A novel N-terminal motif for palmitoylation of G-protein alpha subunits

Cysteine-containing peptide sequences exhibit facile uncatalyzed transacylation and acyl-CoA-dependent acylation at the lipid bilayer interface

A variety of simple cysteine-containing lipopeptides, with sequences modeled on those found in naturally occurring S-acylated proteins, undergo spontaneous S-acylation in phospholipid vesicles at physiological pH when either long-chain acyl-CoAs or other S-acylated peptides are added as acyl donors. Fluorescent or radiolabeled lipopeptides with the sequence myristoyl-GCX- (X = G, L, R, T, or V), a motif found to undergo S-acylation in several intracellular regulatory proteins, and the prenylated peptide -SCRC(farnesyl)-OMe, modeled on the carboxyl terminus of p21H-ras, were all found to be suitable acyl acceptors for such uncatalyzed S-acyl transfer reactions at physiological pH. Acylation of these cysteinyl-containing lipopeptides to high stoichiometry was observed, on time scales ranging from a few hours to a few tens of minutes, in vesicles containing relatively low concentrations (< or = mol %) and only a modest molar excess (2.5:1) of the acyl donor species. No evidence was obtained for acyl transfer to peptide serine or threonine hydroxyl groups under the same conditions. These observations may have significant implications both for the design of in vitro studies of the S-acylation of membrane-associated proteins and for our understanding of the mechanisms of S-acylation of these species in vivo.

The short and long forms of the alpha subunit of the stimulatory guanine-nucleotide-binding protein are unequally redistributed during (-)-isoproterenol-mediated desensitization of intact S49 lymphoma cells

We report here that desensitization of the beta-adrenergic receptor-triggered transmembrane signalling in S49 wild-type lymphoma cells, induced by (-)-isoproterenol (1 microM), results in unequal intracellular redistribution of the splicing variants of the alpha subunit of the stimulatory guanine-nucleotide-binding regulatory (Gs alpha) protein (Gs alpha-short and Gs alpha-long) and alters the functional characteristics of the membrane-associated signal transduction complex. We found that two cellular pools of membranes, light-density membranes and plasma membranes prepared by sucrose-density-gradient centrifugation of cell homogenates differed in their content of Gs alpha splicing subforms and, moreover, that prolonged activation of the beta-adrenergic pathway induced intermembrane redistribution of the splicing variants of Gs alpha. Short (10 min) as well as prolonged (1 h) (-)-isoproterenol treatment of the cells shifted Gs alpha-short from light-density membranes to plasma membranes and increased the total amount of light-density membrane-bound Gs alpha-long; in parallel, the maximal (-)-isoproterenol-stimulated or AlF4(-)-stimulated adenylyl cyclase activities measured in the plasma membrane pools prepared from treated cells decreased. The functional characteristics of the membrane-bound Gs alpha pools were examined by a cyc(-)-reconstitutive adenylyl cyclase assay where extracts of the plasma membrane and light-density-membrane pools, respectively, were mixed with plasma membranes derived from the mutant S49 cell line, cyc-, lacking Gs alpha. The maximal cyc(-)-reconstitutive activities of the extracts prepared from light-density membranes of short-term as well as long-term desensitized cells increased compared to control cells. These findings may indicate differences in the functioning of the splicing variants of Gs alpha.

Palmitoylation but not the extreme amino-terminus of Gq alpha is required for coupling to the NK2 receptor

Gq alpha and G11 alpha differ from other G protein alpha subunits in that they have unique, conserved 6 residue amino-terminal extensions. Wild-type and amino-terminal mutants of Gq alpha expressed in COS cells were analyzed for their ability to functionally couple with co-expressed neurokinin NK2 receptor. Wild-type, T2A and delta 2-7 Gq alpha were able to stimulate agonist driven phospholipase C (PLC) activity in identical manners. Other activities of these two amino-terminal mutants including aluminum fluoride stimulated PLC activity, palmitoylation, interaction with G beta gamma subunits and GTP gamma S-induced trypsin resistance are also similar to the wild-type alpha subunit. This demonstrates that the NK2 receptor is able to functionally interact with the alpha subunit of Gq and that the first seven amino-acids of Gq alpha are not required for any of the alpha subunit functions tested. In contrast to the T2A and delta 2-7 mutants, a C9,10A Gq alpha mutant was not able to couple to either the NK2 receptor or PLC, as assessed by high-affinity agonist binding and activation of PLC either in intact cells or in vitro. The C9,10A protein was able to assume a GTP gamma S-induced trypsin-resistant conformation and partitioned primarily to the pelletable fraction in a manner similar to the wild-type protein. However, it was not labeled with [3H]palmitic acid. This suggests that blocking palmitoylation at the amino-terminus of Gq alpha results in a loss of functional activity which reflects an inability to interact with both the receptor and downstream signaling targets.

