Isolation and characterization of Golgi apparatus-specific GODZ with the DHHC zinc finger domain

Lipidation states orchestrate CLICK-III/CaMKIγ's stepwise association with Golgi and rafts-enriched membranes and specify its functional coupling to STEF-Rac1-dependent neurite extension

CLICK-III/CaMKIγ is a lipid-anchored neuronal isoform of multifunctional Ca2+/calmodulin-dependent protein kinases, which mediates BDNF-dependent dendritogenesis in cultured cortical neurons. We found that two distinct lipidation states of CaMKIγ, namely, prenylation and palmitoylation, controlled its association with detergent-resistant microdomains in the dendrites and were essential for its dendritogenic activity. However, the impact of each lipid modification on membrane targeting/trafficking and how it specifies functional coupling leading to polarized changes in neuronal morphology are not clear. Here, we show that prenylation induces membrane anchoring of CaMKIγ, permitting access to the Golgi apparatus, and a subsequent palmitoylation facilitates association with cholesterol-enriched lipid microdomains or lipid rafts, in particular at the Golgi. To specifically test the role of palmitoylated CaMKγ in neurite extension, we identified and took advantage of a cell system, PC12, which, unlike neurons, conveniently lacked CaMKIγ and was deficient in the activity-dependent release of a neuritogenic growth factor while possessing the ability to activate polarized rafts signaling for morphogenesis. This system allowed us to rigorously demonstrate that an activity-dependent, lipid rafts-restricted Rac activation leading to neuritogenesis could be functionally rescued by dually lipidated CaMKIγ expression, revealing that not only prenylation but also palmitoylation is essential for CaMKIγ to activate a compartmentalized STEF-Rac1 pathway. These results shed light on the significance of recruiting prenylated and palmitoylated CaMKIγ into the coalescing signalosomes at lipid rafts together with Rac1 and its specific GEF and STEF and forming a compartmentalized Ca2+ signaling pathway that underlies activity-dependent neuritogenesis and morphogenesis during axodendritic polarization critical for brain development and circuitogenesis.

Structural Exploration on Palmitoyltransferase DHHC3 from Homo sapiens

DHHC3 belongs to a family of DHHC palmitoyltransferase, which catalyzes the S-palmitoylation of target proteins by attaching a fatty acyl group to a cysteine. Recently, DHHC3 has been demonstrated to be a promising antitumor target in cancer therapeutics. However, the detailed structure and catalysis mechanism of DHHC3 remain elusive, considering its sequence diversity from the DHHC homologues with known crystal structures. Here, we described the expression and purification of human DHHC3 (hDHHC3) and truncated hDHHC3 with the flexible N-terminal domain (NTD) removed. Purified hDHHC3 proteins were used under various conditions for protein crystallization. LAMTOR1, one of the interacting proteins of hDHHC3 to facilitate the crystallization, was further identified by mass spectrometry and co-immunoprecipitation assay. The structural exploration using cryogenic electronic microscopy (cryo-EM) on the inactive hDHHS3 mutant showed a typical sideview of membrane proteins. These results provide a preliminary guidance for the structural determination of DHHC3.

Recent progress of palmitoyl transferase DHHC3 as a novel antitumor target

DHHC3 is a DHHC-family palmitoyl acyltransferase that is responsible for many mammalian palmitoylation events. By regulating the posttranslational modification of its specific substrates, DHHC3 has shown a strong protumor effect in various cancers. In this review, the authors introduce the research progress of DHHC3 as a new antitumor target through the expression of DHHC3 in patients with tumors, substrate proteins and potential mechanisms. Recent advances in the search for protein structures and inhibitors are also reviewed. Several design strategies to facilitate the optimization of the process of drug design based on DHHC3 are also discussed.

Golgi Apparatus Regulates Plasma Membrane Composition and Function

Golgi apparatus is the central component of the mammalian secretory pathway and it regulates the biosynthesis of the plasma membrane through three distinct but interacting processes: (a) processing of protein and lipid cargoes; (b) creation of a sharp transition in membrane lipid composition by non-vesicular transport of lipids; and (c) vesicular sorting of proteins and lipids at the trans-Golgi network to target them to appropriate compartments. We discuss the molecules involved in these processes and their importance in physiology and development. We also discuss how mutations in these molecules affect plasma membrane composition and signaling leading to genetic diseases and cancer.

A Not-So-Ancient Grease History: Click Chemistry and Protein Lipid Modifications

Protein lipid modification involves the attachment of hydrophobic groups to proteins via ester, thioester, amide, or thioether linkages. In this review, the specific click chemical reactions that have been employed to study protein lipid modification and their use for specific labeling applications are first described. This is followed by an introduction to the different types of protein lipid modifications that occur in biology. Next, the roles of click chemistry in elucidating specific biological features including the identification of lipid-modified proteins, studies of their regulation, and their role in diseases are presented. A description of the use of protein-lipid modifying enzymes for specific labeling applications including protein immobilization, fluorescent labeling, nanostructure assembly, and the construction of protein-drug conjugates is presented next. Concluding remarks and future directions are presented in the final section.

Palmitoylation-mediated synaptic regulation of AMPA receptor trafficking and function

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) is a major glutamate-gated ion channel in the brain and is important for synaptic transmission, synaptic plasticity, and learning. Palmitoylation, a post-translational modification, is a critical process regulating AMPAR trafficking, synaptic function and plasticity, and learning and memory in health and diseases. In this review, we discuss current knowledge on the palmitoylation-dependent regulation of AMPAR trafficking and functions. We focus on the palmitoylation of AMPARs and other synaptic proteins that directly or indirectly interact with AMPARs, including postsynaptic density 95, glutamate receptor-interacting protein/AMPAR-binding protein, A-kinase anchoring protein 79/150, and protein interacting with C kinase 1. Finally, we discuss what future studies should address in the field of palmitoylation-dependent AMPAR trafficking and function with regard to physiology and neurodegenerative diseases.

The Absence of DHHC3 Affects Primary and Latent Herpes Simplex Virus 1 Infection

UL20, an essential herpes simplex virus 1 (HSV-1) protein, is involved in cytoplasmic envelopment of virions and virus egress. We reported recently that UL20 can bind to a host protein encoded by the zinc finger DHHC-type containing 3 (ZDHHC3) gene (also known as Golgi-specific DHHC zinc finger protein [GODZ]). Here, we show for the first time that HSV-1 replication is compromised in murine embryonic fibroblasts (MEFs) isolated from GODZ^-/- mice. The absence of GODZ resulted in blocking palmitoylation of UL20 and altered localization and expression of UL20 and glycoprotein K (gK); the expression of gB and gC; and the localization and expression of tegument and capsid proteins within HSV-1-infected MEFs. Electron microscopy revealed that the absence of GODZ limited the maturation of virions at multiple steps and affected the localization of virus and endoplasmic reticulum morphology. Virus replication in the eyes of ocularly HSV-1-infected GODZ^-/- mice was significantly lower than in HSV-1-infected wild-type (WT) mice. The levels of UL20, gK, and gB transcripts in the corneas of HSV-1-infected GODZ^-/- mice on day 5 postinfection were markedly lower than in WT mice, whereas only UL20 transcripts were reduced in trigeminal ganglia (TG). In addition, HSV-1-infected GODZ^-/- mice showed notably lower levels of corneal scarring, and HSV-1 latency reactivation was also reduced. Thus, normal HSV-1 infectivity and viral pathogenesis are critically dependent on GODZ-mediated palmitoylation of viral UL20.IMPORTANCE HSV-1 infection is widespread. Ocular infection can cause corneal blindness; however, approximately 70 to 90% of American adults exposed to the virus show no clinical symptoms. In this study, we show for the first time that the absence of a zinc finger protein called GODZ affects primary and latent infection, as well as reactivation, in ocularly infected mice. The reduced virus infectivity is due to the absence of the GODZ interaction with HSV-1 UL20. These results strongly suggest that binding of UL20 to GODZ promotes virus infectivity in vitro and viral pathogenesis in vivo.

