We purified an intracellular pyranose oxidase from mycelial extracts of the white rot fungus Trametes multicolor by using ammonium sulfate fractionation, hydrophobic interaction, ion-exchange chromatography, and gel filtration. The native enzyme has a molecular mass of 270 kDa as determined by equilibrium ultracentrifugation and is composed of four identical 68-kDa subunits as determined by matrix-assisted laser desorption ionization mass spectrometry. Each subunit contains one covalently bound flavin adenine dinucleotide as its prosthetic group. The enzyme oxidizes several aldopyranoses specifically at position C-2, and its preferred electron donor substrates are D-glucose, D-xylose, and L-sorbose. During this oxidation reaction electrons are transferred to oxygen, yielding hydrogen peroxide. In addition, the enzyme catalyzes the twoelectron reduction of 1,4-benzoquinone, several substituted benzoquinones, and 2,6-dichloroindophenol, as well as the one-electron reduction of the ABTS [2,2-azinobis(3-ethylbenzthiazolinesulfonic acid)] cation radical. As judged by the catalytic efficiencies (k cat /K m ), some of these quinone electron acceptors are much better substrates for pyranose oxidase than oxygen. The optimum pH of the pyranose oxidase-catalyzed reaction depends strongly on the electron acceptor employed and varies from 4 to 8. It has been proposed that the main metabolic function of pyranose oxidase is as a constituent of the ligninolytic system of white rot fungi that provides peroxidases with H 2 O 2 . An additional function could be reduction of quinones, key intermediates that are formed during mineralization of lignin.The enzyme pyranose oxidase (P2O) (pyranose:oxygen 2-oxidoreductase; EC 1.1.3.10), which catalyzes the oxidation of several aldopyranoses at position C-2 to yield the corresponding 2-ketoaldoses (aldos-2-uloses, osones), is widely distributed among wood-degrading basidiomycetes (14,32,44). It has been purified and characterized from several microorganisms, including Phanerochaete chrysosporium (2, 45), Phlebiopsis gigantea (39), Pleurotus ostreatus (40), Polyporus obtusus (28), Trametes (Coriolus) versicolor (35), and unidentified basidiomycete no. 52 (26). The currently available data reveal some general similarities among P2Os from these different fungi. Typically, P2O is a rather large, homotetrameric protein that contains covalently bound flavin adenine dinucleotide (FAD). The in vivo substrates of P2O probably are D-glucose, D-galactose, and D-xylose, which are abundant in lignocellulose and which are oxidized to 2-keto-D-glucose (D-arabino-hexos-2-ulose, 2-dehydro-D-glucose), 2-keto-D-galactose (D-lyxo-hexos-2-ulose, 2-dehydro-D-galactose), and 2-keto-D-xylose (D-threopentos-2-ulose, 2-dehydro-D-xylose), respectively. In addition, P2O also exhibits significant activity with a number of other carbohydrates, including L-sorbose, D-glucono-1,5-lactone, and D-allose (20). The substrate selectivity, however, varies to some extent among P2Os isolated from different fungi. During the oxi...

␥ -Tubulin is assumed to participate in microtubule nucleation in acentrosomal plant cells, but the underlying molecular mechanisms are still unknown. Here, we show that ␥ -tubulin is present in protein complexes of various sizes and different subcellular locations in Arabidopsis and fava bean. Immunoprecipitation experiments revealed an association of ␥ -tubulin with ␣␤ -tubulin dimers. ␥ -Tubulin cosedimented with microtubules polymerized in vitro and localized along their whole length. Large ␥ -tubulin complexes resistant to salt treatment were found to be associated with a highspeed microsomal fraction. Blue native electrophoresis of detergent-solubilized microsomes showed that the molecular mass of the complexes was Ͼ 1 MD. Large ␥ -tubulin complexes were active in microtubule nucleation, but nucleation activity was not observed for the smaller complexes. Punctate ␥ -tubulin staining was associated with microtubule arrays, accumulated with short kinetochore microtubules interacting in polar regions with membranes, and localized in the vicinity of nuclei and in the area of cell plate formation. Our results indicate that the association of ␥ -tubulin complexes with dynamic membranes might ensure the flexibility of noncentrosomal microtubule nucleation. Moreover, the presence of other molecular forms of ␥ -tubulin suggests additional roles for this protein species in microtubule organization.

Electrical wiring of different types of pyranose oxidase (P2O) (fungal wild type, recombinant wild type with a hexahistidine tag, mutant form E542K with a hexa-histidine tag) from Trametes multicolor, and recombinant P2O from Coriolus sp. overexpressed in Escherichia coli as well as of pyranose dehydrogenase (PDH) from Agaricus meleagris and Agaricus xanthoderma with an osmium redox polymer (poly(1-vinylimidazole) 12 -[Os(4,4'-dimethyl-2,2'-dipyridyl) 2 -Cl 2 ] 2þ/þ ) on graphite electrodes was carried out. After optimization studies using glucose as substrate, the biosensors, which showed the best characteristics in terms of linear range, detection limit and sensitivity were selected, viz. wild type P2O from T. multicolor and PDH from A. meleagris. These two enzymes were used and investigated for their selectivity for a number of different sugars.

