Inhibition of Gl cyclin activity by the Ras/cAMP pathway in yeast

IN the yeast Saccharomyces cerevisiae commitment to cell division (Start) requires growth to a critical cell size^1-3. The Gl cyclins Clnl, Cln2 and Cln3 activate the Cdc28 protein kinase and are rate-limiting activators of Start^4-6. When glucose is added to cells growing in a poor carbon source, the critical cell size required for Start is reset from a small to a large size^2,3,7. In yeast glucose acts through Ras proteins to stimulate adenylyl cyclase activating the three cyclic AMP-dependent protein kinases Tpkl, Tpk2 and Tpk3 (refs 8, 9). We find that stimulation of the Ras/cAMP pathway represses expression of CLN1, CLN2 and co-regulated genes, inhibiting Start. This helps explain the increase in critical size when cells are shifted from poor to rich medium. This connection between the molecules controlling growth (Ras/cAMP) and those controlling division (cyclins) helps explain how division is co-ordinated with growth.