Binding of Herpes Simplex Virus 1 UL20 to GODZ (DHHC3) Affects Its Palmitoylation and Is Essential for Infectivity and Proper Targeting and Localization of UL20 and Glycoprotein K

Herpes simplex virus 1 (HSV-1) UL20 plays a crucial role in the envelopment of the cytoplasmic virion and its egress. It is a nonglycosylated envelope protein that is regulated as a γ1 gene. Two-hybrid and pulldown assays demonstrated that UL20, but no other HSV-1 gene-encoded proteins, binds specifically to GODZ (also known as DHHC3), a cellular Golgi apparatus-specific Asp-His-His-Cys (DHHC) zinc finger protein. A catalytically inactive dominant-negative GODZ construct significantly reduced HSV-1 replication in vitro and affected the localization of UL20 and glycoprotein K (gK) and their interactions but not glycoprotein C (gC). GODZ is involved in palmitoylation, and we found that UL20 is palmitoylated by GODZ using a GODZ dominant-negative plasmid. Blocking of palmitoylation using 2-bromopalmitate (2-BP) affected the virus titer and the interaction of UL20 and gK but did not affect the levels of these proteins. In conclusion, we have shown that binding of UL20 to GODZ in the Golgi apparatus regulates trafficking of UL20 and its subsequent effects on gK localization and virus replication. We also have demonstrated that GODZ-mediated UL20 palmitoylation is critical for UL20 membrane targeting and thus gK cell surface expression, providing new mechanistic insights into how UL20 palmitoylation regulates HSV-1 infectivity.IMPORTANCE HSV-1 UL20 is a nonglycosylated essential envelope protein that is highly conserved among herpesviruses. In this study, we show that (i) HSV-1 UL20 binds to GODZ (also known as DHHC3), a Golgi apparatus-specific Asp-His-His-Cys (DHHC) zinc finger protein; (ii) a GODZ dominant-negative mutant and an inhibitor of palmitoylation reduced HSV-1 titers and altered the localization of UL20 and glycoprotein K; and (iii) UL20 is palmitoylated by GODZ, and this UL20 palmitoylation is required for HSV-1 infectivity. Thus, blocking of the interaction of UL20 with GODZ, using a GODZ dominant-negative mutant or possibly GODZ shRNA, should be considered a potential alternative therapy in not only HSV-1 but also other conditions in which GODZ processing is an integral component of pathogenesis.

Protein lipid modifications--More than just a greasy ballast

Covalent lipid modifications of proteins are crucial for regulation of cellular plasticity, since they affect the chemical and physical properties and therefore protein activity, localization, and stability. Most recently, lipid modifications on proteins are increasingly attracting important regulatory entities in diverse signaling events and diseases. In all cases, the lipid moiety of modified proteins is essential to allow water-soluble proteins to strongly interact with membranes or to induce structural changes in proteins that are critical for elemental processes such as respiration, transport, signal transduction, and motility. Until now, roughly about ten lipid modifications on different amino acid residues are described at the UniProtKB database and even well-known modifications are underrepresented. Thus, it is of fundamental importance to develop a better understanding of this emerging and so far under-investigated type of protein modification. Therefore, this review aims to give a comprehensive and detailed overview about enzymatic and nonenzymatic lipidation events, will report their role in cellular biology, discuss their relevancy for diseases, and describe so far available bioanalytical strategies to analyze this highly challenging type of modification.

Dissociation of Golgi-associated DHHC-type Zinc Finger Protein (GODZ)- and Sertoli Cell Gene with a Zinc Finger Domain-β (SERZ-β)-mediated Palmitoylation by Loss of Function Analyses in Knock-out Mice

The γ2 subunit of GABA type A receptors (GABA_ARs) is thought to be subject to palmitoylation by both Golgi-associated DHHC-type zinc finger protein (GODZ; also known as DHHC3) and its paralog Sertoli cell gene with a zinc finger domain-β (SERZ-β; DHHC7) based on overexpression of enzymes and substrates in heterologous cells. Here we have further investigated the substrate specificity of these enzymes by characterization of GODZ and SERZ-β knock-out (KO) mice as well as double KO (DKO) neurons. Palmitoylation of γ2 and a second substrate, growth-associated protein of 43 kDa, that is independently implicated in trafficking of GABA_ARs was significantly reduced in brain of GODZ KO versus wild-type (WT) mice but unaltered in SERZ-β KO mice. Accumulation of GABA_ARs at synapses, GABAergic innervation, and synaptic function were reduced in GODZ KO and DKO neurons to a similar extent, indicating that SERZ-β does not contribute to palmitoylation or trafficking of GABA_ARs even in the absence of GODZ. Notably, these effects were seen only when mutant neurons were grown in competition with WT neurons, thereby mimicking conditions of shRNA-transfected neurons previously used to characterize GODZ. However, GABA-evoked whole-cell currents of DKO neurons and the GABA_AR cell surface expression in DKO neurons and GODZ or SERZ-β KO brain slices were unaltered, indicating that GODZ-mediated palmitoylation selectively controls the pool of receptors at synapses. The different substrate specificities of GODZ and SERZ-β in vivo were correlated with their differential localization to cis- versus trans-Golgi compartment, a mechanism that was compromised by overexpression of GODZ.

Dissection of Thrombospondin-4 Domains Involved in Intracellular Adaptive Endoplasmic Reticulum Stress-Responsive Signaling

Thrombospondins are a family of stress-inducible secreted glycoproteins that underlie tissue remodeling. We recently reported that thrombospondin-4 (Thbs4) has a critical intracellular function, regulating the adaptive endoplasmic reticulum (ER) stress pathway through activating transcription factor 6α (Atf6α). In the present study, we dissected the domains of Thbs4 that mediate interactions with ER proteins, such as BiP (Grp78) and Atf6α, and the domains mediating activation of the ER stress response. Functionally, Thbs4 localized to the ER and post-ER vesicles and was actively secreted from cardiomyocytes, as were the type III repeat (T3R) and TSP-C domains, while the LamG domain localized to the Golgi apparatus. We also mutated the major calcium-binding motifs within the T3R domain of full-length Thbs4, causing ER retention and secretion blockade. The T3R and TSP-C domains as well as wild-type Thbs4 and the calcium-binding mutant interacted with Atf6α, induced an adaptive ER stress response, and caused expansion of intracellular vesicles. In contrast, overexpression of a related secreted oligomeric glycoprotein, Nell2, which lacks only the T3R and TSP-C domains, did not cause these effects. Finally, deletion of Atf6α abrogated Thbs4-induced vesicular expansion. Taken together, these data identify the critical intracellular functional domains of Thbs4, which was formerly thought to have only extracellular functions.

