Timing of palmitoylation of influenza virus hemagglutinin

Palmitoylation of the hemagglutinin of influenza B virus by ER-localized DHHC enzymes 1, 2, 4, and 6 is required for efficient virus replication

IMPORTANCE

Influenza viruses are a public health concern since they cause seasonal outbreaks and occasionally pandemics. Our study investigates the importance of a protein modification called "palmitoylation" in the replication of influenza B virus. Palmitoylation involves attaching fatty acids to the viral protein hemagglutinin and has previously been studied for influenza A virus. We found that this modification is important for the influenza B virus to replicate, as mutating the sites where palmitate is attached prevented the virus from generating viable particles. Our experiments also showed that this modification occurs in the endoplasmic reticulum. We identified the specific enzymes responsible for this modification, which are different from those involved in palmitoylation of HA of influenza A virus. Overall, our research illuminates the similarities and differences in fatty acid attachment to HA of influenza A and B viruses and identifies the responsible enzymes, which might be promising targets for anti-viral therapy.

Palmitoylation of the envelope membrane proteins GP5 and M of porcine reproductive and respiratory syndrome virus is essential for virus growth

Porcine reproductive and respiratory syndrome virus (PRRSV), an enveloped positive-strand RNA virus in the Arteiviridae family, is a major pathogen affecting pigs worldwide. The membrane (glyco)proteins GP5 and M form a disulfide-linked dimer, which is a major component of virions. GP5/M are required for virus budding, which occurs at membranes of the exocytic pathway. Both GP5 and M feature a short ectodomain, three transmembrane regions, and a long cytoplasmic tail, which contains three and two conserved cysteines, respectively, in close proximity to the transmembrane span. We report here that GP5 and M of PRRSV-1 and -2 strains are palmitoylated at the cysteines, regardless of whether the proteins are expressed individually or in PRRSV-infected cells. To completely prevent S-acylation, all cysteines in GP5 and M have to be exchanged. If individual cysteines in GP5 or M were substituted, palmitoylation was reduced, and some cysteines proved more important for efficient palmitoylation than others. Neither infectious virus nor genome-containing particles could be rescued if all three cysteines present in GP5 or both present in M were replaced in a PRRSV-2 strain, indicating that acylation is essential for virus growth. Viruses lacking one or two acylation sites in M or GP5 could be rescued but grew to significantly lower titers. GP5 and M lacking acylation sites form dimers and GP5 acquires Endo-H resistant carbohydrates in the Golgi apparatus suggesting that trafficking of the membrane proteins to budding sites is not disturbed. Likewise, GP5 lacking two acylation sites is efficiently incorporated into virus particles and these viruses exhibit no reduction in cell entry. We speculate that multiple fatty acids attached to GP5 and M in the endoplasmic reticulum are required for clustering of GP5/M dimers at Golgi membranes and constitute an essential prerequisite for virus assembly.

Porcine reproductive and respiratory syndrome virus (PRRSV), an arterivirus in the order Nidovirales, is an important pathogen for pigs. Despite its importance in veterinary medicine, basic structural and functional features of its membrane proteins have not been elucidated. Here, we provide evidence for palmitoylation of the PRRSV major membrane proteins GP5 and M at a cluster of membrane-near cysteines. Fatty acid attachment is required for virus growth, since removal of all acylation sites from either M or GP5 prevents recue of infectious particles. Furthermore, viruses lacking individual acylation sites in M and GP5 grow to significantly lower titers in cell culture. The specific infectivity and cell entry of viruses lacking two acylation sites in Gp5 is, however, not reduced. Likewise, these viruses revealed no effect on dimerization of GP5 with M, its transport to budding sites, and incorporation into virus particles. Since cells transfected with a cDNA expressing non-acylated GP5, or non-acylated M release no virus-like particles into the supernatant we propose that the fatty acids are required for the budding process. They might trigger assembly of GP5/M dimers to form a coat inside the lipid bilayer that induces membrane curvature.

Hemagglutinin of Influenza A, but not of Influenza B and C viruses is acylated by ZDHHC2, 8, 15 and 20

Hemagglutinin (HA), a glycoprotein of Influenza A viruses and its proton channel M2 are site-specifically modified with fatty acids. Whereas two cysteines in the short cytoplasmic tail of HA contain only palmitate, stearate is exclusively attached to one cysteine located at the cytoplasmic border of the transmembrane region (TMR). M2 is palmitoylated at a cysteine positioned in an amphiphilic helix near the TMR. The enzymes catalyzing acylation of HA and M2 have not been identified, but zinc finger DHHC domain-containing (ZDHHC) palmitoyltransferases are candidates. We used a siRNA library to knockdown expression of each of the 23 human ZDHHCs in HA-expressing HeLa cells. siRNAs against ZDHHC2 and 8 had the strongest effect on acylation of HA as demonstrated by Acyl-RAC and confirmed by 3H-palmitate labeling. CRISPR/Cas9 knockout of ZDHHC2 and 8 in HAP1 cells, but also of the phylogenetically related ZDHHCs 15 and 20 strongly reduced acylation of group 1 and group 2 HAs and of M2, but individual ZDHHCs exhibit slightly different substrate preferences. These ZDHHCs co-localize with HA at membranes of the exocytic pathway in a human lung cell line. ZDHHC2, 8, 15 and 20 are not required for acylation of the HA-esterase-fusion protein of Influenza C virus that contains only stearate at one transmembrane cysteine. Knockout of these ZDHHCs also did not compromise acylation of HA of Influenza B virus that contains two palmitoylated cysteines in its cytoplasmic tail. Results are discussed with respect to the acyl preferences and possible substrate recognition features of the identified ZDHHCs.

