Meiosis: Organizing Microtubule Organizers
In certain types of eukaryotic cells, the microtubule cytoskeleton is coopted to create specialized structures, such as axons in neurons, or flagella in spermatozoa. Even in nonspecialized cells, however, the microtubule cytoskeleton can be transiently altered in interesting ways. The archetypical example of this is the mitotic spindle, which rapidly assembles during mitosis to effect chromosome segregation and then disassembles equally quickly. Although spindle formation involves a wholesale
... organization of cellular components and is, appropriately, incredibly complex, smaller organizational changes in the cytoskeleton can similarly illuminate our understanding of cytoskeletal regulation. Recent studies in fission yeast [1,2] have now revealed that a single protein can be responsible for a major change in microtubule organization during the early stages of meiosis. During fission yeast meiosis, after fusion of the cell nuclei of the two mating-partners, the resulting zygote nucleus undergoes sweeping oscillatory movements in the cell [3-5]. These 'horsetail' movements -so called because the nucleus, led by the spindle pole body, the yeast centrosome equivalent, resembles the shape of a horsetail -are driven by microtubules and the forcegenerating enzyme cytoplasmic dynein. Strikingly, mutants in which the movements do not occur, including mutants in dynein itself , show much reduced recombination between homologous chromosomes, indicating that these movements are more than a cytological curiosity. The horsetail movements are accompanied by changes in nuclear organization of chromosomes: in vegetative (mitotically growing) cells, the interphase centromeres are associated with the spindle pole body, but in meiotic cells it is the telomeres that associate with the spindle pole body [3,6]. Collectively, these modifications and movements within the nucleus and the cytoplasm are thought to allow for a more efficient homology search among the Schizosaccharomyces pombe chromosomes, and thus to promote proper recombination. A key feature of horsetail movement is that it involves a rearrangement of the microtubule cytoskeleton. To appreciate this properly requires a brief explanation of interphase microtubule organizing centers in fission yeast. In higher eukaryotic cells, microtubules are centrally organized in a radial array, nucleated from the centrosome, where microtubule 'minus' ends are associated with the conserved microtubule-nucleating γ γ-tubulin complex , while microtubule 'plus' ends are more likely found towards the cell periphery. By contrast, interphase microtubules in vegetative fission yeast run along the long axis of the cylindrically shaped cells , and are nucleated from sites in the cell middle that may include the spindle pole body, additional sites on the surface of the nucleus, and satellite sites on microtubules themselves [9-11] (Figure 1) . Nucleation from all of these sites involves the coiled-coil protein mto1p (previously known In eukaryotic cells, the 3′ ′ poly(A) tails found on mRNA influence their stability and translation. The discovery of a second nuclear poly(A) polymerase complex has fueled a series of reports defining a new and unexpected role for 3′ ′ end poly(A) tails in the nuclear surveillance and turnover of noncoding RNAs and intergenic mRNAs of unknown function.