Dual lipidation of the brain-specific Cdc42 isoform regulates its functional properties

Cdc42 (cell division cycle 42) is a member of the Rho GTPase family which regulates a variety of cellular activities by controlling actin cytoskeleton and gene expression. Cdc42 is expressed in the form of two splice variants. The canonical Cdc42 isoform is prenylated (Cdc42-prenyl), whereas the brainspecific isoform can be palmitoylated (Cdc42-palm). In the present study we have demonstrated palmitoylation of endogenous Cdc42 in rodent and human brains and identified Cys(188) and Cys(189) as acylation sites of Cdc42-palm. Moreover, we have shown that Cys(188) can also be prenylated. Analysis of acylation-deficient mutants revealed that lipidation of Cys(188) is essential for proper membrane binding of Cdc42-palm as well as for Cdc42-mediated regulation of gene transcription and induction of densely packed filopodia in neuroblastoma cells. We also found that Cdc42-prenyl is a dominant splice variant in a wide range of commonly used cell lines as well as in the cerebellum, whereas Cdc42-palm is the main Cdc42 isoform in hippocampus, where it is critically involved in the formation of dendritic filopodia and spines. Replacement of endogenous Cdc42 by its acylation-deficient mutants revealed the importance of Cdc42-palm lipidation for its morphogenic and synaptogenic effects in neurons. These findings demonstrate that dual lipidation of Cdc42-palm represents an important regulator of morphogenic signalling in hippocampal neurons.

Palmitoylation-dependent regulation of glutamate receptors and their PDZ domain-containing partners

In recent years, it has become clear that both AMPA (α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid)- and NMDA (N-methyl-D-aspartate)-type glutamate receptors, and many of their interacting partners, are palmitoylated proteins. Interfering with palmitoylation dramatically affects receptor trafficking and distribution and, in turn, can profoundly alter synaptic transmission. Increased knowledge of synaptic palmitoylation not only will aid our understanding of physiological neuronal regulation, but also may provide insights into, and even novel treatments for, neuropathological conditions. In the present paper, we review recent advances regarding the regulation of ionotropic glutamate receptor trafficking and function by palmitoylation.

Regulation in the targeting of TRAIL receptor 1 to cell surface via GODZ for TRAIL sensitivity in tumor cells

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and its receptors, TRAIL-R1 (DR4) and TRAIL-R2 (DR5), promote the selective clearing of various malignancies by inducing apoptosis, holding the promise as a potent therapeutic agent for anticancer. Though DR4 and DR5 have high sequence similarity, differential regulation of both receptors in human tumor cells remains largely unexplored. Here, we repot that golgi-specific Asp-His-His-Cys (DHHC) zinc finger protein (GODZ) regulates TRAIL/DR4-mediated apoptosis. Using the SOS protein recruitment-yeast two-hybrid screening, we isolated GODZ that interacted with the death domain of DR4. GODZ binds to DR4, but not to DR5, through the DHHC and the C-terminal transmembrane domain. Expression level of GODZ affects apoptosis of tumor cells triggered by TRAIL, but not that induced by TNF-α/cycloheximide (CHX) or DNA-damaging drugs. In parallel, GODZ functions to localize DR4 to the plasma membrane (PM) via DHHC motif. Also, introduction of mutation into the cysteine-rich motif of DR4 results in its mistargeting and attenuates TRAIL- or GODZ-mediated apoptosis. Interestingly, GODZ expression is highly downregulated in Hep-3B tumor cells, which show resistance to TRAIL. However, reconstitution of GODZ expression enhances the targeting of DR4 to cell surface and sensitizes Hep-3B cells to TRAIL. Taken together, these data establish that GODZ is a novel DR4-selective regulator responsible for targeting of DR4 to the PM, and thereby for TRAIL-induced apoptosis.

Reversible palmitoylation regulates surface stability of AMPA receptors in the nucleus accumbens in response to cocaine in vivo

BACKGROUND

Palmitoylation is emerging as one of the most important posttranslational modifications of excitatory synaptic proteins in mammalian brain cells. As a reversible and regulatable modification sensitive to changing synaptic inputs, palmitoylation of ionotropic glutamate receptors contributes not only to the modulation of normal receptor and synaptic activities but also to the pathogenesis of various neuropsychiatric disorders. Here we report that palmitoylation of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor is regulated by the psychostimulant, cocaine, and such regulation is involved in cocaine action.

METHODS

We tested palmitoylation and surface expression of AMPA receptors in striatal neurons and psychomotor behavior in response to cocaine in rats.

RESULTS

All four AMPA receptor subunits (GluA1-4 or GluR1-4) are palmitoylated in the nucleus accumbens (NAc) of adult rats. Among them, GluA1 and GluA3 are preferentially upregulated in their palmitoylation levels by a systemic injection of cocaine. The upregulated GluA1 and 3 palmitoylation is a transient and reversible event. Consequently, it increases the susceptibility of surface-expressed GluA1 and 3 to internalization trafficking, leading to a temporal loss of surface receptor expression. Blockade of the regulated GluA1/3 palmitoylation with a palmitoylation inhibitor in the local NAc reverses the loss of surface GluA1/3. The inhibition of palmitoylation concurrently sustains behavioral responsivity to cocaine as well.

CONCLUSIONS

Our data identify a novel drug-palmitoylation coupling in the center of limbic reward circuits. Through palmitoylating selective AMPA receptor subunits, cocaine activity dependently regulates trafficking and subcellular localization of the receptor in NAc neurons and dynamically controls psychomotor sensitivity to the psychoactive drug in vivo.

Dynamic palmitoylation and the role of DHHC proteins in T cell activation and anergy

Although protein S-palmitoylation was first characterized >30 years ago, and is implicated in the function, trafficking, and localization of many proteins, little is known about the regulation and physiological implications of this posttranslational modification. Palmitoylation of various signaling proteins required for TCR-induced T cell activation is also necessary for their proper function. Linker for activation of T cells (LAT) is an essential scaffolding protein involved in T cell development and activation, and we found that its palmitoylation is selectively impaired in anergic T cells. The recent discovery of the DHHC family of palmitoyl acyl transferases and the establishment of sensitive and quantitative proteomics-based methods for global analysis of the palmitoyl proteome led to significant progress in studying the biology and underlying mechanisms of cellular protein palmitoylation. We are using these approaches to explore the palmitoyl proteome in T lymphocytes and, specifically, the mechanistic basis for the impaired palmitoylation of LAT in anergic T cells. This chapter reviews the history of protein palmitoylation and its role in T cell activation, the DHHC family and new methodologies for global analysis of the palmitoyl proteome, and summarizes our recent work in this area. The new methodologies will accelerate the pace of research and provide a greatly improved mechanistic and molecular understanding of the complex process of protein palmitoylation and its regulation, and the substrate specificity of the novel DHHC family. Reversible protein palmitoylation will likely prove to be an important posttranslational mechanism that regulates cellular responses, similar to protein phosphorylation and ubiquitination.

