S-acylation of the Insulin-Responsive Aminopeptidase (IRAP): Quantitative analysis and Identification of Modified Cysteines

Smad7 palmitoylation by the S-acyltransferase zDHHC17 enhances its inhibitory effect on TGF-β/Smad signaling

Intracellular signaling by the pleiotropic cytokine transforming growth factor-β (TGF-β) is inhibited by Smad7 in a feedback control mechanism. The activity of Smad7 is tightly regulated by multiple post-translational modifications. Using resin-assisted capture and metabolic labeling methods, we show here that Smad7 is S-palmitoylated in mammary epithelial cell models that are widely studied because of their strong responses to TGF-β and their biological relevance to mammary development and tumor progression. S-palmitoylation of Smad7 is mediated by zDHHC17, a member of a family of 23 S-acyltransferase enzymes. Moreover, we identified four cysteine residues (Cys202, Cys225, Cys415, and Cys417) in Smad7 as palmitoylation acceptor sites. S-palmitoylation of Smad7 on Cys415 and Cys417 promoted the translocation of Smad7 from the nucleus to the cytoplasm, enhanced the stability of the Smad7 protein, and enforced its inhibitory effect on TGF-β-induced Smad transcriptional response. Thus, our findings reveal a new post-translational modification of Smad7, and highlight an important role of S-palmitoylation to enhance inhibition of TGF-β/Smad signaling by Smad7.

The role of an amphiphilic helix and transmembrane region in the efficient acylation of the M2 protein from influenza virus

Protein palmitoylation, a cellular process occurring at the membrane-cytosol interface, is orchestrated by members of the DHHC enzyme family and plays a pivotal role in regulating various cellular functions. The M2 protein of the influenza virus, which is acylated at a membrane-near amphiphilic helix serves as a model for studying the intricate signals governing acylation and its interaction with the cognate enzyme, DHHC20. We investigate it here using both experimental and computational assays. We report that altering the biophysical properties of the amphiphilic helix, particularly by shortening or disrupting it, results in a substantial reduction in M2 palmitoylation, but does not entirely abolish the process. Intriguingly, DHHC20 exhibits an augmented affinity for some M2 mutants compared to the wildtype M2. Molecular dynamics simulations unveil interactions between amino acids of the helix and the catalytically significant DHHC and TTXE motifs of DHHC20. Our findings suggest that the binding of M2 to DHHC20, while not highly specific, is mediated by requisite contacts, possibly instigating the transfer of fatty acids. A comprehensive comprehension of protein palmitoylation mechanisms is imperative for the development of DHHC-specific inhibitors, holding promise for the treatment of diverse human diseases.

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.

Dynamics of CLIMP-63 S-acylation control ER morphology

The complex architecture of the endoplasmic reticulum (ER) comprises distinct dynamic features, many at the nanoscale, that enable the coexistence of the nuclear envelope, regions of dense sheets and a branched tubular network that spans the cytoplasm. A key player in the formation of ER sheets is cytoskeleton-linking membrane protein 63 (CLIMP-63). The mechanisms by which CLIMP-63 coordinates ER structure remain elusive. Here, we address the impact of S-acylation, a reversible post-translational lipid modification, on CLIMP-63 cellular distribution and function. Combining native mass-spectrometry, with kinetic analysis of acylation and deacylation, and data-driven mathematical modelling, we obtain in-depth understanding of the CLIMP-63 life cycle. In the ER, it assembles into trimeric units. These occasionally exit the ER to reach the plasma membrane. However, the majority undergoes S-acylation by ZDHHC6 in the ER where they further assemble into highly stable super-complexes. Using super-resolution microscopy and focused ion beam electron microscopy, we show that CLIMP-63 acylation-deacylation controls the abundance and fenestration of ER sheets. Overall, this study uncovers a dynamic lipid post-translational regulation of ER architecture.

