Because Ca2+ signaling of budding yeast, through the activation of calcineurin and the Mpk1/Slt2 mitogen-activated protein kinase cascade, performs redundant function(s) in the events essential for growth, the simultaneous deletion of both these pathways (Deltacnb1 Deltampk1) leads to lethality. A PTC4 cDNA that encodes a protein phosphatase belonging to the PP2C family was obtained as a high dosage suppressor of the lethality of Deltacnb1 Deltampk1 strain. Overexpression of PTC4 led to a decrease in the high osmolarity-induced Hog1 phosphorylation, and HOG1 deletion remarkably suppressed the synthetic lethality, indicating an antagonistic role of the high osmolarity glycerol (HOG) pathway and the Ca2+ signaling pathway in growth regulation. The calcineurin-Crz1 pathway was required for the down-regulation of the HOG pathway. Analysis of the time course of actin polarization, bud formation, and the onset of mitosis in synchronous cell cultures demonstrated that calcineurin negatively regulates actin polarization at the bud site, whereas the HOG pathway positively regulates bud formation at a later step after actin has polarized.