A complex genetic interaction implicates that phospholipid asymmetry and phosphate homeostasis regulate Golgi functions

In eukaryotic cells, phospholipid flippases translocate phospholipids from the exoplasmic to the cytoplasmic leaflet of the lipid bilayer. Budding yeast contains five flippases, of which Cdc50p-Drs2p and Neo1p are primarily involved in membrane trafficking in endosomes and Golgi membranes. TheANY1/CFS1gene was identified as a suppressor of growth defects in theneo1 Delta andcdc50 Delta mutants. Cfs1p is a membrane protein of the PQ-loop family and is localized to endosomal/Golgi membranes, but its relationship to phospholipid asymmetry remains unknown. Theneo1 Delta cfs1 Delta mutant appears to function normally in membrane trafficking but may function abnormally in the regulation of phospholipid asymmetry. To identify a gene that is functionally relevant toNEO1andCFS1, we isolated a mutation that is synthetically lethal withneo1 Delta cfs1 Delta and identifiedERD1. Erd1p is a Golgi membrane protein that is involved in the transport of phosphate (Pi) from the Golgi lumen to the cytoplasm. The Neo1p-depletedcfs1 Delta erd1 Delta mutant accumulated plasma membrane proteins in the Golgi, perhaps due to a lack of phosphatidylinositol 4-phosphate. The Neo1p-depletedcfs1 Delta erd1 Delta mutant also exhibited abnormal structure of the endoplasmic reticulum (ER) and induced an unfolded protein response, likely due to defects in the retrieval pathway from thecis-Golgi region to the ER. Genetic analyses suggest that accumulation of Pi in the Golgi lumen is responsible for defects in Golgi functions in the Neo1p-depletedcfs1 Delta erd1 Delta mutant. Thus, the luminal ionic environment is functionally relevant to phospholipid asymmetry. Our results suggest that flippase-mediated phospholipid redistribution and luminal Pi concentration coordinately regulate Golgi membrane functions.