Proteolipids and Protein Lipidation
In 1951, proteins that were soluble in organic solvents such as chloroform-methanol were found in rat brain myelin by Folch and Lees (perhaps better known for devising the most common method for lipid extraction), who coined the term ‘proteolipid’. However it was another twenty years before it was shown that these contained covalently bound fatty acids and so differed from the plasma lipoproteins. Such lipid-modified proteins are now known to be widespread in nature with many important functions in animals, plants and bacteria. Proteolipids can be defined as all proteins containing covalently bound lipid moieties such as fatty acids, isoprenoids, cholesterol and glycosylphosphatidylinositols (the last have their own web page). Protein lipidation provides an interface between the lipophilic membranes of cells and otherwise hydrophilic proteins that enables signalling and many other functions at the membrane surface. Here proteolipids from animals and plants are discussed, but bacteria also produce many distinctive lipopeptides and proteolipids, and these have there own web page. The term ‘lipoprotein’ is also used to describe such compounds on occasion, but to avoid confusion this might be better reserved for the non-covalently linked lipid-protein complexes of the type found in plasma.
1. Introducing Protein-Lipid Modifications
It is a curious but important fact that the two main types of protein with fatty acid modifications to have been described from eukaryotic organisms contain saturated fatty acid components, i.e. those with only myristoyl and those with predominantly palmitoyl moieties, each with a distinctive type of linkage, amide or thiol ester, respectively. Prenylated lipids contain an isoprenoid group linked via a sulfur atom (thiol ether bond) to the protein. Other protein-lipid links exist and are discussed below, but they tend to be less frequent.
A further important class of proteolipids contains a linkage to cholesterol in addition to N-palmitoylation, the ‘hedgehog’ signalling proteins, while bacteria contain proteolipids with N-acyl- and S-diacylglycerol groups attached to an N-terminal cysteine. Some O-acylated proteins with important biological functions are known also. Often, N-myristoylated and prenylated proteins have one or more additional S-acylation residue. Such proteolipids are important for the functioning of all classes of eukaryotes (animals, plants and fungi) and of bacteria.
Covalent modification of proteins with lipids has the effect of changing them from a generally hydrophilic nature to one that is hydrophobic at one end at least, thus facilitating the interaction with membranes. Protein S-palmitoylation occurs post-translationally at the membrane surface, while N‑myristoylation takes place at the ribosomal level and S-prenylation is brought about by cytoplasmic enzymes. It is now clear that such modifications are important in determining the activities of proteins and in targeting them to specific subcellular membrane domains, including the rafts or caveolae in plasma membranes. Thus, both myristoylated and palmitoylated proteins are targeted to rafts (as are the GPI-anchored proteins), but prenylated lipids are not. It is significant that many signalling proteins (e.g. receptors, G-proteins, protein tyrosine kinases) and often their substrates are modified by lipids with implications for the relevant signalling events at the cell surface. Many of these proteolipids influence human disease states and are potential pharmacological targets. For example, deregulation of palmitoylation has been associated with heart disease, cancer, mental retardation and schizophrenia. In addition, some pathogenic organisms can hijack the protein acylation mechanism to increase the susceptibility of the host to infection.
The reversible nature of S-palmitoylation in particular as a regulator of cellular functions has drawn comparisons with other important reversible post-translational modifications such as phosphorylation and ubiquitination.
2. N-Myristoylated Proteins
In most N-myristoylated proteins, myristic acid (14:0) specifically, which is a ubiquitous but usually minor component of cellular lipids (only 1% of the total fatty acids), is bound to the amino-terminal glycine residue of a relatively conserved sequence of the protein via an amide linkage that is comparatively stable to hydrolysis. Therefore, N-terminal acylation is believed to be an irreversible modification, although there may be exceptions. These proteolipids constitute a large family of essential eukaryotic proteins (~2% of the total) with many different functions, and they are located either in the cytosol or in the cytosolic (inner) membrane of cells, or both. Indeed, N-glycine myristoylation mediates the targeting of the modified proteins to various membranes in the cell, including the plasma membrane, endoplasmic reticulum, Golgi, mitochondria and nuclei. The acyl group anchors the protein to membranes, although simultaneous binding to phospholipids or other membrane constituents is necessary to increase the strength of the interaction. For example, a single N‑myristoylation is not sufficient to ensure membrane association and a second signal is necessary, e.g. hydrophobic residues, another membrane-bound binding partner or further N‑myristoylation or S‑palmitoylation (see below).
In vivo, the acyl group as the CoA ester is attached by the action of myristoyl-CoA:protein N-myristoyltransferases (NMT1 or NMT2 in humans) to the N‑terminal glycine of the growing peptide as it begins to emerge from the ribosome, i.e. it is mainly a co-translational rather than a post-translational event. Most eukaryotes have such enzymes but not prokaryotes, although lower eukaryotes have only one isoform. NMT1, but not NMT2, is essential for cell proliferation, but both NMT1 and NMT2 are required for cell survival. While they are both cytosolic, they can bind to the ribosomes and so may facilitate the co-translational timing of the reaction. Both have an N-terminal region with polybasic amino acid sequences, which are required for targeting to the ribosomes. The crystal structure of NMT1 has shown that it has a characteristic binding cleft that is involved in the recognition of potential substrates for myristoylation (with some overlap with targets for N-acetylation). There is an absolute requirement for the N-terminal glycine on the target protein, while amino acid residues two through eight are the consensus motif (Met-Glc-X-X-X-Ser/Thr/Cys) used in substrate recognition.
