NLRP3 Cys126 palmitoylation by ZDHHC7 promotes inflammasome activation

Nucleotide oligomerization domain (NOD)-like receptor protein 3 (NLRP3) inflammasome hyperactivation contributes to many human chronic inflammatory diseases, and understanding how NLRP3 inflammasome is regulated can provide strategies to treat inflammatory diseases. Here, we demonstrate that NLRP3 Cys126 is palmitoylated by zinc finger DHHC-type palmitoyl transferase 7 (ZDHHC7), which is critical for NLRP3-mediated inflammasome activation. Perturbing NLRP3 Cys126 palmitoylation by ZDHHC7 knockout, pharmacological inhibition, or modification site mutation diminishes NLRP3 activation in macrophages. Furthermore, Cys126 palmitoylation is vital for inflammasome activation in vivo. Mechanistically, ZDHHC7-mediated NLRP3 Cys126 palmitoylation promotes resting NLRP3 localizing on the trans-Golgi network (TGN) and activated NLRP3 on the dispersed TGN, which is indispensable for recruitment and oligomerization of the adaptor ASC (apoptosis-associated speck-like protein containing a CARD). The activation of NLRP3 by ZDHHC7 is different from the termination effect mediated by ZDHHC12, highlighting versatile regulatory roles of S-palmitoylation. Our study identifies an important regulatory mechanism of NLRP3 activation that suggests targeting ZDHHC7 or the NLRP3 Cys126 residue as a potential therapeutic strategy to treat NLRP3-related human disorders.

GCP16 stabilizes the DHHC9 subfamily of protein acyltransferases through a conserved C-terminal cysteine motif

Protein S-acylation is a reversible lipid post-translational modification that allows dynamic regulation of processes such as protein stability, membrane association, and localization. Palmitoyltransferase ZDHHC9 (DHHC9) is one of the 23 human DHHC acyltransferases that catalyze protein S-acylation. Dysregulation of DHHC9 is associated with X-linked intellectual disability and increased epilepsy risk. Interestingly, activation of DHHC9 requires an accessory protein-GCP16. However, the exact role of GCP16 and the prevalence of a requirement for accessory proteins among other DHHC proteins remain unclear. Here, we report that one role of GCP16 is to stabilize DHHC9 by preventing its aggregation through formation of a protein complex. Using a combination of size-exclusion chromatography and palmitoyl acyltransferase assays, we demonstrate that only properly folded DHHC9-GCP16 complex is enzymatically active in vitro. Additionally, the ZDHHC9 mutations linked to X-linked intellectual disability result in reduced protein stability and DHHC9-GCP16 complex formation. Notably, we discovered that the C-terminal cysteine motif (CCM) that is conserved among the DHHC9 subfamily (DHHC14, -18, -5, and -8) is required for DHHC9 and GCP16 complex formation and activity in vitro. Co-expression of GCP16 with DHHCs containing the CCM improves DHHC protein stability. Like DHHC9, DHHC14 and DHHC18 require GCP16 for their enzymatic activity. Furthermore, GOLGA7B, an accessory protein with 75% sequence identity to GCP16, improves protein stability of DHHC5 and DHHC8, but not the other members of the DHHC9 subfamily, suggesting selectivity in accessory protein interactions. Our study supports a broader role for GCP16 and GOLGA7B in the function of human DHHCs.

DHHC9 and GCP16 constitute a human protein fatty acyltransferase with specificity for H- and N-Ras

Covalent lipid modifications mediate the membrane attachment and biological activity of Ras proteins. All Ras isoforms are farnesylated and carboxyl-methylated at the terminal cysteine; H-Ras and N-Ras are further modified by palmitoylation. Yeast Ras is palmitoylated by the DHHC cysteine-rich domain-containing protein Erf2 in a complex with Erf4. Here we report that H- and N-Ras are palmitoylated by a human protein palmitoyltransferase encoded by the ZDHHC9 and GCP16 genes. DHHC9 is an integral membrane protein that contains a DHHC cysteine-rich domain. GCP16 encodes a Golgi-localized membrane protein that has limited sequence similarity to yeast Erf4. DHHC9 and GCP16 co-distribute in the Golgi apparatus, a location consistent with the site of mammalian Ras palmitoylation in vivo. Like yeast Erf2.Erf4, DHHC9 and GCP16 form a protein complex, and DHHC9 requires GCP16 for protein fatty acyltransferase activity and protein stability. Purified DHHC9.GCP16 exhibits substrate specificity, palmitoylating H- and N-Ras but not myristoylated G (alphai1) or GAP-43, proteins with N-terminal palmitoylation motifs. Hence, DHHC9.GCP16 displays the properties of a functional human ortholog of the yeast Ras palmitoyltransferase.

