While a normal cell (left) has finished cytokinesis, a UPR mutant (right) is still stuck.

A stress-coping strategy of the ER is needed in every cell cycle, show Bicknell et al. (page 1017). The ER's unfolded protein response (UPR), previously associated only with unusual or stressful conditions, is needed for the routine event of cytokinesis.

The UPR is activated when the ER is overloaded with misfolded proteins. It increases ER capacity and helps reduce the load. But the authors wondered whether everyday ER fluctuations might also tweak the UPR.

One particularly taxing process, the group reasoned, is cell division. Sure enough, cytokinesis was slowed in budding yeast cells that lacked the UPR. The stalls required no additional stress; division was enough. No other mitotic stages were affected.

The ER supports cytokinesis by making new phospholipids for the pinching process and as the first stage of the secretory pathway, which provides necessary vesicles to the bud neck. Either of these cytokinetic duties might put enough strain on the ER to activate the UPR slightly.

Artificially induced ER stress prevented yeast cells from completing cytokinesis. This blockade might stem from the absence of the required secretory vesicles or phospholipids. Or it might be activated as a checkpoint, stalling division until the ER is in better shape.

Unlike yeast, the mammalian UPR dampens translation, which stalls cells in G1 by limiting cyclin D1 levels. If pushed through this blockade, however, the cells might reveal a need for the UPR in breaking apart mammalian daughter cells.