Cell growth and nutrient availability control the mitotic exit signaling network in budding yeast

Cell growth is required for cell cycle progression. The amount of growth required for cell cycle progression is reduced in poor nutrients, which leads to a reduction in cell size. In budding yeast, nutrients influence cell size by modulating the duration and extent of bud growth, which occurs predominantly in mitosis. However, the mechanisms are unknown. Here, we used mass spectrometry to identify proteins that mediate the effects of nutrients on bud growth. This led to the discovery that nutrients regulate numerous components of the Mitotic Exit Network (MEN), which controls exit from mitosis. A key component of the MEN undergoes gradual multi-site phosphorylation during bud growth that is dependent upon growth and correlated with the extent of growth. Furthermore, activation of the MEN is sufficient to over-ride a growth requirement for mitotic exit. The data suggest a model in which the MEN integrates signals regarding cell growth and nutrient availability to ensure that mitotic exit occurs only when sufficient growth has occurred.