Prenylation of Ras proteins is required for efficient hSOS1-promoted guanine nucleotide exchange

Protein lipidation in health and disease: molecular basis, physiological function and pathological implication

Posttranslational modifications increase the complexity and functional diversity of proteins in response to complex external stimuli and internal changes. Among these, protein lipidations which refer to lipid attachment to proteins are prominent, which primarily encompassing five types including S-palmitoylation, N-myristoylation, S-prenylation, glycosylphosphatidylinositol (GPI) anchor and cholesterylation. Lipid attachment to proteins plays an essential role in the regulation of protein trafficking, localisation, stability, conformation, interactions and signal transduction by enhancing hydrophobicity. Accumulating evidence from genetic, structural, and biomedical studies has consistently shown that protein lipidation is pivotal in the regulation of broad physiological functions and is inextricably linked to a variety of diseases. Decades of dedicated research have driven the development of a wide range of drugs targeting protein lipidation, and several agents have been developed and tested in preclinical and clinical studies, some of which, such as asciminib and lonafarnib are FDA-approved for therapeutic use, indicating that targeting protein lipidations represents a promising therapeutic strategy. Here, we comprehensively review the known regulatory enzymes and catalytic mechanisms of various protein lipidation types, outline the impact of protein lipidations on physiology and disease, and highlight potential therapeutic targets and clinical research progress, aiming to provide a comprehensive reference for future protein lipidation research.

Insights into the post-translational modification and its emerging role in shaping the tumor microenvironment

More and more in-depth studies have revealed that the occurrence and development of tumors depend on gene mutation and tumor heterogeneity. The most important manifestation of tumor heterogeneity is the dynamic change of tumor microenvironment (TME) heterogeneity. This depends not only on the tumor cells themselves in the microenvironment where the infiltrating immune cells and matrix together forming an antitumor and/or pro-tumor network. TME has resulted in novel therapeutic interventions as a place beyond tumor beds. The malignant cancer cells, tumor infiltrate immune cells, angiogenic vascular cells, lymphatic endothelial cells, cancer-associated fibroblastic cells, and the released factors including intracellular metabolites, hormonal signals and inflammatory mediators all contribute actively to cancer progression. Protein post-translational modification (PTM) is often regarded as a degradative mechanism in protein destruction or turnover to maintain physiological homeostasis. Advances in quantitative transcriptomics, proteomics, and nuclease-based gene editing are now paving the global ways for exploring PTMs. In this review, we focus on recent developments in the PTM area and speculate on their importance as a critical functional readout for the regulation of TME. A wealth of information has been emerging to prove useful in the search for conventional therapies and the development of global therapeutic strategies.

Protein Lipidation: Occurrence, Mechanisms, Biological Functions, and Enabling Technologies

Protein lipidation, including cysteine prenylation, N-terminal glycine myristoylation, cysteine palmitoylation, and serine and lysine fatty acylation, occurs in many proteins in eukaryotic cells and regulates numerous biological pathways, such as membrane trafficking, protein secretion, signal transduction, and apoptosis. We provide a comprehensive review of protein lipidation, including descriptions of proteins known to be modified and the functions of the modifications, the enzymes that control them, and the tools and technologies developed to study them. We also highlight key questions about protein lipidation that remain to be answered, the challenges associated with answering such questions, and possible solutions to overcome these challenges.

A novel role of farnesylation in targeting a mitotic checkpoint protein, human Spindly, to kinetochores

The mitotic checkpoint protein Spindly is farnesylated in vivo and this modification is required for its interaction with the RZZ complex and its localization to kinetochores.

Kinetochore (KT) localization of mitotic checkpoint proteins is essential for their function during mitosis. hSpindly KT localization is dependent on the RZZ complex and hSpindly recruits the dynein–dynactin complex to KTs during mitosis, but the mechanism of hSpindly KT recruitment is unknown. Through domain-mapping studies we characterized the KT localization domain of hSpindly and discovered it undergoes farnesylation at the C-terminal cysteine residue. The N-terminal 293 residues of hSpindly are dispensable for its KT localization. Inhibition of farnesylation using a farnesyl transferase inhibitor (FTI) abrogated hSpindly KT localization without affecting RZZ complex, CENP-E, and CENP-F KT localization. We showed that hSpindly is farnesylated in vivo and farnesylation is essential for its interaction with the RZZ complex and hence KT localization. FTI treatment and hSpindly knockdown displayed the same mitotic phenotypes, indicating that hSpindly is a key FTI target in mitosis. Our data show a novel role of lipidation in targeting a checkpoint protein to KTs through protein–protein interaction.

Latent Membrane Protein 2 (LMP2)

LMP2A is an EBV-encoded protein with three domains: (a) an N-terminal cytoplasmic domain, which has PY motifs that bind to WW domain-containing E3 ubiquitin ligases and an ITAM that binds to SH2 domain-containing proteins, (b) a transmembrane domain with 12 transmembrane segments that localizes LMP2A in cellular membranes, and (c) a 27-amino acid C-terminal domain which mediates homodimerization and heterodimerization of LMP2 protein isoforms. The most prominent two isoforms of the protein are LMP2A and LMP2B. The LMP2B isoform lacks the 19-amino acid N-terminal domain found in LMP2A, which modulates cellular signaling resulting in a baseline activation of B cells and degradation of cellular kinases leading to the downregulation of normal B cell signaling pathways. These two seemingly contradictory processes allow EBV to establish and maintain latency. LMP2 is expressed in many EBV-associated malignancies. While its antigenic properties may be useful in developing LMP2-specific immunity, the LMP2A N-terminal motifs also provide a basis to target LMP2A-modulated cellular kinases for the development of treatment strategies.

Assessing the subcellular distribution of oncogenic phosphoinositide 3-kinase using microinjection into live cells

Oncogenic mutations in PIK3CA lead to an increase in intrinsic phosphoinositide kinase activity, but it is thought that increased access of PI3Kα (phosphoinositide 3-kinase α) to its PM (plasma membrane) localized substrate is also required for increased levels of downstream PIP₃/Akt [phosphoinositide-3,4,5-trisphosphate/also called PKB (protein kinase B)] signalling. We have studied the subcellular localization of wild-type and the two most common oncogenic mutants of PI3Kα in cells maintained in growth media, and starved or stimulated cells using a novel method in which PI3Kα is pre-formed as a 1:1 p110α:p85α complex in vitro then introduced into live cells by microinjection. Oncogenic E545K and H1047R mutants did not constitutively interact with membrane lipids in vitro or in cells maintained in 10% (v/v) FBS. Following stimulation of RTKs (receptor tyrosine kinases), microinjected PI3Kα was recruited to the PM, but oncogenic forms of PI3Kα were not recruited to the PM to a greater extent and did not reside at the PM longer than the wild-type PI3Kα. Instead, the E545K mutant specifically bound activated Cdc42 in vitro and microinjection of E545K was associated with the formation of cellular protrusions, providing some preliminary evidence that changes in protein–protein interactions may play a role in the oncogenicity of the E545K mutant in addition to the well-known changes in lipid kinase activity.