Selenoprotein K contributes to CD36 subcellular trafficking in hepatocytes by accelerating nascent COPII vesicle formation and aggravates hepatic steatosis

SelenoproteinK (SelK), an endoplasmic reticulum (ER) - resident protein, possesses the property of mediate oxidation resistance and ER - associated protein degradation (ERAD) in several tissues. Here, we found that increased SelK markedly promotes fatty acid translocase (CD36) subcellular trafficking and aggravates lipid accumulation in hepatocytes. We demonstrated that SelK is required for the assembly of COPII vesicles and accelerates transport of palmitoylated-CD36 from the ER to Golgi, thus facilitating CD36 plasma membrane distribution both in vivo and in vitro. The mechanism is that SelK increases the stability of Sar1B and triggers CD36-containing nascent COPII vesicle formation, consequently, promotes CD36 subcellular trafficking. Furthermore, we verified that the intervention of SelK SH3 binding domain can inhibit the vesicle formation and CD36 subcellular trafficking, significantly ameliorates NAFLD in mice. Collectively, our findings disclose an unexpected role of SelK in regulating NAFLD development, suggesting that targeting the SelK of hepatocytes may be a new therapeutic strategy for the treatment of NAFLD.

DHHC17 Is a New Regulator of AMPK Signaling

The association of AMP-activated protein kinase (AMPK) with membranes plays a critical role in the regulation of AMPK activation and function. Protein lipid modification, including palmitoylation, myristoylation, and farnesyation, constitutes a crucial mechanism in the regulation of protein dynamic interactions with membranes. Among the three subunits of the AMPK heterotrimeric complex, the structural subunit AMPKβ is myristoylated and the catalytic subunit AMPKα is palmitoylated. Here, we report the characterization of AMPKα palmitoylation. We found that AMKPα was palmitoylated at Cys209 and Cys543, and this was required for AMPK activation and subcellular membrane compartmentalization. To understand the regulation of AMPKα palmitoylation, we have identified DHHC17 as a candidate palmitoyltransferase for AMPKα and found that DHHC17, by palmitoylating AMPKα, modulated AMPK membrane association and activation in response to energy stress. To determine the role of DHHC17 in cell function, we generated DHHC17 liver-specific knockout mice and found that inactivation of DHHC17 in the mouse liver impaired AMPK activation and hepatic autophagy and caused a type 2 diabetes-like syndrome. Overall, our studies demonstrate that AMPKα palmitoylation plays a critical role in AMPK activation and that DHHC17, through its modulation of AMPK signaling, constitutes a new regulator of hepatic metabolism.

Molecular Dynamics of DHHC20 Acyltransferase Suggests Principles of Lipid and Protein Substrate Selectivity

Lipid modification of viral proteins with fatty acids of different lengths (S-acylation) is crucial for virus pathogenesis. The reaction is catalyzed by members of the DHHC family and proceeds in two steps: the autoacylation is followed by the acyl chain transfer onto protein substrates. The crystal structure of human DHHC20 (hDHHC20), an enzyme involved in the acylation of S-protein of SARS-CoV-2, revealed that the acyl chain may be inserted into a hydrophobic cavity formed by four transmembrane (TM) α-helices. To test this model, we used molecular dynamics of membrane-embedded hDHHC20 and its mutants either in the absence or presence of various acyl-CoAs. We found that among a range of acyl chain lengths probed only C16 adopts a conformation suitable for hDHHC20 autoacylation. This specificity is altered if the small or bulky residues at the cavity's ceiling are exchanged, e.g., the V185G mutant obtains strong preferences for binding C18. Surprisingly, an unusual hydrophilic ridge was found in TM helix 4 of hDHHC20, and the responsive hydrophilic patch supposedly involved in association was found in the 3D model of the S-protein TM-domain trimer. Finally, the exchange of critical Thr and Ser residues in the spike led to a significant decrease in its S-acylation. Our data allow further development of peptide/lipid-based inhibitors of hDHHC20 that might impede replication of Corona- and other enveloped viruses.

S-acylation controls SARS-CoV-2 membrane lipid organization and enhances infectivity

SARS-CoV-2 virions are surrounded by a lipid bilayer that contains membrane proteins such as spike, responsible for target-cell binding and virus fusion. We found that during SARS-CoV-2 infection, spike becomes lipid modified, through the sequential action of the S-acyltransferases ZDHHC20 and 9. Particularly striking is the rapid acylation of spike on 10 cytosolic cysteines within the ER and Golgi. Using a combination of computational, lipidomics, and biochemical approaches, we show that this massive lipidation controls spike biogenesis and degradation, and drives the formation of localized ordered cholesterol and sphingolipid-rich lipid nanodomains in the early Golgi, where viral budding occurs. Finally, S-acylation of spike allows the formation of viruses with enhanced fusion capacity. Our study points toward S-acylating enzymes and lipid biosynthesis enzymes as novel therapeutic anti-viral targets.