N-terminal fatty acylation of the alpha-subunit of the G-protein Gi1: only the myristoylated protein is a substrate for palmitoylation

The alpha-subunit of the G-protein Gi1 carries two fatty acyl moieties covalently bound to its N-terminal region: myristic acid is linked to glycine-2 and palmitic acid is linked to cysteine-3. Using site-directed mutagenesis on a cDNA construct of alpha i1 we have generated an alpha i1-G2A mutant, carrying alanine instead of glycine at position 2, and alpha i1-C3S mutant, in which serine replaced cysteine-3 and a double mutant with both substitutions (alpha i1-G2A/C3S). These constructs were individually expressed by transfection in Cos-7 cells, and incorporation of fatty acids into the various mutants was compared with wild-type alpha i1 monitoring metabolic labelling with [3H]palmitate or [3H]myristate. The disruption of the palmitoylation site in alpha i1-C3S did not influence myristoylation, whereas prevention of myristoylation in alpha i1-G2A also abolished palmitoylation. Co-translational myristoylation is thus an absolute requirement for alpha i1 to be post-translationally palmitoylated. The non-palmitoylated alpha i1-C3S showed reduced membrane binding to the same extent as the non-myristoylated/non-palmitoylated alpha i1-G2A and alpha i1-G2A/C3S mutants, indicating that the attachment of palmitic acid is necessary for proper interaction with the membrane.

Palmitoylation of multiple Src-family kinases at a homologous N-terminal motif

We have recently identified a novel N-terminal cysteine-containing motif which specifies the palmitoylation of several G-protein alpha-subunits [Parenti, Viganó, Newman, Milligan and Magee (1993) Biochem. J. 291, 349-353]. A related motif occurs at the N-terminus of members of the Src family of protein tyrosine kinases except for Src itself and Blk. We have investigated whether the Src, Fyn, Yes and Lck gene products are palmitoylated. Src was not labelled with [3H]palmitate when endogenously expressed in COS cells. In contrast, endogenous Yes immunoprecipitated from COS cells was palmitoylated. Fyn was palmitoylated in insect cells infected with a recombinant baculovirus and the palmitoylation was independent of protein synthesis, suggesting a dynamic turnover of this lipid. Fatty acid analysis indicated that most of the label was incorporated as palmitate. Lck was palmitoylated when expressed by transfection in COS cells. All of these protein tyrosine kinases were also detectably myristoylated in each of the systems tested. Experiments performed with mutants of Lck expressed by transfection in COS cells indicated that cysteines at positions 3 and 5 were both palmitoylation sites and that myristoylation was required for palmitoylation. To confirm that palmitoylation was occurring on cysteines in the N-terminal region of Fyn, site-directed mutagenesis was used to replace the cysteines at positions 3 and 6 with alanine. The resulting protein was not palmitoylated but was still myristoylated when expressed in COS cells. A glycine to alanine mutant at position 2 was also not palmitoylated, showing that myristoylation is a prerequisite for palmitoylation. Our data indicate that Src family members containing the N-terminal cysteine motif are indeed palmitoylated. By analogy with Ras, it is possible that palmitoylation may play an important role in the localization and function of Src family protein tyrosine kinases.

