Highlights d The g-tubulin ring complex (gTuRC) nucleates microtubules and caps their minus ends d Microtubule nucleation from purified gTuRC is highly cooperative, yet inefficient d A partly open, asymmetric structure of gTuRC explains inefficient nucleation d Actin and MZT2 stabilize the closed part of the gTuRC structure

Background:The causal relationship between tubulin acetylation and microtubule stability has remained poorly understood. Results: Pharmacological inhibition of HDAC6 increased HDAC6 binding to microtubules, enhanced microtubule stability, and suppressed dynamics. Conclusion: Increased binding of HDAC6, rather than acetylation per se, causes microtubule stability. Significance: The study indicates a MAP-like function of HDAC6 that extends beyond its tubulin deacetylase function.

BackgroundGriseofulvin, an antifungal drug, has recently been shown to inhibit proliferation of various types of cancer cells and to inhibit tumor growth in athymic mice. Due to its low toxicity, griseofulvin has drawn considerable attention for its potential use in cancer chemotherapy. This work aims to understand how griseofulvin suppresses microtubule dynamics in living cells and sought to elucidate the antimitotic and antiproliferative action of the drug.MethodsThe effects of griseofulvin on the dynamics of individual microtubules in live MCF-7 cells were measured by confocal microscopy. Immunofluorescence microscopy, western blotting and flow cytometry were used to analyze the effects of griseofulvin on spindle microtubule organization, cell cycle progression and apoptosis. Further, interactions of purified tubulin with griseofulvin were studied in vitro by spectrophotometry and spectrofluorimetry. Docking analysis was performed using autodock4 and LigandFit module of Discovery Studio 2.1.ResultsGriseofulvin strongly suppressed the dynamic instability of individual microtubules in live MCF-7 cells by reducing the rate and extent of the growing and shortening phases. At or near half-maximal proliferation inhibitory concentration, griseofulvin dampened the dynamicity of microtubules in MCF-7 cells without significantly disrupting the microtubule network. Griseofulvin-induced mitotic arrest was associated with several mitotic abnormalities like misaligned chromosomes, multipolar spindles, misegregated chromosomes resulting in cells containing fragmented nuclei. These fragmented nuclei were found to contain increased concentration of p53. Using both computational and experimental approaches, we provided evidence suggesting that griseofulvin binds to tubulin in two different sites; one site overlaps with the paclitaxel binding site while the second site is located at the αβ intra-dimer interface. In combination studies, griseofulvin and vinblastine were found to exert synergistic effects against MCF-7 cell proliferation.ConclusionsThe study provided evidence suggesting that griseofulvin shares its binding site in tubulin with paclitaxel and kinetically suppresses microtubule dynamics in a similar manner. The results revealed the antimitotic mechanism of action of griseofulvin and provided evidence suggesting that griseofulvin alone and/or in combination with vinblastine may have promising role in breast cancer chemotherapy.

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We have synthesized eight analogues (D1-D8) of dolastatin 10 containing several unique amino acid subunits. Of these agents, D5 was found to be most effective in inhibiting both HeLa cell proliferation and microtubule assembly in vitro. At low nanomolar concentrations, D5 inhibited the proliferation of several types of cancer cells in culture. D5 bound to tubulin with a dissociation constant of 29.4 ± 6 μM. D5 depolymerized microtubules in cultured cells and produced mulitpolar spindles. At its half-maximal inhibitory concentration (15 nM), D5 strongly suppressed the dynamics of individual microtubules in live MCF-7 cells. D5 increased the accumulation of checkpoint proteins BubR1 and Mad2 at the kinetochoric region and caused G2/M block in these cells. The blocked cells underwent apoptosis with the activation of Jun N-terminal kinase. The results suggested that D5 exerts its antiproliferative action by dampening microtubule dynamics.

