Translational control of CAK and Cdk T-loop phosphorylation in response to growth in yeast

Cyclin-dependent kinases (Cdks) require activating T-loop phosphorylation, a modification that is considered constitutive. Here, we examined the regulation of the Cdk-activating kinase, Cak1, in budding yeast. We measured Cak1 levels and the activating T169 phosphorylation of Cdc28 (the budding yeast Cdk) across various nutrient environments. We found that the abundance of Cak1 and the T169 phosphorylation is significantly reduced in cells that are proliferating very slowly or have entered quiescence. A small upstream open reading frame (uORF) in the CAK1 transcript represses Cak1 synthesis, especially in poor growth conditions. Eliminating the uORF increased Cak1 levels but did not alter proliferation kinetics under most laboratory contexts. Instead, it reduced the viability of quiescent cells and the fitness of slowly proliferating chemostat cultures. In cells lacking several type 2C protein phosphatases, which remove the T169 phosphorylation, there was a pronounced acceleration of initiation of cell division in the absence of the uORF in CAK1. Our results suggest an unexpected layer of control, impinging on the activating phosphorylation of the Cdk. The uORF-mediated repression of Cak1 synthesis directly couples protein synthesis to the activity of the core cell cycle machinery.