Toward the identification of ZDHHC enzymes required for palmitoylation of viral protein as potential drug targets

Introduction: S-acylation is the attachment of fatty acids not only to cysteines of cellular, but also of viral proteins. The modification is often crucial for the protein´s function and hence for virus replication. Transfer of fatty acids is mediated by one or several of the 23 members of the ZDHHC family of proteins. Since their genes are linked to various human diseases, they represent drug targets.Areas covered: The authors explore whether targeting acylation of viral proteins might be a strategy to combat viral diseases. Many human pathogens contain S-acylated proteins; the ZDHHCs involved in their acylation are currently identified. Based on the 3D structure of two ZDHHCs, the regulation and the biochemistry of the palmitolyation reaction and the lipid and protein substrate specificities are discussed. The authors then speculate how ZDHHCs might recognize S-acylated membrane proteins of Influenza virus.Expert opinion: Although many viral diseases can now be treated, the available drugs bind to viral proteins that rapidly mutate and become resistant. To develop inhibitors for the genetically more stable cellular ZDHHCs, their binding sites for viral substrates need to be identified. If only a few cellular proteins are recognized by the same binding site, development of specific inhibitors may have therapeutic potential.

Matrix proteins of enveloped viruses: a case study of Influenza A virus M1 protein

Influenza A virus, a member of the Orthomyxoviridae family of enveloped viruses, is one of the human and animal top killers, and its structure and components are therefore extensively studied during the last decades. The most abundant component, M1 matrix protein, forms a matrix layer (scaffold) under the viral lipid envelope, and the functional roles as well as structural peculiarities of the M1 protein are still under heavy debate. Despite multiple attempts of crystallization, no high resolution structure is available for the full length M1 of Influenza A virus. The likely reason for the difficulties lies in the intrinsic disorder of the M1 C-terminal part preventing diffraction quality crystals to be grown. Alternative structural methods including synchrotron small-angle X-ray scattering (SAXS), atomic force microscopy, cryo-electron microscopy/tomography are therefore widely applied to understand the structure of M1, its self-association and interactions with the lipid membrane and the viral nucleocapsid. These methods reveal striking similarities in the behavior of M1 and matrix proteins of other enveloped RNA viruses, with the differences accompanied by the specific features of the viral lifecycles, thus suggesting common interaction principles and, possibly, common evolutional ancestors. The structural information on the Influenza A virus M1 protein obtained to the date strongly suggests that the intrinsic disorder in the C-terminal domain has important functional implications.

Hemagglutinin-esterase-fusion (HEF) protein of influenza C virus

Influenza C virus, a member of the Orthomyxoviridae family, causes flu-like disease but typically only with mild symptoms. Humans are the main reservoir of the virus, but it also infects pigs and dogs. Very recently, influenza C-like viruses were isolated from pigs and cattle that differ from classical influenza C virus and might constitute a new influenza virus genus. Influenza C virus is unique since it contains only one spike protein, the hemagglutinin-esterase-fusion glycoprotein HEF that possesses receptor binding, receptor destroying and membrane fusion activities, thus combining the functions of Hemagglutinin (HA) and Neuraminidase (NA) of influenza A and B viruses. Here we briefly review the epidemiology and pathology of the virus and the morphology of virus particles and their genome. The main focus is on the structure of the HEF protein as well as on its co- and post-translational modification, such as N-glycosylation, disulfide bond formation, S-acylation and proteolytic cleavage into HEF1 and HEF2 subunits. Finally, we describe the functions of HEF: receptor binding, esterase activity and membrane fusion.

Site-specific S-acylation of influenza virus hemagglutinin: the location of the acylation site relative to the membrane border is the decisive factor for attachment of stearate

S-Acylation of hemagglutinin (HA), the main glycoprotein of influenza viruses, is an essential modification required for virus replication. Using mass spectrometry, we have previously demonstrated specific attachment of acyl chains to individual acylation sites. Whereas the two cysteines in the cytoplasmic tail of HA contain only palmitate, stearate is exclusively attached to a cysteine positioned at the end of the transmembrane region (TMR). Here we analyzed recombinant viruses containing HA with exchange of conserved amino acids adjacent to acylation sites or with a TMR cysteine shifted to a cytoplasmic location to identify the molecular signal that determines preferential attachment of stearate. We first developed a new protocol for sample preparation that requires less material and might thus also be suitable to analyze cellular proteins. We observed cell type-specific differences in the fatty acid pattern of HA: more stearate was attached if human viruses were grown in mammalian compared with avian cells. No underacylated peptides were detected in the mass spectra, and even mutations that prevented generation of infectious virus particles did not abolish acylation of expressed HA as demonstrated by metabolic labeling experiments with [(3)H]palmitate. Exchange of conserved amino acids in the vicinity of an acylation site had a moderate effect on the stearate content. In contrast, shifting the TMR cysteine to a cytoplasmic location virtually eliminated attachment of stearate. Thus, the location of an acylation site relative to the transmembrane span is the main signal for stearate attachment, but the sequence context and the cell type modulate the fatty acid pattern.

Palmitoylation of influenza virus proteins

Influenza viruses contain two palmitoylated (S-acylated) proteins: the major spike protein HA (haemagglutinin) and the proton-channel M2. The present review describes the fundamental biochemistry of palmitoylation of HA: the location of palmitoylation sites and the fatty acid species bound to HA. Finally, the functional consequences of palmitoylation of HA and M2 are discussed regarding association with membrane rafts, entry of viruses into target cells by HA-mediated membrane fusion as well as the release of newly assembled virus particles from infected cells.