Purification of recombinant G protein alpha subunits from Escherichia coli

The purification of recombinant G protein a subunits expressed in Escherichia coli (E. coli) is a convenient and inexpensive method to obtain homogeneous preparations of protein for biochemical and biophysical analyses. Wild-type and mutant forms of G alpha are easily produced for analysis of their intrinsic biochemical properties, as well as for reconstitution with receptors, effectors, regulators, and G protein beta gamma subunits. Methods are described for the expression of Gi alpha and Gs alpha proteins in E. coli. Protocols are provided for the purification of untagged G protein a subunits using conventional chromatography and histidine (His)-tagged subunits using metal chelate chromatography. Modification of G alpha with myristate can be recapitulated in E. coli by expressing N-myristoyltransferase (NMT) with its G protein substrate. Protocols for the production and purification of myristoylated G alpha are presented.

Identification of a Ras palmitoyltransferase in Saccharomyces cerevisiae

Most Ras proteins are posttranslationally modified by a palmitoyl lipid moiety through a thioester linkage. However, the mechanism by which this occurs is not known. Here, evidence is presented that the Ras2 protein of Saccharomyces cerevisiae is palmitoylated by a Ras protein acyltransferase (Ras PAT) encoded by the ERF2 and ERF4 genes. Erf2p is a 41-kDa protein localized to the membrane of the endoplasmic reticulum and contains a conserved DHHC cysteine-rich domain (DHHC-CRD). Erf2p co-purifies with Erf4p (26 kDa) when it is expressed in yeast or in Escherichia coli. The Erf2p/Erf4p complex is required for Ras PAT activity, and mutations within conserved residues (Cys(189), His(201), and Cys(203)) of the Erf2p DHHC-CRD domain abolish Ras PAT activity. Furthermore, a palmitoyl-Erf2p intermediate is detected suggesting that Erf2p is directly involved in palmitate transfer. ERF2 and ERF4 are the first genes identified that encode a palmitoyltransferase for a Ras GTPase.

Enrichment of G-protein palmitoyltransferase activity in low density membranes: in vitro reconstitution of Galphai to these domains requires palmitoyltransferase activity

Many signaling proteins are targeted to low density, sphingomyelin- and cholesterol-enriched membranes, also called lipid rafts. These domains organize receptor-mediated signaling events at the plasma membrane. Fatty acylation is one mechanism for targeting proteins to rafts. It was therefore of interest to determine if protein palmitoyltransferase activity is also present in these domains. In this study, protein palmitoyltransferase activity, assayed using G-protein alpha subunits as a substrate, was found to be highly enriched in low density membranes derived from cells that express caveolin as well as those that do not. Depletion of cellular cholesterol with the drug methyl-beta-cyclodextrin resulted in inhibition of palmitoyltransferase activity and a redistribution of the remaining activity to membranes of higher density. This effect was reversed by adding cholesterol to cyclodextrin-treated cells. When reconstituted into cell membranes, the population of purified recombinant G(alphai) that was palmitoylated was highly enriched in the low density membrane fractions, whereas the bulk unmodified G(alphai)-protein was largely excluded. This effect required palmitoyltransferase activity and was abolished if the palmitoylated cysteine was mutated. Thus, palmitoyltransferase facilitates the enrichment of fatty acylated signaling molecules in plasma membrane subdomains.