Multiple palmitoyltransferases are required for palmitoylation-dependent regulation of large conductance calcium- and voltage-activated potassium channels

Palmitoylation is emerging as an important and dynamic regulator of ion channel function; however, the specificity with which the large family of acyl palmitoyltransferases (zinc finger Asp-His-His-Cys type-containing acyl palmitoyltransferase (DHHCs)) control channel palmitoylation is poorly understood. We have previously demonstrated that the alternatively spliced stress-regulated exon (STREX) variant of the intracellular C-terminal domain of the large conductance calcium- and voltage-activated potassium (BK) channels is palmitoylated and targets the STREX domain to the plasma membrane. Using a combined imaging, biochemical, and functional approach coupled with loss-of-function (small interfering RNA knockdown of endogenous DHHCs) and gain-of-function (overexpression of recombinant DHHCs) assays, we demonstrate that multiple DHHCs control palmitoylation of the C terminus of STREX channels, the association of the STREX domain with the plasma membrane, and functional channel regulation. Cysteine residues 12 and 13 within the STREX insert were the only endogenously palmitoylated residues in the entire C terminus of the STREX channel. Palmitoylation of this dicysteine motif was controlled by DHHCs 3, 5, 7, 9, and 17, although DHHC17 showed the greatest specificity for this site upon overexpression of the cognate DHHC. DHHCs that palmitoylated the channel also co-assembled with the channel in co-immunoprecipitation experiments, and knockdown of any of these DHHCs blocked regulation of the channel by protein kinase A-dependent phosphorylation. Taken together our data reveal that a subset of DHHCs controls STREX palmitoylation and function and suggest that DHHC17 may preferentially target cysteine-rich domains. Finally, our approach may prove useful in elucidating the specificity of DHHC palmitoylation of intracellular domains of other ion channels and transmembrane proteins.

Molecular identification of ancient and modern mammalian magnesium transporters

A large number of mammalian Mg2+ transporters have been hypothesized on the basis of physiological data, but few have been investigated at the molecular level. The recent identification of a number of novel proteins that mediate Mg2+ transport has enhanced our understanding of how Mg2+ is translocated across mammalian membranes. Some of these transporters have some similarity to those found in prokaryocytes and yeast cells. Human Mrs2, a mitochondrial Mg2+ channel, shares many of the properties of the bacterial CorA and yeast Alr1 proteins. The SLC41 family of mammalian Mg2+ transporters has a similarity with some regions of the bacterial MgtE transporters. The mammalian ancient conserved domain protein (ACDP) Mg2+ transporters are found in prokaryotes, suggesting an ancient origin. However, other newly identified mammalian transporters, including TRPM6/7, MagT, NIPA, MMgT, and HIP14 families, are not represented in prokaryotic genomes, suggesting more recent development. MagT, NIPA, MMgT, and HIP14 transporters were identified by differential gene expression using microarray analysis. These proteins, which are found in many different tissues and subcellular organelles, demonstrate a diversity of structural properties and biophysical functions. The mammalian Mg2+ transporters have no obvious amino acid similarities, indicating that there are many ways to transport Mg2+ across membranes. Most of these proteins transport a number of divalent cations across membranes. Only MagT1 and NIPA2 are selective for Mg2+. Many of the identified mammalian Mg2+ transporters are associated with a number of congenital disorders encompassing a wide range of tissues, including intestine, kidney, brain, nervous system, and skin. It is anticipated that future research will identify other novel Mg2+ transporters and reveal other diseases.

DHHC5 interacts with PDZ domain 3 of post-synaptic density-95 (PSD-95) protein and plays a role in learning and memory

A family of integral membrane proteins containing a signature DHHC motif has been shown to display protein S-acyltransferase activity, modifying cysteine residues in proteins with fatty acids. The physiological roles of these proteins have largely been unexplored. Here we report that mice homozygous for a hypomorphic allele of a previously uncharacterized member, DHHC5, are born at half the expected rate, and survivors show a marked deficit in contextual fear conditioning, an indicator of defective hippocampal-dependent learning. DHHC5 is highly enriched in a post-synaptic density preparation and co-immunoprecipitates with post-synaptic density protein-95 (PSD-95), an interaction that is mediated through binding of the carboxyl terminus of DHHC5 and the PDZ3 domain of PSD-95. Immunohistochemistry demonstrated that DHHC5 is expressed in the CA3 and dentate gyrus in the hippocampus. These findings point to a previously unsuspected role for DHHC5 in post-synaptic function affecting learning and memory.

Golgi-specific DHHC zinc finger protein GODZ mediates membrane Ca2+ transport

The Golgi-specific zinc finger protein GODZ (palmitoyl acyltransferase/DHHC-3) mediates the palmitoylation and post-translational modification of many protein substrates that regulate membrane-protein interactions. Here, we show that GODZ also mediates Ca(2+) transport in expressing Xenopus laevis oocytes. Two-electrode voltage-clamp, fluorescence, and (45)Ca(2+) isotopic uptake determinations demonstrated voltage- and concentration-dependent, saturable, and substrate-inhibitable Ca(2+) transport in oocytes expressing GODZ cRNA but not in oocytes injected with water alone. Moreover, we show that GODZ-mediated Ca(2+) transport is regulated by palmitoylation, as the palmitoyl acyltransferase inhibitor 2-bromopalmitate or alteration of the acyltransferase DHHC motif (GODZ-DHHS) diminished GODZ-mediated Ca(2+) transport by approximately 80%. The GODZ mutation V61R abolished Ca(2+) transport but did not affect palmitoyl acyltransferase activity. Coexpression of GODZ-V61R with GODZ-DHHS restored GODZ-DHHS-mediated Ca(2+) uptake to values observed with wild-type GODZ, excluding an endogenous effect of palmitoylation. Coexpression of an independent palmitoyl acyltransferase (HIP14) with the GODZ-DHHS mutant also rescued Ca(2+) transport. HIP14 did not mediate Ca(2+) transport when expressed alone. Immunocytochemistry studies showed that GODZ and HIP14 co-localized to the Golgi and the same post-Golgi vesicles, suggesting that heteropalmitoylation might play a physiological role in addition to a biochemical function. We conclude that GODZ encodes a Ca(2+) transport protein in addition to its ability to palmitoylate protein substrates.

Dual palmitoylation of NR2 subunits regulates NMDA receptor trafficking

Modification of NMDA receptor function and trafficking contributes to the regulation of synaptic transmission and is important for several forms of synaptic plasticity. Here, we report that NMDA receptor subunits NR2A and NR2B have two distinct clusters of palmitoylation sites in their C-terminal region. Palmitoylation within the first cluster on a membrane-proximal region increases tyrosine phosphorylation of tyrosine-based internalization motifs by Src family protein tyrosine kinases, leading to enhanced stable surface expression of NMDA receptors. In addition, palmitoylation of these sites regulates constitutive internalization of the NMDA receptor in developing neurons. In marked contrast, palmitoylation of the second cluster in the middle of C terminus by distinct palmitoyl transferases causes receptors to accumulate in the Golgi apparatus and reduces receptor surface expression. These data suggest that regulated palmitoylation of NR2 subunits differentially modulates receptor trafficking and might be important for NMDA-receptor-dependent synaptic plasticity.

GABA(A) receptors and their associated proteins: implications in the etiology and treatment of schizophrenia and related disorders

Gamma-aminobutyric acid type A (GABA(A)) receptors play an important role in mediating fast synaptic inhibition in the brain. They are ubiquitously expressed in the CNS and also represent a major site of action for clinically relevant drugs. Recent technological advances have greatly clarified the molecular and cellular roles played by distinct GABA(A) receptor subunit classes and isoforms in normal brain function. At the same time, postmortem and genetic studies have linked neuropsychiatric disorders including schizophrenia and bipolar disorder with GABAergic neurotransmission and various specific GABA(A) receptor subunits, while evidence implicating GABA(A)R-associated proteins is beginning to emerge. In this review we discuss the mounting genetic, molecular, and cellular evidence pointing toward a role for GABA(A) receptor heterogeneity in both schizophrenia etiology and therapeutic development. Finally, we speculate on the relationship between schizophrenia-related disorders and selected GABA(A) receptor associated proteins, key regulators of GABA(A) receptor trafficking, targeting, clustering, and anchoring that often carry out these functions in a subtype-specific manner.

