A single-step protein affinity purification protocol using Aspergillus nidulans is described. Detailed protocols for cell breakage, affinity purification, and depending on the application, methods for protein release from affinity beads are provided. Examples defining the utility of the approaches, which should be widely applicable, are included.Over 50 years ago, Richards and colleagues found that cleavage of RNase A by subtilisin resulted in two peptides, the S-peptide (residues 1 to 20) and the S-protein (residues 21 to 124), which bind tightly to each other with high affinity (dissociation constant, ϳ10 Ϫ9

Septal pores—the intercellular bridges of fungi—are open during interphase but closed at mitosis. The NIMA kinase mitotically regulates septal pore closing and opening potentially via mechanisms analogous to how it regulates mitotic nuclear pores. The findings explain how and why physically connected Aspergillus cells can maintain mitotic autonomy.

NDR (nuclear Dbf-2-related) kinases constitute key regulatory nodes in signaling networks that control multiple biological processes such as growth, proliferation, mitotic exit, morphogenesis, and apoptosis. Two NDR pathways called the septation initiation network (SIN) and the morphogenesis Orb6 network (MOR) exist in the fission yeast Schizosaccharomyces pombe. The SIN promotes cytokinesis, and the MOR drives cell separation at the end of cytokinesis and polarized growth during interphase. We showed previously that cross talk exists between these two pathways, with the SIN inhibiting the MOR during cytokinesis through phosphorylation of the MOR component Nak1 by the SIN Sid2 kinase. The reason for this inhibition remained uncertain. We show here that failure to inhibit MOR signaling during cytokinesis results in cell lysis at the site of septum formation. Time-lapse analysis revealed that MOR signaling during cytokinesis causes cells to prematurely initiate septum degradation/cell separation. The cell lysis phenotype is due to premature initiation of cell separation because it can be rescued by mutations in genes required for cell separation/septum degradation. We also shed further light on how the SIN inhibits the MOR. Sid2 phosphorylation of the MOR proteins Sog2 and Nak1 is required to prevent cell lysis during cytokinesis. Together, these results show that SIN inhibition of the MOR enforces proper temporal ordering of cytokinetic events. In the fission yeast Schizosaccharomyces pombe, cytokinesis and other late mitotic events are regulated by the septation initiation network (SIN) pathway, an NDR (nuclear Dbf-2-related) kinase signaling network analogous to the hippo pathway in mammalian cells and the mitotic exit network (MEN) in the budding yeast Saccharomyces cerevisiae (1-4). SIN signaling is essential for actomyosin ring assembly and constriction as well as for septum formation. The SIN also regulates mitotic entry, spindle checkpoint inactivation, spindle elongation, telophase nuclear positioning, and inhibition of polarized growth during cytokinesis (5-9). Recent work has begun to identify targets of the SIN, which include cytoskeletal and signaling proteins (5,8,(10)(11)(12)(13). SIN signaling is under precise temporal control; the SIN is activated in the anaphase of mitosis and inactivated upon completion of actomyosin ring constriction and septum formation (14). Regulation of other cell cycle signaling networks by the SIN may be important for properly coordinating cell cycle events. For example, the SIN inhibits a second conserved NDR kinase pathway called the morphogenesis Orb6 network (MOR) (9), which is analogous to the RAM (regulation of Ace2 and morphogenesis) network in budding yeast (3). The MOR normally drives septum degradation and cell separation following actomyosin ring constriction as well as polarized growth during the interphase by promoting localization of the actin cytoskeleton to cell tips (15). In mitosis, the SIN and other proteins cause actin to reorganize at the cell divisio...