Differential effects of acyl-CoA binding protein on enzymatic and non-enzymatic thioacylation of protein and peptide substrates

Both enzymatic and autocatalytic mechanisms have been proposed to account for protein thioacylation (commonly known as palmitoylation). Acyl-CoA binding proteins (ACBP) strongly suppress non-enzymatic thioacylation of cysteinyl-containing peptides by long-chain acyl-CoAs. At physiological concentrations of ACBP, acyl-CoAs, and membrane lipids, the rate of spontaneous acylation is expected to be too slow to contribute significantly to thioacylation of signaling proteins in mammalian cells (Leventis et al., Biochemistry 36 (1997) 5546-5553). Here we characterized the effects of ACBP on enzymatic thioacylation. A protein S-acyltransferase activity previously characterized using G-protein alpha-subunits as a substrate (Dunphy et al., J. Biol. Chem., 271 (1996) 7154-7159), was capable of thioacylating short lipid-modified cysteinyl-containing peptides. The minimum requirements for substrate recognition were a free cysteine thiol adjacent to a hydrophobic lipid anchor, either myristate or farnesyl isoprenoid. PAT activity displayed specificity for the acyl donor, efficiently utilizing long-chain acyl-CoAs, but not free fatty acid or S-palmitoyl-N-acetylcysteamine. ACBP only modestly inhibited enzymatic thioacylation of a myristoylated peptide or G-protein alpha-subunits under conditions where non-enzymatic thioacylation was reduced to background. Thus, protein S-acyltransferase remains active in the presence of physiological concentrations of ACBP and acyl-CoA in vitro and is likely to represent the predominant mechanism of thioacylation in vivo.

SNAP-25 is targeted to the plasma membrane through a novel membrane-binding domain

SNAP-25, syntaxin, and synaptobrevin are SNARE proteins that mediate fusion of synaptic vesicles with the plasma membrane. Membrane attachment of syntaxin and synaptobrevin is achieved through a C-terminal hydrophobic tail, whereas SNAP-25 association with membranes appears to depend upon palmitoylation of cysteine residues located in the center of the molecule. This process requires an intact secretory pathway and is inhibited by brefeldin A. Here we show that the minimal plasma membrane-targeting domain of SNAP-25 maps to residues 85-120. This sequence is both necessary and sufficient to target a heterologous protein to the plasma membrane. Palmitoylation of this domain is sensitive to brefeldin A, suggesting that it uses the same membrane-targeting mechanism as the full-length protein. As expected, the palmitoylated cysteine cluster is present within this domain, but surprisingly, membrane anchoring requires an additional five-amino acid sequence that is highly conserved among SNAP-25 family members. Significantly, the membrane-targeting module coincides with the protease-sensitive stretch (residues 83-120) that connects the two alpha-helices that SNAP-25 contributes to the four-helix bundle of the synaptic SNARE complex. Our results demonstrate that residues 85-120 of SNAP-25 represent a protein module that is physically and functionally separable from the SNARE complex-forming domains.

G-protein palmitoyltransferase activity is enriched in plasma membranes

Heterotrimeric G proteins are covalently modified by lipids. Myristoylation of G-protein alpha subunits and prenylation of gamma subunits are stable modifications. In contrast, palmitoylation of alpha subunits is dynamic and thus has the potential for regulating protein function. Indeed, receptor activation of Gs increases palmitate turnover on the alpha subunit, presumably by stimulating deacylation. The enzymes that catalyze reversible palmitoylation of G-protein alpha subunits have not been characterized. Here we report the identification of a palmitoyl-CoA:protein S-palmitoyltransferase activity that acylates G-protein alpha subunits in vitro. Palmitoyltransferase activity is membrane-associated and requires detergent for solubilization. The preferred G-protein substrate for the enzyme activity is the alpha subunit in the context of the heterotrimer. Both myristoylated and nonmyristoylated G-protein alpha subunits are recognized as substrates. The palmitoyltransferase activity demonstrates a modest preference for palmitoyl-CoA over other fatty acyl-CoA substrates. Palmitoyltransferase activity is high in plasma membrane and present at low or undetectable levels in Golgi, endoplasmic reticulum, and mitochondria of rat liver. The subcellular localization of this enzyme activity is consistent with a role for regulated cycles of acylation and deacylation accompanying activation of G-protein signal transduction pathways.