Neuronal palmitoyl acyl transferases exhibit distinct substrate specificity

Palmitoylation, a post-translational modification of cysteine residues with the lipid palmitate, has recently emerged as an important mechanism for regulating protein trafficking and function. With the identification of 23 DHHC mammalian palmitoyl acyl transferases (PATs), a key question was the nature of substrate-enzyme specificity for these PATs. Using the acyl-biotin exchange palmitoylation assay, we compared the substrate specificity of four neuronal PATs, namely DHHC-3, DHHC-8, HIP14L (DHHC-13), and HIP14 (DHHC-17). Exogenous expression of enzymes and substrates in COS cells reveals that HIP14L and HIP14 modulate huntingtin palmitoylation, DHHC-8 modulates paralemmin-1 palmitoylation, and DHHC-3 shows the least substrate specificity. These in vitro data were validated by lentiviral siRNA-mediated knockdown of endogenous HIP14 and DHHC-3 in cultured rat cortical neurons. PATs require the presence of palmitoylated cysteines in order to interact with their substrates. To understand the elements that influence enzyme/substrate specificity further, we fused the HIP14 ankryin repeat domain to the N terminus of DHHC-3, which is not a PAT for huntingtin. This modification enabled DHHC-3 to behave similarly to HIP14 by modulating palmitoylation and trafficking of huntingtin. Taken together, this study indicates that individual PATs have specific substrate preference, determined by regulatory domains outside the DHHC domain of the enzymes.

Huntingtin-interacting proteins, HIP14 and HIP14L, mediate dual functions, palmitoyl acyltransferase and Mg2+ transport

Polyglutamine expansions of huntingtin protein are responsible for the Huntington neurological disorder. HIP14 protein has been shown to interact with huntingtin. HIP14 and a HIP14-like protein, HIP14L, with a 69% similarity reside in the Golgi and possess palmitoyl acyltransferase activity through innate cysteine-rich domains, DHHC. Here, we used microarray analysis to show that reduced extracellular magnesium concentration increases HIP14L mRNA suggesting a role in cellular magnesium metabolism. Because HIP14 was not on the microarray platform, we used real-time reverse transcriptase-PCR to show that HIP14 and HIP14L transcripts were up-regulated 3-fold with low magnesium. Western analysis with a specific HIP14 antibody also showed that endogenous HIP14 protein increased with diminished magnesium. Furthermore, we demonstrate that when expressed in Xenopus oocytes, HIP14 and HIP14L mediate Mg2+ uptake that is electrogenic, voltage-dependent, and saturable with Michaelis constants of 0.87 +/- 0.02 and 0.74 +/- 0.07 mm, respectively. Diminished magnesium leads to an apparent increase in HIP14-green fluorescent protein and HIP14L-green fluorescent fusion proteins in the Golgi complex and subplasma membrane post-Golgi vesicles of transfected epithelial cells. We also show that inhibition of palmitoylation with 2-bromopalmitate, or deletion of the DHHC motif HIP14DeltaDHHC, diminishes HIP14-mediated Mg2+ transport by about 50%. Coexpression of an independent protein acyltransferase, GODZ, with the deleted HIP14DeltaDHHC mutant restored Mg2+ transport to values observed with wild-type HIP14. Although we did not directly measure palmitoylation of HIP14 in these studies, the data are consistent with a regulatory role of autopalmitoylation in HIP14-mediated Mg2+ transport. We conclude that the huntingtin interacting protein genes, HIP14 and HIP14L, encode Mg2+ transport proteins that are regulated by their innate palmitoyl acyltransferases thus fulfilling the characteristics of "chanzymes."

Discovery of protein-palmitoylating enzymes

Posttranslational modification provides proteins with additional function and regulatory control beyond genomic information, allowing cells to maintain homeostasis and respond to extracellular signals. Protein palmitoylation, the common posttranslational modification with the lipid palmitate, plays a pivotal role in protein trafficking and function. Palmitoylation is unique in that it is reversible and dynamically regulated by specific extracellular signals. The reversible nature of protein palmitoylation enables proteins to shuttle between intracellular compartments upon extracellular signals. However, the molecular mechanisms of protein palmitoylation have long been elusive, mostly because the enzymes responsible for protein palmitoylation were unknown. Recently, genetically conserved DHHC family proteins have emerged as palmitoyl-acyl transferases. With the identification of specific enzymes for palmitoylated proteins, including H-Ras, PSD-95, and eNOS, the specificity and regulatory mechanism of DHHC enzymes are beginning to be clarified.

The cell biology of synaptic plasticity: AMPA receptor trafficking

The cellular processes that govern neuronal function are highly complex, with many basic cell biological pathways uniquely adapted to perform the elaborate information processing achieved by the brain. This is particularly evident in the trafficking and regulation of membrane proteins to and from synapses, which can be a long distance away from the cell body and number in the thousands. The regulation of neurotransmitter receptors, such as the AMPA-type glutamate receptors (AMPARs), the major excitatory neurotransmitter receptors in the brain, is a crucial mechanism for the modulation of synaptic transmission. The levels of AMPARs at synapses are very dynamic, and it is these plastic changes in synaptic function that are thought to underlie information storage in the brain. Thus, understanding the cellular machinery that controls AMPAR trafficking will be critical for understanding the cellular basis of behavior as well as many neurological diseases. Here we describe the life cycle of AMPARs, from their biogenesis, through their journey to the synapse, and ultimately through their demise, and discuss how the modulation of this process is essential for brain function.

Regulation of dendritogenesis via a lipid-raft-associated Ca2+/calmodulin-dependent protein kinase CLICK-III/CaMKIgamma

Ca(2+) signaling plays a central role in activity-dependent regulation of dendritic arborization, but key molecular mechanisms downstream of calcium elevation remain poorly understood. Here we show that the C-terminal region of the Ca(2+)/calmodulin-dependent protein kinase CLICK-III (CL3)/CaMKIgamma, a membrane-anchored CaMK, was uniquely modified by two sequential lipidification steps: prenylation followed by a kinase-activity-regulated palmitoylation. These modifications were essential for CL3 membrane anchoring and targeting into detergent-resistant lipid microdomains (or rafts) in the dendrites. We found that CL3 critically contributed to BDNF-stimulated dendritic growth. Raft insertion of CL3 specifically promoted dendritogenesis of cortical neurons by acting upstream of RacGEF STEF and Rac, both present in lipid rafts. Thus, CL3 may represent a key element in the Ca(2+)-dependent and lipid-raft-delineated switch that turns on extrinsic activity-regulated dendrite formation in developing cortical neurons.

