Aim44p regulates phosphorylation of Hof1p to promote contractile ring closure during cytokinesis in budding yeast

Whereas actomyosin and septin ring organization and function in cytokinesis are thoroughly described, little is known regarding the mechanisms by which the actomyosin ring interacts with septins and associated proteins to coordinate cell division. Here we show that the protein product of YPL158C, Aim44p, undergoes septin-dependent recruitment to the site of cell division. Aim44p colocalizes with Myo1p, the type II myosin of the contractile ring, throughout most of the cell cycle. The Aim44p ring does not contract when the actomyosin ring closes. Instead, it forms a double ring that associates with septin rings on mother and daughter cells after cell separation. Deletion of AIM44 results in defects in contractile ring closure. Aim44p coimmunoprecipitates with Hof1p, a conserved F-BAR protein that binds both septins and type II myosins and promotes contractile ring closure. Deletion of AIM44 results in a delay in Hof1p phosphorylation and altered Hof1p localization. Finally, overexpression of Dbf2p, a kinase that phosphorylates Hof1p and is required for relocalization of Hof1p from septin rings to the contractile ring and for Hof1p-triggered contractile ring closure, rescues the cytokinesis defect observed in aim44∆ cells. Our studies reveal a novel role for Aim44p in regulating contractile ring closure through effects on Hof1p.     

Two distinct modes of myosin assembly and dynamics during epithelial wound closure

Actomyosin contraction powers the sealing of epithelial sheets during embryogenesis and wound closure; however, the mechanisms are poorly understood. After laser ablation wounding of Madin-Darby canine kidney cell monolayers, we observed distinct steps in wound closure from time-lapse images of myosin distribution during resealing. Immediately upon wounding, actin and myosin II regulatory light chain accumulated at two locations: (1) in a ring adjacent to the tight junction that circumscribed the wound and (2) in fibers at the base of the cell in membranes extending over the wound site. Rho-kinase activity was required for assembly of the myosin ring, and myosin II activity was required for contraction but not for basal membrane extension. As it contracted, the myosin ring moved toward the basal membrane with ZO-1 and Rho-kinase. Thus, we suggest that tight junctions serve as attachment points for the actomyosin ring during wound closure and that Rho-kinase is required for localization and activation of the contractile ring.     

DNA bending induced by the catabolite activator protein allows ring formation of a 144 bp DNA

The effect of the catabolite activator protein, CAP, on the ligation of a 144 bp DNA was examined. This DNA has EcoRI ends and contains the lac operon CAP site and promoter-operator region. At low DNA concentrations (nM) and 37 degrees C the presence of CAP and cAMP enables T4 ligase to convert the linear duplex to a covalently closed ring. Nuclease digestion and sedimentation equilibrium studies show that the ring is a monomer circle. Ring formation does not occur in the absence of either CAP or cAMP. The kinetics of ring closure, and the bimolecular joining of two fragments were measured. The presence of CAP decreased the rate of bimolecular joining of the EcoRI ends of linear DNAs. Thus the measured rates of ring closure are likely to be a lower limit for this process. Closure reactions carried out with ethidium bromide indicate that CAP induced bending rather than twisting is responsible for ring formation. The all or none nature of the closure reaction suggests that persistence length DNAs may be useful in a simple assay for protein induced DNA bending.     

A new synthesis of inosine from 5-amino-1-beta-D-ribofuranosyl-4-imidazole-carboxamide.

Inosine was prepared (15% yield) by treatment of 5-amino-1-beta-D-ribofuranosyl-4-imidazolecarboxamide (AICA-riboside) with chloroform in the presence of sodium methoxide. This ring closure can be reasonably explained by assuming the formation of dichlorocarbene from chloroform and alkali. Carbon tetrachloride or hexachloroethane as a carbene source was more effective for the ring closure of AICA-riboside, giving inosine in 48% and 51% yields respectively.     

Closing the ring: historical biogeography of the salamander ring species Ensatina eschscholtzii

