9.A.19 The Lipid Intermediate Transporter (Arv1) Family
Glycosylphosphatidylinositol (GPI), covalently attached to many eukaryotic proteins, not only acts as a membrane anchor but is also thought to be a sorting signal for GPI-anchored proteins that are associated with sphingolipid and sterol-enriched domains. GPI anchors contain a conserved core structure. The core structure is synthesized in two topologically distinct stages on the leaflets of the endoplasmic reticulum (ER). Early GPI intermediates are assembled on the cytoplasmic side of the ER and then are flipped into the ER lumen where a complete GPI precursor is synthesized and transferred to protein. The flipping process is predicted to be mediated by a flippase. Yeast Arv1p is required for the delivery of an early GPI intermediate, GlcN-acylPI, to the first mannosyltransferase of GPI synthesis in the ER lumen (Kajiwara et al., 2008). ARV1 deletion, and mutations in other proteins involved in GPI anchor synthesis, affect inositol phosphorylceramide synthesis as well as the intracellular distribution and amounts of sterols, suggesting a role of GPI anchor synthesis in lipid flow from the ER.
ER membrane cholesterol is maintained at an optimal concentration of ∼5 mol % by the net impact of sterol synthesis, modification, and export. Arv1p in S. cerevisiae is a key component of this homeostasis due to its probable role in intracellular sterol transport (Tong et al., 2010). Mammalian ARV1, which can fully complement the yeast lesion, encodes a ubiquitously expressed, resident ER protein. Repeated dosing of specific antisense oligonucleotides to ARV1 produced a marked reduction of ARV1 transcripts in liver, adipose, and to a lesser extent, intestine. This resulted in marked hypercholesterolemia, elevated serum bile acids, and activation of the hepatic farnesoid X receptor (FXR) regulatory pathway. Knockdown of ARV1 in murine liver and HepG2 cells was associated with accumulation of cholesterol in the ER at the expense of the plasma membrane and suppression of sterol regulatory element-binding proteins and their targets (Tong et al., 2010). Arv1 knockout mice exhibited a dramatic lean phenotype, with major reductions in white adipose tissue mass and body weight. This loss was accompanied by improved glucose tolerance, higher adiponectin levels, increased energy expenditure and greater rates of whole-body fatty acid oxidation (Lagor et al. 2015).
The ARV1-encoded protein mediates sterol transport from the endoplasmic reticulum (ER) to the plasma membrane. Yeast ARV1 mutants accumulate multiple lipids in the ER and are sensitive to pharmacological modulators of both sterol and sphingolipid metabolism. Shechtman et al. (2011) demonstrated sterol accumulation, subcellular membrane expansion, elevated lipid droplet formation, and vacuolar fragmentation in ARV1 mutants. Loss of subcellular lipid transport due to ARV1 deficiency disrupts organelle homeostasis and activates the unfolded protein response. Arv1 might regulate membrane insertion of tail-anchored proteins involved in membrane homoeostasis (Georgiev et al. 2013).
In S. cerevisiae, ARV1 encodes a 321 amino acid transmembrane protein localized to the ER and Golgi. Arv1 cells harbor defects in sphingolipid and glycosylphosphatidylinositol biosyntheses, and may harbor sterol trafficking defects. Villasmil and Nickels (2011) determined the orientation of full-length Arv1 in the ER membrane. Although 4-6 TMSs are predicted, they concluded that the protein has a 3TMS topology. They also determined the minimal protein length required for function. Arv1 has putative lipid and glycosylphosphatidylinositol intermediate transport activities. It contains a conserved N-terminal cytosolic zinc ribbon motif known as the ARV1 homology domain, followed by multiple transmembrane regions, anchoring it in the ER. Deletion of ARV1 in yeast results in defective sterol trafficking, aberrant lipid synthesis, ER stress, membrane disorganization and hypersensitivity to fatty acids (FAs).