GABAAreceptor associated proteins: a key factor regulating GABAAreceptor function

gamma-Aminobutyric acid (GABA), an important inhibitory neurotransmitter in both vertebrates and invertebrates, acts on GABA receptors that are ubiquitously expressed in the CNS. GABA(A) receptors also represent a major site of action of clinically relevant drugs, such as benzodiazepines, barbiturates, ethanol, and general anesthetics. It has been shown that the intracellular M3-M4 loop of GABA(A) receptors plays an important role in regulating GABA(A) receptor function. Therefore, studies of the function of receptor intracellular loop associated proteins become important for understanding mechanisms of regulating receptor activity. Recently, several labs have used the yeast two-hybrid assay to identify proteins interacting with GABA(A) receptors, for example, the interaction of GABA(A) receptor associated protein (GABARAP) and Golgi-specific DHHC zinc finger protein (GODZ) with gamma subunits, PRIP, phospholipase C-related, catalytically inactive proteins (PRIP-1) and (PRIP-2) with GABARAP and receptor gamma2 and beta subunits, Plic-1 with some alpha and beta subunits, radixin with the alpha5 subunit, HAP1 with the beta1 subunit, GABA(A) receptor interacting factor-1 (GRIF-1) with the beta2 subunit, and brefeldin A-inhibited GDP/GTP exchange factor 2 (BIG2) with the beta3 subunit. These proteins have been shown to play important roles in modulating the activities of GABA(A) receptors ranging from enhancing trafficking, to stabilizing surface and internalized receptors, to regulating modification of GABA(A) receptors. This article reviews the current studies of GABA(A) receptor intracellular loop-associated proteins.

GODZ-mediated palmitoylation of GABA(A) receptors is required for normal assembly and function of GABAergic inhibitory synapses

Golgi-specific DHHC (Asp-His-His-Cys) zinc finger protein (GODZ) is a DHHC family palmitoyl acyltransferase that is implicated in palmitoylation and regulated trafficking of diverse substrates that function either at inhibitory or excitatory synapses. Of particular interest is the gamma2 subunit of GABA(A) receptors, which is required for targeting these receptors to inhibitory synapses. Here, we report that GODZ and, to a lesser extent, its close paralog sertoli cell gene with a zinc finger domain-beta (SERZ-beta) are the main members of the DHHC family of enzymes that are able to palmitoylate the gamma2 subunit in heterologous cells. Yeast two-hybrid and colocalization assays in human embryonic kidney 293T (HEK293T) cells indicate that GODZ and SERZ-beta show indistinguishable palmitoylation-dependent interaction with the gamma2 subunit. After coexpression in HEK293T cells, they form homomultimers and heteromultimers, as shown by coimmunoprecipitation and in vivo cross-linking experiments. Analyses in neurons transfected with dominant-negative GODZ (GODZ(C157S)) or plasmid-based GODZ-specific RNAi indicate that GODZ is required for normal accumulation of GABA(A) receptors at synapses, for normal whole-cell and synaptic GABAergic inhibitory function and, indirectly, for GABAergic innervation. Unexpectedly, GODZ was found to be dispensable for normal postsynaptic AMPA receptor-mediated glutamatergic transmission. We conclude that GODZ-mediated palmitoylation of GABA(A) receptors and possibly other substrates contributes selectively to the formation and normal function of GABAergic inhibitory synapses.

In vitro and cellular assays for palmitoyl acyltransferases using fluorescent lipidated peptides

Protein palmitoylation is emerging as an important post-translational modification in development as well as in the establishment and progression of diseases such as cancer. This chapter describes the use of fluorescent lipidated peptides to characterize palmitoyl acyltransferase (PAT) activities in vitro and in intact cells. The peptides mimic two motifs that are enzymatically palmitoylated, i.e. C-terminal farnesyl and N-terminal myristoyl sequences. These substrate peptides can be separated from the palmitoylated product peptides by reversed-phase HPLC, detected and quantified by the fluorescence of their NBD label. Through these methods, the activities of PATs toward these alternate substrates in isolated membranes or intact cells can be quantified. The in vitro assay has been used to characterize human PATs and to identify inhibitors of these enzymes. The cellular assay has been useful in elucidating the kinetics of protein palmitoylation by PATs in situ, and the sub-cellular distribution of the palmitoylated products.

Molecular cloning and characterization of human Aph2 gene, involved in AP-1 regulation by interaction with JAB1

A human Aph2 gene (hAph2) was identified and cloned from a human placenta cDNA library. Bioinformatics analysis revealed hAPH2 protein shares 96% identity with mouse APH2 and contains a zf-DHHC domain (148-210aa), which is always involved in protein-protein or protein-DNA interaction. Differential expression patterns of hAph2 mRNA were observed in normal human tissues. Yeast two-hybrid screening found another hAPH2-interacting protein JAB1. The zf-DHHC domain of hAPH2 and the C-terminal of JAB1 were confirmed to be critical for the interaction. Fused with GFP and expressed in COS-7, NIH/3T3 and SMMC-7721 cell lines, hAPH2 showed predominant distribution in the cytoplasm and co-localized with JAB1 around the nucleus. Furthermore, overexpression of hAPH2 could increase apoptosis of COS-7 cells and negatively regulate JAB1-induced activation of AP-1 in a concentration dependent manner. The expression level of c-jun was also down-regulated by overexpression of hAPH2 in COS-7 cells. These data showed some basic characterization and function of hAph2 (hAPH2), dependent or independent with JAB1.

Discovery and characterization of inhibitors of human palmitoyl acyltransferases

The covalent attachment of palmitate to specific proteins by the action of palmitoyl acyltransferases (PAT) plays critical roles in the biological activities of several oncoproteins. Two PAT activities are expressed by human cells: type 1 PATs that modify the farnesyl-dependent palmitoylation motif found in H- and N-Ras, and type 2 PATs that modify the myristoyl-dependent palmitoylation motif found in the Src family of tyrosine kinases. We have previously shown that the type 1 PAT HIP14 causes cellular transformation. In the current study, we show that mRNA encoding HIP14 is up-regulated in a number of types of human tumors. To assess the potential of HIP14 and other PATs as targets for new anticancer drugs, we developed three cell-based assays suitable for high-throughput screening to identify inhibitors of these enzymes. Using these screens, five chemotypes, with activity toward either type 1 or type 2 PAT activity, were identified. The activity of the hits were confirmed using assays that quantify the in vitro inhibition of PAT activity, as well as a cell-based assay that determines the abilities of the compounds to prevent the localization of palmitoylated green fluorescent proteins to the plasma membrane. Representative compounds from each chemotype showed broad antiproliferative activity toward a panel of human tumor cell lines and inhibited the growth of tumors in vivo. Together, these data show that PATs, and HIP14 in particular, are interesting new targets for anticancer compounds, and that small molecules with such activity can be identified by high-throughput screening.

Identification of Golgi-localized acyl transferases that palmitoylate and regulate endothelial nitric oxide synthase

Lipid modifications mediate the subcellular localization and biological activity of many proteins, including endothelial nitric oxide synthase (eNOS). This enzyme resides on the cytoplasmic aspect of the Golgi apparatus and in caveolae and is dually acylated by both N-myristoylation and S-palmitoylation. Palmitoylation-deficient mutants of eNOS release less nitric oxide (NO). We identify enzymes that palmitoylate eNOS in vivo. Transfection of human embryonic kidney 293 cells with the complementary DNA (cDNA) for eNOS and 23 cDNA clones encoding the Asp-His-His-Cys motif (DHHC) palmitoyl transferase family members showed that five clones (2, 3, 7, 8, and 21) enhanced incorporation of [³H]-palmitate into eNOS. Human endothelial cells express all five of these enzymes, which colocalize with eNOS in the Golgi and plasma membrane and interact with eNOS. Importantly, inhibition of DHHC-21 palmitoyl transferase, but not DHHC-3, in human endothelial cells reduces eNOS palmitoylation, eNOS targeting, and stimulated NO production. Collectively, our data describe five new Golgi-targeted DHHC enzymes in human endothelial cells and suggest a regulatory role of DHHC-21 in governing eNOS localization and function.

