The target of rapamycin complex 1 (TORC1) pathway couples nutrient， energy and hormonal signals with eukaryotic cell growth and division. In yeast， TORC1 coordinates growth with G1-S cell cycle progression， also coined as START， by favouring the expression of G1 cyclins that activate cyclin-dependent protein kinases (CDKs) and by destabilizing the CDK inhibitor Sic1. Following TORC1 downregulation by rapamycin treatment or nutrient limitation， clearance of G1 cyclins and C-terminal phosphorylation of Sic1 by unknown protein kinases are both required for Sic1 to escape ubiquitin-dependent proteolysis prompted by its flagging via the SCF(Cdc4) (Skp1/Cul1/F-box protein) ubiquitin ligase complex. Here we show that the stabilizing phosphorylation event within the C-terminus of Sic1 requires stimulation of the mitogen-activated protein kinase， Mpk1， and inhibition of the Cdc55 protein phosphatase 2A (PP2A(Cdc55)) by greatwall kinase-activated endosulfines. Thus， Mpk1 and the greatwall kinase pathway serve TORC1 to coordinate the phosphorylation status of Sic1 and consequently START with nutrient availability.