Palmitoylation of virus proteins

The article summarises the results of more than 30 years of research on palmitoylation (S‐acylation) of viral proteins, the post‐translational attachment of fatty acids to cysteine residues of integral and peripheral membrane proteins. Analysing viral proteins is not only important to characterise the cellular pathogens but also instrumental to decipher the palmitoylation machinery of cells. This comprehensive review describes methods to identify S‐acylated proteins and covers the fundamental biochemistry of palmitoylation: the location of palmitoylation sites in viral proteins, the fatty acid species found in S‐acylated proteins, the intracellular site of palmitoylation and the enzymology of the reaction. Finally, the functional consequences of palmitoylation are discussed regarding binding of proteins to membranes or membrane rafts, entry of enveloped viruses into target cells by spike‐mediated membrane fusion as well as assembly and release of virus particles from infected cells. The topics are described mainly for palmitoylated proteins of influenza virus, but proteins of other important pathogens, such as the causative agents of AIDS and severe acute respiratory syndrome, and of model viruses are discussed.

Mutation of a raft-targeting signal in the transmembrane region retards transport of influenza virus hemagglutinin through the Golgi

Inclusion of proteins into membrane-rafts favours interactions required for virus assembly but has also been proposed to facilitate vesicular transport of proteins. The hemagglutinin (HA) of influenza virus contains a raft-targeting sequence in the outer leaflet of its transmembrane region. We report that its mutation enhances co-localization of HA with a cis-Golgi marker and retards Golgi-localized processing, such as acquisition of Endo-H resistant carbohydrates and proteolytic cleavage. In contrast, trimerization of the molecule in the ER and transport to the apical membrane were not affected. The second signal for raft-targeting, S-acylation at cytoplasmic cysteines, did not retard HA transport.

Intrinsic membrane association of the cytoplasmic tail of influenza virus M2 protein and lateral membrane sorting regulated by cholesterol binding and palmitoylation

The influenza virus transmembrane protein M2 is a proton channel, but also plays a role in the scission of nascent virus particles from the plasma membrane. An amphiphilic helix in the CT (cytoplasmic tail) of M2 is supposed to insert into the lipid bilayer, thereby inducing curvature. Palmitoylation of the helix and binding to cholesterol via putative CRAC (cholesterol recognition/interaction amino acid consensus) motifs are believed to target M2 to the edge of rafts, the viral-budding site. In the present study, we tested pre-conditions of this model, i.e. that the CT interacts with membranes, and that acylation and cholesterol binding affect targeting of M2. M2-CT, purified as a glutathione transferase fusion protein, associated with [3H]photocholesterol and with liposomes. Mutation of tyrosine residues in the CRAC motifs prevented [(3)H]photocholesterol labelling and reduced liposome binding. M2-CT fused to the yellow fluorescent protein localized to the Golgi in transfected cells; membrane targeting was dependent on CRAC and (to a lesser extent) on palmitoylation. Preparation of giant plasma membrane vesicles from cells expressing full-length M2-GFP (green fluorescent protein) showed that the protein is partly present in the raft domain. Raft targeting required palmitoylation, but not the CRAC motifs. Thus palmitoylation and cholesterol binding differentially affect the intrinsic membrane binding of the amphiphilic helix.

Linker and/or transmembrane regions of influenza A/Group-1, A/Group-2, and type B virus hemagglutinins are packed differently within trimers

Influenza virus hemagglutinin is a homotrimeric spike glycoprotein crucial for virions' attachment, membrane fusion, and assembly reactions. X-ray crystallography data are available for hemagglutinin ectodomains of various types/subtypes but not for anchoring segments. To get structural information for the linker and transmembrane regions of hemagglutinin, influenza A (H1-H16 subtypes except H8 and H15) and B viruses were digested with bromelain or subtilisin Carlsberg, either within virions or in non-ionic detergent micelles. Proteolytical fragments were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Within virions, hemagglutinins of most influenza A/Group-1 and type B virus strains were more susceptible to digestion with bromelain and/or subtilisin compared to A/Group-2 hemagglutinins. The cleavage sites were always located in the hemagglutinin linker sequence. In detergent, 1) bromelain cleaved hemagglutinin of every influenza A subtype in the linker region; 2) subtilisin cleaved Group-2 hemagglutinins in the linker region; 3) subtilisin cleaved Group-1 hemagglutinins in the transmembrane region; 4) both enzymes cleaved influenza B virus hemagglutinin in the transmembrane region. We propose that the A/Group-2 hemagglutinin linker and/or transmembrane regions are more tightly associated within trimers than type A/Group-1 and particularly type B ones. This hypothesis is underpinned by spatial trimeric structure modeling performed for transmembrane regions of both Group-1 and Group-2 hemagglutinin representatives. Differential S-acylation of the hemagglutinin C-terminal anchoring segment with palmitate/stearate residues possibly contributes to fine tuning of transmembrane trimer packing and stabilization since decreased stearate amount correlated with deeper digestion of influenza B and some A/Group-1 hemagglutinins.

The lack of an inherent membrane targeting signal is responsible for the failure of the matrix (M1) protein of influenza A virus to bud into virus-like particles

The matrix protein (M1) of influenza A virus is generally viewed as a key orchestrator in the release of influenza virions from the plasma membrane during infection. In contrast to this model, recent studies have indicated that influenza virus requires expression of the envelope proteins for budding of intracellular M1 into virus particles. Here we explored the mechanisms that control M1 budding. Similarly to previous studies, we found that M1 by itself fails to form virus-like-particles (VLPs). We further demonstrated that M1, in the absence of other viral proteins, was preferentially targeted to the nucleus/perinuclear region rather than to the plasma membrane, where influenza virions bud. Remarkably, we showed that a 10-residue membrane targeting peptide from either the Fyn or Lck oncoprotein appended to M1 at the N terminus redirected M1 to the plasma membrane and allowed M1 particle budding without additional viral envelope proteins. To further identify a functional link between plasma membrane targeting and VLP formation, we took advantage of the fact that M1 can interact with M2, unless the cytoplasmic tail is absent. Notably, native M2 but not mutant M2 effectively targeted M1 to the plasma membrane and produced extracellular M1 VLPs. Our results suggest that influenza virus M1 may not possess an inherent membrane targeting signal. Thus, the lack of efficient plasma membrane targeting is responsible for the failure of M1 in budding. This study highlights the fact that interactions of M1 with viral envelope proteins are essential to direct M1 to the plasma membrane for influenza virus particle release.

