Cohesin cleavage by separase is enhanced by a substrate motif distinct from the cleavage site.

Chromosome segregation begins when the cysteine protease, separase, cleaves the Scc1 subunit of cohesin at the metaphase-to-anaphase transition. Separase is inhibited prior to metaphase by the tightly bound securin protein, which contains a pseudosubstrate motif that blocks the separase active site. To investigate separase substrate specificity and regulation, here we develop a system for producing recombinant, securin-free human separase. Using this enzyme, we identify an LPE motif on the Scc1 substrate that is distinct from the cleavage site and is required for rapid and specific substrate cleavage. Securin also contains a conserved LPE motif, and we provide evidence that this sequence blocks separase engagement of the Scc1 LPE motif. Our results suggest that rapid cohesin cleavage by separase requires a substrate docking interaction outside the active site. This interaction is blocked by securin, providing a second mechanism by which securin inhibits cohesin cleavage.

Condensin II Regulates Interphase Chromatin Organization Through the Mrg-Binding Motif of Cap-H2.

The spatial organization of the genome within the eukaryotic nucleus is a dynamic process that plays a central role in cellular processes such as gene expression, DNA replication, and chromosome segregation. Condensins are conserved multi-subunit protein complexes that contribute to chromosome organization by regulating chromosome compaction and homolog pairing. Previous work in our laboratory has shown that the Cap-H2 subunit of condensin II physically and genetically interacts with the Drosophila homolog of human MORF4-related gene on chromosome 15 (MRG15). Like Cap-H2, Mrg15 is required for interphase chromosome compaction and homolog pairing. However, the mechanism by which Mrg15 and Cap-H2 cooperate to maintain interphase chromatin organization remains unclear. Here, we show that Cap-H2 localizes to interband regions on polytene chromosomes and co-localizes with Mrg15 at regions of active transcription across the genome. We show that co-localization of Cap-H2 on polytene chromosomes is partially dependent on Mrg15. We have identified a binding motif within Cap-H2 that is essential for its interaction with Mrg15, and have found that mutation of this motif results in loss of localization of Cap-H2 on polytene chromosomes and results in partial suppression of Cap-H2-mediated compaction and homolog unpairing. Our data are consistent with a model in which Mrg15 acts as a loading factor to facilitate Cap-H2 binding to chromatin and mediate changes in chromatin organization.

Two Polo-like kinase 4 binding domains in Asterless perform distinct roles in regulating kinase stability.

Plk4 (Polo-like kinase 4) and its binding partner Asterless (Asl) are essential, conserved centriole assembly factors that induce centriole amplification when overexpressed. Previous studies found that Asl acts as a scaffolding protein; its N terminus binds Plk4's tandem Polo box cassette (PB1-PB2) and targets Plk4 to centrioles to initiate centriole duplication. However, how Asl overexpression drives centriole amplification is unknown. In this paper, we investigated the Asl-Plk4 interaction in Drosophila melanogaster cells. Surprisingly, the N-terminal region of Asl is not required for centriole duplication, but a previously unidentified Plk4-binding domain in the C terminus is required. Mechanistic analyses of the different Asl regions revealed that they act uniquely during the cell cycle: the Asl N terminus promotes Plk4 homodimerization and autophosphorylation during interphase, whereas the Asl C terminus stabilizes Plk4 during mitosis. Therefore, Asl affects Plk4 in multiple ways to regulate centriole duplication. Asl not only targets Plk4 to centrioles but also modulates Plk4 stability and activity, explaining the ability of overexpressed Asl to drive centriole amplification.

Autoinhibition and relief mechanism for Polo-like kinase 4.

Polo-like kinase 4 (Plk4) is a master regulator of centriole duplication, and its hyperactivity induces centriole amplification. Homodimeric Plk4 has been shown to be ubiquitinated as a result of autophosphorylation, thus promoting its own degradation and preventing centriole amplification. Unlike other Plks, Plk4 contains three rather than two Polo box domains, and the function of its third Polo box (PB3) is unclear. Here, we performed a functional analysis of Plk4's structural domains. Like other Plks, Plk4 possesses a previously unidentified autoinhibitory mechanism mediated by a linker (L1) near the kinase domain. Thus, autoinhibition is a conserved feature of Plks. In the case of Plk4, autoinhibition is relieved after homodimerization and is accomplished by PB3 and by autophosphorylation of L1. In contrast, autophosphorylation of the second linker promotes separation of the Plk4 homodimer. Therefore, autoinhibition delays the multiple consequences of activation until Plk4 dimerizes. These findings reveal a complex mechanism of Plk4 regulation and activation which govern the process of centriole duplication.

Drosophila casein kinase I alpha regulates homolog pairing and genome organization by modulating condensin II subunit Cap-H2 levels.

