Georjana Barnes - US grants

DESCRIPTION (provided by applicant): The yeast mitotic spindle is less complex than its counterparts in larger eukaryotes and has been intensively studied using genetics, biochemistry, cell biology and ultrastructure approaches, providing an opportunity to develop an understanding of its function and regulation at a level that is not currently achievable in any other organism. The proposed studies are critical for attaining this goal. Correct establishment, function and checkpoint monitoring of kinetochore-microtubule attachments are central to mitotic fidelity. Despite much progress identifying key proteins involved in this physical attachment, and assessing their activities in vivo and in vitro, little is understood about mechanisms regulating attachments, about how microtubule-binding outer kinetochore proteins associate with the central kinetochore, or about emergent properties that result when separate subcomplexes like Dam1 and Ndc80 are together in kinetochores. Building upon discovery and analysis of the Dam1 complex, and structure studies of the Ndc80 complex, new studies will identify their binding partners, determine how Dam1 and Ndc80 complex structures relate to function, how post-translational modifications and binding partners affect function, and how ensembles of kinetochore complexes interact dynamically with microtubules. Previous discoveries that Aurora kinase regulates kinetochore attachment via Dam1 complex phosphorylation, identification of an Aurora kinase consensus site, identification of the fourth yeast Aurora kinase complex subunit, and novel implication of casein kinase 2 in inner kinetochore regulation, provide a robust foundation for studies to reveal how protein kinases regulate critical mitotic functions. Proposed studies will determine how the Aurora kinase complex is targeted to specific cellular locations, will identify its targets at each location, and will determine how phosphorylation affects activity of its targets. Molecular genetics and biochemical analysis of the fully reconstituted, four-protein Aurora kinase complex will determine how this complex is regulated. Having recently identified casein kinase 2 as a mitotic regulator of the inner kinetochore protein Ndc10 and the widely conserved Mif2 (CENP-C) linker protein, and obtained in vivo evidence for dual-regulation of these proteins by CK2 and Aurora kinase, powerful in vivo and in vitro tests of how these two kinases regulate kinetochore function will be conducted. Thanks to unique advantages of yeast, strong inroads into a full molecular dissection of mitotic spindle disassembly have already been made, and will be built upon to fully elucidate pathways and mechanisms. As cells exit mitosis, the enormously complex mitotic spindle must be disassembled rapidly, which involves taking apart stabilized microtubule bundles, disassembling protein subcomplexes, reversing post-translational modifications and destroying proteins. Because spindle disassembly mechanisms are largely unexplored, this is an extremely fertile and important area for investigation. Comprehensive genetic screens and interaction analyses will identify genes and pathways, and focused phenotypic and biochemical studies will reveal detailed mechanisms. PUBLIC HEALTH RELEVANCE: These studies will increase understanding of fundamental aspects of mitotic spindle function and regulation, and since many mitotic proteins and mechanisms are evolutionarily conserved, will provide a framework for elucidating mitotic mechanisms in humans. Chromosome instability is a key contributing factor in cancer and birth defects. Therefore, understanding principles of spindle function may suggest novel strategies for prevention, detection and treatment of human diseases.

microtubule associated protein; intermolecular interaction; mass spectrometry; chemical structure function; biomedical resource;

DESCRIPTION (provided by applicant): The yeast mitotic spindle has been intensively studied using genetics, biochemistry, cell biology and ultrastructure approaches, providing an opportunity to understand its function and regulation at a level not currently achievable in any other organism. The proposed studies are critical for attaining this goal. The 10- protein Dam1 complex provides critical functions in spindle integrity and kinetochore-spindle attachment, and it is a crucial target of the budding yeast Aurora kinase. The Dam1 complex assembles into rings on microtubules that associate preferentially with the GTP-tubulin lattice, it stabilizes microtubules against disassembly, and it remains attached to the disassembling microtubule ends. These properties are ideally suited for a mechanical linkage between chromosomes and spindle microtubules. Using a recently developed real-time fluorescence assay for Dam1 ring movement and microtubule stabilization, effects of protein phosphorylation and interacting proteins on biochemical activities of the Dam1 complex will be tested. By attaching microbeads to the Dam1 complex, forces generated on the ring complex by microtubule disassembly will be measured using laser tweezers. Cryo-EM studies of Dam1 complex-coated microtubules that are already quite advanced will be used for helical reconstructions to obtain a 3D structure of the complex that can be docked onto the known structure of the microtubule surface. This analysis promises to reveal structural determinants for the function of the complex as a dynamic microtubule-binding interface, and will be extended to post-translationally modified Dam1 complex, and to the complex with associated interacting proteins. Understanding of how the Aurora kinase Ipl1 regulates mitotic spindle functions will be increased by combining bioinformatics, mass spectrometry and chemical genetics to identify additional Ipl1 target proteins. Molecular genetics will determine how phosphorylation affects target protein functions. These studies will increase understanding of fundamental aspects of mitotic spindle function and regulation, and will provide a framework for elucidating mitotic mechanisms in humans. Chromosome instability is a key contributing factor in cancer and birth defects. Therefore, understanding principles of spindle function may suggest novel strategies for prevention, detection and treatment of human diseases.