Oncogenic forms of PI3Kα (phosphoinositide 3-kinase α) microinjected into live cells are not recruited to the PM (plasma membrane) to a greater extent, and do not reside at the PM longer, than wild-type PI3Kα.

Immunopurification of adenomatous polyposis coli (APC) proteins

Background

The adenomatous polyposis coli (APC) tumour suppressor gene encodes a 2843 residue (310 kDa) protein. APC is a multifunctional protein involved in the regulation of β-catenin/Wnt signalling, cytoskeletal dynamics and cell adhesion. APC mutations occur in most colorectal cancers and typically result in truncation of the C-terminal half of the protein.

Results

In order to investigate the biophysical properties of APC, we have generated a set of monoclonal antibodies which enable purification of recombinant forms of APC. Here we describe the characterisation of these anti-APC monoclonal antibodies (APC-NT) that specifically recognise endogenous APC both in solution and in fixed cells. Full-length APC(1–2843) and cancer-associated, truncated APC proteins, APC(1–1638) and APC(1–1311) were produced in Sf9 insect cells.

Conclusions

Recombinant APC proteins were purified using a two-step affinity approach using our APC-NT antibodies. The purification of APC proteins provides the basis for detailed structure/function analyses of full-length, cancer-truncated and endogenous forms of the protein.

Prenylation and membrane localization of Cdc42 are essential for activation by DOCK7

The unconventional guanine nucleotide exchange factor (GEF) family comprising 11 DOCK180 related proteins is classified into four subfamilies, A through D, based on their relative GEF activity toward the closely related Rac and Cdc42 GTPases. DOCK proteins participate in the remodeling of the actin cytoskeleton and are key regulators of cell motility, phagocytosis, and adhesion. Here we show that the guanine nucleotide exchange domain of DOCK7, DHR2 (for DOCK homology region 2), is a potent GEF for prenylated Cdc42 and Rac1 in a model liposome system, demonstrating that the prenylation and membrane localization of Cdc42 or Rac1 are necessary for their activation by DOCK7. Additionally, we identify DOCK7 residues that confer GTPase GEF specificity. Finally, using our liposome reconstitution assay, we show that a more narrowly defined GEF domain of DHR2 (designated DHR2s) harbors an N-terminal site distinct from the GEF active site that binds preferentially to the active, GTP-bound forms of Cdc42 and Rac1 and thereby recruits free DHR2s from solution to the membrane surface. This recruitment results in a progressive increase in the effective concentration of DHR2s at the membrane surface that in turn provides for an accelerated rate of guanine nucleotide exchange on Cdc42. The positive cooperativity observed in our reconstituted system suggests that the action of DOCK7 in vivo may involve the coordinated integration of Cdc42/Rac signaling in the context of the membrane recruitment of a DOCK7 GEF complex.

Autophosphorylation of serine 608 in the p85 regulatory subunit of wild type or cancer-associated mutants of phosphoinositide 3-kinase does not affect its lipid kinase activity

Background

The α-isoform of the Type 1A Phosphoinositide 3-kinases (PI3Kα) has protein kinase activity as well as phosphoinositide lipid kinase activity. The best described substrate for its protein kinase activity is its regulatory subunit, p85α, which becomes phosphorylated on Serine 608. Phosphorylation of Serine 608 has been reported to down-regulate its lipid kinase activity.

Results

We have assessed whether oncogenic mutants of PI3Kα, which have up-regulated lipid kinase activity, have altered levels of Serine 608 phosphorylation compared to wild type PI3Kα, and whether differential phosphorylation of Serine 608 contributes to increased activity of oncogenic forms of PI3Kα with point mutations in the helical or the kinase domains. Despite markedly increased lipid kinase activity, protein kinase activity was not altered in oncogenic compared to wild type forms of PI3Kα. By manipulating levels of phosphorylation of Serine 608 in vitro, we found no evidence that the protein kinase activity of PI3Kα affects its phosphoinositide lipid kinase activity in either wild-type or oncogenic mutants of PI3Kα.

Conclusions

Phosphorylation of p85α S608 is not a significant regulator of wild-type or oncogenic PI3Kα lipid kinase activity.

Osteoclasts in multiple myeloma are derived from Gr-1+CD11b+myeloid-derived suppressor cells

Osteoclasts play a key role in the development of cancer-associated osteolytic lesions. The number and activity of osteoclasts are often enhanced by tumors. However, the origin of osteoclasts is unknown. Myeloid-derived suppressor cells (MDSCs) are one of the pre-metastatic niche components that are induced to expand by tumor cells. Here we show that the MDSCs can differentiate into mature and functional osteoclasts in vitro and in vivo. Inoculation of 5TGM1-GFP myeloma cells into C57BL6/KaLwRij mice led to a significant expansion of MDSCs in blood, spleen, and bone marrow over time. When grown in osteoclastogenic media in vitro, MDSCs from tumor-challenged mice displayed 14 times greater potential to differentiate into mature and functional osteoclasts than those from non-tumor controls. Importantly, MDSCs from tumor-challenged LacZ transgenic mice differentiated into LacZ+osteoclasts in vivo. Furthermore, a significant increase in tumor burden and bone loss accompanied by increased number of osteoclasts was observed in mice co-inoculated with tumor-challenged MDSCs and 5TGM1 cells compared to the control animals received 5TGM1 cells alone. Finally, treatment of MDSCs from myeloma-challenged mice with Zoledronic acid (ZA), a potent inhibitor of bone resorption, inhibited the number of osteoclasts formed in MDSC cultures and the expansion of MDSCs and bone lesions in mice. Collectively, these data provide in vitro and in vivo evidence that tumor-induced MDSCs exacerbate cancer-associated bone destruction by directly serving as osteoclast precursors.