During biosynthesis, the leader methionine residue is first removed from the nascent peptide chain by a methionine aminopeptidase (MAP) to expose the N-terminal glycine, before the N‑myristoyl transferase catalyses the formation of the stable amide bond. Myristoyl-CoA is supplied by an acyl-CoA binding protein (ACBD6), and this controls the specificity of the reaction by forming a complex with the transferase thereby stimulating it and preventing it from utilizing the competitor palmitoyl-CoA. In addition, it has become apparent that myristoylation can also occur post-translationally in apoptotic cells on internal glycine residues exposed by caspase cleavage in partially hydrolysed proteins. The latter process may have implications for health and disease.
Exceptions to these generalities are photoreceptor proteins, which are modified heterogeneously with the uncommon 12:0, 5‑14:1 and 5,8‑14:2 fatty acids as well as 14:0. N-Palmitoylated proteins have been found on occasion, usually where there is a dual lipid modification, for example, the cholesterol-linked hedgehog proteins (see below). In addition, myristoylation occurs on the ε-amine group of internal lysines in interleukin 1α and tumor necrosis factor α, and certain histone proteins can be modified similarly by various fatty acids. With these, different enzymes or iso-enzymes from those for N-terminal myristoylation are involved.
In humans, 569 proteins are now known to be myristoylated, and these include proteins that play important roles in signalling networks, apoptosis, oncogenesis and viral replication. The functions of such proteolipids are steadily being unravelled, and it is evident that they are of critical physiological importance, for example as participants in cellular signalling. The lipid moieties are involved in regulating protein activity perhaps by modifying or stabilizing their conformations, by facilitating protein-protein interactions, and in targeting otherwise soluble proteins to the membranes and to appropriate receptors. N-Myristoylation is essential for erythrocyte formation and myelination, it is necessary for calcium release in mitochondria and is an important factor in T cell function and thence in the immune response. During apoptosis, NMTs are substrates for caspases, which are believed to regulate their location from the ribosomal and membrane regions to the cytosol and vice versa. Increased levels of N-myristoylation have been observed in certain cancers, and there have been suggestions that NMTs could be a target for therapeutic intervention.
N-Myristoyl transferase activity is believed to play a role in innate immunity, the evolutionarily conserved host defense mechanism that responds immediately to pathogens but does not provide long-lasting immunity to the host (in contrast to adaptive immunity). It is important in infections by viruses, including human immunodeficiency virus-1 (HIV-1), bacteria, e.g. Shigella flexneri, fungi and parasitic protozoa, including such diseases as malaria. For example, the shigella virulence factor IpaJ has been identified as an irreversible demyristoylase.
Myristoylation is an irreversible stable modification in essence, and the half-life of a myristoylated protein is similar to that of the nascent polypeptide chain. On the other hand, N-myristoylated proteins can be dissociated from membranes by de-S-palmitoylation, by phosphorylation, by binding of the hydrophobic moieties to competing cytosolic proteins, if the myristoyl moiety is relatively exposed, or by conformational changes, and these reactions may be forms of regulation. The low levels of myristic acid in tissues may also be a controlling factor. Macrophages contain at least one protease that targets certain N-myristoylated proteins, and similar protease activity has been reported from other tissues.
In plants, 650 Arabidopsis proteins are potentially myristoylated, including many protein kinases, phosphatases, thioredoxins and transcription factors. As an example, the calcium-sensing molecule CBL1 requires N-myristoylation for association with the endoplasmic reticulum followed by S-acylation to promote trafficking to the plasma membrane; in contrast CBL2 is not N-myristoylated but rather is triply S-acylated to direct it to the tonoplast.
Mass spectrometry is currently a key method for characterization of N-myristoylated proteins, although the fatty acyl group can be released for analysis by conventional chromatographic means (e.g. gas chromatography) by the acidic hydrolysis conditions commonly employed to cleave peptide bonds. For example, treatment with 6M HCl or 2M HCl in 83% methanol at 100°C for several hours is required to release the N-acyl group as the free fatty acid or methyl ester, respectively. New methods involving specific chemical probes are now facilitating detection and analysis of N-myristoylated proteins and other proteolipids.
Lysine fatty acylation: It is now recognized that a second form of protein N-acylation exists in which lysine residues are acylated (mainly with 14:0), again with the effect of increasing the affinity of proteins for specific cellular membranes. So far, a relatively limited number of proteins are known to be modified in this way including tumor necrosis factor α (TNF-α), interleukin 1-α and some of the Ras family of GTPases. Although human lysine fatty acyl transferases are known, their molecular targets have apparently not yet been determined. In this instance, the modification is reversible as sirtuins 2 and 6 (SIRT2/6), mammalian nicotinamide adenine dinucleotide (NAD)-dependent lysine deacylases, catalyse the removal of fatty acyl groups from lysine residues, and they are thus able to regulate the activities of the relevant proteins. Among reported effects are tumorigenesis and increased pathogenesis of bacteria.
