Microtubules support nuclear nonproliferation arrangement

Microtubules suppor t nuclear nonproliferation arrangement L aporte et al. describe how a mi-crotubule array reorganizes the nucleus to aid the survival of qui-escent budding yeast. Eukaryotic cells undergo a dramatic rearrangement when they temporarily exit the cell cycle and become qui-escent. Budding yeast, for example, reorganize their actin cyto-skeleton into immobile structures called actin bodies when they stop proliferating, but what happens to microtubules in these cells is unknown. Laporte et al. found that, when yeast ran out of nutrients and entered quiescence, they formed an array of stable microtubules emanating across the nucleus from the spindle pole body (the yeast equivalent of the centrosome). These microtubules caused a drastic reorganization of the cell nucleus. The nucleolus was pushed to the side instead of lying opposite the spindle pole body as it does in proliferating yeast. And the centromeres of each chromo-some—attached to microtubule plus-ends—were spread out along the nuclear microtubule array, instead of clustering near the spindle pole body as they do in G1. Quiescent yeast required the kinesin Kar3 and the dynein/ dynactin motor complex to form the nuclear microtubule array. In the absence of these proteins, yeast no longer rearranged their nuclei and were less able to survive quiescence and reenter the cell cycle when nutrients became available again. Senior author Isabelle Sagot speculates that microtubule-induced changes in nuclear organization might alter the expression of genes that help establish and maintain quiescence. She now wants to investigate how quiescent yeast assemble the nuclear microtubule array and to examine its affects on gene expression. H álová et al. identify a new mechanism by which yeast and mammalian cells regulate the activity of TOR protein kinases. In response to environmental stimuli, TOR kinases regulate a range of cellular processes, from growth and proliferation to differentiation and cytoskeletal organization. The kinases assemble into two distinct complexes, TORC1 and TORC2, whose activities are tightly regulated by multiple signaling pathways. Hálová et al. wondered whether any of these pathways might regulate TOR by controlling the kinase's phosphorylation. The researchers initially focused on fi ssion yeast Tor1. This kinase, which assembles into the TORC2 complex, was phosphory-lated on a conserved residue, threonine 1972, in the enzyme's ATP-binding domain. A nonphosphorylatable version of Tor1 showed increased kinase activity, and yeast expressing this mutant showed increased resistance to oxidative and osmotic stress. Compared with wild-type cells, however, yeast expressing nonphosphorylatable Tor1 were less able …