G2 cyclins are required for the degradation of G1 cyclins in yeast

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Abstract
PROGRESSION of the eukaryotic cell cycle is controlled by cyclin-dependent kinases (CDKs)1. Cdc28, the budding yeast homologue of Cdc2 (Cdkl), is required for both the Gl/S and G2/M transitions of the cell cycle2. The functional specificity of the Cdc28 kinase is determined by its association with Gl or G2 cyclins. Alternation of cell cycle phases is thus mainly due to mechanisms that ensure that one cyclin family succeeds another. Here we show that the G2 cyclins Clbl, Clb2, Clb3 and Clb4 are required for the proteolysis of the Gl cyclins Clnl and Cln2, providing a mechanism for coupling synthesis of G2 cyclins with the disappearance of Gl cyclins. Our data indicate that this pathway involves the Ubc9 ubiquitin-conjugating enzyme3. The Cdc34 ubiquitin-conjugating activity4 may function redundantly with Ubc9, or it may only be involved in Clnl,2 turnover through its role in promoting the degradation of Sicl, a specific inhibitor of Cdc28-Clb complexes5.