Solubilisation of the armadillo-repeat protein β-catenin using a zwitterionic detergent allows resolution of phosphorylated forms by 2DE

β-catenin is a member of the armadillo repeat family of proteins and has important functions in cell-cell adhesion and Wnt signalling. Different protein species of β-catenin have been shown to exist in the cell and the relative proportions of these species are altered upon stimulation of cells with Wnt-3a (Gottardi and Gumbiner, 2004). In order to determine whether posttranslational modifications (PTMs) of β-catenin underlie these different protein species, we have used 2DE separation and immunoblotting with an antibody specific for β-catenin. High-resolution separation of differentially modified species of β-catenin in 2DE required the addition of ASB-16, a zwitterionic detergent that can solubilise integral membrane proteins. ASB-16 was also necessary for focussing of other armadillo repeat proteins, such as γ-catenin and p120-catenin. 2DE using ASB-16 allowed detection of a previously unreported phosphorylation site in the transcriptionally active form of β-catenin that binds to GST-Tcf in response to Wnt signalling.

The tumor suppressor DiRas3 forms a complex with H-Ras and C-RAF proteins and regulates localization, dimerization, and kinase activity of C-RAF

The maternally imprinted Ras-related tumor suppressor gene DiRas3 is lost or down-regulated in more than 60% of ovarian and breast cancers. The anti-tumorigenic effect of DiRas3 is achieved through several mechanisms, including inhibition of cell proliferation, motility, and invasion, as well as induction of apoptosis and autophagy. Re-expression of DiRas3 in cancer cells interferes with the signaling through Ras/MAPK and PI3K. Despite intensive research, the mode of interference of DiRas3 with the Ras/RAF/MEK/ERK signal transduction is still a matter of speculation. In this study, we show that DiRas3 associates with the H-Ras oncogene and that activation of H-Ras enforces this interaction. Furthermore, while associated with DiRas3, H-Ras is able to bind to its effector protein C-RAF. The resulting multimeric complex consisting of DiRas3, C-RAF, and active H-Ras is more stable than the two protein complexes H-Ras·C-RAF or H-Ras·DiRas3, respectively. The consequence of this complex formation is a DiRas3-mediated recruitment and anchorage of C-RAF to components of the membrane skeleton, suppression of C-RAF/B-RAF heterodimerization, and inhibition of C-RAF kinase activity.

Secondary structure of lipidated Ras bound to a lipid bilayer

Ras proteins are small guanine nucleotide binding proteins that regulate many cellular processes, including growth control. They undergo distinct post-translational lipid modifications that are required for appropriate targeting to membranes. This, in turn, is critical for Ras biological function. However, most in vitro studies have been conducted on nonlipidated truncated forms of Ras proteins. Here, for the first time, attenuated total reflectance-FTIR studies of lipid-modified membrane-bound N-Ras are performed, and compared with nonlipidated truncated Ras in solution. For these studies, lipidated N-Ras was prepared by linking a farnesylated and hexadecylated N-Ras lipopeptide to a truncated N-Ras protein (residues 1-181). It was then bound to a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayer tethered on an attenuated total reflectance crystal. The structurally sensitive amide I absorbance band in the IR was detected and analysed to determine the secondary structure of the protein. The NMR three-dimensional structure of truncated Ras was used to calibrate the contributions of the different secondary structural elements to the amide I absorbance band of truncated Ras. Using this novel approach, the correct decomposition was selected from several possible solutions. The same parameter set was then used for the membrane-bound lipidated Ras, and provided a reliable decomposition for the membrane-bound form in comparison with truncated Ras. This comparison indicates that the secondary structure of membrane-bound Ras is similar to that determined for the nonlipidated truncated Ras protein for the highly conserved G-domain. This result validates the multitude of investigations of truncated Ras without anchor in vitro. The novel attenuated total reflectance approach opens the way for detailed studies of the interaction network of the membrane-bound Ras protein.

High glucose induced endothelial cell growth inhibition is associated with an increase in TGFbeta1 secretion and inhibition of Ras prenylation via suppression of the mevalonate pathway

OBJECTIVE

Ras proteins are known to affect cellular growth and function. The influence of the prenylation status of Ras on the observed changes in endothelial cell growth under high glucose conditions has not previously been examined.

METHODS

Human umbilical vein endothelial cells were exposed to normal or high glucose conditions for 72 h. They were then examined for proliferative and hypertrophic effects, transforming growth factor beta(1) (TGFbeta(1)) release, and phosphorylated p38 expression. The importance of prenylation was explored by the addition of mevalonate, isoprenoids or farnesyltransferase inhibitors to control the high glucose media and by measuring changes induced by high glucose and exogenous TGFbeta(1) in Ras prenylation and farnesyltransferase activity. Kidneys from diabetic rats treated with atorvastatin were also compared to specimens from untreated animals and the expression of the Ras effector p-Akt examined.

RESULTS

High glucose conditions caused a reduction in cell number. This was reversed in the presence of mevalonate or farnesylpyrophosphate (FPP), suggesting that the cell growth abnormalities observed are due to high glucose induced inhibition of the mevalonate pathway and subsequent prenylation of proteins. Endothelial cells exposed to high glucose increased their secretion of TGFbeta(1) and the phosphorylation of p38 both of which were reversed by concurrent exposure to FPP. A reduction in farnesyltransferase activity was observed after exposure to both high glucose and TGFbeta(1). Exposure to a farnesyltransferase inhibitor in control conditions mimicked the growth response observed with high glucose exposure and prenylated Ras was reduced by exposure to both high glucose and TGFbeta(1). Finally, interruption of the mevalonate pathway with a statin reduced the expression of p-Akt in diabetic rat kidneys.

CONCLUSION

This study demonstrates that high glucose induced significant alterations in endothelial cell growth by inhibition of the mevalonate pathway, which subsequently mediates the increase in TGFbeta(1) and inhibition of Ras prenylation.