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.

Palmitoylated acyl protein thioesterase APT2 deforms membranes to extract substrate acyl chains

Many biochemical reactions require controlled recruitment of proteins to membranes. This is largely regulated by posttranslational modifications. A frequent one is S-acylation, which consists of the addition of acyl chains and can be reversed by poorly understood acyl protein thioesterases (APTs). Using a panel of computational and experimental approaches, we dissect the mode of action of the major cellular thioesterase APT2 (LYPLA2). We show that soluble APT2 is vulnerable to proteasomal degradation, from which membrane binding protects it. Interaction with membranes requires three consecutive steps: electrostatic attraction, insertion of a hydrophobic loop and S-acylation by the palmitoyltransferases ZDHHC3 or ZDHHC7. Once bound, APT2 is predicted to deform the lipid bilayer to extract the acyl chain bound to its substrate and capture it in a hydrophobic pocket to allow hydrolysis. This molecular understanding of APT2 paves the way to understand the dynamics of APT2-mediated deacylation of substrates throughout the endomembrane system.

NS1-mediated upregulation of ZDHHC22 acyltransferase in influenza a virus infected cells

Influenza A viruses contain two S-acylated proteins, the ion channel M2 and the glycoprotein hemagglutinin (HA). Acylation of the latter is essential for virus replication. Here we analysed the expression of each of the 23 members of the family of ZDHHC acyltransferases in human airway cells, the site of virus replication. RT-PCR revealed that every ZDHHC acyltransferase (except ZDHHC19) is expressed in A549 and Calu cells. Interestingly, expression of one ZDHHC, ZDHHC22, is upregulated in virus-infected cells; this effect is more pronounced after infection with an avian compared to a human virus strain. The viral protein NS1 triggers ZDHHC22 expression in transfected cells, whereas recombinant viruses lacking a functional NS1 gene did not cause ZDHHC22 upregulation. CRISPR/Cas9 technology was then used to knock-out the ZDHHC22 gene in A549 cells. However, acylation of M2 and HA was not reduced, as analysed for intracellular HA and M2 and the stoichiometry of S-acylation of HA incorporated into virus particles did not change according to MALDI-TOF mass spectrometry analysis. Comparative mass spectrometry of palmitoylated proteins in wt and ΔZDHHC22 cells identified 25 potential substrates of ZDHHC22 which might be involved in virus replication.

Regulatory effects of protein S-acylation on insulin secretion and insulin action

Post-translational modifications (PTMs) such as phosphorylation and ubiquitination are well-studied events with a recognized importance in all aspects of cellular function. By contrast, protein S-acylation, although a widespread PTM with important functions in most physiological systems, has received far less attention. Perturbations in S-acylation are linked to various disorders, including intellectual disability, cancer and diabetes, suggesting that this less-studied modification is likely to be of considerable biological importance. As an exemplar, in this review, we focus on the newly emerging links between S-acylation and the hormone insulin. Specifically, we examine how S-acylation regulates key components of the insulin secretion and insulin response pathways. The proteins discussed highlight the diverse array of proteins that are modified by S-acylation, including channels, transporters, receptors and trafficking proteins and also illustrate the diverse effects that S-acylation has on these proteins, from membrane binding and micro-localization to regulation of protein sorting and protein interactions.

The Role of Insulin Regulated Aminopeptidase in Endocytic Trafficking and Receptor Signaling in Immune Cells

Insulin regulated aminopeptidase (IRAP) is a type II transmembrane protein with broad tissue distribution initially identified as a major component of Glut4 storage vesicles (GSV) in adipocytes. Despite its almost ubiquitous expression, IRAP had been extensively studied mainly in insulin responsive cells, such as adipocytes and muscle cells. In these cells, the enzyme displays a complex intracellular trafficking pattern regulated by insulin. Early studies using fusion proteins joining the IRAP cytosolic domain to various reporter proteins, such as GFP or the transferrin receptor (TfR), showed that the complex and regulated trafficking of the protein depends on its cytosolic domain. This domain contains several motifs involved in IRAP trafficking, as demonstrated by mutagenesis studies. Also, proteomic studies and yeast two-hybrid experiments showed that the IRAP cytosolic domain engages in multiple protein interactions with cytoskeleton components and vesicular trafficking adaptors. These findings led to the hypothesis that IRAP is not only a cargo of GSV but might be a part of the sorting machinery that controls GSV dynamics. Recent work in adipocytes, immune cells, and neurons confirmed this hypothesis and demonstrated that IRAP has a dual function. Its carboxy-terminal domain located inside endosomes is responsible for the aminopeptidase activity of the enzyme, while its amino-terminal domain located in the cytosol functions as an endosomal trafficking adaptor. In this review, we recapitulate the published protein interactions of IRAP and summarize the increasing body of evidence indicating that IRAP plays a role in intracellular trafficking of several proteins. We describe the impact of IRAP deletion or depletion on endocytic trafficking and the consequences on immune cell functions. These include the ability of dendritic cells to cross-present antigens and prime adaptive immune responses, as well as the control of innate and adaptive immune receptor signaling and modulation of inflammatory responses.

