ABSTRACT
Class A G protein-coupled receptors (GPCRs), a superfamily of cell membrane signaling receptors, moonlight as constitutively active phospholipid scramblases. The plasma membrane of metazoan cells is replete with GPCRs yet has a strong resting trans-bilayer phospholipid asymmetry, with the signaling lipid phosphatidylserine confined to the cytoplasmic leaflet. To account for the persistence of this lipid asymmetry in the presence of GPCR scramblases, we hypothesized that GPCR-mediated lipid scrambling is regulated by cholesterol, a major constituent of the plasma membrane. We now present a technique whereby synthetic vesicles reconstituted with GPCRs can be supplemented with cholesterol to a level similar to that of the plasma membrane and show that the scramblase activity of two prototypical GPCRs, opsin and the ß1-adrenergic receptor, is impaired upon cholesterol loading. Our data suggest that cholesterol acts as a switch, inhibiting scrambling above a receptor-specific threshold concentration to disable GPCR scramblases at the plasma membrane.
Subject(s)
Phospholipids , Receptors, G-Protein-Coupled , Animals , Biological Transport , Cholesterol , Phospholipid Transfer Proteins/metabolism , Phospholipids/metabolism , Receptors, G-Protein-Coupled/metabolism , Signal Transduction , Cattle , TurkeysABSTRACT
Genome-wide association studies (GWASs) of serum metabolites have the potential to uncover genes that influence human metabolism. Here, we combined an integrative genetic analysis that associates serum metabolites to membrane transporters with a coessentiality map of metabolic genes. This analysis revealed a connection between feline leukemia virus subgroup C cellular receptor 1 (FLVCR1) and phosphocholine, a downstream metabolite of choline metabolism. Loss of FLVCR1 in human cells strongly impairs choline metabolism due to the inhibition of choline import. Consistently, CRISPR-based genetic screens identified phospholipid synthesis and salvage machinery as synthetic lethal with FLVCR1 loss. Cells and mice lacking FLVCR1 exhibit structural defects in mitochondria and upregulate integrated stress response (ISR) through heme-regulated inhibitor (HRI) kinase. Finally, Flvcr1 knockout mice are embryonic lethal, which is partially rescued by choline supplementation. Altogether, our findings propose FLVCR1 as a major choline transporter in mammals and provide a platform to discover substrates for unknown metabolite transporters.