3. S-Acylated (Palmitoylated) Proteins
In the S-acylated proteins, long-chain fatty acids are linked to one or more (up to four) internal cysteine residues via labile high-energy thioester bonds. As palmitic acid (16:0) is the most common acyl component, these are often termed S-palmitoylated proteins, but this is something of a misnomer, as other fatty acids are often present, including 16:1, 18:0 and 18:1, so the term 'S-acylation' is preferable. For example, S-18:0 is as common as S-16:0 in plants, and viral proteins are often linked to C18 fatty acids. There is some evidence that the nature of the acylating fatty acid can affect the functional outcome. S-Palmitoylation is also observed in conjunction with N-terminal myristoylation or C-terminal prenylation sites. In contrast to N-myristoylation and isoprenylation, there does not appear to be any specific peptide target sequence, although proteomic methods now enable prediction of which of these cysteine residues is likely to be palmitoylated.
The archetypal proteolipid protein (‘PLP’) or lipophilin is the main protein in the myelin of the central nervous system; it has multiple functions and was the first of its type to be identified and properly characterized. It is a highly conserved hydrophobic protein of 276 to 280 amino acids with four transmembrane segments that binds at least six palmitate groups via thioester bonds. However, a wide variety of different palmitoylated proteins, with many different functions are now known, and this is most prevalent type of protein modification by lipids. For example, well over 500 different palmitoylated proteins have been identified in humans (10% of the proteome) and more than 50 in the yeast Saccharomyces cerevisiae. These can be grouped into three broad categories - poly-acylated membrane proteins (e.g. some receptors and rhodopsin), mono-acylated membrane proteins (some receptors and viral proteins), and hydrophilic proteins (such as certain protein kinases). For example in brain, 490 palmitoylation sites have been identified on 342 synaptic proteins, 44% of which are integral membrane proteins. It is now apparent that protein palmitoylation is essential for intracellular signalling and for the folding, trafficking and function of such disparate proteins as Src-family kinases, Ras family GTPases, G-proteins and G-protein coupled receptors.
Thio-acylation occurs post-translation of the protein, and is catalysed by specific endomembrane-bound acyltransferases. Although there is some evidence for occasional non-enzymatic palmitoylation, enzymatic mechanisms predominate and palmitoyl S-acyltransferases were identified definitively first from yeasts and subsequently from mammalian cells. Families of such enzymes (23 in humans coded by more than 20 separate genes, a similar number in Arabidopsis, and 7 in yeasts) have now been characterized with a conserved cysteine-rich domain containing zinc and a distinctive aspartate-histidine-histidine-cysteine (DHHC) motif, which is required for activity. They are membrane proteins with a number of subcellular locations that span the bilayer at least four times with the DHHC domain and the N- and C-terminal domains on the cytosolic face. Additionally, there is a conserved C-terminus (PaCCT) domain in most palmitoyl transferases and this is important for their function. Each of the mammalian DHHC proteins appear to be associated with specific subcellular locations but mainly the endoplasmic reticulum and Golgi, with a few at the plasma membrane and some at more than one location; there is also some preference for particular tissues. Simplistically, Golgi-located palmitoyl acyltransferases promote the palmitoylation and forward trafficking of substrates to the plasma membrane, while those in other organelles tend to generate the products for local activity. In plants, a high proportion of the enzymes are located at the plasma membrane. While many palmitoyl transferases have overlapping substrate specificities, some are highly specific for particular proteins.
Cysteine palmitoylation forms a thioester bond that is similar in energy to that in the palmitoyl donor, palmitoyl-CoA, so the reaction does not require an energy source such as ATP. The protein transacylases are all palmitoylated spontaneously when incubated with palmitoyl-CoA with release of CoA, suggesting that the auto-palmitoylated acyl-enzyme intermediate is involved in the transfer of the palmitoyl moiety to a substrate. Indeed, it is now established that the cysteine residue in the DHHC motif is palmitoylated for transfer to the target protein, as part of a two-step catalytic mechanism. There is an absolute requirement for long-chain acyl-CoA esters, mainly 16:0, as fatty acyl donors, though some members of the DHHC protein family have different acyl specificities and at least one can use a wide variety of fatty acyl-CoA substrates. The acyl chain binds in a cavity formed by the transmembrane domain, and the acyl chain-length selectivity is believed to depend upon cavity mutants with preferences for specific acyl chains. In general, there does not appear to be a preferred substrate-sequence motif in target proteins, but the active site on the enzyme resides at the membrane-cytosol interface so that membrane-proximal cysteines are candidates for palmitoylation. Prior to S-acylation, modification by N-myristoylation or prenylation is often required, and with the former, specific target sequences for subsequent palmitoylation have been identified.
