A Nup133-dependent NPC-anchored network tethers centrosomes to the nuclear envelope in prophase

Centrosomes are closely associated with the nuclear envelope (NE) throughout the cell cycle and this association is maintained in prophase when they separate to establish the future mitotic spindle. At this stage, the kinetochore constituents CENP-F, NudE, NudEL, dynein, and dynactin accumulate at the NE. We demonstrate here that the N-terminal domain of the nuclear pore complex (NPC) protein Nup133, although largely dispensable for NPC assembly, is required for efficient anchoring of the dynein/dynactin complex to the NE in prophase. Nup133 exerts this function through an interaction network via CENP-F and NudE/EL. We show that this molecular chain is critical for maintaining centrosome association with the NE at mitotic entry and contributes to this process without interfering with the previously described RanBP2-BICD2-dependent pathway of centrosome anchoring. Finally, our study reveals that tethering of centrosomes to the nuclear surface at the G2/M transition contributes, along with other cellular mechanisms, to early stages of bipolar spindle assembly.

Gamma-tubulin regulates the anaphase-promoting complex/cyclosome during interphase

Activation of the APC/C requires microtubule-nucleating independent aspects of γ-tubulin function.

A cold-sensitive γ-tubulin allele of Aspergillus nidulans, mipAD159, causes defects in mitotic and cell cycle regulation at restrictive temperatures that are apparently independent of microtubule nucleation defects. Time-lapse microscopy of fluorescently tagged mitotic regulatory proteins reveals that cyclin B, cyclin-dependent kinase 1, and the Ancdc14 phosphatase fail to accumulate in a subset of nuclei at restrictive temperatures. These nuclei are permanently removed from the cell cycle, whereas other nuclei, in the same multinucleate cell, cycle normally, accumulating and degrading these proteins. After each mitosis, additional daughter nuclei fail to accumulate these proteins, resulting in an increase in noncycling nuclei over time and consequent inhibition of growth. Extensive analyses reveal that these noncycling nuclei result from a nuclear autonomous, microtubule-independent failure of inactivation of the anaphase-promoting complex/cyclosome. Thus, γ-tubulin functions to regulate this key mitotic and cell cycle regulatory complex.

Timely septation requires SNAD-dependent spindle pole body localization of the septation initiation network components in the filamentous fungus Aspergillus nidulans

In the filamentous fungus Aspergillus nidulans, cytokinesis/septation is triggered by the septation initiation network (SIN), which first appears at the spindle pole body (SPB) during mitosis. The coiled-coil protein SNAD is associated with the SPB and is required for timely septation and conidiation. We have determined that SNAD acted as a scaffold protein that is required for the localization of the SIN proteins of SIDB and MOBA to the SPB. Another scaffold protein SEPK, whose localization at the SPB was dependent on SNAD, was also required for SIDB and MOBA localization to the SPB. In the absence of either SEPK or SNAD, SIDB/MOBA successfully localized to the septation site, indicating that their earlier localization at SPB was not essential for their later appearance at the division site. Unlike their functional counterparts in fission yeast, SEPK and SNAD were not required for vegetative growth but only for timely septation. Furthermore, down-regulation of negative regulators of the SIN suppressed the septation and conidiation phenotypes due to the loss of SNAD. Therefore, we conclude that SPB localization of SIN components is not essential for septation per se, but critical for septation to take place in a timely manner in A. nidulans.

Mlp1 acts as a mitotic scaffold to spatially regulate spindle assembly checkpoint proteins in Aspergillus nidulans

During open mitosis several nuclear pore complex (NPC) proteins have mitotic specific localizations and functions. We find that the Aspergillus nidulans Mlp1 NPC protein has previously unrealized mitotic roles involving spatial regulation of spindle assembly checkpoint (SAC) proteins. In interphase, An-Mlp1 tethers the An-Mad1 and An-Mad2 SAC proteins to NPCs. During a normal mitosis, An-Mlp1, An-Mad1, and An-Mad2 localize similarly on, and around, kinetochores until telophase when they transiently localize near the spindle but not at kinetochores. During SAC activation, An-Mlp1 remains associated with kinetochores in a manner similar to An-Mad1 and An-Mad2. Although An-Mlp1 is not required for An-Mad1 kinetochore localization during early mitosis, it is essential to maintain An-Mad1 in the extended region around kinetochores in early mitosis and near the spindle in telophase. Our data are consistent with An-Mlp1 being part of a mitotic spindle matrix similar to its Drosophila orthologue and demonstrate that this matrix localizes SAC proteins. By maintaining SAC proteins near the mitotic apparatus, An-Mlp1 may help monitor mitotic progression and coordinate efficient mitotic exit. Consistent with this possibility, An-Mad1 and An-Mlp1 redistribute from the telophase matrix and associate with segregated kinetochores when mitotic exit is prevented by expression of nondegradable cyclin B.

