ABSTRACT
Phosphoinositide phosphatases play an essential but as yet not well-understood role in lipid-based signal transduction. Members of a subfamily of these enzymes share a specific domain that was first identified in the yeast Sac1 protein [1]. Sac1 homology domains were shown to exhibit 3- and 4-phosphatase activity in vitro [2, 3] and were also found, in addition to rat and yeast Sac1p, in yeast Inp/Sjl proteins [4, 5] and mammalian synaptojanins [6]. Despite the detailed in vitro characterization of the enzymatic properties of yeast Sac1p, the exact cellular function of this protein has remained obscure. We report here that Sac1p has a specific role in secretion and acts as an antagonist of the phosphatidylinositol 4-kinase Pik1p in Golgi trafficking. Elimination of Sac1p leads to excessive forward transport of chitin synthases and thus causes specific cell wall defects. Similar defects in membrane trafficking are caused by the overexpression of PIK1. Taken together, these findings provide strong evidence that the generation of PtdIns(4)P is sufficient to trigger forward transport from the Golgi to the plasma membrane and that Sac1p is critically required for the termination of this signal.
