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REVIEW G-Protein-Coupled Receptors, Cholesterol and Palmitoylation: Facts

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REVIEW G-Protein-Coupled Receptors, Cholesterol and Palmitoylation: Facts 371 REVIEW G-protein-coupled receptors, cholesterol and palmitoylation: facts about fats Bice Chini and Marco Parenti1 Cellular and Molecular Pharmacology Section, CNR Institute of Neuroscience, Via Vanvitelli 32, 20129 Milan, Italy 1Department of Experimental Medicine, University of Milano-Bicocca, Monza, Italy (Correspondence should be addressed to B Chini; Email: [email protected]) Abstract G-protein-coupled receptors (GPCRs) are integral membrane proteins, hence it is not surprising that a number of their structural and functional features are modulated by both proteins and lipids. The impact of interacting proteins and lipids on the assembly and signalling of GPCRs has been extensively investigated over the last 20–30 years, and a further impetus has been given by the proposal that GPCRs and/or their immediate signalling partners (G proteins) can partition within plasma membrane domains, termed rafts and caveolae, enriched in glycosphingolipids and cholesterol. The high content of these specific lipids, in particular of cholesterol, in the vicinity of GPCR transmembranes can affect GPCR structure and/or function. In addition, most GPCRs are post-translationally modified with one or more palmitic acid(s), a 16-carbon saturated fatty acid, covalently bound to cysteine(s) localised in the carboxyl-terminal cytoplasmic tail. The insertion of palmitate into the cytoplasmic leaflet of the plasma membrane can create a fourth loop, thus profoundly affecting GPCR structure and hence the interactions with intracellular partner proteins. This review briefly highlights how lipids of the membrane and the receptor themselves can influence GPCR organisation and functioning. Journal of Molecular Endocrinology (2009) 42, 371–379 G-protein-coupled receptors–cholesterol of phospholipids. By increasing the packing, and hence interactions the lateral assembly of phospholipids, cholesterol reduces membrane fluidity and increases rigidity Eighty-ninety per cent of whole-cell cholesterol is (Mouritsen & Zuckermann 2004). The ‘rafts/caveolae contained within the plasma membrane, where it signalling hypothesis’ suggests that plasma membrane accounts for 20–25% of lipid molecules (Lange 1991, domains enriched in cholesterol and sphingolipids Simons & Ikonen 2000). Cholesterol, which is amphi- compartmentalise the proteins involved in specific philic and has a small KOH head group and a rigid, signalling tasks. By regulating lateral mobility and planar four-ring backbone, influences the functional compartmentalisation, cholesterol affects the prob- properties of the plasma membrane by interacting with ability with which receptors interact with their other resident lipids (Ohvo-Rekila et al. 2002, Ikonen downstream signalling partners, thus optimising their 2008) and proteins (Lee 2004). In particular, it may act spatial–temporal interactions, hence signalling effi- on the conformation of membrane proteins: i) by ciency and regulation (Simons & Toomre 2000). The alterations in the physico-chemical properties of the trafficking of receptors between regions with different bilayer; ii) through specific and localised molecular lipid compositions is still unclear, but an interesting interactions; or iii) by a combination of both. hypothesis has recently been advanced for the human d-opioid receptor based on results obtained with plasmon-waveguide resonance spectroscopy experi- Alterations in the physico-chemical properties of the ments on solid-supported bilayers (Alves et al. 2005). bilayer It has been found that the unbound inactive receptor is The effects of cholesterol on the fluidity and rigidity of enriched in phosphatidylcholine bilayers, whereas the the plasma membrane are well known: a cholesterol activated receptor is concentrated in sphingomyelin molecule can span approximately half a bilayer, bilayers, characterised by a greater thickness. Since it is preferentially interacting with the saturated fatty chains known that receptor activation involves changes in the Journal of Molecular Endocrinology (2009) 42, 371–379 DOI: 10.1677/JME-08-0114 0952–5041/09/042–371 q 2009 Society for Endocrinology Printed in Great Britain Online version via http://www.endocrinology-journals.org Downloaded from Bioscientifica.com at 09/29/2021 03:12:47PM via free access 372 B CHINI and M PARENTI . GPCRs, palmitoylation and cholesterol orientation of transmembrane helices with an increase has a farnesyl isoprenoid chain attached to the in receptor vertical length (i.e. perpendicular to the cytoplasmic carboxyl terminal (Miggin et al. 2003). plane of the plasma membrane; Salamon et al. 2000), it Cysteines are by far the most frequent acceptor sites is hypothesised