ABSTRACT
Lysophosphatidic acid (LPA) stimulates heterotrimeric G protein signaling by activating three closely related receptors, termed LPA(1), LPA(2) and LPA(3). Here we show that in addition to promoting LPA(1) signaling, membrane cholesterol is essential for the association of LPA(1) with beta-arrestin, which leads to signal attenuation and clathrin-dependent endocytosis of LPA(1). Reduction of clathrin heavy chain expression, using small interfering RNAs, inhibited LPA(1) endocytosis. LPA(1) endocytosis was also inhibited in beta-arrestin 1 and 2-null mouse embryo fibroblasts (beta-arrestin 1/2 KO MEFs), but was restored upon re-expression of wild-type beta-arrestin 2. beta-arrestin attenuates LPA signaling as LPA(1)-dependent phosphoinositide hydrolysis was significantly elevated in beta-arrestin 1/2 KO MEFs and was reduced to wild-type levels upon re-expression of wild-type beta-arrestin. Interestingly, extraction of membrane cholesterol with methyl-beta-cyclodextrin inhibited LPA(1) signaling, beta-arrestin membrane recruitment and LPA(1) endocytosis. Cholesterol repletion restored all of these functions. However, neither the stimulation of phosphoinositide hydrolysis by the M(1) acetylcholine receptor nor its endocytosis was affected by cholesterol extraction. LPA treatment increased the detergent resistance of LPA(1) and this was inhibited by cholesterol extraction, suggesting that LPA(1) localizes to detergent-resistant membranes upon ligand stimulation. These data indicate that although LPA(1) is internalized by clathrin- and beta-arrestin dependent endocytosis, membrane cholesterol is critical for LPA(1) signaling, membrane recruitment of beta-arrestins and LPA(1) endocytosis.
Subject(s)
Arrestins/metabolism , Cell Membrane/metabolism , Cholesterol/metabolism , Clathrin/metabolism , Endocytosis , Receptors, Lysophosphatidic Acid/metabolism , Animals , Arrestins/deficiency , Cytosol/metabolism , Detergents/pharmacology , Endocytosis/drug effects , Fibroblasts/cytology , HeLa Cells , Humans , Hydrolysis/drug effects , Lysophospholipids/agonists , Lysophospholipids/pharmacology , Membrane Microdomains/drug effects , Mice , Phosphatidylinositols/metabolism , Receptors, Lysophosphatidic Acid/agonists , Receptors, Muscarinic/metabolism , Signal Transduction , beta-Arrestin 1 , beta-Arrestin 2 , beta-ArrestinsABSTRACT
Golgi-localized gamma-ear homology domain, ADP-ribosylation factor (ARF)-binding proteins (GGAs) facilitate distinct steps of post-Golgi traffic. Human and yeast GGA proteins are only ~25% identical, but all GGA proteins have four similar domains based on function and sequence homology. GGA proteins are most conserved in the region that interacts with ARF proteins. To analyze the role of ARF in GGA protein localization and function, we performed mutational analyses of both human and yeast GGAs. To our surprise, yeast and human GGAs differ in their requirement for ARF interaction. We describe a point mutation in both yeast and mammalian GGA proteins that eliminates binding to ARFs. In mammalian cells, this mutation disrupts the localization of human GGA proteins. Yeast Gga function was studied using an assay for carboxypeptidase Y missorting and synthetic temperature-sensitive lethality between GGAs and VPS27. Based on these assays, we conclude that non-Arf-binding yeast Gga mutants can function normally in membrane trafficking. Using green fluorescent protein-tagged Gga1p, we show that Arf interaction is not required for Gga localization to the Golgi. Truncation analysis of Gga1p and Gga2p suggests that the N-terminal VHS domain and C-terminal hinge and ear domains play significant roles in yeast Gga protein localization and function. Together, our data suggest that yeast Gga proteins function to assemble a protein complex at the late Golgi to initiate proper sorting and transport of specific cargo. Whereas mammalian GGAs must interact with ARF to localize to and function at the Golgi, interaction between yeast Ggas and Arf plays a minor role in Gga localization and function.