IRAP-dependent endosomal T cell receptor signalling is essential for T cell responses

T cell receptor (TCR) activation is modulated by mechanisms such as TCR endocytosis, which is thought to terminate TCR signalling. Here we show that, upon internalization, TCR continues to signal from a set of specialized endosomes that are crucial for T cell functions. Mechanistically, TCR ligation leads to clathrin-mediated internalization of the TCR-CD3ζ complex, while maintaining CD3ζ signalling, in endosomal vesicles that contain the insulin responsive aminopeptidase (IRAP) and the SNARE protein Syntaxin 6. Destabilization of this compartment through IRAP deletion enhances plasma membrane expression of the TCR-CD3ζ complex, yet compromises overall CD3ζ signalling; moreover, the integrity of this compartment is also crucial for T cell activation and survival after suboptimal TCR activation, as mice engineered with a T cell-specific deletion of IRAP fail to develop efficient polyclonal anti-tumour responses. Our results thus reveal a previously unappreciated function of IRAP-dependent endosomal TCR signalling in T cell activation.

T cell receptors (TCR) are internalized when activated by their ligands. Here the authors show that the internalized TCRs are localized to endosomes expressing IRAP and Syntaxin 6 to maintain intracellular signalling capacity, whose importance is shown by the absence of efficient polyclonal anti-tumour response in mice with T-specific conditional deletion of IRAP.

S-acylation modulates the function of the apical sodium-dependent bile acid transporter in human cells

The ileal apical sodium-dependent bile acid transporter (ASBT) is crucial for the enterohepatic circulation of bile acids. ASBT function is rapidly regulated by several posttranslational modifications. One reversible posttranslational modification is S-acylation, involving the covalent attachment of fatty acids to cysteine residues in proteins. However, whether S-acylation affects ASBT function and membrane expression has not been determined. Using the acyl resin-assisted capture method, we found that the majority of ASBT (∼80%) was S-acylated in ileal brush border membrane vesicles from human organ donors, as well as in HEK293 cells stably transfected with ASBT (2BT cells). Metabolic labeling with alkyne-palmitic acid (100 μm for 15 h) also showed that ASBT is S-acylated in 2BT cells. Incubation with the acyltransferase inhibitor 2-bromopalmitate (25 μm for 15 h) significantly reduced ASBT S-acylation, function, and levels on the plasma membrane. Treatment of 2BT cells with saturated palmitic acid (100 μm for 15 h) increased ASBT function, whereas treatment with unsaturated oleic acid significantly reduced ASBT function. Metabolic labeling with alkyne-oleic acid (100 μm for 15 h) revealed that oleic acid attaches to ASBT, suggesting that unsaturated fatty acids may decrease ASBT's function via a direct covalent interaction with ASBT. We also identified Cys-314 as a potential S-acylation site. In conclusion, these results provide evidence that S-acylation is involved in the modulation of ASBT function. These findings underscore the potential for unsaturated fatty acids to reduce ASBT function, which may be useful in disorders in which bile acid toxicity is implicated.

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.

Detection of Heterogeneous Protein S-Acylation in Cells

The use of synthetically synthesized azide and alkyne fatty acid analogs coupled with bioorthogonal Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction-based detection methods to study protein S-acylation reactions has replaced the traditional method of using in vivo metabolic radiolabeling with tritiated palmitic acid and has greatly facilitated our understanding of this essential cellular process. Here, we describe the chemical synthesis of myristic (C:14), palmitic (C16:0), and stearic (C18:0) acid-azide probes and detail how they may be utilized as chemical reporters for the analysis of S-acylation of exogenously expressed proteins in cells.