Differential palmitoylation of the endosomal SNAREs syntaxin 7 and syntaxin 8

Palmitoylation is a posttranslational modification that regulates protein trafficking and stability. In this study we investigated whether the endosomal soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) proteins syntaxin 7 and syntaxin 8 are modified with palmitate. Using metabolic labeling and site-directed mutagenesis, we show that human syntaxins 7 and 8 are modified with palmitate through a thioester linkage. Palmitoylation is dependent upon cysteine 239 of human syntaxin 7 and cysteine 214 of syntaxin 8, residues that are located on the cytoplasmic face of the transmembrane domain (TMD). Palmitoylation of syntaxin 8 is minimally affected by the Golgi-disturbing agent brefeldin A (BFA), whereas BFA dramatically inhibits palmitoylation of syntaxin7. The differential effect of BFA suggests that palmitoylation of syntaxins 7 and 8 occurs in distinct subcellular compartments. Palmitoylation does not affect the rate of protein turnover of syntaxins 7 and 8 nor does it influence the steady-state localization of syntaxin 8 in late endosomes. Syntaxin 7 actively cycles between endosomes and the plasma membrane. Palmitoylation-defective syntaxin 7 is selectively retained on the plasma membrane, suggesting that palmitoylation is important for intercompartmental transport of syntaxin 7.

S acylation of the hemagglutinin of influenza viruses: mass spectrometry reveals site-specific attachment of stearic acid to a transmembrane cysteine

S acylation of cysteines located in the transmembrane and/or cytoplasmic region of influenza virus hemagglutinins (HA) contributes to the membrane fusion and assembly of virions. Our results from using mass spectrometry (MS) show that influenza B virus HA possessing two cytoplasmic cysteines contains palmitate, whereas HA-esterase-fusion glycoprotein of influenza C virus having one transmembrane cysteine is stearoylated. HAs of influenza A virus having one transmembrane and two cytoplasmic cysteines contain both palmitate and stearate. MS analysis of recombinant viruses with deletions of individual cysteines, as well as tandem-MS sequencing, revealed the surprising result that stearate is exclusively attached to the cysteine positioned in the transmembrane region of HA.

The nutritional flavanone naringenin triggers antiestrogenic effects by regulating estrogen receptor alpha-palmitoylation

Naringenin (Nar) is a component of fruits and vegetables associated with healthful benefits, such as in osteoporosis, cancer, and cardiovascular diseases. These protective effects have been linked with Nar antiestrogenic as well as estrogenic activities. Previous studies indicate that Nar impaired estrogen receptor (ER) alpha signaling by interfering with ERalpha-mediated activation of ERK and phosphoinositide 3-kinase signaling pathways in the absence of effects at the transcriptional level. The present studies evaluated the hypothesis that these Nar antagonistic effects occur at the level of the plasma membrane. Our results indicate that Nar induces ERalpha depalmitoylation faster than 17beta-estradiol, which results in receptor rapid dissociation from caveolin-1. Furthermore, Nar impedes ERalpha to bind adaptor (modulator of nongenomic actions of the ER) and signaling (c-Src) proteins involved in the activation of the mitogenic signaling cascades (i.e. ERK and phosphoinositide 3-kinase). On the other hand, Nar induces the ER-dependent, but palmitoylation-independent, activation of p38 kinase, which in turn is responsible for Nar-mediated antiproliferative effects in cancer cells. Altogether, these data highlight new ER-dependent mechanisms on the root of antiproliferative and antiestrogenic effects of Nar. Moreover, the different modulation of ERalpha palmitoylation exerted by different ligands represents a pivotal mechanism that drives cancer cell to proliferation or apoptosis.

Importance of conserved cysteine residues in the coronavirus envelope protein

Coronavirus envelope (E) proteins play an important, not fully understood role(s) in the virus life cycle. All E proteins have conserved cysteine residues located on the carboxy side of the long hydrophobic domain, suggesting functional significance. In this study, we confirmed that mouse hepatitis coronavirus A59 E protein is palmitoylated. To understand the role of the conserved residues and the necessity of palmitoylation, three cysteines at positions 40, 44, and 47 were changed singly and in various combinations to alanine. Double- and triple-mutant E proteins resulted in decreased virus-like particle output when coexpressed with the membrane (M) protein. Mutant E proteins were also studied in the context of a full-length infectious clone. Single-substitution viruses exhibited growth characteristics virtually identical to those of the wild-type virus, while the double-substitution mutations gave rise to viruses with less robust growth phenotypes indicated by smaller plaques and decreased virus yields. In contrast, replacement of all three cysteines resulted in crippled virus with significantly reduced yields. Triple-mutant viruses did not exhibit impairment in entry. Mutant E proteins localized properly in infected cells. A comparison of intracellular and extracellular virus yields suggested that release is only slightly impaired. E protein lacking all three cysteines exhibited an increased rate of degradation compared to that of the wild-type protein, suggesting that palmitoylation is important for the stability of the protein. Altogether, the results indicate that the conserved cysteines and presumably palmitoylation are functionally important for virus production.

