uORF-mediated translational control of CAK and Cdk activation in response to low nutrients in yeast.

Cyclin-dependent kinases (Cdks) require activating T-loop phosphorylation, a modification considered constitutive. Here, we examine the regulation of the Cdk-activating kinase, Cak1, in budding yeast. We measure Cak1 levels and the activating T169 phosphorylation of Cdc28 (the budding yeast Cdk) in different nutrients. The abundance of Cak1 and T169 phosphorylation is reduced in cells that proliferate very slowly or enter quiescence. A small upstream open reading frame (uORF) in CAK1 represses Cak1 synthesis, especially in poor growth conditions. Eliminating the uORF increases Cak1 levels but does not alter proliferation kinetics under most laboratory contexts. Instead, it reduces the viability of quiescent cells. In cells lacking several type 2 C protein phosphatases, which remove the T169 phosphorylation, initiation of cell division is accelerated 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.