The palmitoylation status of the G-protein G(o)1 alpha regulates its activity of interaction with the plasma membrane

Plasmids containing cDNAs encoding either the wild-type guanine-nucleotide-binding protein G(o)1 alpha or the palmitoylation-negative cysteine-3-to-serine (C3S) mutant of G(o)1 alpha were transfected into Rat 1 cells, and clones stably expressing immunoreactivity corresponding to these polypeptides were isolated. Clones C5B (expressing wild-type G(o)1 alpha) and D3 (expressing the mutant form) were selected for detailed study. Immunoprecipitation of whole cell lysates of each clone labelled with either [3H]palmitate or [3H]myristate demonstrated incorporation of [3H]myristate into both wild-type and the C3S mutant of G(o)1 alpha, but that incorporation of hydroxylamine-sensitive [3H]palmitate was restricted to the wild type. When membrane and cytoplasmic fractions were prepared from cells of either the C5B or D3 clones, although immunodetection of wild-type G(o)1 alpha was observed only in the membrane fraction, the C3S mutant was present in both membrane and cytoplasmic fractions. Furthermore, a significant proportion of the C3S G(o)1 alpha immunoreactivity was also detected in the cytoplasmic fraction if immunoprecipitation of recently synthesized G(o)1 alpha was performed from fractions derived from cells pulse-labelled with [35S]Trans label. Pretreatment of cells of both clones C5B and D3 with pertussis toxin led to complete ADP-ribosylation of the cellular population of G(o)1 alpha in both cell types, irrespective of whether the polypeptide was subsequently found in the membrane or cytoplasmic fraction following cellular disruption. By contrast, separation of membrane and cytoplasmic fractions before pertussis-toxin-catalysed [32P]ADP-ribosylation allowed modification only of the membrane-associated G(o)1 alpha (whether wild-type or the C3S mutant). This labelling was decreased substantially by incubation of the membranes with guanosine 5'-[beta gamma-imido]triphosphate. No cytoplasmic G-protein beta subunit was detected immunologically, and the non-membrane-associated C3S G(o)1 alpha from D3 cells migrated as an apparently monomeric 40 kDa protein on a Superose 12 gel-filtration column. Membrane-associated wild-type and C3S G(o)1 alpha appeared to interact with guanine nucleotides with similar affinity, as no alteration in the dose-response curves for guanine-nucleotide-induced maintenance of a stable 37 kDa tryptic fragment was noted for the two forms of G(o)1 alpha. Chemical depalmitoylation of membranes of clone C5B with neutral 1 M hydroxylamine caused a release of some 25-30% of each of G(o)1 alpha, Gi2 alpha and Gq alpha/G11 alpha from the membranes. Equivalent treatment of D3 cells caused an equivalent release of Gi2 alpha and Gq alpha/G11 alpha, but was unable to cause any appreciable release of the CS3 form of G(o)1 alpha, which was membrane-bound.

Overview: protein palmitoylation in the nervous system: current views and unsolved problems

Palmitoylation refers to a dynamic post-translational modification of proteins involving the covalent attachment of long-chain fatty acids to the side chains of cysteine, threonine or serine residues. In recent years, palmitoylation has been identified as a widespread modification of both viral and cellular proteins. Because of its dynamic nature, protein palmitoylation, like phosphorylation, appears to have a crucial role in the functioning of the nervous system. Several important questions regarding the post-translational acylation of cysteine residues in proteins are briefly discussed: (a) What are the molecular mechanisms involved in dynamic acylation? (b) What are the determinants of the fatty acid specificity and the structural requirements of the acceptor proteins? (c) What are the physiological signals regulating this type of protein modification, and (d) What is the biological role(s) of this reaction with respect to the functioning of specific nervous system proteins? We also present the current experimental obstacles that have to be overcome to fully understand the biology of this dynamic modification.

Cysteine3 of Src family protein tyrosine kinase determines palmitoylation and localization in caveolae

Recent work has demonstrated that p56lck, a member of the Src family of protein tyrosine kinases (PTKs), is modified by palmitoylation of a cysteine residue(s) within the first 10 amino acids of the protein (in addition to amino-terminal myristoylation that is a common modification of the Src family of PTKs). This is now extended to three other members of this family by showing incorporation of [3H]palmitate into p59fyn, p55fgr, and p56hck, but not into p60src. The [3H]palmitate was released by treatment with neutral hydroxylamine, indicating a thioester linkage to the protein. Individual replacement of the two cysteine residues within the first 10 amino acids of p59fyn and p56lck with serine indicated that Cys3 was the major determinant of palmitoylation, as well as association of the PTK with glycosyl-phosphatidylinositol-anchored proteins. Introduction of Cys3 into p60src led to its palmitoylation. p59fyn but not p60src partitioned into Triton-insoluble complexes that contain caveolae, microinvaginations of the plasma membrane. Mapping of the requirement for partitioning into caveolae demonstrated that the amino-terminal sequence Met-Gly-Cys is both necessary and sufficient within the context of a Src family PTK to confer localization into caveolae. Palmitoylation of this motif in p59fyn also modestly increased its overall avidity for membranes. These results highlight the role of the amino-terminal motif Met-Gly-Cys in determining the structure and properties of members of the Src family of PTKs.