Palmitoylation: policing protein stability and traffic

Palmitate modifies both peripheral and integral membrane proteins and its addition can be permanent or transient, which makes it unique among the lipid modifications of proteins. The presence of palmitate on a protein affects how the protein interacts with lipids and proteins in a membrane compartment, and the reversibility of palmitoylation allows different modes of trafficking between membrane compartments. Here, we review recent studies that have provided insights into the mechanisms that mediate the functional consequences of this versatile modification.

Distinct subcellular localization for constitutive and agonist-modulated palmitoylation of the human delta opioid receptor

Protein palmitoylation is a reversible lipid modification that plays important roles for many proteins involved in signal transduction, but relatively little is known about the regulation of this modification and the cellular location where it occurs. We demonstrate that the human delta opioid receptor is palmitoylated at two distinct cellular locations in human embryonic kidney 293 cells and undergoes dynamic regulation at one of these sites. Although palmitoylation could be readily observed for the mature receptor (Mr 55,000), [3H]palmitate incorporation into the receptor precursor (Mr 45,000) could be detected only following transport blockade with brefeldin A, nocodazole, and monensin, indicating that the modification occurs initially during or shortly after export from the endoplasmic reticulum. Blocking of palmitoylation with 2-bromopalmitate inhibited receptor cell surface expression, indicating that it is needed for efficient intracellular transport. However, cell surface biotinylation experiments showed that receptors can also be palmitoylated once they have reached the plasma membrane. At this location, palmitoylation is regulated in a receptor activation-dependent manner, as was indicated by the opioid agonist-promoted increase in the turnover of receptor-bound palmitate. This agonist-mediated effect did not require receptor-G protein coupling and occurred at the cell surface without the need for internalization or recycling. The activation-dependent modulation of receptor palmitoylation may thus contribute to the regulation of receptor function at the plasma membrane.

Biochemical and functional characterization of the membrane association and membrane permeabilizing activity of the severe acute respiratory syndrome coronavirus envelope protein

A diverse group of cytolytic animal viruses encodes small, hydrophobic proteins to modify host cell membrane permeability to ions and small molecules during their infection cycles. In this study, we show that expression of the SARS-CoV E protein in mammalian cells alters the membrane permeability of these cells. Immunofluorescent staining and cell fractionation studies demonstrate that this protein is an integral membrane protein. It is mainly localized to the ER and the Golgi apparatus. The protein can be translocated to the cell surface and is partially associated with lipid rafts. Further biochemical characterization of the protein reveals that it is posttranslationally modified by palmitoylation on all three cysteine residues. Systematic mutagenesis studies confirm that the membrane permeabilizing activity of the SARS-CoV E protein is associated with its transmembrane domain.

The palmitoyltransferase of the cation-dependent mannose 6-phosphate receptor cycles between the plasma membrane and endosomes

The cation-dependent mannose 6-phosphate receptor (CD-MPR) mediates the transport of lysosomal enzymes from the trans-Golgi network to endosomes. Evasion of lysosomal degradation of the CD-MPR requires reversible palmitoylation of a cysteine residue in its cytoplasmic tail. Because palmitoylation is reversible and essential for correct trafficking, it presents a potential regulatory mechanism for the sorting signals within the cytoplasmic domain of the CD-MPR. Characterization of the palmitoylation performing an in vitro palmitoylation assay by using purified full-length CD-MPR revealed that palmitoylation of the CD-MPR occurs enzymatically by a membrane-bound palmitoyltransferase. In addition, analysis of the localization revealed that the palmitoyltransferase cycles between endosomes and the plasma membrane. This was identified by testing fractions from HeLa cell homogenate separated on a density gradient in the in vitro palmitoylation assay and further confirmed by in vivo labeling experiments by using different treatments to block specific protein trafficking steps within the cell. We identified a novel palmitoyltransferase activity in the endocytic pathway responsible for palmitoylation of the CD-MPR. The localization of the palmitoyltransferase not only fulfills the requirement of our hypothesis to be a regulator of the intracellular trafficking of the CD-MPR but also may affect the sorting/activity of other receptors cycling through endosomes.

Intracellular trafficking of a palmitoylated membrane-associated protein component of enveloped vaccinia virus

The F13L protein of vaccinia virus, an essential and abundant palmitoylated peripheral membrane component of intra- and extracellular enveloped virions, associates with Golgi, endosomal, and plasma membranes in the presence or absence of other viral proteins. In the present study, the trafficking of a fully functional F13L-green fluorescent protein (GFP) chimera in transfected and productively infected cells was analyzed using specific markers and inhibitors. We found that Sar1(H79G), a trans-dominant-negative protein inhibitor of cargo transport from the endoplasmic reticulum, had no apparent effect on the intracellular distribution of F13L-GFP, suggesting that the initial membrane localization occurs at a downstream compartment of the secretory pathway. Recycling of F13L-GFP from the plasma membrane was demonstrated by partial colocalization with FM4-64, a fluorescent membrane marker of endocytosis. Punctate F13L-GFP fluorescence overlapped with clathrin and Texas red-conjugated transferrin, suggesting that endocytosis occurred via clathrin-coated pits. The inhibitory effects of chlorpromazine and trans-dominant-negative forms of dynamin and Eps15 protein on the recycling of F13L-GFP provided further evidence for clathrin-mediated endocytosis. In addition, the F13L protein was specifically coimmunoprecipitated with alpha-adaptin, a component of the AP-2 complex that interacts with Eps15. Nocodazole and wortmannin perturbed the intracellular trafficking of F13L-GFP, consistent with its entry into late and early endosomes through the secretory and endocytic pathways, respectively. The recycling pathway described here provides a mechanism for the reutilization of the F13L protein following its deposition in the plasma membrane during the exocytosis of enveloped virions.

