Insider information: how palmitoylation of Ras makes it a signaling double agent

Physiological relevance of ACOT8-Nef interaction in HIV infection

During human immunodeficiency virus (HIV) infection, Nef viral protein plays a crucial role in viral pathogenesis and progression of acquired immunodeficiency syndrome. Nef is expressed in the early stages of infection and alters the cellular environment increasing infectivity, viral replication, and the evasion of host immune response through several mechanisms. Nef has numerous functional domains that allow it to interact with a number of proteins, interfering with intracellular traffic. Among these proteins, human peroxisomal thioesterase 8, ACOT8, has been shown to be an important cellular partner of Nef. It has been suggested that this interaction may be involved in Nef-dependent endocytosis and also in the modulation of lipid composition in membrane rafts. However, the actual role of this interaction, as well as the mechanisms involved, has not yet been fully elucidated. In this review, we focused on the interplay between Nef and ACOT8 proteins, highlighting the possible physiological relevance in HIV infection.

Specificity in S-nitrosylation: a short-range mechanism for NO signaling?

SIGNIFICANCE

Nitric oxide (NO) classical and less classical signaling mechanisms (through interaction with soluble guanylate cyclase and cytochrome c oxidase, respectively) operate through direct binding of NO to protein metal centers, and rely on diffusibility of the NO molecule. S-Nitrosylation, a covalent post-translational modification of protein cysteines, has emerged as a paradigm of nonclassical NO signaling.

RECENT ADVANCES

Several nonenzymatic mechanisms for S-nitrosylation formation and destruction have been described. Enzymatic mechanisms for transnitrosylation and denitrosylation have been also studied as regulators of the modification of specific subsets of proteins. The advancement of modification-specific proteomic methodologies has allowed progress in the study of diverse S-nitrosoproteomes, raising clues and questions about the parameters for determining the protein specificity of the modification.

CRITICAL ISSUES

We propose that S-nitrosylation is mainly a short-range mechanism of NO signaling, exerted in a relatively limited range of action around the NO sources, and tightly related to the very controlled regulation of subcellular localization of nitric oxide synthases. We review the nonenzymatic and enzymatic mechanisms that support this concept, as well as physiological examples of mammalian systems that illustrate well the precise compartmentalization of S-nitrosylation.

FUTURE DIRECTIONS

Individual and proteomic studies of protein S-nitrosylation-based signaling should take into account the subcellular localization in order to gain further insight into the functional role of this modification in (patho)physiological settings.

Driven to death: Inhibition of farnesylation increases Ras activity and promotes growth arrest and cell death [corrected]

To improve cancer outcomes, investigators are turning increasingly to small molecule medicines that disrupt vital signaling cascades, inhibit malignant growth, or induce apoptosis. One vital signaling molecule is Ras, and a key step in Ras activation is membrane anchoring of Ras through prenylation, the C-terminal addition of a lipid anchor. Small molecule inhibitors of farnesyltransferase (FTI), the enzyme most often responsible for prenylating Ras, showed clinical promise, but development of FTIs such as tipifarnib has been stalled by uncertainty about their mechanism of action, because Ras seemed unimpeded in tipifarnib-treated samples. Interpretation was further complicated by the numerous proteins that may be farnesylated, as well as availability of an alternate prenylation pathway, geranylgeranylation. Our initial observations of varied response by cancer cell lines to tipifarnib led us to evaluate the role of FTI in Ras signal alteration using various tumor models. We describe our novel counterintuitive finding that endogenous Ras activity increases in cancer cell lines with low endogenous Ras activity when farnesyltransferase is inhibited by either tipifarnib or short hairpin RNA. In response to tipifarnib, variable growth arrest and/or cell death correlated with levels of activated extracellular signal–regulated kinase (ERK) and p38 mitogenactivated protein kinase (MAPK). Sensitivity to tipifarnib treatment was shown by growth inhibition and by an increase in subdiploid cell numbers; cells with such sensitivity had increased activation of ERK and p38 MAPK. Because Ras must be prenylated to be active, our findings suggest that geranylgeranylated N-Ras or K-Ras B interacts differently with downstream effector proteins in sensitive cancer cells responding to tipifarnib, switching the balance from cell proliferation to growth inhibition [corrected].