Activation of calcium-dependent kinases and epidermal growth factor receptor regulate muscarinic acetylcholine receptor-mediated MAPK/ERK activation in thyroid epithelial cells

We previously showed that stimulation of muscarinic acetylcholine receptors (mAChR) by carbachol (Cch) caused a time- and dose-dependent increase of mitogen-activated protein kinase/extracellular signal-regulated kinases (MAPK/ERK) phosphorylation in thyroid epithelial cells. In this study, we demonstrated that mAChR stimulation also induced a time-dependent increase in the tyrosine phosphorylation of proline-rich tyrosine kinase 2 (Pyk2), which was prevented by pretreatment of thyroid epithelial cells with the specific Src-family tyrosine kinase inhibitor PP2. Besides, phosphorylation of Pyk2 was attenuated by chelation of extracellular Ca(2+) or inhibition of phospholipase C (PLC), and was evoked by thapsigargin, a specific microsomal Ca(2+)-ATPase inhibitor. Incorporation of Pyk2 antisense oligonucleotides in thyroid epithelial cells to down-regulated Pyk2 expression or pretreatment of cells with the Ca(2+)/calmodulin protein kinase II (CaM kinase II) inhibitor KN-62 significantly reduced Cch-induced MAPK/ERK phosphorylation. In addition, Cch-induced MAPK/ERK phosphorylation was partially inhibited by LY294002 and wortmannin, two selective inhibitors of phosphatidylinositol 3-kinase (PI3K), tyrphostin AG1478, a specific inhibitor of epidermal growth factor receptor (EGFR) kinase, and (-)-perillic acid, a post-translational inhibitor of small G-proteins isoprenylation. Taken together, our data suggest that Pyk2, CaM kinase II and Src-family tyrosine kinases are key molecules for the activation of MAPK/ERK cascade through the EGFR/Ras/Raf pathway in thyroid epithelial cells in response to mAChR stimulation.

Lipidated ras and rab peptides and proteins--synthesis, structure, and function

Chemical biology can be defined as the study of biological phenomena from a chemical approach. Based on the analysis of relevant biological phenomena and their structural foundation, unsolved problems are identified and tackled through a combination of chemistry and biology. Thus, new synthetic methods and strategies are developed and employed for the construction of compounds that are used to investigate biological procedures. Solid-phase synthesis has emerged as the preferred method for the synthesis of lipidated peptides, which can be chemoselectively ligated to proteins of the Ras superfamily. The generated peptides and proteins have solved biological questions in the field of the Ras-superfamily GTPases that are not amendable to chemical or biological techniques alone.

Prenylation of Ras facilitates hSOS1-promoted nucleotide exchange, upon Ras binding to the regulatory site

The oncoprotein Ras is anchored in lipid membranes due to its C-terminal lipid modification. The ubiquitously expressed Ras nucleotide exchange-factor hSOS1 promotes nucleotide exchange and thus Ras activation. This reaction is enhanced by a positive feedback loop whereby activated Ras binds to an allosteric site of SOS to enhance GEF activity. Here we present biochemical data showing that prenylation of both active site bound and allosterically bound N-Ras is required for efficient hSOS1-promoted nucleotide exchange. Our results indicate that prenyl sensitivity of the allosteric feedback-activation is mediated by the PH domain of hSOS1. Farnesylation of Ras thereby allows hSOS1 to bind even GDP-loaded allosteric regulator to maintain basal hSOS1-activity.

Cyclic AMP signalling in Dictyostelium: G-proteins activate separate Ras pathways using specific RasGEFs

In general, mammalian Ras guanine nucleotide exchange factors (RasGEFs) show little substrate specificity, although they are often thought to regulate specific pathways. Here, we provide in vitro and in vivo evidence that two RasGEFs can each act on specific Ras proteins. During Dictyostelium development, RasC and RasG are activated in response to cyclic AMP, with each regulating different downstream functions: RasG regulates chemotaxis and RasC is responsible for adenylyl cyclase activation. RasC activation was abolished in a gefA- mutant, whereas RasG activation was normal in this strain, indicating that RasGEFA activates RasC but not RasG. Conversely, RasC activation was normal in a gefR- mutant, whereas RasG activation was greatly reduced, indicating that RasGEFR activates RasG. These results were confirmed by the finding that RasGEFA and RasGEFR specifically released GDP from RasC and RasG, respectively, in vitro. This RasGEF target specificity provides a mechanism for one upstream signal to regulate two downstream processes using independent pathways.

Purification and membrane reconstitution of catalytically active Menkes copper-transporting P-type ATPase (MNK; ATP7A)

The MNK (Menkes disease protein; ATP7A) is a major copper- transporting P-type ATPase involved in the delivery of copper to cuproenzymes in the secretory pathway and the efflux of excess copper from extrahepatic tissues. Mutations in the MNK (ATP7A) gene result in Menkes disease, a fatal neurodegenerative copper deficiency disorder. Currently, detailed biochemical and biophysical analyses of MNK to better understand its mechanisms of copper transport are not possible due to the lack of purified MNK in an active form. To address this issue, we expressed human MNK with an N-terminal Glu-Glu tag in Sf9 [Spodoptera frugiperda (fall armyworm) 9] insect cells and purified it by antibody affinity chromatography followed by size-exclusion chromatography in the presence of the non-ionic detergent DDM (n-dodecyl beta-D-maltopyranoside). Formation of the classical vanadate-sensitive phosphoenzyme by purified MNK was activated by Cu(I) [EC50=0.7 microM; h (Hill coefficient) was 4.6]. Furthermore, we report the first measurement of Cu(I)-dependent ATPase activity of MNK (K0.5=0.6 microM; h=5.0). The purified MNK demonstrated active ATP-dependent vectorial 64Cu transport when reconstituted into soya-bean asolectin liposomes. Together, these data demonstrated that Cu(I) interacts with MNK in a co-operative manner and with high affinity in the sub-micromolar range. The present study provides the first biochemical characterization of a purified full-length mammalian copper-transporting P-type ATPase associated with a human disease.

Hydrophobic modifications of Ras proteins by isoprenoid groups and fatty acids--More than just membrane anchoring

During the last years, post-translational modification of peripheral membrane proteins with hydrophobic side groups has been attributed to a couple of additional functions than just simple anchoring into lipid bilayers. In particular isoprenylation and N- and S-acylation did quicken interest in terms of specific recognition elements for protein-protein interactions and as hydrophobic switches that allow for temporal regulated association with distinct target structures. Furthermore new insights into the heterogeneity of natural membranes have connected the physical properties of e.g. farnesyl or palmitoyl side chains with a preference for such sub-compartments as lipid rafts or caveolae. In this review the impact of the two frequently realized modifications by isoprenylation and S-acylation on the process of cellular signal transduction is exemplified with proteins of the Ras and Rab family of small GTP-binding proteins.

