Vesicle trafficking maintains nuclear shape in Saccharomyces cerevisiae during membrane proliferation

The parameters that control nuclear size and shape are poorly understood. In yeast, unregulated membrane proliferation, caused by deletion of the phospholipid biosynthesis inhibitor SPO7, leads to a single nuclear envelope "flare" that protrudes into the cytoplasm. This flare is always associated with the asymmetrically localized nucleolus, which suggests that the site of membrane expansion is spatially confined by an unknown mechanism. Here we show that in spo7Δ cells, mutations in vesicle-trafficking genes lead to multiple flares around the entire nucleus. These mutations also alter the distribution of small nucleolar RNA-associated nucleolar proteins independently of their effect on nuclear shape. Both single- and multi-flared nuclei have increased nuclear envelope surface area, yet they maintain the same nuclear/cell volume ratio as wild-type cells. These data suggest that, upon membrane expansion, the spatial confinement of the single nuclear flare is dependent on vesicle trafficking. Moreover, flares may facilitate maintenance of a constant nuclear/cell volume ratio in the face of altered membrane proliferation.