Differential expression of DHHC9 in microsatellite stable and instable human colorectal cancer subgroups

Microarray analysis on pooled samples has previously identified ZDHHC9 (DHHC9) to be upregulated in colon adenocarcinoma compared to normal colon mucosa. Analyses of 168 samples from proximal and distal adenocarcinomas using U133plus2.0 microarrays validated these findings, showing a significant two-fold (log 2) upregulation of DHHC9 transcript (P<10⁻⁶). The upregulation was more striking in microsatellite stable (MSS), than in microsatellite instable (MSI), tumours. Genes known to interact with DHHC9 as H-Ras or N-Ras did not show expression differences between MSS and MSI. Immunohistochemical analysis was performed on 60 colon adenocarcinomas, previously analysed on microarrays, as well as on tissue microarrays with 40 stage I–IV tumours and 46 tumours from different organ sites. DHHC9 protein was strongly expressed in MSS compared to MSI tumours, readily detectable in premalignant lesions, compared to the rare expression seen in normal mucosa. DHHC9 was specific for tumours of the gastrointestinal tract and localised to the Golgi apparatus, in vitro and in vivo. Overexpression of DHHC9 decreased the proliferation of SW480 and CaCo2 MSS cell lines significantly. In conclusion, DHHC9 is a gastrointestinal-related protein highly expressed in MSS colon tumours. The palmitoyl transferase activity, modifying N-Ras and H-Ras, suggests DHHC9 as a target for anticancer drug design.

Signal transduction in prostate cancer progression

Prostate cancer is the most frequently diagnosed cancer among men and the second leading cause of male cancer deaths in the United States. When prostate cancer initially presents in the clinic, the tumour is dependent on androgen for growth and, therefore, responsive to the surgical or pharmacological ablation of circulating androgens. However, there is a high rate of treatment failure because the disease often recurs as androgen-independent metastases. Surprisingly, this late-stage androgen-independent prostate cancer almost always retains expression of the AR (androgen receptor), despite the near absence of circulating androgens. Although late-stage prostate cancer is androgen-independent, the AR still seems to play a role in cancer cell growth at this stage of disease. Therefore a key to understanding hormone-independent prostate cancer is to determine the mechanism(s) by which the AR can function even in the absence of physiological levels of circulating androgen. This review will focus on the role of growth factor signalling in prostate cancer progression to androgen independence and thus outline potential molecular areas of intervention to treat prostate cancer progression.

The human SNARE protein Ykt6 mediates its own palmitoylation at C-terminal cysteine residues

The yeast SNARE (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor) protein Ykt6 was shown to mediate palmitoylation of the fusion factor Vac8 in a reaction essential for the fusion of vacuoles. Here I present evidence that hYkt6 (human Ykt6) has self-palmitoylating activity. Incubation of recombinant hYkt6 with [3H]Pal-CoA ([3H]palmitoyl-CoA) leads to covalent attachment of palmitate to C-terminal cysteine residues. The N-terminal domain of human Ykt6 contains a Pal-CoA binding site and is required for the reaction.

The human Kv1.1 channel is palmitoylated, modulating voltage sensing: Identification of a palmitoylation consensus sequence