Prenylcysteine methylesterase in Arabidopsis thaliana

Prenylated proteins undergo a series of post-translational modifications, including prenylation, proteolysis, and methylation. Collectively, these modifications generate a prenylcysteine methylester at the carboxyl terminus and modulate protein targeting and function. Prenylcysteine methylation is the only reversible step in this series of modifications. However, prenylcysteine alpha-carboxyl methylesterase (PCME) activity has not been described in plants. We have detected a specific PCME activity in Arabidopsis thaliana membranes that discriminates between biologically relevant and irrelevant prenylcysteine methylester substrates. Furthermore, we have identified an Arabidopsis gene (At5g15860) that encodes measurable PCME activity in recombinant yeast cells with greater specificity for biologically relevant prenylcysteine methylesters than the activity found in Arabidopsis membranes. These results suggest that specific and non-specific esterases catalyze the demethylation of prenylcysteine methylesters in Arabidopsis membranes. Our findings are discussed in the context of prenylcysteine methylation/demethylation as a potential regulatory mechanism for membrane association and function of prenylated proteins in Arabidopsis.

P2Y receptors activate MAPK/ERK through a pathway involving PI3K/PDK1/PKC-zeta in human vein endothelial cells

AIMS

In this study we investigated the effects of P2 receptors in the regulation of mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) in human umbilical vein endothelial cells (HUVEC).

METHODS

Cytosolic Ca(2+) concentration ([Ca(2+)](i)) was measured using fura-2/AM, and MAPK/ ERK phosphorylation using Western blot analysis.

RESULTS

ATP, 2-meSATP, UTP and UDP cause a rapid and transitory increase in the phosphorylation of MAPK/ERK. In contrast, negligible response was seen for a,Beta-meATP, a general P2X receptors agonist. ATP-dependent activation of MAPK/ERK was prevented by pretreatment of HUVEC with pertussis toxin or MEK inhibitor PD98059. In addition, activation of the MAPK/ ERK cascade by ATP was blocked in cells pretreated with wortmannin and LY294002, but not by U73122, BAPTA or a Ca(2+)-free medium. Furthermore, an inhibition of ATP-dependent MAPK/ERK phosphorylation was observed in HUVEC pretreated with high doses of GF109203X or myristoylated PKC- zeta pseudosubstrate. Similar results were observed when cells were pretreated with the Src tyrosine kinase inhibitor PP2. However, ATP-stimulated MAPK/ERK activation was unaffected in cells pretreated with AG1478 or perillic acid. We also found that ATP stimulates both the phosphorylation of 3- phosphoinositide-dependent protein kinase-1 (PDK1) and its translocation to plasma membrane in a time-dependent manner.

CONCLUSION

These observations suggest that the effects mediated by ATP in HUVEC occur via PTX-sensitive G-protein-coupled P2Y receptors through PI3K-dependent mechanisms, in which PDK1 and PKC-zeta are two key molecules within signal cascade leading to MAPK/ERK activation.

In situ phosphorylation of immobilized receptors on biosensor surfaces: application to E-cadherin/beta-catenin interactions

Phosphorylation is a key posttranslational modification for modulating biological interactions. Biosensor technology is ideally suited for examining in real time the role of phosphorylation on protein-protein interactions in signaling pathways. We have developed processes for on-chip phosphorylation of immobilized receptors on biosensor surfaces. These processes have been used to analyze E-cadherin/beta-catenin interactions. Phosphorylation of the intracellular domain (ICD) of E-cadherin modulates its affinity to beta-catenin and consequently the strength of cell-cell adhesion. We have phosphorylated immobilized E-cadherin ICD in situ using casein kinase 1 (CK1), casein kinase 2 (CK2), and src. On-chip phosphorylation of E-cadherin was confirmed using anti-phosphoserine and anti-phosphotyrosine antibodies. The binding of beta-catenin to E-cadherin was analyzed quantitatively. CK1 phosphorylation of E-cadherin increased the binding affinity to beta-catenin from approximately 230 to 4 nM. A similar increase in affinity, from 260 to 4 nM, was obtained with CK2 phosphorylation of E-cadherin. However, phosphorylation by src kinase decreased the affinity constant from approximately 260 nM to 4 microM. Interestingly, phosphorylation of E-cadherin by CK1 or CK2 prevented the inhibition of beta-catenin binding by src phosphorylation.

Farnesylation or geranylgeranylation? Efficient assays for testing protein prenylation in vitro and in vivo

BACKGROUND

Available in vitro and in vivo methods for verifying protein substrates for posttranslational modifications via farnesylation or geranylgeranylation (for example, autoradiography with 3H-labeled anchor precursors) are time consuming (weeks/months), laborious and suffer from low sensitivity.

RESULTS

We describe a new technique for detecting prenyl anchors in N-terminally glutathione S-transferase (GST)-labeled constructs of target proteins expressed in vitro in rabbit reticulocyte lysate and incubated with 3H-labeled anchor precursors. Alternatively, hemagglutinin (HA)-labeled constructs expressed in vivo (in cell culture) can be used. For registration of the radioactive marker, we propose to use a thin layer chromatography (TLC) analyzer. As a control, the protein yield is tested by Western blotting with anti-GST- (or anti-HA-) antibodies on the same membrane that has been previously used for TLC-scanning. These protocols have been tested with Rap2A, v-Ki-Ras2 and RhoA (variant RhoA63L) including the necessary controls. We show directly that RasD2 is a farnesylation target.

CONCLUSION

Savings in time for experimentation and the higher sensitivity for detecting 3H-labeled lipid anchors recommend the TLC-scanning method with purified GST- (or HA-) tagged target proteins as the method of choice for analyzing their prenylation capabilities in vitro and in vivo and, possibly, also for studying the myristoyl and palmitoyl posttranslational modifications.

Visualizing Ras signalling in real-time

Ras GTPases are universal molecular switches that act as kinetic timers of signal transduction events. They are post-translationally modified by the addition of lipid groups to their hypervariable carboxyl termini, which plug the proteins to membranes and influence their dynamic sorting and trafficking. For the past twenty years, the plasma membrane has been considered to be the predominant platform from which Ras operates. Recent work using live-cell imaging and novel probes to visualize where and when Ras is active has supported this long-held belief. However, an equally fascinating aspect of these imaging studies has been the discovery of dynamic Ras activity, as well as distinct signal output, from intracellular organelles. Activation of Ras on the Golgi exhibits kinetics different from Ras activation on the plasma membrane, and compartmentalized Ras signalling seems particularly prominent in lymphocytes. However, data on the spatial and temporal regulation of Ras activity has frequently differed depending on the nature of the probe, the cell type and the stimulus. Nevertheless, because Ras traffics through endomembranes en route to the plasma membrane, it seems likely that Ras can signal from such compartments. The burning question in this field concerns the significance of this observation for endogenous Ras signalling output.

