How yeast cells control separation

During cell division, complex signalling pathways ensure that the two daughter cells separate only when each has received a single complete set of chromosomes. Carlos Vázquez de Aldana and co-workers describe how, at high temperatures, the protein Swm1p is needed for the final stages of cell separation in Saccharomyces cerevisiae (see p. 545). Swm1p is a core subunit in the anaphase-promoting complex (APC), the E3 ubiquitin ligase that initiates the metaphase-anaphase transition and promotes mitotic exit once chromosome segregation is complete. The authors report that, at 38°C, swm1 cells – mutants lacking SWM1 – form chains instead of separating into single cells after chromosome segregation. This behaviour results from reduced expression of chitinase and other proteins involved in separation, all of which require the transcription factor Ace2p for their expression. The authors show that at 38°C Ace2p fails to move from the cytoplasm to the nucleus (as happens at 28°C) at the end of mitosis, and they speculate that this failure to relocalise is linked to incorrect inactivation of cyclin-dependent kinases in swm1 cells.