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The kinetochores is a multi-protein complex that assemblies on the centromere and attaches chromosomes to spindle microtubules during cell division. Its attachment to chromosomes is tightly monitored to ensure faithful chromosome segregation. A recent systematic yeast two-hybrid screen focusing on mitotic spindle function identified interactions between the kinetochore protein Mif2 and two Casein Kinase 2 (CK2) subunits, Cka2 and Ckb2. CK2 is a ubiquitous and highly conserved Ser/Thr kinase in eukaryotes. Although CK2 has been linked to cell cycle regulation, it has remained largely unclear how CK2 regulates this event. Additionally, Mif2 and another kinetochore protein Ndc10 (Cbf2) were previously identified as putative substrates of CK2. Ndc10 is a component of the CBF3 complex. Mif2 is the S. cerevisiae homolog of human CENP-C. Both proteins bind to centromeres and recruit other inner and outer kinetochore proteins. Previous studies have demonstrated that both Mif2 and Ndc10 are phosphorylated by Ipl1, the yeast Aurora B kinase. However, the role of phosphorylation has not been fully explored. The overall goal of my project is to determine how CK2 regulates kinetochore function. I hypothesize that CK2 regulates mitotic progression by regulating Aurora B phosphorylation of Mif2 and Ndc10. To test these hypotheses, I will first determine if loss of Cka2 kinase activity affects mitotic spindle and chromosome segregation. Genetic interaction between cka2-ts and ipl1-ts will be examined. In addition to cka2- ts mutants, cka2-as (analogue sensitive) mutants will be created and tested. I will also test if Mif2 andNdc10 are phosphorylated by CK2 in vitro and in vivo, and their phosphorylation sites will be mapped by mass spectrometry. Both Mif2 and Ndc10 have multiple putative Ipl1 phosphorylation sites. How phosphorylation by one kinase affects phosphorylation by the other will be investigated in vitro and in vivo. Moreover, phosphorylation sites of Mif2 and Ndc10 will be mutagenized and the role of these modifications will be examined. Finally, using cka2-as mutants, a screen focusing on kinetochores and their regulatory proteins will be conducted to identify additionalCK2 substrates. Understanding how CK2 regulates kinetochores will provide novel insights into the role of CK2 in cell cycle regulation.

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. To maintain their genetic integrity, eukaryotic cells must segregate their chromosomes properly to opposite poles during mitosis. Errors in this process give rise to aneuploidy that is implicated in cancer progression and birth defects. In mammals, Aurora B complex is responsible for detection and correction of faulty centromere/microtubules attachments. The aim of this project is to study the structure, function and regulation of the Ipl1 complex in yeast that is the equivalent of the Aurora B complex. Ipl1 complex downstream targets are pretty well known but the mechanisms responsible for its activation and localization are poorly understood, so I am focusing on upstream activation and localization of Ipl1 complex.

Abstract - NO CHANGES FROM ORIGINAL Microtubules (MTs) exhibit dynamic instability in which they exist in growing, pausing, or shrinking states that interconvert stochastically. This behavior provides the mechanism by which microtubules assemble into a seemingly infinite variety of structures that provide countless cellular functions including cell motility, mitosis, and axonal transport. Numerous microtubule-associated proteins (MAPs) and motors bind to the microtubule lattice or to the growth-favored ?plus? end to regulate microtubule assembly dynamics, organization, and interactions. Important questions that are still unanswered are: how the ensemble behavior of these proteins collectively regulates microtubule dynamics and how these activities are regulated through cell-cycle stages and in different cellular subcellular-compartments. A biochemical cell-extract assay recently developed in the Barnes laboratory unifies, for the first time, two of the most powerful approaches for studies of microtubule dynamics: biochemical extract studies and genetics. Cell extracts are made from budding yeast mutants and dynamics of single microtubules are observed. Since mitosis is a highly conserved process, lessons learned from these studies are likely to apply generally. Unlike many other assays, this assay uses homologous sources of tubulin and MAPs, avoiding species incompatibility. Moreover, dynamics of single microtubules are quantitatively analyzed by highly sensitive Total Internal Fluorescence Microscopy. The three aims are: (1) To determine how specific MAPs and motors affect microtubule assembly dynamics and to reveal emergent properties that arise from their combined activities. Extracts will be prepared from wild-type yeast and mutants of different microtubule dynamics regulators, singly or in combinations, and microtubule dynamics in the extracts will be quantitatively analyzed to parse the contributions of individual proteins to collective microtubule dynamics regulation. Using cell-cycle-staged extracts from mutants of MT dynamics regulators, proteins responsible for programmed changes in microtubule dynamics through the cell cycle will be identified. (2) To determine how activities of these proteins are coordinately regulated through the cell cycle. For four different cell-cycle stages, phosphorylation sites on MT dynamics regulators will be mapped by mass spectrometry of the regulators. Functional importance of the identified cell-cycle-specific phosphorylations will be tested by site- directed mutagenesis of the target proteins. (3) To analyze dynamic properties of kinetochores on microtubules in yeast extracts and to determine how kinetochores affect microtubule dynamics. The extract system was successfully adopted for studies of kinetochore association with, and effects on, microtubules. The kinetochore proteins Mtw1 and Spc105 associate with microtubules in the assay. Moreover, they are directionally transported toward microtubule plus ends, and maintain association with disassembling microtubules, all in a cell cycle-dependent manner. This assay will allow mechanisms for kinetochore attachment to MTs, kinetochore activities on MTs, and kinetochore regulation to be revealed.