In contrast to irreversible N-myristoylation, hydrolysis of S-palmitoylated proteins occurs readily and is catalysed by thioesterases, two of which have been now been characterized in the cytosol, i.e. acyl protein thioesterase 1 (APT1) and LYPLA2 (APT2), each with specificities for particular classes of protein conjugates. Two lysosomal enzymes, palmitoyl protein thioesterases 1 and 2 (PPT1 & 2), and a mitochondrial enzyme (ABHD10) have related activities. Thus, most proteolipids of this type undergo cycles of acylation-deacylation, with a half-life that is much shorter than that of the peptide per se, and this permits proteins to shuttle between membranes and other cellular compartments, for example between the plasma membrane and Golgi in both directions. For example, Ras-proteins, which transmit signals within cells, are palmitoylated for transport to the plasma membrane, then de-palmitoylated for transport back to the Golgi (in this instance by a family of ABHD17 serine hydrolases), so at the steady state these proteins are active and can signal at both organelles, a continual cycle that is essential for proper functioning. Such acylation-deacylation cycles are believed to have regulatory functions, and for example, some proteins are prenylated and S-acylated in an active form, while the inactive form is only prenylated. The activities of synthetic and hydrolytic enzymes are regulated dynamically by extracellular stimuli, like phosphorylation, and the level of palmitoylation is determined by a finely tuned balance between the activities of these enzymes in specific cellular locations.
As with the myristoylated proteins, palmitoylation is believed to modify protein function partly by modifying their conformations, but mainly in targeting otherwise soluble proteins to specific membranes or to appropriate receptors. The number of bound fatty acyl groups may control the strength of the interaction with membranes. For example, a hydrophobic protein with a single acylation can bind only loosely to membranes and is easily displaced. However, a second or further acylation ensures strong targeting of a protein to the cytoplasmic face of the membrane, and guarantees that it is firmly bound to a specific site on the membrane where an appropriate receptor may be located. Palmitoylation of integral membrane proteins may be a regulatory function, changing their conformation to increase their stability through protecting them from degradation by preventing ubiquitinylation and increasing their resistance to proteases.
In addition, palmitoylation is believed to be an important factor in the process of trafficking proteins between organelles and in directing them to specific membrane compartments. For example, in neurons, palmitoylation targets proteins for transport to nerve terminals and may regulate trafficking at synapses; it is essential for the growth and integrity of neuronal axons and for conveying axonal signals. Protein S-palmitoylation is also a basic mechanism for control of the properties and functions of ion channels, both directly and indirectly via other signalling pathways. Thus, protein lipidation is a potent regulator of apoptotic signaling via activation of calcium channels in the plasma membrane and endoplasmic reticulum. In the heart, S-palmitoylation is involved in the regulation of cardiac electrophysiology, including the activity of the sodium-calcium exchanger, phospholemman, the cardiac sodium pump and the voltage-gated sodium channel. Mitochondrial morphology and function is regulated by stearoylation of human transferrin receptor 1. Among many further examples that could be listed, S-acylation regulates key components of the insulin secretion and insulin response pathways, including channels, transporters and receptors. Dysregulation of DHHC palmitoyl acyltransferases is known to be associated with many human diseases, including schizophrenia, X-linked mental retardation, Huntington's disease and cancer, and these are now seen as targets for drug development.
More generally, the saturated acyl moiety in palmitoylation facilitates transfer of proteins to lipid rafts, subdomains of the plasma membrane that are enriched in sphingolipids and cholesterol. S-Palmitoylation enables flotillins to bind to the cytosolic leaflet of the plasma membrane in raft domains, where they oligomerize to serve as scaffolds that facilitate the assembly of multiprotein complexes at the membrane–cytosol interface. Proteolipids can then participate in cell signalling events, such as in T-cells of the immune system, where the unique feature of reversibility in lipid S-palmitoylation modifications is advantageous. As a further example, Ras proteins are activated by extracellular stimuli and produce signals that lead to cell proliferation, differentiation and apoptosis. They undergo palmitoylation on the Golgi membrane, which enables them to be transported to the plasma membrane where they exert their signalling function. Eventually, the signal is attenuated by depalmitoylation and thence dissociation of the protein from the plasma membrane. Following recaptured by the Golgi, the protein can undergo a new sequence of plasma membrane targeting and signalling. S-palmitoylation of Ras proteins is also important in fungal pathogens.
In addition, palmitoylation is involved in lipoprotein metabolism. Lipoprotein particles containing apolipoprotein B (apo B), such as chylomicrons, very-low-density and low-density lipoproteins, are essential for the transport of triacylglycerols and cholesterol esters in plasma. It has been established that palmitoylation of apo B regulates the biogenesis of the nascent lipoprotein particles that contain this apolipoprotein and may regulate the amount available for lipid transport.
Infections: The innate immune system responds to invasion by microorganisms by signalling through pattern recognition receptors (PRRs) that bind conserved microbial features. Many of these receptors,including the Toll-like receptors, are S-palmitoylated, this is believed to be an activating modification for innate immune signalling.