Aim The salamander Ensatina eschscholtzii Gray is a classic example of a ring species, or a species that has expanded around a central barrier to form a secondary contact characterized by species‐level divergence. In the original formulation of the ring species scenario, an explicit biogeographical model was proposed to account for the occurrence of intraspecific sympatry between two subspecies in southern California (the ‘southern closure’ model). Here we develop an alternative ring species model that is informed by the geomorphological development of the California Coast Ranges, and which situates the point of ring closure in the Monterey Bay region of central coastal California (the ‘Monterey closure’ model). Our study has two aims. The first is to use phylogenetic methods to evaluate the two competing biogeographical models. The second is to describe patterns of phylogeographical diversity throughout the range of the Ensatina complex, and to compare these patterns with previously published molecular systematic data. Location Western North America, with a focus on the state of California, USA. Methods We obtained mitochondrial DNA sequence data from 385 individuals from 224 populations. A phylogeny was inferred using Bayesian techniques, and the geographical distributions of haplotypes and clades were mapped. The two biogeographical ring species models were tested against our Bayesian topology, including the associated Bayesian 95% credible set of trees. Results High levels of phylogeographical diversity were revealed, especially in central coastal and northern California. Our Bayesian topology contradicts the Monterey closure model; however, 0.08% of the trees in our Bayesian 95% credible set are consistent with this model. In contrast, the classic ring species biogeographical model (the southern closure model) is consistent with our Bayesian topology, as were 99.92% of the trees in our 95% credible set. Main conclusions Our Bayesian phylogenetic analysis most strongly supports the classic ring species model, modified to accommodate an improved understanding of the complex geomorphological evolution of the California Coast Ranges. In addition, high levels of phylogeographical diversity in central and northern California were identified, which is consistent with the striking levels of allozymic differentiation reported previously from those regions.     

Bond-optimized ring closure for proline: comparison of conformations and semiempirical energies with small molecule X-ray structures

A method is described for generating proline ring structures by successive addition of atoms, wherein ring closure is achieved by optimizing the fit to known ring bond-angles and one closing bond-length ("bond-optimized ring closure"). Two ring torsion angles are fixed independently within broad, allowed ranges, and the remaining torsion angles are determined uniquely in most cases. The independent torsion angles are chosen as phi and chi 2, and ring closure is achieved without prohibitive strain through most of the ranges -130 degrees less than phi less than -20 degrees and -60 degrees less than chi 2 less than 60 degrees. Comparisons of predicted ring structures to 191 X-ray diffraction structures from the literature, starting with the known values of phi and chi 2, yielded root-mean-square deviations of 4.8 degrees in chi 1, 4.7 degrees in chi 3, 8.3 degrees in chi 4, and 0.3-2% in the ring bond angles and the N-C delta distance. Semiempirical energies were calculated for the optimized structures using three sets of energy parameters from the literature. The energy surfaces show broad minima coinciding with the torsion angle regions in which the highest concentrations of observed structures are found. Two of the sets of energy parameters produce double minima corresponding to the "up" and "down" puckered conformations.     

Actomyosin tube formation in polar body cytokinesis requires Anillin in C. elegans

Polar body extrusion (PBE) is the specialized asymmetric division by which oocytes accomplish reduction in ploidy and retention of cytoplasm. During maternal gametogenesis, as in male meiosis and mitosis, cytokinesis is accomplished by a ring rich in active Rho, myosin, and formin-nucleated F-actin [1-7]. However, unlike mitosis, wherein the contractile ring encircles the cell equator, the polar body ring assembles as a discoid cortical washer. Here we show that in Caenorhabditis elegans, the meiotic contractile ring transforms during closure from a disc above the spindle to a cylinder around the spindle midzone. The meiotic midbody tube comprises stacked cytoskeletal rings. This topological transition suggests a novel mechanism for constriction of an initially discoid cytokinetic ring. Analysis of mouse PBE indicates that midbody tube formation is a conserved process. Depletion of the scaffold protein anillin (ANI-1) from C. elegans results in large and unstable polar bodies that often fuse with the oocyte. Anillin is dispensable for contractile ring assembly, initiation, and closure but is required for the meiotic contractile ring to transform from a disc into a tube. We propose that cytoskeletal bundling by anillin promotes formation of the midbody tube, which ensures the fidelity of PBE.     

src64 and tec29 are required for microfilament contraction during Drosophila cellularization

Formation of the Drosophila cellular blastoderm involves both membrane invagination and cytoskeletal regulation. Mutations in src64 and tec29 reveal a novel role for these genes in controlling contraction of the actin-myosin microfilament ring during this process. Although membrane invagination still proceeds in mutant embryos, its depth is not uniform, and basal closure of the cells does not occur during late cellularization. Double-mutant analysis between scraps, a mutation in anillin that eliminates microfilament rings, and bottleneck suggests that microfilaments can still contract even though they are not organized into rings. However, the failure of rings to contract in the src64 bottleneck double mutant suggests that src64 is required for microfilament ring contraction even in the absence of Bottleneck protein. Our results suggest that src64-dependent microfilament ring contraction is resisted by Bottleneck to create tension and coordinate membrane invagination during early cellularization. The absence of Bottleneck during late cellularization allows src64-dependent microfilament ring constriction to drive basal closure.     