Palmitoylation sites and processing of synaptotagmin I, the putative calcium sensor for neurosecretion

Synaptotagmin I, the calcium sensor for neurotransmission, is palmitoylated. We have identified the palmitoylation sites as five cysteine residues located between the transmembrane and cytoplasmic regions. In contrast to wild-type synaptotagmin, the non-acylated mutant is not converted to the endoglycosidase-H-resistant form after expression in CV-1 cells. This indicates a block in transport through the Golgi complex. However, when expressed in PC-12 and RBL cells non-acylated synaptotagmin is targeted to the plasma membrane and to secretory granules. No significant cleavage of [(3)H]palmitate from synaptotagmin was observed in pulse-chase experiments. This indicates that the majority of fatty acids are structural rather than dynamic components.

Palmitoylation and processing of the lipopeptide surfactant protein C

Pulmonary surfactant, a mixture of lipids and proteins, reduces the surface tension at the air-water interface of the lung alveoli by forming a surface active film. This way, it prevents alveoli from collapsing and facilitates the work of breathing. Surfactant protein C (SP-C) plays an important role in this surfactant function. SP-C is expressed as a proprotein (proSP-C), which becomes posttranslationally modified with palmitate and undergoes several rounds of proteolytical cleavage. This results in the formation of mature SP-C, which is stored in the lamellar bodies (LB) and finally secreted into the alveolar space. Recently, new insights into the sorting, processing and palmitoylation of proSP-C have been obtained by mutagenesis studies. Moreover, reports on the association of development of lung disease with SP-C deficiency have led to new insights into the importance of SP-C for proper surfactant homeostasis. In addition, new information has become available on the role of the palmitoyl chains of SP-C in surface activity. This review summarizes these recent developments in the processing and function of SP-C, with particular emphasis on the signals for and role of palmitoylation of SP-C.

Fatty acid modification of the coxsackievirus and adenovirus receptor

Membrane-proximal cysteines 259 and 260 in the cytoplasmic tail of the coxsackievirus and adenovirus receptor (CAR) are known to be essential for the tumor suppression activity of CAR. We demonstrate that these residues provide an S-acylation motif for modification of CAR with the fatty acid palmitate. Substitution of alanine for cysteines 259 and 260 results in the additional localization of CAR in perinuclear compartments with no effect on the efficiency of adenovirus infection. The results indicate that palmitylation is important for stable plasma membrane expression and biological activity of CAR but is not critical for adenovirus receptor performance.

Enzymatic depalmitoylation of viral glycoproteins with acyl-protein thioesterase 1 in vitro

Many glycoproteins of enveloped viruses as well as cellular proteins are covalently modified with fatty acids. Palmitoylation is often reversible, but the enzymology of this hydrophobic protein modification is not understood. Recently a cytosolic enzyme designated acyl-protein thioesterase 1 (APT1) was purified, which depalmitoylates several cellular proteins. Since hitherto no transmembrane proteins have been tested as substrates for APT1 we have investigated whether palmitoylated viral membrane glycoproteins can be deacylated by use of this enzyme. Recombinant APT1 was purified from Escherichia coli, and depalmitoylation of [3H]palmitate-labeled glycoproteins present in virus particles was measured by SDS-PAGE, fluorography, and scanning densitometry. We find that APT1 causes rapid and almost complete cleavage of fatty acids from the G-protein of vesicular stomatitis virus, hemagglutinin proteins of influenza A and C virus, and E2 of Semliki Forest virus (SFV). In contrast, E1 of SFV is largely resistant against APT1 activity. This substrate specificity of APT1 was also observed using microsomes prepared from SFV-infected cells. Our data emphasize the potential of APT1 as a tool for functional analysis of protein-bound fatty acids.

The juxtamembrane lysine and arginine residues of surfactant protein C precursor influence palmitoylation via effects on trafficking

Surfactant protein (SP)-C propeptide (proSP-C) becomes palmitoylated on cysteines 5 and 6 before mature SP-C is formed by several proteolytic steps. To study the structural requirements for the palmitoylation of proSP-C, his-tagged human proSP-C (his-proSP-C) and his-proSP-C mutants were expressed in Chinese hamster ovary cells and analyzed by metabolic labeling with [(3)H]palmitate and immunocytochemistry. Substitution of cysteines 5 and 6 by serines showed that these were the only two cysteine residues palmitoylated in his-proSP-C. Substitution of the juxtamembrane basic residues lysine and arginine by uncharged glutamines led to a large decrease in palmitoylation level of proSP-C. The addition of brefeldin A nearly abolished this decrease for the lysine and double mutant; the palmitoylation of the arginine mutant increased also, but not to wild-type (WT) levels. Fluorescence immunocytochemistry showed that WT proSP-C was localized in punctate vesicles throughout the cell, whereas the mutant lacking the juxtamembrane positive charges was found more perinuclear, probably in the endoplasmic reticulum (ER). This indicates that the two basic juxtamembrane residues influence palmitoylation of proSP-C by preventing the transport of proSP-C out of the ER, implying that proSP-C becomes palmitoylated normally in a compartment distal to the ER.