Similarly, protein S-palmitoylation is critical for activity of many human and plant pathogens, including fungal, bacterial and viral infections. While parasitic protozoa and fungi possess their own palmitoyltransferases, viruses and bacteria hijack the enzymes in their hosts in order to favour their internalization, survival, and replication inside the cells. Palmitoylation of viral proteins is essential for their life cycle, and three types of membrane proteins in viruses, including many that are highly pathogenic, are known to be S-acylated, a factor that is important for the immune response. For example, palmitoylated 'spike' proteins are the main transmembrane proteins in the viral envelope, and they are involved in the entry of viruses into cells by catalysing receptor binding and/or membrane fusion. The viroporins are a second group, which are freely expressed in infected cells but not into virus particle per se to any appreciable extent. They possess one or two membrane-spanning regions, which amongst other functions serve as hydrophilic pores in membranes. A third diverse group of palmitoylated proteins produced by viruses are peripheral membrane proteins in which the fatty acid component simply anchors the modified protein to a membrane. In contrast to other proteins, glycoproteins of viral membrane in that they are palmitoylated at or near the cytoplasmic face and then remain palmitoylated. S-Palmitoylation is essential for the survival, growth and infectivity of the trypanosomatids (protozoal parasites) in humans.>
Plants: Numerous S-acylated proteins are known to be present in higher plants, perhaps as many as 50% of the total, and many functions are now being reported although progress appears to have been slower than with mammalian systems. As in animals, S-acylation alters protein interactions with membranes, together with their activation state, conformations and sub-cellular distributions. SNARE and G proteins and the cellulose synthase complex are among several proteins known to be modified in this way with effects on fertility, Ca2+ signalling, movement of potassium ions, stress signalling (e.g. the immune response), and the growth of root hairs and pollen tubes. For example, the 18 subunits of the cellulose synthase A family are multiply S-acylated (70–110 S-acyl groups), rendering it the most heavily S-acylated complex ever described. As this complex is integral to the plasma membrane and extrudes cellulose microfibrils into the extracellular environment to form the cell wall, the extrusion process is believed to enable the complex to pass through the plane of the plasma membrane to form microdomains and recruit accessory proteins for unhindered transport across the membrane.
Few S-acylated proteins have been matched to the any of the 24 S-acyl transferases in Arabidopsis, so each enzyme may have multiple substrates (as many as 100) involved in diverse processes. Protein thioesterases comparable to those in animals, i.e. with de-S-acylation activity, have not yet been characterized in plants although two candidate enzymes have been identified that unexpectedly may not be in the serine hydrolase superfamily. The regulatory functions of S-palmitoylation may be more important at the plasma membrane in plants than in animals.
Analysis by modern mass spectrometric methods in concert with a site-specific acyl-biotin-exchange reaction permits location of the acyl group to specific cysteine residues. In contrast to the N-acylated proteins, the fatty acids are easily released from the thiol linkage by base-catalysed transesterification for analysis by gas chromatography.
4. Prenylated Proteins
Prenylated proteins are formed by attachment of isoprenoid lipid units, farnesyl (C15) or geranylgeranyl (C20), via cysteine thio-ether bonds, which are more stable than ester or thioester bonds, at or near the carboxyl terminus. Such proteins were first detected in fungi, but they are ubiquitous in mammalian cells where they can amount to up to 2% of the total proteins, and they are increasingly being found in other eukaryotic organisms. In addition, some proteins from pathogenic bacteria and viruses can be prenylated by their hosts, apparently as a protective measure. Isoprenylation is a stable (non-reversible) modification, which targets specific proteins to membranes and aids protein-protein interactions; it is essential for their functions. However, in contrast to the acylated proteins, the bulky branched nature of the lipid moiety of isoprenylated proteins ensures that the latter cannot be incorporated into ordered raft microdomains.
Whether a protein is prenylated is determined by specific amino acid sequence motifs at the carboxyl terminus, principally a CAAX sequence with cysteine (C) attached to two aliphatic amino acids (A) then to a variable carboxyl-terminal amino acid residue (X). The nature of the X residue determines whether a protein will be farnesylated or geranylgeranylated; with a few exception, proteins ending in serine, methionine, alanine or glutamine are farnesylated, while proteins ending in leucine or isoleucine are geranylgeranylated. A single farnesylation is usual, but some dual geranylgeranylation can occur. Proteins involved in cellular signalling and trafficking pathways are most involved, and the most important of these are probably the Ras super-family (low-molecular weight G-proteins, or guanosine 5’‑triphosphate (GTP) hydrolases), which act as molecular switches for many different signal pathways including those controlling cell proliferation, adhesion, apoptosis and migration, and the integrity of the cytoskeleton.
The isoprenoid units are produced by the mevalonate pathway, as discussed on our web page dealing with cholesterol with farnesyl pyrophosphate as the branch-point in sterol/isoprene synthesis. Subsequent biosynthesis of prenylated proteins involves a concerted series of reactions in which the proteins are transported through various cellular organelles, ending mainly but not only at the plasma membrane. Prenylation occurs in the cytoplasm of the cell after synthesis of the protein per se, with farnesyl or geranylgeranyl pyrophosphate as the isoprenoid substrate, each catalysed by its own transferases, i.e. protein farnesyltransferase and protein geranylgeranyl transferase (two types GGT-1 and 2), respectively. The enzymes transfer the isoprenoid group to the cysteine residue in the CAAX box, and enzyme-bound Zn2+ is necessary for the proper transfer of the lipid anchor. Cleavage of the terminal tri-peptide (AAX) then occurs in the endoplasmic reticulum via a specific protease, before the new terminal cysteine is enzymically methylated at the carboxyl group with S-adenosyl methionine as the methyl donor (further increasing the hydrophobicity of the proteolipid). GGT-2 is known to transfer two geranylgeranyl units to the C-terminal double-cysteine motif (CC or CXC) of the 'Rab' family of proteins.