Recently, we found that divalent calcium has no detectable effect on the assembly of Mycobacterium tuberculosis FtsZ (MtbFtsZ), whereas it strongly promoted the assembly of Escherichia coli FtsZ (EcFtsZ). While looking for potential calcium binding residues in EcFtsZ, we found a mutation (E93R) that strongly promoted the assembly of EcFtsZ. The mutation increased the stability and bundling of the FtsZ protofilaments and produced a dominating effect on the assembly of the wild type FtsZ (WT-FtsZ). Although E93R-FtsZ was found to bind to GTP similarly to the WT-FtsZ, it displayed lower GTPase activity than the WT-FtsZ. E93R-FtsZ complemented for its wild type counterpart as observed by a complementation test using JKD7-1/pKD3 cells. However, the bacterial cells became elongated upon overexpression of the mutant allele. We modeled the structure of E93R-FtsZ using the structures of MtbFtsZ/Methanococcus jannaschi FtsZ (MjFtsZ) dimers as templates. The MtbFtsZ-based structure suggests that the Arg 93 -Glu 138 salt bridge provides the additional stability, whereas the effect of mutation appears to be indirect (allosteric) if the EcFtsZ dimer is similar to that of MjFtsZ. The data presented in this study suggest that an increase in the stability of the FtsZ protofilaments is detrimental for the bacterial cytokinesis.Cell division in almost all bacteria is primarily driven by FtsZ, a protein considered as the homologue of the eukaryotic cytoskeletal protein tubulin (1, 2). FtsZ is a highly conserved protein in prokaryotes and plays a key role in the spatiotemporal regulation of the divisome assembly (2). Polymerization of FtsZ results in the formation of a ring-like structure, known as the Z-ring. The Z-ring is a highly dynamic structure (3), and the dynamics of the Z-ring is attributed to the assembly and disassembly of FtsZ protofilaments (4). Perturbation of the FtsZ assembly has been reported to impair bacterial cell division (5-9).Spatiotemporal regulation of the Z-ring at the midcell is tightly regulated by several effectors of FtsZ assembly (10 -12). Although the arrangement of FtsZ polymers in the bacterial Z-ring has not yet been visualized, the reconstitution of the FtsZ polymers in vitro has contributed a lot to our understanding of the bacterial cell division. Recently, an attempt was made to reconstitute the Z-ring in a liposome in vitro, and it was found that neither FtsZ assembly nor the generation of the constriction force requires FtsA or other downstream division proteins (13). Further, FtsZ polymers have been found to have intrinsic ability to form cytoplasmic ringlike structures in yeast cells, indicating that FtsZ assembly into the ring may not require any other regulatory protein (14).FtsZ protein of different bacteria exhibit different assembly properties in vitro. For example, the polymerization of Escherichia coli FtsZ (EcFtsZ) 2 has been observed to be more rapid than that of the Mycobacterium tuberculosis FtsZ (MtbFtsZ) (15). Recently, we have shown that divalent calcium strongly influenced ...

Background: Dynein Light Chain 1 (LC8) has been shown to pull down tubulin subunits, suggesting that it interacts with microtubules. Results: LC8 decorates microtubules in vitro and in Drosophila embryos, promotes microtubule assembly, and stabilizes microtubules both in vitro and in tissue-cultured cells. Conclusion: LC8 stabilizes microtubules. Significance: Data provide the first evidence of a novel MAP-like function of LC8.

The γ-tubulin ring complex (γTuRC) is the major microtubule nucleator in cells. However, the mechanism of its regulation is not understood. Here, we purified human γTuRC and quantitatively characterized its nucleation properties in a TIRF microscopy-based real-time nucleation assay. We find that microtubule nucleation by γTuRC is kinetically inhibited compared to microtubule elongation. Determining the cryo-EM structure of γTuRC at 4 Å resolution reveals an asymmetric conformation with only part of the complex in a 'closed' conformation matching the microtubule geometry. Several factors stabilise the closed conformation. One is actin in the core of the complex and others, likely MZT1 or MZT2, line the outer perimeter of the closed part of γTuRC. The opposed side of γTuRC is in an 'open', nucleation-incompetent conformation, leading to a structural asymmetry, explaining the kinetic inhibition of nucleation by human γTuRC. Our data suggest possible regulatory mechanisms for microtubule nucleation by γTuRC closure.