Question

Yeast CDK (cdc2) has been fused with the mitotic cyclin (cdc13) and are together expressed from the cdc13 promoter (see figure below). Cells with this fusion go through the first cell stage but die in the next Gl. Explain hypothesize why cells with this fusion die in GI. If Cdc13-Cdc2 fused protein is injected in the cells described in "4" at the Gl from the second cell cycle they attempt to enter straight into M phase without passing through the intervening Sand G2 phases. Explain hypothesize why the rescue sends them straight into M-phase. Inject cdc13-cdc2 fused protein S M Arrest G1- cell cycle 1" cell cycle

Answer 1

Answer a-

The cdc2-13 complex regulates replication and mitosis by promoting cdc25 activity and also promotes amino-acid synthesis during G1 phase. Activity of this fusion proteins can be regulated by reversible association with Rum1, an cdk inhibitor as shown in the following figure.

Taken from- Gérard, Claude, et al. "Cell cycle control by a minimal Cdk network." PLoS computational biology 11.2 (2015).

The low activity state of cdc2-13 fusion proteis is achieved by Rum1-dependent inhibition and their degradation depends on APC. Whereas the intermediate fusion protein activity is maintained by Wee1-dependent inhibitory mechanism. Cells with this fusion proteins pass first cell cycle stages but die at G1 phase of next cycle may be due to higher level of inhibition of this fusion protein by Rum1 at the G1 stage.

Answer b-

When cells are injected with cdc2-13 fusion proteins cells can directly enter M phase. It is shown by Coudreuse, D. & Nurse, P. that, cells with higher CDK activity can enter mitosis from any stages of  cell cycle. Higher CDK activity does not even activate the S phase checkpoint. This suggests that M phase progression is independent of passage through S phase checkpoint which requires intermediate or low CDK levels.