Protein palmitoylation by a family of DHHC protein S-acyltransferases

Protein palmitoylation refers to the posttranslational addition of a 16 carbon fatty acid to the side chain of cysteine, forming a thioester linkage. This acyl modification is readily reversible, providing a potential regulatory mechanism to mediate protein-membrane interactions and subcellular trafficking of proteins. The mechanism that underlies the transfer of palmitate or other long-chain fatty acids to protein was uncovered through genetic screens in yeast. Two related S-palmitoyltransferases were discovered. Erf2 palmitoylates yeast Ras proteins, whereas Akr1 modifies the yeast casein kinase, Yck2. Erf2 and Akr1 share a common sequence referred to as a DHHC (aspartate-histidine-histidine-cysteine) domain. Numerous genes encoding DHHC domain proteins are found in all eukaryotic genome databases. Mounting evidence is consistent with this signature motif playing a direct role in protein acyltransferase (PAT) reactions, although many questions remain. This review presents the genetic and biochemical evidence for the PAT activity of DHHC proteins and discusses the mechanism of protein-mediated palmitoylation.

spe-10 encodes a DHHC-CRD zinc-finger membrane protein required for endoplasmic reticulum/Golgi membrane morphogenesis during Caenorhabditis elegans spermatogenesis

C. elegans spermatogenesis employs lysosome-related fibrous body-membranous organelles (FB-MOs) for transport of many cellular components. Previous work showed that spe-10 mutants contain FB-MOs that prematurely disassemble, resulting in defective transport of FB components into developing spermatids. Consequently, spe-10 spermatids are smaller than wild type and contain defective FB-MO derivatives. In this article, we show that spe-10 encodes a four-pass integral membrane protein that has a DHHC-CRD zinc-finger motif. The DHHC-CRD motif is found in a large, diverse family of proteins that have been implicated in palmitoyl transfer during protein lipidation. Seven spe-10 mutants were analyzed, including missense, nonsense, and deletion mutants. An antiserum to SPE-10 showed significant colocalization with a known marker for the FB-MOs during wild-type spermatogenesis. In contrast, the spe-10(ok1149) deletion mutant lacked detectable SPE-10 staining; this mutant lacks a spe-10 promoter and most coding sequence. The spe-10(eb64) missense mutation, which changes a conserved residue within the DHHC-CRD domain in all homologues, behaves as a null mutant. These results suggest that wild-type SPE-10 is required for the MO to properly deliver the FB to the C. elegans spermatid and the DHHC-CRD domain is essential for this function.

The vacuolar DHHC-CRD protein Pfa3p is a protein acyltransferase for Vac8p

Palmitoylation of the vacuolar membrane protein Vac8p is essential for vacuole fusion in yeast (Veit, M., R. Laage, L. Dietrich, L. Wang, and C. Ungermann. 2001. EMBO J. 20:3145–3155; Wang, Y.X., E.J. Kauffman, J.E. Duex, and L.S. Weisman. 2001. J. Biol. Chem. 276:35133–35140). Proteins that contain an Asp-His-His-Cys (DHHC)–cysteine rich domain (CRD) are emerging as a family of protein acyltransferases, and are therefore candidates for mediators of Vac8p palmitoylation. Here we demonstrate that the DHHC-CRD proteins Pfa3p (protein fatty acyltransferase 3, encoded by YNL326c) and Swf1p are important for vacuole fusion. Cells lacking Pfa3p had fragmented vacuoles when stressed, and cells lacking both Pfa3p and Swf1p had fragmented vacuoles under normal growth conditions. Pfa3p promoted Vac8p membrane association and palmitoylation in vivo and partially purified Pfa3p palmitoylated Vac8p in vitro, establishing Vac8p as a substrate for palmitoylation by Pfa3p. Vac8p is the first N-myristoylated, palmitoylated protein identified as a substrate for a DHHC-CRD protein.

Differential Regulation of AMPA Receptor Subunit Trafficking by Palmitoylation of Two Distinct Sites

Modification of AMPA receptor function is a major mechanism for the regulation of synaptic transmission and underlies several forms of synaptic plasticity. Post-translational palmitoylation is a reversible modification that regulates localization of many proteins. Here, we report that palmitoylation of the AMPA receptor regulates receptor trafficking. All AMPA receptor subunits are palmitoylated on two cysteine residues in their transmembrane domain (TMD) 2 and in their C-terminal region. Palmitoylation on TMD 2 is upregulated by the palmitoyl acyl transferase GODZ and leads to an accumulation of the receptor in the Golgi and a reduction of receptor surface expression. C-terminal palmitoylation decreases interaction of the AMPA receptor with the 4.1N protein and regulates AMPA- and NMDA-induced AMPA receptor internalization. Moreover, depalmitoylation of the receptor is regulated by activation of glutamate receptors. These data suggest that regulated palmitoylation of AMPA receptor subunits modulates receptor trafficking and may be important for synaptic plasticity.

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.

Transmembrane Topology of the Protein Palmitoyl Transferase Akr1

The two recently identified protein acyl transferases (PATs), Akr1p and Erf2p/Erf4p, point toward the DHHC protein family as a likely PAT family. The DHHC protein family, defined by the novel, zinc finger-like DHHC cysteine-rich domain (DHHC-CRD), is a diverse collection of polytopic membrane proteins extending through all eukaryotes. To define the PAT domains that are oriented to the cytoplasm and are thus available to effect the cytoplasmically limited palmitoyl modification, we have determined the transmembrane topology of the yeast PAT Akr1p. Portions of the yeast protein invertase (Suc2p) were inserted in-frame at 10 different hydrophilic sites within the Akr1 polypeptide. Three of the Akr1-Suc2-Akr1 insertion proteins were found to be extensively glycosylated, indicating that the invertase segment inserted at these Akr1p sites is luminally oriented. The remaining seven insertion proteins were not glycosylated, consistent with a cytoplasmic orientation for these sites. The results support a model in which the Akr1 polypeptide crosses the bilayer six times with the bulk of its hydrophilic domains disposed toward the cytoplasm. Cytoplasmic domains include both the relatively large, ankyrin repeat-containing N-terminal domain and the DHHC-CRD, which maps to a cytosolic loop segment. Functionality of the different Akr1-Suc2-Akr1 proteins also was examined. Insertions at only 4 of the 10 sites were found to disrupt Akr1p function. Interestingly, these four sites all map cytoplasmically, suggesting key roles for these cytoplasmic domains in Akr1 PAT function. Finally, extrapolating from the Akr1p topology, topology models are proposed for other DHHC protein family members.