Prenylation alone tends to target proteins from the cytosol to endomembranes, such as the endoplasmic reticulum and Golgi, a process facilitated by several binding or chaperone proteins. However, S-acylation (palmitoylation) of prenylated proteins can occur also, increasing the affinity for membranes and then proteins are directed from the endomembranes to the plasma membrane. As the second modification is reversible, it may function as part of a control mechanism. After they have been fully processed, these proteolipids have a high affinity for cellular membranes and possess a unique structure at their carboxyl termini, which functions as a specific recognition motif in some protein-protein interactions.
The "Ras" proteins in mammalian cells are farnesylated, while a subfamily of "Rho" proteins are usually geranylgeranylated. Prenylated proteins of the Ras family are attached to the cytoplasmic face of the cell membranes, where they transfer the signal from surface receptors to transcription factors that effect gene expression in the nucleus. An additional palmitoylation is required to bind to membranes and aids the transfer of Ras proteins from the Golgi and thence to the plasma membrane. Rho proteins can be anchored to the plasma membrane, endomembranes or endosomes by geranylgeranylation alone. Although the prenylation reaction is irreversible, some geranylgeranylated Rho proteins can be removed from membranes and in effect de-activated by binding via specific protein-protein interactions; prenylated proteins can also be trafficked between membranes by this means. However, binding between prenylated and non-prenylated proteins can serve to increase the activity of the latter, and many examples of this have now been documented.
Ultimately, degradation of prenylated proteins in animals occurs in the lysosomal compartment of the cell and is catalysed by a prenylcysteine lyase, which is a flavin-containing monooxygenase that converts prenylcysteine to prenyl aldehyde by a novel mechanism.
As many of these proteins are involved in the development of cancer, they are the subject of much pharmaceutical interest, focusing especially on the inhibition of the prenylation reaction with Ras proteins as the main target; a farnesyltransferase inhibitor for patients with acute myeloid leukemia is undergoing clinical trials. Defects in prenylation or its regulation have also been implicated in the rare disease progeria, but also in cardiovascular disease, neurodegenerative disorders and metabolic diseases. In addition, inhibitors of protein farnesyltransferase have been shown to be efficacious in the treatment of protozoal pathogens and other parasitic diseases in animal models, and they appear to be of value in the treatment of viral (e.g. hepatitis D) and fungal infections.
In plants, protein prenylation is required for plant growth, development and environmental (stress) responses, including the control of abscisic acid and auxin signalling and for meristem development. For example, at least 250 proteins in Arabidopsis have the CAAX sequence and have the potential to be prenylated, although this has been demonstrated experimentally for only a few. It appears that some prenylated proteins do not use the prenyl group to bind to membranes, but it does permit otherwise hydrophilic proteins to operate as peripheral lipid membrane proteins or as a signal for interaction with other proteins, for example. The genome of Arabidopsis encodes two proteases and two methyltransferases, which process prenylated proteins in the endoplasmic reticulum.
5. O-Acylated Proteins/Peptides
Wnt proteins: In a few proteolipids, serine or threonine residues are acylated such that an O-acyl rather than an S-acyl linkage is formed, and the family of 'Wingless' or Wnt proteins in animals (19 members in humans) falls into this category. These are central mediators of embryonic development and tissue renewal that influence cell proliferation, differentiation and migration and require O-acylation for secretion and activity. They are secreted glycoproteins, which share a conserved sequence of cysteine residues and an N‑terminal signal sequence that targets them for secretion. In all, there is an unusual O-acyl modification with palmitoleic acid (9-cis-hexadecenoic or 9-16:1), which has been termed a lipokine (i.e. a lipid hormone), at a conserved serine residue (and not with S-palmitoylation on a conserved cysteine residue as originally reported, although some members of the Wnt family undergo an additional S-acylation with palmitate at a cysteine residue). Following synthesis, Wnt proteins are processed in the endoplasmic reticulum, where the conserved signal sequence is cleaved before glycosylation and fatty acylation. Following desaturation of palmitoyl-CoA to generate a dedicated pool of palmitoleoyl-CoA in the endoplasmic reticulum by stearoyl-CoA desaturase (SCD1), the subsequent O-acylation requires an O-acyltransferase termed 'porcupine' (PORCN), another MBOAT family member, which is entirely distinct from the S-acyltransferases (one inhibitor of this enzyme is undergoing clinical trials for treatment of Wnt-driven solid tumors). The specificity of the reaction is due to a structural feature in the enzyme that will only permit the use of the cis-9-monoenoic fatty acid; only those Wnt proteins esterified in this way can separate from the PORCN-Wnt complex.
The O-palmitoleoyl residue is essential for intracellular trafficking and activity of Wnt proteins within cells, and it is crucial for maintaining their structural integrity and assists their secretion by facilitating movement from the trans Golgi network to the plasma membrane in signalling cells. The transport mechanism involves transfer to the co-chaperone 'Wntless' or 'WLS' (G protein-coupled receptor 177 (GPR177)), a conserved membrane protein that binds to the palmitoleoyl residue of Wnt proteins and facilitates transport by vesicular or non-vesicular means to the plasma membrane and then across it. In addition, serine acylation may be important for extracellular long-range transport of Wnt proteins in lipoprotein particles, with protein chaperones or in extracellular vesicles. To initiate signaling, Wnt proteins bind to cell-surface receptors of the 'Frizzled' receptor family, where the palmitoleoyl residue fits neatly within a hydrophobic cleft to which it binds to enable ligand-receptor interactions. Wnt proteins are deactivated by an extracellular carboxylesterase ('Notum'), which removes the palmitoleic acid group.
