Mitogen-dependent regulation of SAC1 and PI(4)P signaling at the Golgi. Schematic illustration of cell growth–dependent control of PI(4)P signaling and Golgi trafficking by SAC1 lipid phosphatase. In quiescent cells, SAC1 oligomerizes and accumulates in the Golgi, which in turn down-regulates Golgi PI(4)P and constitutive secretion. After growth factor stimulation, p38 MAPK activity is required for dissociation of SAC1 complexes, which triggers retrograde traffic and redistribution of SAC1 to the ER. Reduction of SAC1 levels at the Golgi allows for elevated concentration of PI(4)P at this organelle, thus accelerating constitutive secretion.