Microtubules support nuclear nonproliferation arrangement

Laporte et al. describe how a microtubule array reorganizes the nucleus to aid the survival of quiescent budding yeast.

Eukaryotic cells undergo a dramatic rearrangement when they temporarily exit the cell cycle and become quiescent. Budding yeast, for example, reorganize their actin cytoskeleton 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 chromosome—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.