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γ-Tubulin is required for microtubule (MT) nucleation at MT organizing centers such as centrosomes or spindle pole bodies, but little is known about its noncentrosomal functions. We conditionally downregulated γ-tubulin by inducible expression of RNA interference (RNAi) constructs in Arabidopsis thaliana. Almost complete RNAi depletion of γ-tubulin led to the absence of MTs and was lethal at the cotyledon stage. After induction of RNAi expression, γ-tubulin was gradually depleted from both cytoplasmic and microsomal fractions. In RNAi plants with partial loss of γ-tubulin, MT recovery after drug-induced depolymerization was impaired. Similarly, immunodepletion of γ-tubulin from Arabidopsis extracts severely compromised in vitro polymerization of MTs. Reduction of γ-tubulin protein levels led to randomization and bundling of cortical MTs. This finding indicates that MT-bound γ-tubulin is part of a cortical template guiding the microtubular network and is essential for MT nucleation. Furthermore, we found that cells with decreased levels of γ-tubulin could progress through mitosis, but cytokinesis was strongly affected. Stepwise diminution of γ-tubulin allowed us to reveal roles for MT nucleation in plant development, such as organization of cell files, anisotropic and polar tip growth, and stomatal patterning. Some of these functions of γ-tubulin might be independent of MT nucleation.

A novel alcohol oxidase (AOX) has been purified from mycelial pellets of the wood-degrading basidiomycete Gloeophyllum trabeum and characterized as a homooctameric nonglycosylated protein with native and subunit molecular masses of 628 and 72.4 kDa, containing noncovalently bonded flavin adenine dinucleotide. The isolated AOX cDNA contained an open reading frame of 1,953 bp translating into a polypeptide of 651 amino acids displaying 51 to 53% identity with other published fungal AOX amino acid sequences. The enzyme catalyzed the oxidation of short-chain primary aliphatic alcohols with a preference for methanol (K m ‫؍‬ 2.3 mM, k cat ‫؍‬ 15.6 s ؊1 ). Using polyclonal antibodies and immunofluorescence staining, AOX was localized on liquid culture hyphae and extracellular slime in sections from degraded wood and on cotton fibers. Transmission electron microscopy immunogold labeling localized the enzyme in the hyphal periplasmic space and wall and on extracellular tripartite membranes and slime, while there was no labeling of hyphal peroxisomes. AOX was further shown to be associated with membranous or slime structures secreted by hyphae in wood fiber lumina and within the secondary cell walls of degraded wood fibers. The differences in AOX targeting compared to the known yeast peroxisomal localization were traced to a unique C-terminal sequence of the G. trabeum oxidase, which is apparently responsible for the protein's different translocation. The extracellular distribution and the enzyme's abundance and preference for methanol, potentially available from the demethylation of lignin, all point to a possible role for AOX as a major source of H 2 O 2 , a component of Fenton's reagent implicated in the generally accepted mechanisms for brown rot through the production of highly destructive hydroxyl radicals.

Pyranose dehydrogenase is a fungal flavin-dependent sugar oxidoreductase which is structurally and catalytically related to fungal pyranose oxidase and cellobiose dehydrogenase and probably fulfills similar biological functions in lignocellulose breakdown. It is a monomeric secretory glycoprotein and is limited to a rather small group of litter-decomposing basidiomycetes. Compared with pyranose oxidase, it displays broader substrate specificity and a variable regioselectivity and is unable to utilize oxygen as electron acceptor using substituted benzoquinones and (organo) metallic ions instead. Depending on the structure of the sugar in pyranose form (mono/di/oligosaccharide or glycoside) and the enzyme source, selective monooxidations at C-1, C-2, C-3, or dioxidations at C-2,3 or C-3,4 of the molecule to the corresponding aldonolactones (C-1), or (di)dehydrosugars (aldos(di)uloses) can be performed. These features make pyranose dehydrogenase a promising and versatile biocatalyst for production of highly reactive, sometimes unique, di- and tri-carbonyl sugar derivatives that may serve as interesting chiral intermediates for the synthesis of rare sugars, novel drugs, and fine chemicals.

γ-Tubulin is essential for microtubule nucleation and also plays less understood roles in nuclear and cell-cycle-related functions. High abundancy of γ-tubulin in acentrosomal Arabidopsis cells facilitated purification and biochemical characterization of large molecular species of γ-tubulin. TEM, fluorescence, and atomic force microscopy of purified high molecular γ-tubulin forms revealed the presence of linear filaments with a double protofilament substructure, filament bundles and aggregates. Filament formation from highly purified γ-tubulin free of γ-tubulin complex proteins (GCPs) was demonstrated for both plant and human γ-tubulin. Moreover, γ-tubulin associated with porcine brain microtubules formed oligomers. Experimental evidence on the intrinsic ability of γ-tubulin to oligomerize/polymerize was supported by conservation of α- and β-tubulin interfaces for longitudinal and lateral interactions for γ-tubulins. STED (stimulated emission depletion) microscopy of Arabidopsis cells revealed fine, short γ-tubulin fibrillar structures enriched on mitotic microtubular arrays that accumulated at polar regions of acentrosomal spindles and the outer nuclear envelope before mitosis, and were also present in nuclei. Fine fibrillar structures of γ-tubulin representing assemblies of higher order were localized in cell-cycle-dependent manner at sites of dispersed γ-tubulin location in acentrosomal plant cells as well as at sites of local γ-tubulin enrichment after drug treatment. Our findings that γ-tubulin preserves the capability of prokaryotic tubulins to self-organize into filaments assembling by lateral interaction into bundles/clusters help understanding of the relationship between structure and multiple cellular functions of this protein species and suggest that besides microtubule nucleation and organization, γ-tubulin may also have scaffolding or sequestration functions.