The spatial organization of chromosomes within interphase nuclei is important for gene expression and epigenetic inheritance. Although the extent of physical interaction between chromosomes and their degree of compaction varies during development and between different cell-types, it is unclear how regulation of chromosome interactions and compaction relate to spatial organization of genomes. Drosophila is an excellent model system for studying chromosomal interactions including homolog pairing. Recent work has shown that condensin II governs both interphase chromosome compaction and homolog pairing and condensin II activity is controlled by the turnover of its regulatory subunit Cap-H2. Specifically, Cap-H2 is a target of the SCFSlimb E3 ubiquitin-ligase which down-regulates Cap-H2 in order to maintain homologous chromosome pairing, chromosome length and proper nuclear organization. Here, we identify Casein Kinase I alpha (CK1α) as an additional negative-regulator of Cap-H2. CK1α-depletion stabilizes Cap-H2 protein and results in an accumulation of Cap-H2 on chromosomes. Similar to Slimb mutation, CK1α depletion in cultured cells, larval salivary gland, and nurse cells results in several condensin II-dependent phenotypes including dispersal of centromeres, interphase chromosome compaction, and chromosome unpairing. Moreover, CK1α loss-of-function mutations dominantly suppress condensin II mutant phenotypes in vivo. Thus, CK1α facilitates Cap-H2 destruction and modulates nuclear organization by attenuating chromatin localized Cap-H2 protein.

Polo-like kinase 4 autodestructs by generating its Slimb-binding phosphodegron.

Polo-like kinase 4 (Plk4) is a conserved master regulator of centriole assembly. Previously, we found that Drosophila Plk4 protein levels are actively suppressed during interphase. Degradation of interphase Plk4 prevents centriole overduplication and is mediated by the ubiquitin-ligase complex SCF(Slimb/ßTrCP). Since Plk4 stability depends on its activity, we studied the consequences of inactivating Plk4 or perturbing its phosphorylation state within its Slimb-recognition motif (SRM). Mass spectrometry of in-vitro-phosphorylated Plk4 and Plk4 purified from cells reveals that it is directly responsible for extensively autophosphorylating and generating its Slimb-binding phosphodegron. Phosphorylatable residues within this regulatory region were systematically mutated to determine their impact on Plk4 protein levels and centriole duplication when expressed in S2 cells. Notably, autophosphorylation of a single residue (Ser293) within the SRM is critical for Slimb binding and ubiquitination. Our data also demonstrate that autophosphorylation of numerous residues flanking S293 collectively contribute to establishing a high-affinity binding site for SCF(Slimb). Taken together, our findings suggest that Plk4 directly generates its own phosphodegron and can do so without the assistance of an additional kinase(s).

Maintenance of interphase chromosome compaction and homolog pairing in Drosophila is regulated by the condensin cap-h2 and its partner Mrg15.

Dynamic regulation of chromosome structure and organization is critical for fundamental cellular processes such as gene expression and chromosome segregation. Condensins are conserved chromosome-associated proteins that regulate a variety of chromosome dynamics, including axial shortening, lateral compaction, and homolog pairing. However, how the in vivo activities of condensins are regulated and how functional interactors target condensins to chromatin are not well understood. To better understand how Drosophila melanogaster condensin is regulated, we performed a yeast two-hybrid screen and identified the chromo-barrel domain protein Mrg15 to interact with the Cap-H2 condensin subunit. Genetic interactions demonstrate that Mrg15 function is required for Cap-H2-mediated unpairing of polytene chromosomes in ovarian nurse cells and salivary gland cells. In diploid tissues, transvection assays demonstrate that Mrg15 inhibits transvection at Ubx and cooperates with Cap-H2 to antagonize transvection at yellow. In cultured cells, we show that levels of chromatin-bound Cap-H2 protein are partially dependent on Mrg15 and that Cap-H2-mediated homolog unpairing is suppressed by RNA interference depletion of Mrg15. Thus, maintenance of interphase chromosome compaction and homolog pairing status requires both Mrg15 and Cap-H2. We propose a model where the Mrg15 and Cap-H2 protein-protein interaction may serve to recruit Cap-H2 to chromatin and facilitates compaction of interphase chromatin.

SCFSlimb ubiquitin ligase suppresses condensin II-mediated nuclear reorganization by degrading Cap-H2.

Condensin complexes play vital roles in chromosome condensation during mitosis and meiosis. Condensin II uniquely localizes to chromatin throughout the cell cycle and, in addition to its mitotic duties, modulates chromosome organization and gene expression during interphase. Mitotic condensin activity is regulated by phosphorylation, but mechanisms that regulate condensin II during interphase are unclear. Here, we report that condensin II is inactivated when its subunit Cap-H2 is targeted for degradation by the SCF(Slimb) ubiquitin ligase complex and that disruption of this process dramatically changed interphase chromatin organization. Inhibition of SCF(Slimb) function reorganized interphase chromosomes into dense, compact domains and disrupted homologue pairing in both cultured Drosophila cells and in vivo, but these effects were rescued by condensin II inactivation. Furthermore, Cap-H2 stabilization distorted nuclear envelopes and dispersed Cid/CENP-A on interphase chromosomes. Therefore, SCF(Slimb)-mediated down-regulation of condensin II is required to maintain proper organization and morphology of the interphase nucleus.

