A sphingolipid-dependent diffusion barrier confines ER stress to the yeast mother cell.

In many cell types, lateral diffusion barriers compartmentalize the plasma membrane and, at least in budding yeast, the endoplasmic reticulum (ER). However, the molecular nature of these barriers, their mode of action and their cellular functions are unclear. Here, we show that misfolded proteins of the ER remain confined into the mother compartment of budding yeast cells. Confinement required the formation of a lateral diffusion barrier in the form of a distinct domain of the ER-membrane at the bud neck, in a septin-, Bud1 GTPase- and sphingolipid-dependent manner. The sphingolipids, but not Bud1, also contributed to barrier formation in the outer membrane of the dividing nucleus. Barrier-dependent confinement of ER stress into the mother cell promoted aging. Together, our data clarify the physical nature of lateral diffusion barriers in the ER and establish the role of such barriers in the asymmetric segregation of proteotoxic misfolded proteins during cell division and aging.DOI: http://dx.doi.org/10.7554/eLife.01883.001.

The cell biology of open and closed mitosis.

Mitosis is the process of one cell dividing into two daughters, such that each inherits a single and complete copy of the genome of their mother. This is achieved through the equal segregation of the sister chromatids between the daughter cells. However, beyond this simple principle, the partitioning of other cellular components between daughter cells appears to follow a large variety of patterns. We discuss here how the organization of the nuclear envelope during mitosis influences cell division and, subsequently, cellular identity.

Nuclear envelope morphology constrains diffusion and promotes asymmetric protein segregation in closed mitosis.

During vegetative growth, Saccharomyces cerevisiae cells divide asymmetrically: the mother cell buds to produce a smaller daughter cell. This daughter asymmetrically inherits the transcription factor Ace2, which activates daughter-specific transcriptional programs. In this paper, we investigate when and how this asymmetry is established and maintained. We show that Ace2 asymmetry is initiated in the elongated, but undivided, anaphase nucleus. At this stage, the nucleoplasm was highly compartmentalized; little exchange was observed for nucleoplasmic proteins between mother and bud. Using photobleaching and in silico modeling, we show that diffusion barriers compartmentalize the nuclear membranes. In contrast, the behavior of proteins in the nucleoplasm is well explained by the dumbbell shape of the anaphase nucleus. This compartmentalization of the nucleoplasm promoted Ace2 asymmetry in anaphase nuclei. Thus, our data indicate that yeast cells use the process of closed mitosis and the morphological constraints associated with it to asymmetrically segregate nucleoplasmic components.

Budding yeast dma proteins control septin dynamics and the spindle position checkpoint by promoting the recruitment of the Elm1 kinase to the bud neck.

The first step towards cytokinesis in budding yeast is the assembly of a septin ring at the future site of bud emergence. Integrity of this ring is crucial for cytokinesis, proper spindle positioning, and the spindle position checkpoint (SPOC). This checkpoint delays mitotic exit and cytokinesis as long as the anaphase spindle does not properly align with the division axis. SPOC signalling requires the Kin4 protein kinase and the Kin4-regulating Elm1 kinase, which also controls septin dynamics. Here, we show that the two redundant ubiquitin-ligases Dma1 and Dma2 control septin dynamics and the SPOC by promoting the efficient recruitment of Elm1 to the bud neck. Indeed, dma1 dma2 mutant cells show reduced levels of Elm1 at the bud neck and Elm1-dependent activation of Kin4. Artificial recruitment of Elm1 to the bud neck of the same cells is sufficient to re-establish a normal septin ring, proper spindle positioning, and a proficient SPOC response in dma1 dma2 cells. Altogether, our data indicate that septin dynamics and SPOC function are intimately linked and support the idea that integrity of the bud neck is crucial for SPOC signalling.