that the driving force for receptor of palmitoylation, which occurs on the thiol side chain redistribution comes from the increased hydrophobic through a thioester bond. However, it must be kept in match of the elongated receptor for the thicker mind that, although not yet reported for GPCRs, the sphingomyelin-enriched bilayer, a sorting mechanism addition of palmitic acid to proteins can also occur onto that has previously been observed in other subcellular the terminally exposed amides of cysteine (Pepinsky compartments (Jensen & Mouritsen 2004, Lee 2004). et al. 1998) and glycine (Kleuss & Krause 2003), the hydroxyl group of threonine (Branton et al. 1993) and serine (Jing & Trowbridge 1990, Percherancier et al. Specific and localised molecular interactions 2001), and the 3-amino group of lysine via an amide Some membrane proteins directly bind cholesterol, as bond (Stanley et al. 1998). In addition, other fatty acids, firstly shown for caveolin-1 (Murata et al. 1995), whose such as palmitoleate, stearate or oleate, can, much less cholesterol-binding domain has not yet been precisely commonly, bind to cysteines (Liang et al. 2004). mapped, even if its scaffolding domain, originally Rhodopsin was the first GPCR found to be palmitoy- considered to be a protein interaction domain, is now lated at two adjacent cysteines in the carboxyl-terminal thought to play a role in cholesterol binding (Parton cytoplasmic tail (Ovchinnikov et al. 1988) but, since et al. 2006). Some membrane proteins involved in the then, many other GPCRs have been experimentally transport of cholesterol possess a conserved five- shown to be mono-, bis- or tris-palmitoylated at transmembrane helix domain called the sterol-sensing conserved carboxyl-terminal cysteine residues (see domain, which bind cholesterol with a measured Qanbar & Bouvier 2003 for a review), and yet other affinity of around 100 nM (Radhakrishnan et al. 2004). GPCRs containing the canonical cysteines are presum- Others bind cholesterol via a conserved hydrophobic ably palmitoylated. Interestingly, the m-opioid receptor binding pocket, such as OSBP (Suchanek et al. 2007) is not palmitoylated at the two conserved carboxyl- and START proteins (Alpy & Tomasetto 2005), or short terminal cysteines, and cysteine-170 in the second cholesterol-binding motifs (Li & Papadopoulos 1998). intracellular loop (the only other intracellular cysteine) The fact that cholesterol plays a role in stabilising the can be the palmitoylation site; however, as its sub- conformation of specific G-protein-coupled receptors stitution by alanine generates a mutant with very low (GPCRs) has become clear after the first attempts to expression levels, it is has not been possible to measure purify them for pharmacological studies or X-ray [3H]palmitate incorporation (Chen et al. 1998). diffraction analysis (Schertler & Hargrave 2000, Albert Some GPCRs are not modified by palmitic acid, & Boesze-Battaglia 2005). As first noted for rhodopsin, including the a-isoform of the thromboxane A2 cholesterol greatly improves crystal quality and quan- receptor, which lacks any cysteines in its carboxyl tity; furthermore, in the resolved structure of metarho- terminal (Reid & Kinsella 2007), and the GnRH dopsin I, it was found that a cholesterol molecule sits in receptor, which is unique among class A GPCRs as its a pocket formed between adjacent helices of a receptor cytoplasmic carboxyl terminal tail is only one to two dimer, thus suggesting that it may help to form or amino acids long and therefore lacks both palmitoyla- stabilise receptor dimers (Ruprecht et al. 2004). As tion and phosphorylation sites (Navratil et al. 2006). further discussed below, it has also been observed that The thioester bond linking palmitate to cysteine is cholesterol contributes to dimer formation in the labile, thus making the attachment of palmitate to recently resolved structures of the b2-adrenergic proteins reversible and the turnover rate of the bound receptor, thus further supporting its direct role in fatty acid generally shorter than that of the protein. The regulating GPCR conformation (Cherezov et al. 2007). addition and removal of palmitate are respectively enzymatically catalysed by palmitoyltransferases (PATs) and thioesterases. Identifying these enzymes proved to be a very challenging task, and it is only recently that a GPCR palmitoylation: sites and dynamics family of proteins with PAT activity has been identified in yeast by means of genetic screening. They all share a Glycosylation and phosphorylation are certainly the common motif referred to as ‘DHHC’, which is best known post-translational modifications affecting enriched in cysteines and contains a conserved GPCR functions, but lipids are also post-translationally aspartate–histidine–histidine–cysteine signature (see added to most GPCRs in the form of the 16-carbon Mitchell et
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