Preparation of Drosophila S2 cells for light microscopy.

The ideal experimental system would be cheap and easy to maintain, amenable to a variety of techniques, and would be supported by an extensive literature and genome sequence database. Cultured Drosophila S2 cells, the product of disassociated 20-24 hour old embryos, possess all these properties. Consequently, S2 cells are extremely well-suited for the analysis of cellular processes, including the discovery of the genes encoding the molecular components of the process or mechanism of interest. The features of S2 cells that are most responsible for their utility are the ease with which they are maintained, their exquisite sensitivity to double-stranded (ds)RNA-mediated interference (RNAi), and their tractability to fluorescence microscopy as either live or fixed cells. S2 cells can be grown in a variety of media, including a number of inexpensive, commercially-available, fully-defined, serum-free media. In addition, they grow optimally and quickly at 21-24 degrees C and can be cultured in a variety of containers. Unlike mammalian cells, S2 cells do not require a regulated atmosphere, but instead do well with normal air and can even be maintained in sealed flasks. Complementing the ease of RNAi in S2 cells is the ability to readily analyze experimentally-induced phenotypes by phase or fluorescence microscopy of fixed or live cells. S2 cells grow in culture as a single monolayer but do not display contact inhibition. Instead, cells tend to grow in colonies in dense cultures. At low density, S2 cultures grown on glass or tissue culture-treated plastic are round and loosely-attached. However, the cytology of S2 cells can be greatly improved by inducing them to flatten extensively by briefly culturing them on a surface coated with the lectin, concanavalin A (ConA). S2 cells can also be stably transfected with fluorescently-tagged markers to label structures or organelles of interest in live or fixed cells. Therefore, the usual scenario for the microscopic analysis of cells is this: first, S2 cells (which can possess transgenes to express tagged markers) are treated by RNAi to eliminate a target protein(s). RNAi treatment time can be adjusted to allow for differences in protein turn-over kinetics and to minimize cell trauma/death if the target protein is important for viability. Next, the treated cells are transferred to a dish containing a coverslip pre-coated with conA to induce cells to spread and tightly adhere to the glass. Finally, cells are imaged with the researcher's choice of microscopy modes. S2 cells are particularly good for studies requiring extended visualization of live cells since these cells stay healthy at room temperature and normal atmosphere.

FepA- and TonB-dependent bacteriophage H8: receptor binding and genomic sequence.

H8 is derived from a collection of Salmonella enterica serotype Enteritidis bacteriophage. Its morphology and genomic structure closely resemble those of bacteriophage T5 in the family Siphoviridae. H8 infected S. enterica serotypes Enteritidis and Typhimurium and Escherichia coli by initial adsorption to the outer membrane protein FepA. Ferric enterobactin inhibited H8 binding to E. coli FepA (50% inhibition concentration, 98 nM), and other ferric catecholate receptors (Fiu, Cir, and IroN) did not participate in phage adsorption. H8 infection was TonB dependent, but exbB mutations in Salmonella or E. coli did not prevent infection; only exbB tolQ or exbB tolR double mutants were resistant to H8. Experiments with deletion and substitution mutants showed that the receptor-phage interaction first involves residues distributed over the protein's outer surface and then narrows to the same charged (R316) or aromatic (Y260) residues that participate in the binding and transport of ferric enterobactin and colicins B and D. These data rationalize the multifunctionality of FepA: toxic ligands like bacteriocins and phage penetrate the outer membrane by parasitizing residues in FepA that are adapted to the transport of the natural ligand, ferric enterobactin. DNA sequence determinations revealed the complete H8 genome of 104.4 kb. A total of 120 of its 143 predicted open reading frames (ORFS) were homologous to ORFS in T5, at a level of 84% identity and 89% similarity. As in T5, the H8 structural genes clustered on the chromosome according to their function in the phage life cycle. The T5 genome contains a large section of DNA that can be deleted and that is absent in H8: compared to T5, H8 contains a 9,000-bp deletion in the early region of its chromosome, and nine potentially unique gene products. Sequence analyses of the tail proteins of phages in the same family showed that relative to pb5 (Oad) of T5 and Hrs of BF23, the FepA-binding protein (Rbp) of H8 contains unique acidic and aromatic residues. These side chains may promote binding to basic and aromatic residues in FepA that normally function in the adsorption of ferric enterobactin. Furthermore, a predicted H8 tail protein showed extensive identity and similarity to pb2 of T5, suggesting that it also functions in pore formation through the cell envelope. The variable region of this protein contains a potential TonB box, intimating that it participates in the TonB-dependent stage of the phage infection process.