Voltage-dependent K(+) channels rely on precise dynamic protein interactions with surrounding plasma membrane lipids to facilitate complex processes such as voltage sensing and channel gating. Many transmembrane-spanning proteins use palmitoylation to facilitate dynamic membrane interactions. Herein, we demonstrate that the human Kv1.1 ion channel is palmitoylated in the cytosolic portion of the S(2)-S(3) linker domain at residue C243. Through heterologous expression of the human Kv1.1 protein in Sf9 cells, covalent radiolabeling with [(3)H]palmitate, chemical stability studies of the [(3)H]-palmitoylated protein, and site-directed mutagenesis, C243 was identified as the predominant site of palmitoylation. The functional sequelae of palmitoylation were examined by analysis of whole cell currents from WT and mutant channels, which identified a 20-mV leftward shift in the current-voltage relationship when palmitoylation at C243 (but not with other cysteine deletions) is prevented by site-directed mutagenesis, implicating a role for palmitoylated C243 in modulating voltage sensing through protein-membrane interactions. Database searches identified an amino acid palmitoylation consensus motif (ACP/RSKT) that is present in multiple other members of the Shaker subfamily of K(+) channels and in several other unrelated regulatory proteins (e.g., CD36, nitric oxide synthase type 2, and the mannose-6 phosphate receptor) that are known to be palmitoylated by thioester linkages at the predicted consensus site cysteine residue. Collectively, these results (i) identify palmitoylation as a mechanism for K(+) channel interactions with plasma membrane lipids contributing to electric field-induced conformational alterations, and ii) define an amino acid consensus sequence for protein palmitoylation.

On the mechanism of protein palmitoylation

Protein palmitoylation or, more specifically, S-acylation is a reversible post-translational lipid modification. Despite the identification of several proteins that are altered in this way, our understanding of the enzymology of this process has been hampered by the lack of well-characterized acyltransferases. We now know of three proteins in Saccharomyces cerevisiae that promote palmitoylation: effector of Ras function (Erf2), ankyrin-repeat-containing protein (Akr1) and the SNARE protein Ykt6. Erf2 and Akr1 are integral membrane proteins that contain a cysteine-rich domain and an Asp-His-His-Cys motif, both of which catalyse acylation at the carboxyl terminus of their target proteins. Recently, we discovered that Ykt6 mediates the amino-terminal acylation of the fusion protein Vac8. Even though these three proteins differ in sequence, topology, size and substrate specificity, they might function in a similar manner. In this review, we discuss these observations in the context of a potential general mechanism of acylation.

Fatty acid synthase expression is induced by the Epstein-Barr virus immediate-early protein BRLF1 and is required for lytic viral gene expression

The Epstein-Barr virus (EBV) immediate-early (IE) protein BRLF1 (R) is a transcription factor that induces the lytic form of EBV infection. R activates certain early viral promoters through a direct binding mechanism but induces transcription of the other EBV IE gene, BZLF1 (Z), indirectly through cellular factors binding to a CRE motif in the Z promoter (Zp). Here we demonstrate that R activates expression of the fatty acid synthase (FAS) cellular gene through a p38 stress mitogen-activated protein kinase-dependent mechanism. B-cell receptor engagement of Akata cells also increases FAS expression. The FAS gene product is required for de novo synthesis of the palmitate fatty acid, and high-level FAS expression is normally limited to liver, brain, lung, and adipose tissue. We show that human epithelial tongue cells lytically infected with EBV (from oral hairy leukoplakia lesions) express much more FAS than uninfected cells. Two specific FAS inhibitors, cerulenin and C75, prevent R activation of IE (Z) and early (BMRF1) lytic EBV proteins in Jijoye cells. In addition, cerulenin and C75 dramatically attenuate IE and early lytic gene expression after B-cell receptor engagement in Akata cells and constitutive lytic viral gene expression in EBV-positive AGS cells. However, FAS inhibitors do not reduce lytic viral gene expression induced by a vector in which the Z gene product is driven by a strong heterologous promoter. In addition, FAS inhibitors do not reduce R activation of a naked DNA reporter gene construct driven by the Z promoter (Zp). These results suggest that cellular FAS activity is important for induction of Z transcription from the intact latent EBV genome, perhaps reflecting the involvement of lipid-derived signaling pathways or palmitoylated proteins. Furthermore, using FAS inhibitors may be a completely novel approach for blocking the lytic form of EBV replication.