Purification and characterization of smooth muscle cell caveolae

Plasmalemmal caveolae are a membrane specialization that mediates transcytosis across endothelial cells and the uptake of small molecules and ions by both epithelial and connective tissue cells. Recent findings suggest that caveolae may, in addition, be involved in signal transduction. To better understand the molecular composition of this membrane specialization, we have developed a biochemical method for purifying caveolae from chicken smooth muscle cells. Biochemical and morphological markers indicate that we can obtain approximately 1.5 mg of protein in the caveolae fraction from approximately 100 g of chicken gizzard. Gel electrophoresis shows that there are more than 30 proteins enriched in caveolae relative to the plasma membrane. Among these proteins are: caveolin, a structural molecule of the caveolae coat; multiple, glycosylphosphatidylinositol-anchored membrane proteins; both G alpha and G beta subunits of heterotrimeric GTP-binding protein; and the Ras-related GTP-binding protein, Rap1A/B. The method we have developed will facilitate future studies on the structure and function of caveolae.

Lipid modifications of G proteins

Covalent attachment of lipids is a near-universal mechanism through which eukaryotic cells direct and, in some cases, control membrane localization of G proteins. Studies conducted over the past year have substantially advanced our understanding of both the molecular mechanisms and the functional consequences of these modifications. Of particular note are the processes of palmitoylation of the alpha-subunits of heterotrimeric G proteins, and prenylation of members of the Ras superfamily of monomeric G proteins, where recent findings point to unexpected roles for lipid modifications in signaling through these proteins.

Receptor regulation of G-protein palmitoylation

Many alpha subunits of heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins) are palmitoylated. Exposure of cells to the beta-adrenergic agonist isoproterenol increased incorporation of [3H]palmitate specifically into alpha s, the alpha subunit that mediates stimulation of adenylyl cyclase. Pulse-chase experiments suggested that isoproterenol increased turnover of alpha s-bound palmitate. Mutagenesis of Cys-3 in alpha s or alpha o (a homologous alpha subunit) prevented palmitoylation of these proteins. Differing results were obtained when mutations of Cys-3 in alpha s or alpha o were expressed in cells and assayed for their distribution between soluble and membrane fractions. Some alpha subunits, including alpha o, are myristoylated at the amino-terminal glycine residue. Mutation of this glycine prevented both myristoylation and palmitoylation of alpha o, indicating that myristoylation precedes palmitoylation of dually acylated alpha subunits. The amino-terminal sequences and fatty acylation properties of dually acylated alpha subunits are strikingly similar to those of some members of the Src family of protein-tyrosine kinases. The amino-terminal sequence Met-Gly-Cys-Xaa-Xaa-Ser/Cys shared by these proteins may represent a motif for cotranslational and posttranslational processing that includes myristoylation of the glycine residue and reversible palmitoylation of the cysteine residue.

Amino acid residues 24-31 but not palmitoylation of cysteines 30 and 45 are required for membrane anchoring of glutamic acid decarboxylase, GAD65

The smaller isoform of the GABA synthesizing enzyme glutamic acid decarboxylase, GAD65, is synthesized as a soluble protein that undergoes post-translational modification(s) in the NH2-terminal region to become anchored to the membrane of small synaptic-like microvesicles in pancreatic beta cells, and synaptic vesicles in GABA-ergic neurons. A soluble hydrophilic form, a soluble hydrophobic form, and a hydrophobic firmly membrane-anchored form have been detected in beta cells. A reversible and hydroxylamine sensitive palmitoylation has been shown to distinguish the firmly membrane-anchored form from the soluble yet hydrophobic form, suggesting that palmitoylation of cysteines in the NH2-terminal region is involved in membrane anchoring. In this study we use site-directed mutagenesis to identify the first two cysteines in the NH2-terminal region, Cys 30 and Cys 45, as the sites of palmitoylation of the GAD65 molecule. Mutation of Cys 30 and Cys 45 to Ala results in a loss of palmitoylation but does not significantly alter membrane association of GAD65 in COS-7 cells. Deletion of the first 23 amino acids at the NH2 terminus of the GAD65 30/45A mutant also does not affect the hydrophobicity and membrane anchoring of the GAD65 protein. However, deletion of an additional eight amino acids at the NH2 terminus results in a protein which is hydrophilic and cytosolic. The results suggest that amino acids 24-31 are required for hydrophobic modification and/or targeting of GAD65 to membrane compartments, whereas palmitoylation of Cys 30 and Cys 45 may rather serve to orient or fold the protein at synaptic vesicle membranes.

