Adaptation to the spindle checkpoint is regulated by the interplay between Cdc28/Clbs and PP2ACdc55

Abstract

The spindle checkpoint arrests cells in metaphase until all chromosomes are properly attached to the chromosome segregation machinery. Thereafter, the anaphase promoting complex (APC/C) is activated and chromosome segregation can take place. Cells remain arrested in mitosis for hours in response to checkpoint activation, but not indefinitely. Eventually, they adapt to the checkpoint and proceed along the cell cycle. In yeast, adaptation requires the phosphorylation of APC/C. Here, we show that the protein phosphatase PP2ACdc55 dephosphorylates APC/C, ther eby counteracting the activity of the mitotic kinase Cdc28. We also observe that the key regulator of Cdc28, the mitotic cyclin Clb2, increases before cells adapt and is then abruptly degraded at adaptation. Adaptation is highly asynchronous and takes place over a range of several hours. Our data suggest the presence of a double negative loop between PP2ACdc55 and PP2ACdc20 (i.e., a positive feedback loop) that controls PP2ACdc20 activity. The circuit could guarantee sustained PP2ACdc20 activity after Clb2 starts to be degraded. © 2013 Vernieri et al.