RRP22 is a farnesylated, nucleolar, Ras-related protein with tumor suppressor potential

Ras proteins are members of a superfamily of related small GTPases. Some members, such as Ras, are oncogenic. However, other members seem to serve as tumor suppressors, such as Rig and Noey2. We now identify and characterize a novel member of the Ras superfamily, RRP22. Like Ras, RRP22 can be posttranslationally modified by farnesyl. Unlike Ras, RRP22 inhibits cell growth and promotes caspase-independent cell death. Examination of human tumor cells shows that RRP22 is frequently down-regulated due to promoter methylation. Moreover, reexpression of RRP22 in an RRP22-negative neural tumor cell line impairs its growth in soft agar. Unusually for a Ras-related protein, RRP22 localizes to the nucleolus in a GTP-dependent manner, suggesting a novel mechanism of action. Thus, we identify a new member of the Ras superfamily that can serve as a potential tumor suppressor.

NMR characterization of full-length farnesylated and non-farnesylated H-Ras and its implications for Raf activation

The C terminus, also known as the hypervariable region (residues 166-189), of H-, N-, and K-Ras proteins has sequence determinants necessary for full activation of downstream effectors such as Raf kinase and PI-3 kinase as well as for the correct targeting of Ras proteins to lipid rafts and non-raft membranes. There is considerable interest in understanding how residues in the extreme C terminus of the different Ras proteins and farnesylation of the CaaX box cysteine affect Ras membrane localization and allosteric activation of Raf kinase. To provide insights into the structural and dynamic changes that occur in Ras upon farnesylation, we have used NMR spectroscopy to compare the properties of truncated H-Ras (1-166), to non-processed full-length H-Ras (residues 1-185) and full-length (1-189) farnesylated H-Ras. We report that the C-terminal helix alpha-5 extends to residue N172, and the remaining 17 amino acid residues in the C terminus are conformationally averaged in solution. Removal of either 23 or 18 amino acid residues from the C terminus of full length H-Ras generates truncated H-Ras (1-166) and H-Ras (1-171) proteins, respectively, that have been structurally characterized and are biochemical active. Here we report that C-terminal truncation of H-Ras results in minor structural and dynamic perturbations that are propagated throughout the H-Ras protein including increased flexibility of the central beta-sheet and the C-terminal helix alpha-5. Ordering of residues in loop-2, which is involved in Raf CRD binding is also observed. Farnesylation of full-length H-Ras at C186 does not result in detectable conformational changes in H-Ras. Chemical shift mapping studies of farnesylated and non-farnesylated forms of H-Ras with the Raf-CRD show that the farnesyl moiety, the extreme H-Ras C terminus and residues 23-30, contribute to H-Ras:Raf-CRD interactions, thereby increasing the affinity of H-Ras for the Raf-CRD.

Human RAS superfamily proteins and related GTPases

The tumor oncoproteins HRAS, KRAS, and NRAS are the founding members of a larger family of at least 35 related human proteins. Using a somewhat broader definition of sequence similarity reveals a more extended superfamily of more than 170 RAS-related proteins. The RAS superfamily of GTP (guanosine triphosphate) hydrolysis-coupled signal transduction relay proteins can be subclassified into RAS, RHO, RAB, and ARF families, as well as the closely related Galpha family. The members of each family can, in turn, be arranged into evolutionarily conserved branches. These groupings reflect structural, biochemical, and functional conservation. Recent findings have provided insights into the signaling characteristics of representative members of most RAS superfamily branches. The analysis presented here may serve as a guide for predicting the function of numerous uncharacterized superfamily members. Also described are guanosine triphosphatases (GTPases) distinct from members of the RAS superfamily. These related proteins employ GTP binding and GTPase domains in diverse structural contexts, expanding the scope of their function in humans.

Biochemical and functional characterizations of small GTPase Rheb and TSC2 GAP activity

Tuberous sclerosis complex (TSC) is a genetic disease caused by a mutation in either the tsc1 or tsc2 tumor suppressor gene. Recent studies have demonstrated that TSC2 displays GAP (GTPase-activating protein) activity specifically towards the small G protein Rheb and inhibits its ability to stimulate the mTOR signaling pathway. Rheb and TSC2 comprise a unique pair of GTPase and GAP, because Rheb has high basal GTP levels and TSC2 does not have the catalytic arginine finger found in Ras-GAP. To investigate the function of TSC2 and Rheb in mTOR signaling, we analyzed the TSC2-stimulated Rheb GTPase activity. We found that Arg15, a residue equivalent to Gly12 in Ras, is important for Rheb to function as a substrate for TSC2 GAP. In addition, we identified asparagine residues essential for TSC2 GAP activity. We demonstrated a novel catalytic mechanism of the TSC2 GAP and Rheb that TSC2 uses a catalytic "asparagine thumb" instead of the arginine finger found in Ras-GAP. Furthermore, we discovered that farnesylation and membrane localization of Rheb is not essential for Rheb to stimulate S6 kinase (S6K) phosphorylation. Analysis of TSC1 binding defective mutants of TSC2 shows that TSC1 is not required for the TSC2 GAP activity but may function as a regulatory component in the TSC1/TSC2 complex. Our data further demonstrate that GAP activity is essential for the cellular function of TSC2 to inhibit S6K phosphorylation.

Characterization of Saccharomyces cerevisiae Ras1p and chimaeric constructs of Ras proteins reveals the hypervariable region and farnesylation as critical elements in the adenylyl cyclase signaling pathway

Ras1p and Ras2p, from Saccharomyces cerevisiae, are GTP-binding proteins that are essential elements in the signaling cascade leading to the activation of adenylyl cyclase. To overcome proteolytic activities that have hampered biochemical studies of Ras1p so far, its gene was genetically modified after which full-length Ras1p could be obtained. The interaction of farnesylated and unprenylated Ras1p with guanine nucleotides, guanine nucleotide exchange factors, GTPase activating proteins, and adenylyl cyclase was compared to Ras2p and human Ha-Ras interactions. Farnesylation of Ras proteins was demonstrated to be a prerequisite for membrane-bound guanine nucleotide exchange factor dependent formation of Ras-GTP complexes, and for efficient Ras-mediated adenylyl cyclase activation. To relate observed functional deviations with sequence differences between Ras1p and Ras2p, which reside almost exclusively within the hypervariable region, truncated versions and chimaeras of the Ras proteins were made. The characteristics of these constructs point to the presence of the hypervariable region of yeast Ras proteins for an efficient activation of adenylyl cyclase. The importance of the latter was confirmed as inhibition of the activation of adenylyl cyclase by an isolated farnesylated hypervariable region of Ras2p could be shown. This strongly suggests that the hypervariable region of Ras proteins can interact directly with adenylyl cyclase.