The Never in Mitosis A (NIMA) kinase (the founding member of the Nek family of kinases) has been considered a mitotic specific kinase with nuclear restricted roles in the model fungus Aspergillus nidulans. By extending to A. nidulans the results of a synthetic lethal screen performed in Saccharomyces cerevisiae using the NIMA ortholog KIN3, we identified a conserved genetic interaction between nimA and genes encoding proteins of the Endosomal Sorting Complex Required for Transport (ESCRT) pathway. Absence of ESCRT pathway functions in combination with partial NIMA function causes enhanced cell growth defects, including an inability to maintain a single polarized dominant cell tip. These genetic insights suggest NIMA potentially has interphase functions in addition to its established mitotic functions at nuclei. We therefore generated endogenously GFP-tagged NIMA (NIMA-GFP) which was fully functional to follow its interphase locations using live cell spinning disc 4D confocal microscopy. During interphase some NIMA-GFP locates to the tips of rapidly growing cells and, when expressed ectopically, also locates to the tips of cytoplasmic microtubules, suggestive of non-nuclear interphase functions. In support of this, perturbation of NIMA function either by ectopic overexpression or through partial inactivation results in marked cell tip growth defects with excess NIMA-GFP promoting multiple growing cell tips. Ectopic NIMA-GFP was found to locate to the plus ends of microtubules in an EB1 dependent manner, while impairing NIMA function altered the dynamic localization of EB1 and the cytoplasmic microtubule network. Together, our genetic and cell biological analyses reveal novel non-nuclear interphase functions for NIMA involving microtubules and the ESCRT pathway for normal polarized fungal cell tip growth. These insights extend the roles of NIMA both spatially and temporally and indicate that this conserved protein kinase could help integrate cell cycle progression with polarized cell growth.

Mitosis is promoted and regulated by reversible protein phosphorylation catalyzed by the essential NIMA and CDK1 kinases in the model filamentous fungus Aspergillus nidulans. Protein methylation mediated by the Set1/COMPASS methyltransferase complex has also been shown to regulate mitosis in budding yeast with the Aurora mitotic kinase. We uncover a genetic interaction between An-swd1, which encodes a subunit of the Set1 protein methyltransferase complex, with NIMA as partial inactivation of nimA is poorly tolerated in the absence of swd1. This genetic interaction is additionally seen without the Set1 methyltransferase catalytic subunit. Importantly partial inactivation of NIMT, a mitotic activator of the CDK1 kinase, also causes lethality in the absence of Set1 function, revealing a functional relationship between the Set1 complex and two pivotal mitotic kinases. The main target for Set1-mediated methylation is histone H3K4. Mutational analysis of histone H3 revealed that modifying the H3K4 target residue of Set1 methyltransferase activity phenocopied the lethality seen when either NIMA or CDK1 are partially functional. We probed the mechanistic basis of these genetic interactions and find that the Set1 complex performs functions with CDK1 for initiating mitosis and with NIMA during progression through mitosis. The studies uncover a joint requirement for the Set1 methyltransferase complex with the CDK1 and NIMA kinases for successful mitosis. The findings extend the roles of the Set1 complex to include the initiation of mitosis with CDK1 and mitotic progression with NIMA in addition to its previously identified interactions with Aurora and type 1 phosphatase in budding yeast.D URING the transition from interphase into mitosis, chromatin undergoes dramatic global restructuring to go from a relaxed interphase configuration amenable to gene expression to a condensed form characteristic of mitotic chromosomes. Chromatin condensation not only marks mitosis but is also essential for the normal segregation of the duplicated sister chromatids into daughter nuclei. On the other hand, during mitotic exit, decondensation of chromatin needs to be triggered in a correct temporal manner to enable successful transition into interphase. In Aspergillus nidulans, a model filamentous fungus that enabled discovery of cell cycle-specific genes, mitotic initiation requires the function of the essential NIMA mitotic kinase (Oakley and Morris 1983;Osmani et al. 1987). Early evidence pointed to a role for NIMA in regulating chromatin compaction through the cell cycle since overexpression of NIMA was sufficient to induce chromatin condensation independent of cell cycle phase (Osmani et al. 1988;. Importantly, NIMA is required for, and can promote, the phosphorylation of histone H3 at serine 10 (De Souza et al. 2000), a universal marker of mitotic chromatin. In addition, NIMA has been shown to regulate the partial disassembly of nuclear pore complexes, allowing the access of mitotic regulators to the nucleus (Wu et al. 1998;De S...