Synaptobrevin 2 is palmitoylated in synaptic vesicles prepared from adult, but not from embryonic brain

Neuronal SNARE-proteins such as synaptobrevin, SNAP 25, and synaptotagmin are key players during neurosecretion. So far palmitoylation of SNAP-25 and synaptotagmin 1 have been described in vivo. Here we have analyzed palmitoylation of the SNARE-proteins synaptobrevin 2 and synaptotagmin in vitro using synaptosomal and synaptic vesicle preparations from rat brain. Labeling of synaptic vesicles prepared from adult brain with [3H]palmitate revealed synaptobrevin 2 besides synaptotagmin 1 as major palmitoylated proteins. [3H]Palmitoylation of synaptobrevin 2 was resistant to chloroform/methanol extraction, but sensitive to reducing agents indicating a covalent fatty acid bond to cysteine residues. Palmitoylation of synaptobrevin 2 was also confirmed using endogenous synaptobrevin 2 present in PC-12 cells and synaptobrevin 2 expressed with a vacciniavirus system in Cos cells. In contrast to the situation seen with membrane preparations obtained from adult brain, synaptic vesicles prepared from embryonic rat brain did not support [3H]palmitoylation of synaptobrevin and synaptotagmin. These results suggest, that both synaptobrevin 2 and synaptotagmin were efficiently palmitoylated from mature synaptic vesicles. However, at least one component of the palmitoylation machinery is developmentally upregulated.

Palmitoylation of the intracytoplasmic R peptide of the transmembrane envelope protein in Moloney murine leukemia virus

Previously it was reported that the 16-amino-acid (aa) C-terminal cytoplasmic tail of Moloney murine leukemia virus (MoMLV) transmembrane protein Pr15E is cleaved off during virus synthesis, yielding the mature, fusion active transmembrane protein p15E and the 16-aa peptide (R peptide or p2E). It remains to be elucidated how the R peptide impairs fusion activity of the uncleaved Pr15E. The R peptide from MoMLV was analyzed by Tricine-sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunostained with antiserum against the synthetic 16-aa R peptide. The R peptide resolved with an apparent molecular mass of 7 kDa and not the 4 kDa seen with the corresponding synthetic peptide. The 7-kDa R peptide was found to be membrane bound in MoMLV-infected NIH 3T3 cells, showing that cleavage of the 7-kDa R-peptide tail must occur before or during budding of progeny virions, in which only small amounts of the 7-kDa R peptide were found. The 7-kDa R peptide was palmitoylated since it could be labeled with [(3)H]palmitic acid, which explains its membrane association, slower migration on gels, and high sensitivity in immunoblotting. The present results are in contrast to previous findings showing equimolar amounts of R peptide and p15E in virions. The discrepancy, however, can be explained by the presence of nonpalmitoylated R peptide in virions, which were poorly detected by immunoblotting. A mechanistic model is proposed. The uncleaved R peptide can, due to its lipid modification, control the conformation of the ectodomain of the transmembrane protein and thereby govern membrane fusion.

Trafficking of an acylated cytosolic protein: newly synthesized p56(lck) travels to the plasma membrane via the exocytic pathway

The Src-related tyrosine kinase p56(lck) (Lck) is primarily expressed in T lymphocytes where it localizes to the cytosolic side of the plasma membrane and associates with the T cell coreceptors CD4 and CD8. As a model for acylated proteins, we studied how this localization of Lck is achieved. We followed newly synthesized Lck by pulse-chase analysis and found that membrane association of Lck starts soon after synthesis, but is not complete until at least 30-45 min later. Membrane-binding kinetics are similar in CD4/CD8-positive and CD4/CD8-negative cells. In CD4-positive T cells, the interaction with CD4 rapidly follows membrane association of Lck. Studying the route via which Lck travels from its site of synthesis to the plasma membrane, we found that: CD4 associates with Lck within 10 min of synthesis, long before CD4 has reached the plasma membrane; Lck associates with intracellular CD4 early after synthesis and with cell surface CD4 at later times; and transport of CD4-bound Lck to the plasma membrane is inhibited by Brefeldin A. These data indicate that the initial association of newly synthesized Lck with CD4, and therefore with membranes, occurs on intracellular membranes of the exocytic pathway. From this location Lck is transported to the plasma membrane.

Adenovirus death protein, a transmembrane protein encoded in the E3 region, is palmitoylated at the cytoplasmic tail

The 11.6-K protein of human adenovirus 2 (Ad2), which was recently renamed as adenovirus death protein (ADP), is a type III membrane glycoprotein that ultimately localizes to the nuclear membrane. ADP is encoded in the E3 transcription unit of Ad2 and migrates as a set of multiple bands in SDS-PAGE with three major forms. The corresponding gene product of adenovirus 5 (Ad5) has a slightly lower molecular weight and shows the same pattern in SDS-PAGE. We report here the covalent attachment of fatty acids to cysteine residues of ADP. In the case of Ad5-ADP all three major forms of this protein can be labeled by [3H]palmitic acid, but not by [3H]myristic acid, whereas only two [3H]palmitic acid-labeled Ad2-ADP species could be detected. The label is sensitive to treatment with 1 M hydroxylamine at pH 7 and with 20% beta-mercaptoethanol indicating that the fatty acids are linked via a thioester bond. By thin layer chromatography, the vast majority of the incorporated label was identified as palmitic acid. Two cysteine residues at the boundary between transmembrane domain and cytoplasmic tail which could serve as acceptor sites were mutated to alanine residues by site-directed mutagenesis of the cloned Ad5-ADP gene. Expression of wild-type Ad5-ADP and the resulting mutants was performed in HeLa cells using the vaccinia virus T7 expression system. As demonstrated by labeling with [3H]palmitic acid, only the mutants with one remaining cysteine residue in the cytoplasmic tail were able to incorporate [3H]palmitic acid, indicating that either could serve as acceptor site. In contrast the double cysteine mutant could not be labeled by [3H]palmitic acid, clearly demonstrating that cysteines 53 and 54 are required for palmitoylation and probably represent the palmitoylation sites in Ad5-ADP.