Carboxyl methylation of Ras regulates membrane targeting and effector engagement

Post-translational modification of Ras proteins includes prenylcysteine-directed carboxyl methylation. Because Ras participates in Erk activation by epidermal growth factor (EGF), we tested whether Ras methylation regulates Erk activation. EGF stimulation of Erk was inhibited by AFC (N-acetyl-S-farnesyl-L-cysteine), an inhibitor of methylation, but not AGC (N-acetyl-S-geranyl-L-cysteine), an inactive analog of AFC. AFC inhibited Ras methylation as well as the activation of pathway enzymes between Ras and Erk but did not inhibit EGF receptor phosphorylation, confirming action at the level of Ras. Transient transfection of human prenylcysteine-directed carboxyl methyltransferase increased EGF-stimulated Erk activation. AFC but not AGC inhibited movement of transiently transfected green fluorescent protein-Ras from the cytosol to the plasma membrane of COS-1 cells and depleted green fluorescent protein-Ras from the plasma membrane in stably transfected Madin-Darby canine kidney cells, suggesting that methylation regulates Erk by ensuring proper membrane localization of Ras. However, when COS-1 cells were transfected with Ras complexed to CD8, plasma membrane localization of Ras was unaffected by AFC, yet EGF-stimulated Erk activation was inhibited by AFC. Thus, Ras methylation appears to regulate Erk activation both through the localization of Ras as well as the propagation of Ras-dependent signals.

Synthesis and Biological Activity of Photoactivatable N-Ras Peptides and Proteins

A modular strategy for the assembly of farnesylated N-Ras heptapeptides carrying a photoactivatable benzophenone (BP) group within the lipid residue is described. This strategy is based on the fragment condensation of a N-terminal hexapeptide synthesized on the solid support with a cysteine methyl ester which is modified with different farnesyl analogues, incorporating the photophor. At the N-terminus of the peptides different functional groups can be attached, e.g., biotin for product enrichment and detection after photoactivation or a maleimido (MIC) linker, allowing for the coupling to proteins carrying a C-terminal free cysteine. Using this strategy, 24 peptides were synthesized, incorporating farnesyl analogues with four different chain lengths. Two of these photoactivatable conjugates were ligated to oncogenic human N-RasG12V Delta 181. A cellular transformation assay revealed that the semisynthetic proteins retain their biological activity despite the photolabel. The first photolabeling experiments with a geranyl-BP-labeled N-Ras construct and the farnesyl-sensitive guanine nucleotide exchange factor hSos1 indicate that this photoaffinity labeling system can be particularly useful for studying protein-protein interactions, e.g., the participation of the farnesyl group in Ras signaling, which is still discussed with controversy.

A growing family of guanine nucleotide exchange factors is responsible for activation of Ras-family GTPases

GTPases of the Ras subfamily regulate a diverse array of cellular-signaling pathways, coupling extracellular signals to the intracellular response machinery. Guanine nucleotide exchange factors (GEFs) are primarily responsible for linking cell-surface receptors to Ras protein activation. They do this by catalyzing the dissociation of GDP from the inactive Ras proteins. GTP can then bind and induce a conformational change that permits interaction with downstream effectors. Over the past 5 years, approximately 20 novel Ras-family GEFs have been identified and characterized. These data indicate that a variety of different signaling mechanisms can be induced to activate Ras, enabling tyrosine kinases, G-protein-coupled receptors, adhesion molecules, second messengers, and various protein-interaction modules to relocate and/or activate GEFs and elevate intracellular Ras-GTP levels. This review discusses the structure and function of the catalytic or CDC25 homology domain common to almost all Ras-family GEFs. It also details our current knowledge about the regulation and function of this rapidly growing family of enzymes that include Sos1 and 2, GRF1 and 2, CalDAG-GEF/GRP1-4, C3G, cAMP-GEF/Epac 1 and 2, PDZ-GEFs, MR-GEF, RalGDS family members, RalGPS, BCAR3, Smg GDS, and phospholipase C(epsilon).

Modeling of binding modes and inhibition mechanism of some natural ligands of farnesyl transferase using molecular docking

Several natural inhibitors of farnesyl transferase have been reported in the literature: some compounds are competitive with farnesyl pyrophosphate (FPP), whereas other ones are competitive with Ras proteins, even though it is usually hard to highlight their inhibition mechanism, which is still unknown for several natural compounds. The aim of this work is to show that the molecular docking analysis can be successfully used to underline the inhibition mechanism of these natural compounds. First, the selected compounds were subjected to a detailed docking analysis, by means of BioDock, a program able to reveal the most likely binding mode for each ligand. By comparing these results with the binding sites for the natural substrates, earlier determined, it was possible to highlight the site specificity and the inhibition mechanism of the selected compounds. In addition, it is possible to relate the binding mode of these molecules with their lipole values, which is appreciably less for peptidomimetics than for FPP mimetic and reveals a straightforward method to predict and to understand the inhibition mechanism of these natural derivatives.

The linker domain of the Ha-Ras hypervariable region regulates interactions with exchange factors, Raf-1 and phosphoinositide 3-kinase

Ha-Ras and Ki-Ras have different distributions across plasma membrane microdomains. The Ras C-terminal anchors are primarily responsible for membrane micro-localization, but recent work has shown that the interaction of Ha-Ras with lipid rafts is modulated by GTP loading via a mechanism that requires the hypervariable region (HVR). We have now identified two regions in the HVR linker domain that regulate Ha-Ras raft association. Release of activated Ha-Ras from lipid rafts is blocked by deleting amino acids 173-179 or 166-172. Alanine replacement of amino acids 173-179 but not 166-172 restores wild type micro-localization, indicating that specific N-terminal sequences of the linker domain operate in concert with a more C-terminal spacer domain to regulate Ha-Ras raft association. Mutations in the linker domain that confine activated Ha-RasG12V to lipid rafts abrogate Raf-1, phosphoinositide 3-kinase, and Akt activation and inhibit PC12 cell differentiation. N-Myristoylation also prevents the release of activated Ha-Ras from lipid rafts and inhibits Raf-1 activation. These results demonstrate that the correct modulation of Ha-Ras lateral segregation is critical for downstream signaling. Mutations in the linker domain also suppress the dominant negative phenotype of Ha-RasS17N, indicating that HVR sequences are essential for efficient interaction of Ha-Ras with exchange factors in intact cells.

