#023

Gcn4, a yeast transcriptional activator of amino acid and purine biosynthesis genes, is rapidly degraded in rich medium, but stable under starvation conditions. Its degradation depends on the ubiquitin-conjugating enzyme Cdc34 and the ubiquitin ligase SCF^CDC4. Unlike the cell-cycle substrates of Cdc34 / SCF^CDC4, which require phosphorylation by the kinase Cdc28, Gcn4 degradation requires the kinase Pho85. We identified the critical target site of Pho85 on Gcn4; a mutation of this site stabilizes the protein. An extensive analysis of adjacent positions allowed us to distinguish between residues required for phosphorylation by Pho85, and residues required for recognition by SCF^CDC4. The stabilization of Gcn4 under starvation conditions is explained by the observation that a specific Pho85/Pcl complex that is able to phosphorylate Gcn4 on that site is inactive under these conditions. The reduced activity of the Pho85 complex can be explained by the high instability of the cyclin subunit: rapid degradation of specific Pho85 cyclins results in a reduction of their steady-state levels when protein synthesis slows down. Starvation and reduced protein synthesis are also known to result in G1 arrest. We found that increasing the activity of the Pho85 kinase allows to partially overcome this starvation-induced cell cycle arrest, suggesting that Pho85 activity constitutes one of the transducers of physiological signals to the cell cycle machinery.

Return to YGM 2000 Abstract Index