The alpha-subunits of G-proteins G12 and G13 are palmitoylated, but not amidically myristoylated

The alpha-subunits of the G-proteins G12 and G13 were expressed with a baculovirus system in insect cells and analysed for acylation. Both proteins incorporated tritiated palmitic and to a lesser extent also tritiated myristic acid. Radiolabel from both fatty acids was sensitive to treatment with neutral hydroxylamine. This result supports a thioester-type fatty acid bond and argues against amidical N-myristoylation. Fatty acid analysis after labeling with [3H]palmitic acid showed that palmitate represents the predominant fatty acid linked to G alpha 12 and G alpha 13. Separation of cells into cytosolic and membranous fractions revealed that palmitoylated alpha-subunits of G12 were exclusively membrane-bound, whereas [35S]methionine-labeled proteins were detected in soluble and particulate fractions. Inhibition of protein synthesis with cycloheximide did not block palmitoylation of the alpha-subunits, which indicates that palmitoylation occurs independently of protein synthesis.

Novel inhibitory action of tunicamycin homologues suggests a role for dynamic protein fatty acylation in growth cone-mediated neurite extension

In neuronal growth cones, the advancing tips of elongating axons and dendrites, specific protein substrates appear to undergo cycles of posttranslational modification by covalent attachment and removal of long-chain fatty acids. We show here that ongoing fatty acylation can be inhibited selectively by long-chain homologues of the antibiotic tunicamycin, a known inhibitor of N-linked glycosylation. Tunicamycin directly inhibits transfer of palmitate to protein in a cell-free system, indicating that tunicamycin inhibition of protein palmitoylation reflects an action of the drug separate from its previously established effects on glycosylation. Tunicamycin treatment of differentiated PC12 cells or dissociated rat sensory neurons, under conditions in which protein palmitoylation is inhibited, produces a prompt cessation of neurite elongation and induces a collapse of neuronal growth cones. These growth cone responses are rapidly reversed by washout of the antibiotic, even in the absence of protein synthesis, or by addition of serum. Two additional lines of evidence suggest that the effects of tunicamycin on growth cones arise from its ability to inhibit protein long-chain acylation, rather than its previously established effects on protein glycosylation and synthesis. (a) The abilities of different tunicamycin homologues to induce growth cone collapse very systematically with the length of the fatty acyl side-chain of tunicamycin, in a manner predicted and observed for the inhibition of protein palmitoylation. Homologues with fatty acyl moieties shorter than palmitic acid (16 hydrocarbons), including potent inhibitors of glycosylation, are poor inhibitors of growth cone function. (b) The tunicamycin-induced impairment of growth cone function can be reversed by the addition of excess exogenous fatty acid, which reverses the inhibition of protein palmitoylation but has no effect on the inhibition of protein glycosylation. These results suggest an important role for dynamic protein acylation in growth cone-mediated extension of neuronal processes.

The membrane localization of the G protein alpha s subunit is not dependent on its TENIR sequence or effector domain

The GS protein alpha subunit (alpha S) sequences conserved in non-myristoylated alpha subunits (TENIR, residues 369-373) or critical for adenylyl cyclase interaction were investigated as possible sites required for membrane localization. Substitutions were created by site-directed mutagenesis in which the TENIR residues were deleted from alpha S or added to the soluble, non-myristoylated alpha i1. After transfection, COS cells were separated by centrifugation into particulate and soluble fractions. Immunoblots showed that these substitutions did not change the localization: alpha S +/- TENIR in the particulate fraction, non-myristoylated alpha i1 +/- TENIR in the soluble fraction. The constitutively active alpha i/alpha S chimera (CH4A), containing four regions of alpha S sufficient for adenylyl cyclase activation, was mutated to prevent myristoylation (GA-CH4A). Immunoblots of transfected COS cell fractions showed CH4A in the particulate and GA-CH4A in the soluble fraction. While these regions did not lead to membrane localization, the soluble GA-CH4A could activate adenylyl cyclase in the intact cell and after reconstitution with cyc- membranes.

