscopy to show that the elemental step in microtubule severing is the generation of nanoscale damage throughout the microtubule by active extraction of tubulin heterodimers. These damage sites are repaired spontaneously by guanosine triphosphate (GTP)-tubulin incorporation, which rejuvenates and stabilizes the microtubule shaft. Consequently, spastin and katanin increase microtubule rescue rates. Furthermore, newly severed ends emerge with a high density of GTP-tubulin that protects them against depolymerization. The stabilization of the newly severed plus ends and the higher rescue frequency synergize to amplify microtubule number and mass. Thus, severing enzymes regulate microtubule architecture and dynamics by promoting GTP-tubulin incorporation within the microtubule shaft.

Microtubule Hyperacetylation Enhances KL1-Dependent Micronucleation under a Tau Deficiency in Mammary Epithelial Cells

Enhanced microtubule acetylation has been identified as a negative prognostic indicator in breast cancer. We reported previously that primary cultured human mammary epithelial cells manifest breast cancer-related aneuploidization via the activation of severing protein katanin-like (KL)1 when tau is deficient. To address in this current study whether microtubule hyperacetylation is involved in breast carcinogenesis through mitosis, the effects of tubacin on human mammary epithelial cells were tested using immunofluorescence techniques. Tau-knockdown cells showed enhancement of KL1-dependent events, chromosome-bridging and micronucleation in response to tubacin. These enhancements were suppressed by further expression of an acetylation-deficient tubulin mutant. Consistently, using a rat fibroblast-based microtubule sensitivity test, it was confirmed that KL1 also shows enhanced activity in response to microtubule hyperacetylation as well as katanin. It was further observed in rat fibroblasts that exogenously expressed KL1 results in more micronucleation under microtubule hyperacetylation conditions. These data suggest that microtubule acetylation upregulates KL1 and induces more aneuploidy if tau is deficient. It is thus plausible that microtubule hyperacetylation promotes tumor progression by enhancing microtubule sensitivity to KL1, thereby disrupting spindle microtubules and this process could be reversed by the microtubule-binding and microtubule protective octapeptide NAPVSIPQ (NAP) which recruits tau to the microtubules.

Katanin-like 2 (KATNAL2) functions in multiple aspects of haploid male germ cell development in the mouse

The katanin microtubule-severing proteins are essential regulators of microtubule dynamics in a diverse range of species. Here we have defined critical roles for the poorly characterised katanin protein KATNAL2 in multiple aspects of spermatogenesis: the initiation of sperm tail growth from the basal body, sperm head shaping via the manchette, acrosome attachment, and ultimately sperm release. We present data suggesting that depending on context, KATNAL2 can partner with the regulatory protein KATNB1 or act autonomously. Moreover, our data indicate KATNAL2 may regulate δ- and ε-tubulin rather than classical α-β-tubulin microtubule polymers, suggesting the katanin family has a greater diversity of function than previously realised. Together with our previous research, showing the essential requirement of katanin proteins KATNAL1 and KATNB1 during spermatogenesis, our data supports the concept that in higher order species the presence of multiple katanins has allowed for subspecialisation of function within complex cellular settings such as the seminiferous epithelium.

Male infertility affects one in twenty men of reproductive age in western countries. Despite this, the biochemical basis of common defects, including reduced sperm count and abnormal sperm structure and function, remains poorly defined. Microtubules are cellular “scaffolds” that serve critical roles in all cells, including developing male germ cells wherein they facilitate mitosis and meiosis (cell division), sperm head remodelling and sperm tail formation. The precise regulation of microtubule number, length and movement is thus, essential for male fertility. Within this manuscript, we have used spermatogenesis to define the function of the putative microtubule-severing protein katanin-like 2 (KATNAL2). We show that mice with compromised KATNAL2 function are male sterile as a consequence of defects in the structural remodelling of germ cells. Notably, we show the function of microtubule-based structures involved in sperm head shaping and tail formation are disrupted. Further, we show for the first time, that KATNAL2 can function both independently or in concert with the katanin regulatory protein KATNB1 and that it can target the poorly characterized tubulin subunits delta and epsilon. Our research has immediate relevance to the origins of human male infertility and provides novel insights into aspects of microtubule regulation relevant to numerous tissues and species.

[Microtubule severing proteins - structure and activity regulation]

Microtubule severing proteins, katanin, spastin and fidgetin cause local destabilization of the microtubules structure. This ATP-dependent activity leads to the shortening or disassembly of the existing microtubules. The generated short microtubule fragments may serve as templates to polymerize new microtubules and in consequence, the activity of the microtubule severing proteins leads to the reorganization of the microtubular cytoskeleton. This review summarizes current knowledge concerning structural organization of the microtubule severing proteins, the molecular mechanism of their action, factors that regulate the level of the katanin and spastin within the cells and their microtubule severing activity.

[Role of microtubule severing proteins in cytoskeleton reorganization]

ATP-dependent severing activity of microtubule severing proteins leads to the local destabilization of the microtubule structure and causes shortening or disassembly of the existing microtubules or formation of the numerous short microtubule fragments that serve as templates during new microtubule polymerization. Microtubule severing protein-dependent rearrangement of the microtubular cytoskeleton plays an important role in the numerous cellular processes including chromosome segregation during meiosis and mitosis, cells migration, dendrites and axon formation, cilia assembly and arrangement of the cortical microtubules in plant cells.