MAP1S controls microtubule stability throughout the cell cycle in human cells.

Summary Understanding the molecular basis for proper cell division requires a detailed functional analysis of microtubule (MT)-associated proteins. MT-associated protein 1S (MAP1S), the most ubiquitously expressed MAP1 family member, is required for accurate cell division. Here, using quantitative analysis of MT plus-end tracking, we show that MAP1S knockdown alters MT dynamics throughout the cell cycle. Surprisingly, MAP1S downregulation results in faster growing, yet short-lived, MTs in all cell cycle stages and in a global loss of MT acetylation. These aberrations correlate with severe defects in the final stages of cell division. In monopolar cytokinesis assays, we demonstrate that MAP1S guides MT-dependent initiation of cytokinesis. Our data underline the key role of MAP1S as a global regulator of MT stability and demonstrate a new primary function of MAP1S to regulate MT dynamics at the onset of cytokinesis.

Maternal MEMI Promotes Female Meiosis II in Response to Fertilization in Caenorhabditis elegans.

In most animals, female meiosis completes only after fertilization. Sperm entry has been implicated in providing a signal for the initiation of the final meiotic processes; however, a maternal component required for this process has not been previously identified. We report the characterization of a novel family of three highly similar paralogs (memi-1, memi-2, memi-3) that encode oocyte-specific proteins. A hyper-morphic mutation memi-1(sb41) results in failure to exit female meiosis II properly; however, loss of all three paralogs results in a "skipped meiosis II" phenotype. Mutations that prevent fertilization, such as fer-1(hc1), also cause a skipped meiosis II phenotype, suggesting that the MEMI proteins represent a maternal component of a postfertilization signal that specifies the meiosis II program. MEMI proteins are degraded before mitosis and sensitive to ZYG-11, a substrate-specific adapter for cullin-based ubiquitin ligase activity, and the memi-1(sb41) mutation results in inappropriate persistence of the MEMI-1 protein into mitosis. Using an RNAi screen for suppressors of memi-1(sb41), we identified a sperm-specific PP1 phosphatase, GSP-3/4, as a putative sperm component of the MEMI pathway. We also found that MEMI and GSP-3/4 proteins can physically interact via co-immunoprecipitation. These results suggest that sperm-specific PP1 and maternal MEMI proteins act in the same pathway after fertilization to facilitate proper meiosis II and the transition into embryonic mitosis.

Measuring microtubule growth and gliding in Caenorhabditis elegans embryos.

Microtubule plus-tip tracking is a powerful method to measure microtubule growth dynamics in vivo. Here we outline an approach that exploits live confocal microscopy of a GFP-tagged EB1-like protein to measure microtubule growth behavior and minus-end-directed microtubule motor activity at the cortex of Caenorhabditis elegans embryos. The EB1 velocity assay (EVA) provides a method to reproducibly monitor motor- and non-motor-assisted microtubule movements.

The Prp19 complex directly functions in mitotic spindle assembly.

The conserved Prp19 (pre-RNA processing 19) complex is required for pre-mRNA splicing in eukaryotic nuclei. Recent RNAi screens indicated that knockdown of Prp19 complex subunits strongly delays cell proliferation. Here we show that knockdown of the smallest subunit, BCAS2/Spf27, destabilizes the entire complex and leads to specific mitotic defects in human cells. These could result from splicing failures in interphase or reflect a direct function of the complex in open mitosis. Using Xenopus extracts, in which cell cycle progression and spindle formation can be reconstituted in vitro, we tested Prp19 complex functions during a complete cell cycle and directly in open mitosis. Strikingly, immunodepletion of the complex either before or after interphase significantly reduces the number of intact spindles, and increases the percentage of spindles with lower microtubule density and impaired metaphase alignment of chromosomes. Our data identify the Prp19 complex as the first spliceosome subcomplex that directly contributes to mitosis in vertebrates independently of its function in interphase.

The centriolar satellite protein SSX2IP promotes centrosome maturation.

Meiotic maturation in vertebrate oocytes is an excellent model system for microtubule reorganization during M-phase spindle assembly. Here, we surveyed changes in the pattern of microtubule-interacting proteins upon Xenopus laevis oocyte maturation by quantitative proteomics. We identified the synovial sarcoma X breakpoint protein (SSX2IP) as a novel spindle protein. Using X. laevis egg extracts, we show that SSX2IP accumulated at spindle poles in a Dynein-dependent manner and interacted with the γ-tubulin ring complex (γ-TuRC) and the centriolar satellite protein PCM-1. Immunodepletion of SSX2IP impeded γ-TuRC loading onto centrosomes. This led to reduced microtubule nucleation and spindle assembly failure. In rapidly dividing blastomeres of medaka (Oryzias latipes) and in somatic cells, SSX2IP knockdown caused fragmentation of pericentriolar material and chromosome segregation errors. We characterize SSX2IP as a novel centrosome maturation and maintenance factor that is expressed at the onset of vertebrate development. It preserves centrosome integrity and faithful mitosis during the rapid cleavage division of blastomeres and in somatic cells.

EML3 is a nuclear microtubule-binding protein required for the correct alignment of chromosomes in metaphase.

Assembly of the mitotic spindle requires a global change in the activity and constitution of the microtubule-binding-protein array at mitotic onset. An important subset of mitotic microtubule-binding proteins localises to the nucleus in interphase and essentially contributes to spindle formation and function after nuclear envelope breakdown. Here, we used a proteomic approach to selectively identify proteins of this category and revealed 50 poorly characterised human gene products, among them the echinoderm microtubule-associated-protein-like gene product, EML3. Indirect immunofluorescence showed that EML3 colocalises with spindle microtubules throughout all mitotic stages. In interphase, EML3 colocalised with cytoplasmic microtubules and accumulated in interphase nuclei. Using YFP-fusion constructs of EML3, we located a nuclear localisation signal and confirmed the microtubule-binding domain of EML3. Functional analysis of EML3 using time-lapse fluorescence microscopy and detailed end-point analysis of phenotypes after siRNA knockdown demonstrates an important role for EML3 in correct metaphase chromosome alignment. Our proteomic identification screen combined with sensitive phenotypic analysis therefore provides a reliable platform for the identification and characterisation of proteins important for correct cell division.