Analysis of inducible contractile rings suggests a role for protein kinase C in embryonic cytokinesis and wound healing.

A semi-in vitro system derived from Xenopus oocytes which allows induction of contractile ring (CR) formation and closure is described and exploited to elucidate regulatory and structural features of cytokinesis. The inducible CRs (ICRs) are composed of actin filaments and closure is actin filament-dependent as is cytokinesis in vivo. ICR closure in this system is calcium-dependent and pH-sensitive, as is cytokinesis in permeabilized cells (Cande: Journal of Cell Biology 87:326, 1980). Closure of ICRs proceeds at a rate and with a kinetic pattern similar to embryonic cytokinesis. Collectively, these data demonstrate that this system is a faithful mimic of cytokinesis in vivo. ICR formation and closure is protein kinase C (PKC)-dependent and neomycin-sensitive, indicating that the PKC branch of the polyphosphoinositide pathway regulates formation of the actomyosin ring which is the effector of cytokinesis. Kinetic measurements show that the rate of ICR closure reaches a peak of 4-8 microns/sec. Since the maximum measured velocity of actin filament translocation by vertebrate, non-muscle myosins is 0.04 micron/sec, the later observations support a model in which the CR is segmented, containing multiple sites where filaments overlap in a "sliding filament" fashion. Because the rate decreases after reaching a peak, the results also suggest that the number of overlap sites decrease with time.     

Application of combinatorial biocatalysis for a unique ring expansion of dihydroxymethylzearalenone

Combinatorial biocatalysis was applied to generate a diverse set of dihydroxymethylzearalenone analogs with modified ring structure. In one representative chemoenzymatic reaction sequence, dihydroxymethylzearalenone was first subjected to a unique enzyme-catalyzed oxidative ring opening reaction that creates two new carboxylic groups on the molecule. These groups served as reaction sites for further derivatization involving biocatalytic ring closure reactions with structurally diverse bifunctional reagents, including different diols and diamines. As a result, a library of cyclic bislactones and bislactams was created, with modified ring structures covering chemical space and structure activity relationships unattainable by conventional synthetic means.     

Quantitative Analysis of Cytokinesis In Situ during C. elegans Postembryonic Development

The physical separation of a cell into two daughter cells during cytokinesis requires cell-intrinsic shape changes driven by a contractile ring. However, in vivo, cells interact with their environment, which includes other cells. How cytokinesis occurs in tissues is not well understood. Here, we studied cytokinesis in an intact animal during tissue biogenesis. We used high-resolution microscopy and quantitative analysis to study the three rounds of division of the C. elegans vulval precursor cells (VPCs). The VPCs are cut in half longitudinally with each division. Contractile ring breadth, but not the speed of ring closure, scales with cell length. Furrowing speed instead scales with division plane dimensions, and scaling is consistent between the VPCs and C. elegans blastomeres. We compared our VPC cytokinesis kinetics data with measurements from the C. elegans zygote and HeLa and Drosophila S2 cells. Both the speed dynamics and asymmetry of ring closure are qualitatively conserved among cell types. Unlike in the C. elegans zygote but similar to other epithelial cells, Anillin is required for proper ring closure speed but not asymmetry in the VPCs. We present evidence that tissue organization impacts the dynamics of cytokinesis by comparing our results on the VPCs with the cells of the somatic gonad. In sum, this work establishes somatic lineages in post-embryonic C. elegans development as cell biological models for the study of cytokinesis in situ.     

Determination of the extent of DNA bending by an adenine-thymine tract

We determined the magnitude of the bend induced in DNA by an adenine-thymine tract by measuring the rate of cyclization of DNA oligonucleotides containing phased A tracts. A series of linear multimers with 2-bp single-stranded ends, in which the (A.T)6 tracts are separated by CG2-3C sequences and are positioned 10 and 11 bp apart alternately, were prepared from 21 bp long synthetic duplexed deoxyoligonucleotides. The cyclization rates of the multimers (105-210 bp) and the bimolecular association rate of the 84 bp long multimer were measured in the presence of DNA ligase. From the rate constants of the cyclization and bimolecular association reactions, ring closure probabilities were obtained for the multimers. The systematically bent molecules were simulated by Monte Carlo methods, and the ring closure probabilities were calculated for a given set of junction bend angles. By comparing the calculated values of ring closure probabilities to experimental values and adjusting the junction bend angles to fit experimental values, the extent of bending at the junctions (or the extent of bending for an adenine tract) was determined. We conclude that an A6 tract bends the DNA helix by 17-21 degrees.     