Identification of PSD-95 palmitoylating enzymes

Palmitoylation is a lipid modification that plays a critical role in protein trafficking and function throughout the nervous system. Palmitoylation of PSD-95 is essential for its regulation of AMPA receptors and synaptic plasticity. The enzymes that mediate palmitoyl acyl transfer to PSD-95 have not yet been identified; however, proteins containing a DHHC cysteine-rich domain mediate palmitoyl acyl transferase activity in yeast. Here, we isolated 23 mammalian DHHC proteins and found that a subset specifically palmitoylated PSD-95 in vitro and in vivo. These PSD-95 palmitoyl transferases (P-PATs) showed substrate specificity, as they did not all enhance palmitoylation of Lck, SNAP-25b, Galpha(s), or H-Ras in cultured cells. Inhibition of P-PAT activity in neurons reduced palmitoylation and synaptic clustering of PSD-95 and diminished AMPA receptor-mediated neurotransmission. This study suggests that P-PATs regulate synaptic function through PSD-95 palmitoylation.

The gamma2 subunit of GABA(A) receptors is a substrate for palmitoylation by GODZ

The neurotransmitter GABA activates heteropentameric GABA(A) receptors, which are composed mostly of alpha, beta, and gamma2 subunits. Regulated membrane trafficking and subcellular targeting of GABA(A) receptors is important for determining the efficacy of GABAergic inhibitory function. Of special interest is the gamma2 subunit, which is mostly dispensable for assembly and membrane insertion of functional receptors but essential for accumulation of GABA(A) receptors at synapses. In a search for novel receptor trafficking proteins, we have used the SOS-recruitment system and isolated a Golgi-specific DHHC zinc finger protein (GODZ) as a novel gamma2 subunit-interacting protein. GODZ is a member of the superfamily of DHHC cysteine-rich domain (DHHC-CRD) polytopic membrane proteins shown recently in yeast to represent palmitoyltransferases. GODZ mRNA is found in many tissues; however, in brain the protein is detected in neurons only and highly concentrated and asymmetrically distributed in the Golgi complex. GODZ interacts with a cysteine-rich 14-amino acid domain conserved specifically in the large cytoplasmic loop of gamma1-3 subunits but not in other GABA(A) receptor subunits. Coexpression of GODZ and GABA(A) receptors in heterologous cells results in palmitoylation of the gamma2 subunit in a cytoplasmic loop domain-dependent manner. Neuronal GABA(A) receptors are similarly palmitoylated. Thus, GODZ-mediated palmitoylation represents a novel posttranslational modification that is selective for gamma subunit-containing GABA(A) receptor subtypes, a mechanism that is likely to be important for regulated trafficking of these receptors in the secretory pathway.

Palmitoylation of intracellular signaling proteins: regulation and function

Protein S-palmitoylation is the thioester linkage of long-chain fatty acids to cysteine residues in proteins. Addition of palmitate to proteins facilitates their membrane interactions and trafficking, and it modulates protein-protein interactions and enzyme activity. The reversibility of palmitoylation makes it an attractive mechanism for regulating protein activity, and this feature has generated intensive investigation of this modification. The regulation of palmitoylation occurs through the actions of protein acyltransferases and protein acylthioesterases. Identification of the protein acyltransferases Erf2/Erf4 and Akr1 in yeast has provided new insight into the palmitoylation reaction. These molecules work in concert with thioesterases, such as acyl-protein thioesterase 1, to regulate the palmitoylation status of numerous signaling molecules, ultimately influencing their function. This review discusses the function and regulation of protein palmitoylation, focusing on intracellular proteins that participate in cell signaling or protein trafficking.

Model organisms lead the way to protein palmitoyltransferases

The acylation of proteins with palmitate and related fatty acids has been known for over 30 years, but the molecular machinery that carries out palmitoylation has only recently emerged from studies in the model organisms Saccharomyces cerevisiae and Drosophila. Two classes of protein acyltransferases (PATs) have been proposed. In yeast, members of a family of integral membrane proteins harboring a cysteine-rich domain (CRD) containing a conserved DHHC (Asp-His-His-Cys) motif are PATs for cytoplasmic signaling molecules. The DHHC-CRD protein Erf2p, together with an associated subunit Erf4p, palmitoylates yeast Ras proteins, and Akr1p catalyzes the palmitoylation of the yeast casein kinase Yck2p. The existence of a second class of PATs that modify secreted signaling proteins has been suggested from work in Drosophila. Rasp is required in vivo for the production of functional Hedgehog and shares sequence identity with membrane-bound O-acyltransferases, which suggests that it catalyzes the palmitoylation of Hedgehog. With the identification of PATs in model genetic organisms, the field is now poised to uncover their mammalian counterparts and to understand the enzymology of protein palmitoylation.

Direct interaction of GluRδ2 with Shank scaffold proteins in cerebellar Purkinje cells

Glutamate receptor (GluR) delta2 selectively expressed in cerebellar Purkinje cells plays a central role in cerebellar long-term depression (LTD), motor learning, and formation of parallel fiber synapses. By yeast two-hybrid screening, we identified members of the Shank family of scaffold proteins as major GluRdelta2-interacting molecules. GluRdelta2 bound directly to the PDZ domain of Shank proteins through an internal motif in the carboxyl-terminal putative cytoplasmic domain. Shank1 and Shank2 proteins as well as GluRdelta2 proteins were localized in the dendritic spines of cultured Purkinje cells. Anti-GluRdelta2 antibodies immunoprecipitated Shank1, Shank2, Homer, and metabotropic GluR1alpha proteins from the synaptosomal membrane fractions of cerebella. Furthermore, Shank2 interacted with GRIP1 in the cerebellum. These results suggest that through Shank1 and Shank2, GluRdelta2 interacts with the metabotropic GluR1alpha, the AMPA-type GluR, and the inositol 1,4,5-trisphosphate receptor (IP3R) that are essential for cerebellar LTD.

Regulation of GABAA receptor trafficking, channel activity, and functional plasticity of inhibitory synapses

Neural inhibition in the brain is mainly mediated by ionotropic gamma-aminobutyric acid type A (GABA(A)) receptors. Different subtypes of these receptors, distinguished by their subunit composition, are either concentrated at postsynaptic sites where they mediate phasic inhibition or found at perisynaptic and extrasynaptic locations where they prolong phasic inhibition and mediate tonic inhibition, respectively. Of special interest are mechanisms that modulate the stability and function of postsynaptic GABA(A) receptor subtypes and that are implicated in functional plasticity of inhibitory transmission in the brain. We will summarize recent progress on the classification of synaptic versus extrasynaptic receptors, the molecular composition of the postsynaptic cytoskeleton, the function of receptor-associated proteins in trafficking of GABA(A) receptors to and from synapses, and their role in post-translational signaling mechanisms that modulate the stability, density, and function of GABA(A) receptors in the postsynaptic membrane.

New insights into the mechanisms of protein palmitoylation

Since its discovery more than 30 years ago, protein palmitoylation has been shown to have a role in protein-membrane interactions, protein trafficking, and enzyme activity. Until recently, however, the molecular machinery that carries out reversible palmitoylation of proteins has been elusive. In fact, both enzymatic and nonenzymatic S-acylation reaction mechanisms have been proposed. Recent reports of protein palmitoyltransferases in Saccharomyces cerevisiae and Drosophila provide the first glimpse of enzymes that carry out protein palmitoylation. Equally important is the mechanism of depalmitoylation. Two major classes of protein palmitoylthioesterases have been described. One family is lysosomal and is involved in protein degradation. The second is cytosolic and removes palmitoyl moieties preferentially from proteins associated with membranes. This review discusses recent advances in the understanding of mechanisms of addition of palmitate to proteins and removal of palmitate from proteins.