Subcellular localization specified by protein acylation and phosphorylation

Modification of proteins by both lipophilic and hydrophilic moieties is widely documented. Here we present recent insights into how protein targeting is influenced by protein modification, with particular emphasis on dynamic regulation by fatty acylation and phosphorylation of proteins.

Agonist regulation of cellular G protein levels and distribution: mechanisms and functional implications

Exposure of cells to agonists of receptors linked to G proteins can result in downregulation of cellular levels or redistribution of G proteins from membranes to the cytosol. Agonist-induced reductions in G protein levels have been observed for members of each of the Gs, Gi and Gq families of G proteins, are likely to be dependent upon the level of receptor expression, and are generally restricted to the G protein(s) with which the receptor interacts. The mechanisms responsible, reviewed here by Graeme Milligan, vary with cell type and include both second messenger-dependent and -independent enhanced protein degradation. Agonist-induced reduction in cellular G protein levels can provide one mechanism for the development of sustained heterologous desensitization.

Regional distribution and quantitative measurement of the phosphoinositidase C-linked guanine nucleotide binding proteins G11 alpha and Gq alpha in rat brain

Levels of the guanine nucleotide binding proteins G11 alpha and Gq alpha, which produce receptor regulation of phosphoinositidase C, were measured immunologically in 13 regions of rat central nervous system. This was achieved by immunoblotting membranes from these regions with antisera (CQ series) that identify these two polypeptides equally, following separation of the membranes using sodium dodecyl sulphate-polyacrylamide gel electrophoresis conditions that can resolve Gq alpha and G11 alpha. In all regions examined, Gq alpha was more highly expressed than G11 alpha. Ratios of levels of Gq alpha to G11 alpha varied between the regions from 5:1 to 2:1. Quantitative measurements of the levels of Gq alpha and G11 alpha in each region were obtained by comparison with known amounts of purified liver Gq alpha and G11 alpha and with E. coli expressed recombinant Gq alpha. Areas that expressed Gq alpha highly included olfactory bulb (930 ng/mg of membrane protein), frontal cortex (700 ng/mg of membrane protein), parietal occipital cortex (670 ng/mg of membrane protein), caudate putamen (1,003 ng/mg of membrane protein), hippocampus (1,045 ng/mg of membrane protein), hypothalamus (790 ng/mg of membrane protein), and cerebellum (950 ng/mg of membrane protein). More modest levels were observed in thalamus (450 ng/mg of membrane protein), pituitary (480 ng/mg of membrane protein), optic chiasma (330 ng/mg of membrane protein), and spinal cord (350 ng/mg of membrane protein). G11 alpha was more evenly expressed with values ranging from about 170 ng/mg of membrane protein in spinal cord and optic chiasma to close to 300 ng/mg of membrane protein in regions expressing high levels of Gq alpha.(ABSTRACT TRUNCATED AT 250 WORDS)

The G protein alpha s subunit incorporates [3H]palmitic acid and mutation of cysteine-3 prevents this modification

We investigated whether alpha s could be acylated by palmitate by transfecting COS cells with the cDNA for the wild-type, long form of alpha s and metabolically labeling with [3H]palmitate or [35S]methionine. Cells were separated into particulate and soluble fractions and immunoprecipitated with a specific peptide antibody. [3H]Palmitate was incorporated into both endogenous and transfected alpha s. Inhibition of protein synthesis with cycloheximide did not block the radiolabeling of alpha s with [3H]palmitate. Hydroxylamine treatment caused a release of the tritium radiolabel, demonstrating that the incorporation was through a thioester bond. The tritium radiolabel was base-labile and comigrated with [3H]palmitate on thin-layer chromatography. The third residue of the wild-type alpha s was mutated from a cysteine to an alanine by site-directed mutagenesis. This mutant was expressed in COS cells and localized to the particulate fraction as determined by immunoprecipitation of the [35S]methionine-labeled cells. The cysteine-3 mutant did not undergo radiolabeling with [3H]palmitate, indicating that this residue is crucial for the modification.