Wnt proteins are relatively hydrophobic and are stabilized in the aqueous environment of the cell by binding in a hydrophobic space created by specific glypicans, i.e. heparan sulfate proteoglycans that are bound to the outer surface of the plasma membrane by a glycosyl-phosphatidylinositol anchor. These serve as a reservoir from which Wnt proteins can be handed over to signalling receptors.
Ghrelin: A further important examples of an O-acylated proteolipid is ghrelin, a circulating 28-amino acid peptide hormone, which is octanoylated (C8) at a serine residue (third amino acid from the N-terminus). Ghrelin is of particular importance as a hunger-stimulating (anorexigenic) hormone, produced in endocrine cells of the human stomach and pancreas, that increases caloric intake, decreases energy expenditure, and promotes fat deposition.
Only the octanoylated protein has biological activity and binds to the single receptor characterized to date, i.e. growth hormone secretagogue receptor GHS‑R1a, which recruits multiple proteins to initiate a variety of cell signalling cascades associated with the physiological and behavioral effects of ghrelin. For example, this receptor can activate the phospholipase C pathway with accumulation of diacylglycerols and inositol phosphates, thence signalling that leads to the mobilization of intracellular calcium (Ca2+) stores and further downstream effects. Among many other functions that are now known, ghrelin is a potent stimulator of growth hormone from the anterior pituitary gland. The picture is further complicated by a finding that GHS-R1a can dimerize and alter the signalling of other G protein-coupled receptors.
The addition of the octanoyl chain is necessary to induce a conformational change in ghrelin with formation of a hydrophobic core that promotes access to the receptor ligand-binding pocket. The required octanoic acid is obtained by β-oxidation of long-chain fatty acids in ghrelin-producing cells, before the ghrelin O‑acyltransferase (GOAT), a member of the MBOAT family of acyltransferases that is widely involved in glycerolipid metabolism, catalyses octanoylation of proghrelin in the Golgi. After translocation to the endoplasmic reticulum, ghrelin is generated by the action of prohormone convertase 1/3. Mature ghrelin is stored within secretory granules of X/A-like cells, and upon fasting it is released into the circulation to stimulate appetite. The liver-expressed antimicrobial peptide 2 (LEAP2) is an endogenous antagonist for GHS-R1a and is involved in the regulation of ghrelin signalling. GOAT is now perceived as a target for pharmaceutical intervention in the treatment of the metabolic syndrome. The extracellular carboxylesterase ('Notum'), which removes the palmitoleic acid group from Wnt proteins, is also able to deacylate ghrelin.
Other O-acylated proteins: Aside from ghrelin and the Wnt proteins, relatively few proteins appear to be O-acylated with long-chain fatty acids. One further example of an O‑acylated protein is the acyl-CoA:lysophosphatidylcholine acyltransferase I (LPCAT1), which as well as generating the pulmonary surfactant dipalmitoylphosphatidylcholine from lysophosphatidylcholine, catalyses O-palmitoylation of a serine residue on the protein histone H4, presumably as a means of regulating mRNA synthesis that may lead to changes in the global transcriptional activity of the cell. A rare O-acylation mechanism is present also in bacteria of the order Corynebacteriales (see our web page on bacterial proteolipids).
6. Hedgehog Proteins - linked Covalently to Cholesterol and Palmitate
Hedgehog (Hh) proteins have a major role in signalling during the differentiation of cells in the development of all embryos from fruit flies to fish to humans, while after embryogenesis, Hh signalling coordinates reparative and regenerative responses in many tissues. They were first found and studied in the insect model Drosophila melanogaster (and named for an anomalous cuticular feature reminiscent of a hedgehog's spines in a mutant), but they are now known to occur in all higher organisms. Vertebrates, for example, express three hedgehog family proteins designated 'Sonic' (Shh), 'Indian' (Ihh) and 'Desert' (Dhh) hedgehog, of which Shh is most studied. They are required for an extensive range of processes, from the control of left-right asymmetry of the body to the specification of individual cell types within the brain and to limb development. A distinctive feature is that these proteins contain both palmitic acid and cholesterol in covalent linkage, further confirmation if needed of the vital importance of these lipids in animal tissues. Proteins that are functionally analogous but structurally distinct are found in nematodes.
Hedgehog proteolipids are formed post-translationally by attachment of cholesterol via an ester bond to glycine at the C-terminus, a highly conserved region of the protein, while a palmitoyl moiety is attached to a cysteine residue at the N-terminus (N-palmitoylation). The signalling proteins are synthesised initially as ~45 kDa inactive propeptides with two distinct domains, i.e. an N-terminal 'hedge' domain and a C-terminal autoprocessing/cholesterol transferase domain. In a unique reaction in the endoplasmic reticulum, a specific amino acid undergoes an acyl rearrangement in which a peptide bond is replaced with a thioester bond, and this is followed by a transesterification reaction of the thioester bond with the hydroxyl group of a cholesterol moiety. This couples cholesterol covalently to the C-terminus of the N-terminal signalling domain and simultaneously splits the ~45 kDa pro-protein at the cholesteroylation site. Finally, the nascent Hh-N is further modified by the addition of an N-palmitoyl group at Cys-24, a reaction mediated by a palmitoylacyltransferase (Hedgehog acyltransferase or Hhat, another MBOAT family member) to create a highly hydrophobic molecule that is often described as 'Hh-Np for Hh-N-processed'.