Molecular genetic mining of the Aspergillus secondary metabolome: discovery of the emericellamide biosynthetic pathway

The recently sequenced genomes of several Aspergillus species have revealed that these organisms have the potential to produce a surprisingly large range of natural products, many of which are currently unknown. We have found that A. nidulans produces emericellamide A, an antibiotic compound of mixed origins with polyketide and amino acid building blocks. Additionally, we describe the discovery of four previously unidentified, related compounds that we designate emericellamide C-F. Using recently developed gene targeting techniques, we have identified the genes involved in emericellamide biosynthesis. The emericellamide gene cluster contains one polyketide synthase and one nonribosomal peptide synthetase. From the sequences of the genes, we are able to deduce a biosynthetic pathway for the emericellamides. The identification of this biosynthetic pathway opens the door to engineering novel analogs of this structurally complex metabolite.

Fusion PCR and gene targeting in Aspergillus nidulans

We describe a rapid method for the production of fusion PCR products that can be used, generally without band purification, to transform Aspergillus nidulans. This technique can be used to replace genes; tag genes with fluorescent moeties or epitope tags; or replace endogenous promoters with regulatable promoters, by introducing an appropriate selective cassette (e.g., fluorescent protein + selectable marker). The relevant genomic fragments and cassette are first amplified separately by PCR using primers that produce overlapping ends. A second PCR using 'nested' primers fuses the fragments into a single molecule with all sequences in the desired order. This procedure allows a cassette to be amplified once, frozen and used subsequently in many fusion PCRs. Transformation of nonhomologous recombination deficient (nkuADelta) strains of A. nidulans with fusion PCR products results in high frequencies of accurate gene targeting. Fusion PCR takes less than 2 d. Protoplast formation and transformation takes less than 1 d.

A versatile and efficient gene-targeting system for Aspergillus nidulans

Aspergillus nidulans is an important experimental organism, and it is a model organism for the genus Aspergillus that includes serious pathogens as well as commercially important organisms. Gene targeting by homologous recombination during transformation is possible in A. nidulans, but the frequency of correct gene targeting is variable and often low. We have identified the A. nidulans homolog (nkuA) of the human KU70 gene that is essential for nonhomologous end joining of DNA in double-strand break repair. Deletion of nkuA (nkuA delta) greatly reduces the frequency of nonhomologous integration of transforming DNA fragments, leading to dramatically improved gene targeting. We have also developed heterologous markers that are selectable in A. nidulans but do not direct integration at any site in the A. nidulans genome. In combination, nkuA delta and the heterologous selectable markers make up a very efficient gene-targeting system. In experiments involving scores of genes, 90% or more of the transformants carried a single insertion of the transforming DNA at the correct site. The system works with linear and circular transforming molecules and it works for tagging genes with fluorescent moieties, replacing genes, and replacing promoters. This system is efficient enough to make genomewide gene-targeting projects feasible.

gamma-tubulin plays an essential role in the coordination of mitotic events

Recent data from multiple organisms indicate that gamma-tubulin has essential, but incompletely defined, functions in addition to nucleating microtubule assembly. To investigate these functions, we examined the phenotype of mipAD159, a cold-sensitive allele of the gamma-tubulin gene of Aspergillus nidulans. Immunofluorescence microscopy of synchronized material revealed that at a restrictive temperature mipAD159 does not inhibit mitotic spindle formation. Anaphase A was inhibited in many nuclei, however, and after a slight delay in mitosis (approximately 6% of the cell cycle period), most nuclei reentered interphase without dividing. In vivo observations of chromosomes at a restrictive temperature revealed that mipAD159 caused a failure of the coordination of late mitotic events (anaphase A, anaphase B, and chromosomal disjunction) and nuclei reentered interphase quickly even though mitosis was not completed successfully. Time-lapse microscopy also revealed that transient mitotic spindle abnormalities, in particular bent spindles, were more prevalent in mipAD159 strains than in controls. In experiments in which microtubules were depolymerized with benomyl, mipAD159 nuclei exited mitosis significantly more quickly (as judged by chromosomal condensation) than nuclei in a control strain. These data reveal that gamma-tubulin has an essential role in the coordination of late mitotic events, and a microtubule-independent function in mitotic checkpoint control.