Abstract
The lipid phosphatase Sac1 cycles between endoplasmic reticulum and cisternal Golgi compartments. In proliferating mammalian cells, a canonical dilysine motif at the C-terminus of Sac1 is required for coatomer complex-I (COP-I)-binding and continuous retrieval to the ER. Starvation triggers accumulation of Sac1 at the Golgi. The mechanism responsible for Golgi retention of Sac1 is unknown. Here we show that the first of the two transmembrane regions in human SAC1 (TM1) functions in Golgi localization. A minimal construct containing only TM1 and the adjacent flanking sequences is concentrated at the Golgi. Transplanting TM1 into transferrin receptor 2 (TfR2) induces Golgi accumulation of this normally plasma membrane and endosomal protein, indicating that TM1 is sufficient for Golgi localization. In addition, we determined that the N-terminal cytoplasmic domain of SAC1 also promotes Golgi localization, even when TM1 is mutated or absent. We conclude that the distribution of SAC1 within the Golgi is controlled via both passive membrane thickness-dependent partitioning of TM1 and a retention mechanism that requires the N-terminal cytoplasmic region.
Highlights
The lipid phosphatase Sac1 is an evolutionary conserved regulator of phosphatidylinositol-4-phosphate (PtdIns(4)P) and plays vital roles at endoplasmic reticulum (ER)/plasma membrane contact sites and in Golgi organization and trafficking [1,2,3]
Together these data indicate that the short GFP-SAC1(478–549) construct contains all the elements that are needed for efficient Golgi retention
We show that one of the two transmembrane domains in human SAC1 is sufficient for partitioning into the Golgi
Summary
The lipid phosphatase Sac is an evolutionary conserved regulator of phosphatidylinositol-4-phosphate (PtdIns(4)P) and plays vital roles at endoplasmic reticulum (ER)/plasma membrane contact sites and in Golgi organization and trafficking [1,2,3]. Sac is an integral membrane protein containing a large N-terminal cytoplasmic domain and two transmembrane regions near its C terminus [4]. Sac localizes to both ER and cisternal Golgi regions but is absent from the trans-Golgi network (TGN) [7]. The polarized distribution of Sac is critical for maintaining a steep PtdIns(4)P gradient at the Golgi complex with the majority of PtdIns(4)P confined to the TGN [7]. During starvation and in quiescent cells Sac accumulates at the TGN, which reduces Golgi PtdIns(4)P levels and slows anterograde trafficking [1]. A leucine repeat region at the cytoplasmic N-terminal domain promotes oligomerization of Sac during starvation, which is a prerequisite for Golgi accumulation [1]. How the distribution of Sac within the Golgi is controlled remains unclear
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