Microbial transformations of galanthamin-precursors

Summary Galanthamin is a medical important alkaloid. Its chemical synthesis gives a racemic product in low yields. Starting with a belladinderivative an enzymatic ring closure should lead exclusively to a chiral product possibly with the native structure. Although this reactions type is unknown in preparative biotransformations a large number of microorganisms were tested, unfortunately without success. On the other hand in the screen transformation products were found resulting from specific dealkylations of the subtrate. The type of metabolite formed was dependent on the fungi utilized for the transformation. Additionally two N-oxides were formed by Septomyxa affinis, one in good yield. It is possible that the chirality of this compound can direct the ring closure preferentially or exclusively to the desired stereoisomer of narwedine.     

Partial Asymmetric Synthesis in α-Alkylation of Enamine with Electrophilic Olefin

About 30 dipeptides and some tripeptides were led to new benzimidazole derivatives by incorporating their carboxyl groups into benzimidazole ring by the reaction with o-phenylenediamine. The ring closure to benzimidazole was well achieved by heating mildly at a moderate temperature in acetic acid.

Some benzimidazole derivatives of peptides had remarkable phytotoxicities.     

Total synthesis of diazasteroids (1).

Contrary to previous reports, the condensation product of 8-aminoquinoline (1) and 2-carbethoxycyclopentanone (2) undergoes ring closure with polyphosphoric acid to give the 1, 11-diazasteroid 5. Catalytic hydrogenation reduced the A ring and the double bond to produce 6. 8-Aminotetrahydroquinoline (7) and 3-carbethoxy-2-piperidone condense to a tricyclic intermediate (11), which could not be cyclized to a steroid, however.     

New steroid-fused P-heterocycles. Part II. Synthesis and conformational study of oxazaphosphorino[16,17-e]estrone derivatives

16Beta-aminomethyl-17beta-hydroxyestrone 3-methyl ether 6 and its N-propyl (17), N-benzyl (18) and N-arylmethyl derivatives (19-22) were subjected to ring closure reactions with phenylphosphonic dichloride in order to synthetize P-epimeric oxazaphosphorinanes 23a, 24-29 in which the hetero ring is condensed to ring D of the sterane skeleton. The stereostructures of the products were evaluated by 1H, 13C and 31P NMR spectroscopy. The geometry was optimized by utilizing the B3LYP DFT method. The NMR spectral data and the results of the ab initio calculations demonstrated that the stereostructure of the hetero ring was strongly affected by the rigid sterane framework condensed to it, and the phosphoramidate ring proved to adopt predominantly a distorted-boat conformation, regardless of the P-configuration.     

The synthesis and biological activity of certain pentaazaacenaphthylenes, hexaazaacenaphthylenes and their corresponding nucleosides

Synthesis of the tricyclic nucleoside 8-amino-6-N-methyl-2-(beta-D-ribo-furanosyl)-1,2,3,5,6,7-hexaazaacena phthylene (5) has been accomplished by the ring closure of an appropriately substituted pyrazolo[3,4-d]-pyrimidine nucleoside followed by the requisite chemical conversions. The formation, isolation and structural elucidation of two unexpected nucleosides formed by a reductive ring cleavage of the hexaazaacenaphthylene ring system is discussed. A comparison of the antitumor and biological activity of 5 with the structurally related tricyclic pentaazaacenaphthylene nucleoside which is currently in phase II clinical trials at the 5'-phosphate pro-drug is also presented.     

Microwave-assisted synthesis and antimicrobial activities of flavonoid derivatives

Eleven flavonoid derivatives were synthesised using a modified Baker-Venkataraman rearrangement, and subsequent microwave-assisted closure of the heterocyclic ring. All of the synthetic compounds displayed antifungal activity against Aspergillus niger and Fusarium oxysporium, and two of the synthetic flavonoid analogues exhibited significant activity against methicillin-resistant Staphylococcus aureus.     

Wound-induced contractile ring: a model for cytokinesis.

The actomyosin-based contractile ring is required for several biological processes, such as wound healing and cytokinesis of animal cells. Despite progress in defining the roles of this structure in both wound closure and cell division, we still do not fully understand how an actomyosin ring is spatially and temporally assembled, nor do we understand the molecular mechanism of its contraction. Recent results have demonstrated that microtubule-dependent local assembly of F-actin and myosin-II is present in wound closure and is similar to that in cytokinesis in animal cells. Furthermore, signalling factors such as small Rho GTPases have been shown to be involved in the regulation of actin dynamics during both processes. In this review we address recent findings in an attempt to better understand the dynamics of actomyosin contractile rings during wound healing as compared with the final step of animal cell division.