Growth Control of Golgi Phosphoinositides by Reciprocal Localization of Sac1 Lipid Phosphatase and Pik1 4‐Kinase

Abstract

Compartment‐specific control of phosphoinositide lipids is essential for cell function. The Sac1 lipid phosphatase regulates endoplasmic reticulum (ER) and Golgi phosphatidylinositol‐4‐phosphate [PI(4)P] in response to nutrient levels and cell growth stages. During exponential growth, Sac1p interacts with Dpm1p at the ER but shuttles to the Golgi during starvation. Here, we report that a C‐terminal region in Sac1p is required for retention in the perinuclear ER, whereas the N‐terminal domain is responsible for Golgi localization. We also show that starvation‐induced shuttling of Sac1p to the Golgi depends on the coat protein complex II and the Rer1 adaptor protein. Starvation‐induced shuttling of Sac1p to the Golgi specifically eliminates a pool of PI(4)P generated by the lipid kinase Pik1p. In addition, absence of nutrients leads to a rapid dissociation of Pik1p, together with its non‐catalytical subunit Frq1p, from Golgi membranes. Reciprocal rounds of association/dissociation of the Sac1p lipid phosphatase and the Pik1p/Frq1p lipid kinase complex are responsible for growth‐dependent control of Golgi phosphoinositides. Sac1p and Pik1p/Frq1p are therefore elements of a unique machinery that synchronizes ER and Golgi function in response to different growth conditions.

Nucleocytoplasmic Shuttling of the Golgi Phosphatidylinositol 4-Kinase Pik1 Is Regulated by 14-3-3 Proteins and Coordinates Golgi Function with Cell Growth

“…Regulation of the localization of enzymes of PI metabolism has been documented in other studies, e.g., for the PI(4)P 5-kinase Mss4p (Audhya and Emr, 2003) and the lipid phosphatase Sac1p (Faulhammer et al, 2005;Faulhammer et al, 2007). Together with our data showing nucleocytoplasmic shuttling of Pik1p, we propose that relocation of PImetabolizing enzymes between different compartments might be a general mechanism for the rapid adjustment of PI levels on organelles.…”

“…Interestingly, we found Pik1p loss from the TGN under similar conditions that caused relocation of the lipid phosphatase Sac1p from the ER to the Golgi (Faulhammer et al, 2005(Faulhammer et al, , 2007. Because Pik1p generates PI(4)P and Sac1p dephosphorylates this lipid, the coordinated relocation of both enzymes would result in a drastic and rapid reduction of PI(4)P on the TGN membrane.…”

Golgi and plasma membrane pools of PI(4)P contribute to plasma membrane PI(4,5)P 2 and maintenance of KCNQ2/3 ion channel current

“…In addition, transient apposition between organelles can alter phosphoinositide levels by presenting membrane-bound phosphatases in trans. For example, the endoplasmic reticulum (ER) can make contacts with the Golgi, allowing 4-phosphatases of the ER to dephosphorylate Golgi phosphatidylinositol 4-phosphate [PI(4)P] (6)(7)(8).…”

Phosphoregulatory protein 14-3-3 facilitates SAC1 transport from the endoplasmic reticulum

“…All mammalian orthologs of SAC1 possess a C-terminal dilysine ER retention motif that interacts with COPI and is responsible for retrieval of SAC1 from the Golgi to the ER membranes in the presence of growth factors. In contrast, Saccharomyces cerevisiae SAC1 lacks a typical ER retrieval motif and employs Rer1p as an adaptor protein that impairs the interaction between SAC1p and Dpm1p and facilitates ER exit of Sac1 under glucosedeprived conditions (27).…”

“…S1). This species may represent a partially translated cytosolic domain of SAC1 that may associate nonspecifically to membranes as reported earlier in case of yeast SAC1p with microsomal membranes (27). The upper band of , RLSNTSP motif (blue) examined in this study, and SAC1 catalytic motif (dark blue) two transmembrane segments (black) and C-terminal domain with COPI binding motif.…”