Unlike the other lipid-modified proteins discussed above, but like the GPI-anchored proteins, the lipid moieties are located in the exoplasmic or exterior leaflet of membranes with the protein component in the extracellular region. The process is regulated both positively and negatively by various oxy-cholesterol derivatives including vitamin D3 and downstream metabolites of 7-dehydrocholesterol, possibly by competing for binding with sterol-sensing domains (similar to those in proteins involved in cholesterol homeostasis) in receptor and other proteins. The interactions between sterol metabolism and hedgehog signalling are increasingly a focus for research.
Following biosynthesis in the endoplasmic reticulum and Golgi, there are mechanisms to transport the final proteolipid through the membranes and onwards to other cells often for an appreciable distance from the site of synthesis (as much as 15 cell diameters), but much remains to be learned of how this is accomplished. In Drosophila and vertebrates, a protein termed 'dispatched' (DISP), which contains a sterol-sensing domain, is required for transport across the membrane, while cell-surface heparan sulfate proteoglycans, consisting of a glycosylphosphatidyl-inositol(GPI)-linked protein core to which varying numbers of linear glycosaminoglycan chains are attached, may also be required. Extracellular extraction from the membrane and subsequent distribution of cholesterol-modified Shh is enhanced by its interaction with the secreted protein SCUBE2; the cholesterol component is essential for this purpose. Cholesterol-modified Shh is also shed from the surface of producing cells in exovesicles or “exosomes” derived from the budding of cellular membranes. In insect models, these proteolipids are transported in the form of lipoprotein complexes (lipophorins) with the lipid moieties in a phospholipid monolayer that surrounds a core of triacylglycerols and cholesterol esters.
Both lipid components are essential for the proper tissue distribution and function of the attached proteins, and the N-palmitoyl moiety in particular is required to cause the proteins to form multimeric complexes essential for biological activity. The cholesterol modification contributes to the partitioning of hedgehog proteins into plasma lipoprotein complexes for long-range transport. The mammalian Shh receptor termed 'PTCH1 or patched' is a transmembrane protein, which contains a sterol-sensing domain and is located on the primary cilium, an antennae-like projection of the plasma membrane into the cytoplasm of the cell; PTCH1 controls the cholesterol composition of the ciliary membrane. When PTCH1 binds to hedgehog proteins, a second G-protein-coupled receptor designated 'SMO or smoothened' is activated to enter the primary cilium and initiates down-stream signalling by triggering the expression of target genes through a complex network of post-translational processes and translocations. Cholesterol and oxysterols, including 20S‑hydroxycholesterol, 24S,25-epoxycholesterol and (25R)26-hydroxy-7-oxocholesterol, can bind to the extracellular cysteine-rich domain of SMO and induce hedgehog signalling. In this process, it has been determined that human SMO is modified by cholesterol through an ester bond between the 3β-hydroxyl group of the latter and the carboxyl group on the side chain of Asp95 of SMO. Both cholesterol binding and palmitoylation are required for the signalling activity.
Abnormalities in the expression and/or signalling of the Shh hedgehog proteins have been implicated in developmental (morphological) abnormalities, and these can be fatal to embryos and have been linked to disease states in humans. In adults, aberrant metabolism of Hedgehog proteins, especially of Hhat, promotes tumorigenesis in many different cancers, including those of the pancreas, breast and lung, and inhibition of this protein has therapeutic potential.
7. Other Proteolipids
N-Acetylation of the primary amine in the ε-position of lysine residues of histone proteins is a process that leads to neutralization of the position’s positive electrostatic charge in a dynamic mechanism for regulation of chromatin formation and function. The acetylated lysine residues interact with a group of proteins, the so-called “readers”, which contain specific acetyl-lysine binding domains and couple acetylation to various down-stream biological effects including gene transcription. The reaction is reversible, and acetate is added from acetyl-coA by lysine acetyl transferases and removed by lysine deacetylases. In addition, some of the lysine acetyl transferases catalyse reaction with other short-chain acyl moieties, including crotonylation, succinylation and propionylation. Although the histone proteins were the first to be identified, it is now recognized that many other proteins are acetylated in this way in both prokaryotes and eukaryotes. N-Terminal acetylation of proteins by specific acetyl transferases occurs also, but in contrast this is an irreversible reaction. It determines the subcellular localization for certain proteins and modulates protein-protein interactions that are essential for normal development of bone, blood vessels and other tissues.
For practical reasons, the GPI-anchored proteins are discussed elsewhere in these web pages, as are the ceramides and related lipids bound to proteins in skin. In addition, N-terminal acetylation of certain membrane proteins targets them for transfer to the Golgi or lysosomes. In yeasts, a covalent conjugate of phosphatidylethanolamine with a protein designated ‘Atg8’ is involved in the process of autophagy (controlled degradation of cellular components) by promoting the formation of membrane vesicles containing the components to be degraded.
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