Ethanolic Extract of Folium Sennae Mediates the Glucose Uptake of L6 Cells by GLUT4 and Ca2

In today’s world, diabetes mellitus (DM) is on the rise, especially type 2 diabetes mellitus (T2DM), which is characterized by insulin resistance. T2DM has high morbidity, and therapies with natural products have attracted much attention in the recent past. In this paper, we aimed to study the hypoglycemic effect and the mechanism of an ethanolic extract of Folium Sennae (FSE) on L6 cells. The glucose uptake of L6 cells was investigated using a glucose assay kit. We studied glucose transporter 4 (GLUT4) expression and AMP-activated protein kinase (AMPK), protein kinase B (PKB/Akt), and protein kinase C (PKC) phosphorylation levels using western blot analysis. GLUT4 trafficking and intracellular Ca²⁺ levels were monitored by laser confocal microscopy in L6 cells stably expressing IRAP-mOrange. GLUT4 fusion with plasma membrane (PM) was observed by myc-GLUT4-mOrange. FSE stimulated glucose uptake; GLUT4 expression and translocation; PM fusion; intracellular Ca²⁺ elevation; and the phosphorylation of AMPK, Akt, and PKC in L6 cells. GLUT4 translocation was weakened by the AMPK inhibitor compound C, PI3K inhibitor Wortmannin, PKC inhibitor Gö6983, G protein inhibitor PTX/Gallein, and PLC inhibitor U73122. Similarly, in addition to PTX/Gallein and U73122, the IP3R inhibitor 2-APB and a 0 mM Ca²⁺-EGTA solution partially inhibited the elevation of intracellular Ca²⁺ levels. BAPTA-AM had a significant inhibitory effect on FSE-mediated GLUT4 activities. In summary, FSE regulates GLUT4 expression and translocation by activating the AMPK, PI3K/Akt, and G protein–PLC–PKC pathways. FSE causes increasing Ca²⁺ concentration to complete the fusion of GLUT4 vesicles with PM, allowing glucose uptake. Therefore, FSE may be a potential drug for improving T2DM.

Cyclophilin A enables specific HIV-1 Tat palmitoylation and accumulation in uninfected cells

Most HIV-1 Tat is unconventionally secreted by infected cells following Tat interaction with phosphatidylinositol (4,5) bisphosphate (PI(4,5)P₂) at the plasma membrane. Extracellular Tat is endocytosed by uninfected cells before escaping from endosomes to reach the cytosol and bind PI(4,5)P₂. It is not clear whether and how incoming Tat concentrates in uninfected cells. Here we show that, in uninfected cells, the S-acyl transferase DHHC-20 together with the prolylisomerases cyclophilin A (CypA) and FKBP12 palmitoylate Tat on Cys31 thereby increasing Tat affinity for PI(4,5)P₂. In infected cells, CypA is bound by HIV-1 Gag, resulting in its encapsidation and CypA depletion from cells. Because of the lack of this essential cofactor, Tat is not palmitoylated in infected cells but strongly secreted. Hence, Tat palmitoylation specifically takes place in uninfected cells. Moreover, palmitoylation is required for Tat to accumulate at the plasma membrane and affect PI(4,5)P₂-dependent membrane traffic such as phagocytosis and neurosecretion.

It is not clear whether and how incoming HIV-1 Tat accumulates in uninfected cells. Here, Chopard et al. show that, in uninfected cells, incoming Tat is palmitoylated on Cys31 by DHHC-20, which increases its affinity for PI(4,5)P₂ and results in its accumulation at the plasma membrane.

Identification and dynamics of the human ZDHHC16-ZDHHC6 palmitoylation cascade

S-Palmitoylation is the only reversible post-translational lipid modification. Knowledge about the DHHC palmitoyltransferase family is still limited. Here we show that human ZDHHC6, which modifies key proteins of the endoplasmic reticulum, is controlled by an upstream palmitoyltransferase, ZDHHC16, revealing the first palmitoylation cascade. The combination of site specific mutagenesis of the three ZDHHC6 palmitoylation sites, experimental determination of kinetic parameters and data-driven mathematical modelling allowed us to obtain detailed information on the eight differentially palmitoylated ZDHHC6 species. We found that species rapidly interconvert through the action of ZDHHC16 and the Acyl Protein Thioesterase APT2, that each species varies in terms of turnover rate and activity, altogether allowing the cell to robustly tune its ZDHHC6 activity.