Live-cell imaging RNAi screen identifies PP2A-B55alpha and importin-beta1 as key mitotic exit regulators in human cells.

Institute of Biochemistry, Swiss Federal Institute of Technology Zurich (ETHZ), Schafmattstrasse 18, CH-8093 Zurich, Switzerland.
Nature Cell Biology (Impact Factor: 20.06). 09/2010; 12(9):886-93. DOI: 10.1038/ncb2092
Source: PubMed

ABSTRACT When vertebrate cells exit mitosis various cellular structures are re-organized to build functional interphase cells. This depends on Cdk1 (cyclin dependent kinase 1) inactivation and subsequent dephosphorylation of its substrates. Members of the protein phosphatase 1 and 2A (PP1 and PP2A) families can dephosphorylate Cdk1 substrates in biochemical extracts during mitotic exit, but how this relates to postmitotic reassembly of interphase structures in intact cells is not known. Here, we use a live-cell imaging assay and RNAi knockdown to screen a genome-wide library of protein phosphatases for mitotic exit functions in human cells. We identify a trimeric PP2A-B55alpha complex as a key factor in mitotic spindle breakdown and postmitotic reassembly of the nuclear envelope, Golgi apparatus and decondensed chromatin. Using a chemically induced mitotic exit assay, we find that PP2A-B55alpha functions downstream of Cdk1 inactivation. PP2A-B55alpha isolated from mitotic cells had reduced phosphatase activity towards the Cdk1 substrate, histone H1, and was hyper-phosphorylated on all subunits. Mitotic PP2A complexes co-purified with the nuclear transport factor importin-beta1, and RNAi depletion of importin-beta1 delayed mitotic exit synergistically with PP2A-B55alpha. This demonstrates that PP2A-B55alpha and importin-beta1 cooperate in the regulation of postmitotic assembly mechanisms in human cells.

Download full-text


Available from: Veerle Janssens, Jun 23, 2015
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Recent advances in microscope automation provide new opportunities for high-throughput cell biology, such as image-based screening. High-complex image analysis tasks often make the implementation of static and predefined processing rules a cumbersome effort. Machine-learning methods, instead, seek to use intrinsic data structure, as well as the expert annotations of biologists to infer models that can be used to solve versatile data analysis tasks. Here, we explain how machine-learning methods work and what needs to be considered for their successful application in cell biology. We outline how microscopy images can be converted into a data representation suitable for machine learning, and then introduce various state-of-the-art machine-learning algorithms, highlighting recent applications in image-based screening. Our Commentary aims to provide the biologist with a guide to the application of machine learning to microscopy assays and we therefore include extensive discussion on how to optimize experimental workflow as well as the data analysis pipeline.
    Journal of Cell Science 11/2013; 126(24). DOI:10.1242/jcs.123604 · 5.33 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Cytokinesis follows separase activation and chromosome segregation. This order is ensured in budding yeast by the mitotic exit network (MEN), where Cdc14p dephosphorylates key conserved Cdk1-substrates exemplified by the anaphase spindle-elongation protein Ase1p. However, in metazoans, MEN and Cdc14 function is not conserved. Instead, the PP2A-B55α/ENSA/Greatwall (BEG) pathway controls the human Ase1p ortholog PRC1. In this pathway, PP2A-B55 inhibition is coupled to Cdk1-cyclin B activity, whereas separase inhibition is maintained by cyclin B concentration. This creates two cyclin B thresholds during mitotic exit. Simulation and experiments using PRC1 as a model substrate show that the first threshold permits separase activation and chromosome segregation, and the second permits PP2A-B55 activation and initiation of cytokinesis. Removal of the ENSA/Greatwall (EG) timer module eliminates this second threshold, as well as associated delay in PRC1 dephosphorylation and initiation of cytokinesis, by uncoupling PP2A-B55 from Cdk1-cyclin B activity. Therefore, temporal order during mitotic exit is promoted by the metazoan BEG pathway.
    Molecular cell 10/2013; 52(3). DOI:10.1016/j.molcel.2013.09.005 · 14.46 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Correct spindle positioning is fundamental for proper cell division during development and in stem cell lineages. Dynein and an evolutionarily conserved ternary complex (nuclear mitotic apparatus protein [NuMA]-LGN-Gα in human cells and LIN-5-GPR-1/2-Gα in Caenorhabditis elegans) are required for correct spindle positioning, but their relationship remains incompletely understood. By analyzing fixed specimens and conducting live-imaging experiments, we uncovered that appropriate levels of ternary complex components are critical for dynein-dependent spindle positioning in HeLa cells and C. elegans embryos. Moreover, using mutant versions of Gα in both systems, we established that dynein acts at the membrane to direct spindle positioning. Importantly, we identified a region within NuMA that mediates association with dynein. By using this region to target dynein to the plasma membrane, we demonstrated that the mere presence of dynein at that location is sufficient to direct spindle positioning in HeLa cells. Overall, we propose a model in which the ternary complex serves to anchor dynein at the plasma membrane to ensure correct spindle positioning.
    The Journal of Cell Biology 10/2012; 199(1):97-110. DOI:10.1083/jcb.201203166 · 9.69 Impact Factor