Cytoplasmic tail length influences fatty acid selection for acylation of viral glycoproteins

We report remarkable differences in the fatty acid content of thioester-type acylated glycoproteins of enveloped viruses from mammalian cells. The E2 glycoprotein of Semliki Forest virus contains mainly palmitic acid like most other palmitoylated proteins analysed so far. However, the other glycoprotein (E1) of the same virus, as well as the HEF (haemagglutinin esterase fusion) glycoprotein of influenza C virus, are unique in this respect because they are acylated primarily with stearic acid. Comparative radiolabelling of uninfected cells with different fatty acids suggests that stearate may also be the prevailing fatty acid in some cellular acylproteins. To look for further differences between palmitoylated and stearoylated glycoproteins we characterized stearoylation in more detail. We identified the acylation site of HEF as a cysteine residue located at the boundary between the transmembrane region and the cytoplasmic tail. The attachment of stearate to HEF and E1 occurs post-translationally in a pre-Golgi compartment. Thus, stearoylated and palmitoylated proteins cannot be discriminated on the basis of the fatty acid linkage site or the intracellular compartment, where acylation occurs. However, stearoylated acylproteins contain a very short, positively charged cytoplasmic tail, whereas in palmitoylated proteins this molecular region is longer. Replacing the short cytoplasmic tail of stearoylated HEF with the long influenza A virus haemagglutinin (HA) tail in an HEF-HA chimera, and subsequent vaccinia T7 expression in CV-1 cells, yielded proteins with largely palmitic acid bound. The reverse chimera, HA-HEF with a short cytoplasmic tail was not fatty acylated at all during expression, indicating that conformational or topological constraints control fatty acid transfer.

Conformation of the acylation site of palmitoylgramicidin in lipid bilayers of dimyristoylphosphatidylcholine

Gramicidin A(gA) can be palmitoylated by means of an ester linkage to the OH group of the terminal ethanolamine that sits at the membrane-water interface in the functional gA channel. We have investigated palmitoyl-gA as a model transmembrane acylprotein. Ethanolamine-d(4) (NH(2)CD(2)CD(2)OH) was incorporated into gA by total synthesis, and a portion of the labeled gA was palmitoylated. Solid-state (2)H-NMR spectra of acyl- and nonacyl-gA in hydrated dimyristoylphosphatidylcholine (DMPC) bilayers were compared. The spectra for both oriented and nonoriented samples at 4 and at 40 degrees C indicate that the ethanolamine of gA is highly mobile prior to acylation, but essentially immobile after palmitoylation. The (2)H quadrupolar splittings allow the conformation of the ethanolamine group in acyl-gA to be determined. By combining our data with the previously determined quadrupolar splittings for deuterium labels on the palmitoyl chain [Vogt, T.C.B., Killian, J.A., & de Kruijff, B. (1994) Biochemistry 33, 2063-2070], we also propose a model for the acyl chain. The ethanolamine group rotates over Leu(10) and toward the outside of the gA channel's cylinder upon acylation, so that the attached acyl chain passes between the side chains of Trp(9) and Leu(10). To accommodate the acyl chain, the six-membered portion of the indole ring of Trp(9) is displaced by about 0.9 angstroms, by means of 1-2 degree rotations in chi(1) and chi(2).

Palmitoylation of endogenous and viral acceptor proteins by fatty acyltransferase (PAT) present in erythrocyte ghosts and in placental membranes

Human erythrocyte ghosts were shown to have palmitoylating activity which acylates both endogenous ghost polypeptides and exogenous proteins derived from Semliki Forest virus (SFV). Cell-free fatty acid transfer from [3H]palmitoyl-CoA to endogenous protein was greatly enhanced in ghosts when pre-existing fatty acids linked to the endogenous acyl proteins were removed by hydroxylamine treatment prior to the transfer reaction. In contrast to erythrocyte acyl proteins acceptor proteins present in human placental membranes were palmitoylated in vitro to a similar extent with or without prior deacylation by hydroxylamine treatment. This indicates the presence of large pools of non-acylated proteins in placenta and small pools in erythrocytes. In testing for the protein substrate specificity of the palmitoyl transferase (PAT) present in ghosts we found that the SFV acceptor proteins, which are totally unrelated to erythrocytes, competed with the palmitoylation of endogenous ghost protein acceptors. This palmitoylating enzyme is inhibited by Cibacron Blue, SDS, and heat treatment, but stimulated in the presence of low concentrations of mild detergent (TX-100). Since PAT operating at the surface membrane of red blood cells has properties very similar to those of PAT present in human placental microsomes [1], we suggest that only one type of PAT may transfer fatty acids to various acylproteins that occur at multiple locations in different tissues [2].

Determination of the structural requirements for palmitoylation of p63

Palmitoylation of p63, a type II membrane protein localized in the endoplasmic reticulum, is induced in a reversible manner by the drug brefeldin A. To study the requirements for palmitoylation, mutant forms of p63 were expressed in COS cells and analyzed by metabolic labeling with [3H]palmitate, immunoprecipitation, and SDS-polyacrylamide gel electrophoresis. By investigating deletion and point mutations, Cys100 in the 106-amino acid cytoplasmic tail of p63 has been identified as the site of acylation. Site-directed mutagenesis of residues 99-105 together with cytoplasmic tail truncation mutants showed that the amino acids surrounding Cys100 are not critical for palmitoylation of this residue. Analysis of a chimeric construct between p63 and the plasma membrane protein dipeptidylpeptidase IV further revealed that p63 palmitoylation is not dependent on its transmembrane domain. In contrast, the six-amino acid distance between the end of the predicted transmembrane domain and the palmitoylation site was found to be essential for proper acylation of p63.