Fetal hyperinsulinemia increases farnesylation of p21 Ras in fetal tissues

Even though the role of fetal hyperinsulinemia in the pathogenesis of fetal macrosomia in patients with overt diabetes and gestational diabetes mellitus seems plausible, the molecular mechanisms of action of hyperinsulinemia remain largely enigmatic. Recent indications that hyperinsulinemia "primes" various tissues to the mitogenic influence of growth factors by increasing the pool of prenylated Ras proteins prompted us to investigate the effect of fetal hyperinsulinemia on the activitiy of farnesyltransferase (FTase) and the amounts of farnesylated p21 Ras in fetal tissues in the ovine experimental model. Induction of fetal hyperinsulinemia by direct infusion of insulin into the fetus and by either fetal or maternal infusions of glucose resulted in significant increases in the activity of FTase and the amounts of farnesylated p21 Ras in fetal liver, skeletal muscle, fat, and white blood cells. An additional infusion of somatostatin into hyperglycemic fetuses blocked fetal hyperinsulinemia and completely prevented these increases, specifying insulin as the causative factor. We conclude that the ability of fetal hyperinsulinemia to increase the size of the pool of farnesylated p21 Ras may prime fetal tissues to the action of other growth factors and thereby constitute one mechanism by which fetal hyperinsulinemia could induce macrosomia in diabetic pregnancies.

Prenylated Rab acceptor protein is a receptor for prenylated small GTPases

Localization of Ras and Ras-like proteins to the correct subcellular compartment is essential for these proteins to mediate their biological effects. Many members of the Ras superfamily (Ha-Ras, N-Ras, TC21, and RhoA) are prenylated in the cytoplasm and then transit through the endomembrane system on their way to the plasma membrane. The proteins that aid in the trafficking of the small GTPases have not been well characterized. We report here that prenylated Rab acceptor protein (PRA1), which others previously identified as a prenylation-dependent receptor for Rab proteins, also interacts with Ha-Ras, RhoA, TC21, and Rap1a. The interaction of these small GTPases with PRA1 requires their post-translational modification by prenylation. The prenylation-dependent association of PRA1 with multiple GTPases is conserved in evolution; the yeast PRA1 protein associates with both Ha-Ras and RhoA. Earlier studies reported the presence of PRA1 in the Golgi, and we show here that PRA1 co-localizes with Ha-Ras and RhoA in the Golgi compartment. We suggest that PRA1 acts as an escort protein for small GTPases by binding to the hydrophobic isoprenoid moieties of the small GTPases and facilitates their trafficking through the endomembrane system.

Dominant negative farnesyltransferase alpha-subunit inhibits insulin mitogenic effects

Farnesylation of p21Ras is required for translocation to the plasma membrane and subsequent activation by growth factors. Previously we demonstrated that insulin stimulates the phosphorylation of farnesyltransferase (FTase) and its activity, whereby the amount of farnesylated p21Ras anchored at the plasma membrane is increased. Herein we report that substitution of alanine for two serine residues (S60A)(S62A) of the alpha-subunit of FTase creates a dominant negative (DN) mutant. VSMC expressing the FTase alpha-subunit (S60A)(S62A) clone showed a 30% decreased basal FTase activity concurrent with a 15% decrease in the amount of farnesylated p21Ras compared to controls. Expression of alpha-subunit (S60A,S62A) blunted FTase phosphorylation and activity in the presence of hyperinsulinemia, and inhibited insulin-stimulated increases in farnesylated p21Ras. Insulin-stimulated VSMC expressing the FTase alpha-subunit (S60A,S62A) showed decreased (i) phosphorylation of FTase, (ii) FTase activity, (iii) amounts of farnesylated p21Ras, (iv) DNA synthesis, and (v) migration. Thus, down-regulation of FTase activity appears to mitigate the potentially detrimental mitogenic effects of hyperinsulinemia on VSMC.

Disrupting the transforming activity of shrimpras(Q61K) by deleting the CAAX box at the C-terminus

BALB/3T3 cells were transformed by transfection with DNA encoding the mutated ras(Q(61)K) from shrimp Penaeus japonicus. Ras transcription and protein levels had increased significantly in the cells transfected with the S-ras plasmid, compared to cells transfected with a control plasmid pcDNA3.1. The bacterially expressed GTP-locked S-Ras(Q(61)K) is successfully prenylated by rat protein geranylgeranyltransferase I (PGGTase I) and then polymerized with tubulin, in agreement with findings for GTP-locked mammalian K(B)-Ras(Q(61)K) in vitro. Shrimp protein farnesyltransferase (PFTase) of shrimp did not prenylate the GTP-locked shrimp S-Ras(Q(61)K) (Lin and Chuang. 1998. J Exp Zool 281:565-573), whereas rat PFTase efficiently catalyzed the farnesylation of GTP-locked S-Ras(Q(61)K). To investigate the effect of geranylgeranylation on cellular transformation, we generated S-ras(Q(61)K) mutants with deletion of the CAAX box [S-ras(Q(61)K)(-caax)] or replacement of the CAAX box [S-ras(Q(61)K)(Kcaax)] or replacement of the arginine-rich domain [S-ras(Q(61)K)(K-Lys)] with corresponding sequences from rat K(B)-ras(Q(61)K). BALB/3T3 cells transfected with DNA encoding S-ras(Q(61)K), S-ras(Q(61)K)(KCAAX), S-ras(Q(61)K)(K-Lys) were transformed successfully, but S-ras(Q(61)K)(-CAAX) was defective in its ability to transform. Thus, prenylation at CAAX is required for transformation. Either the geranylgeranylated or the farnesylated S-Ras(Q(61)K) was endowed with abilities to transform. The arginine-rich region in S-Ras or the lysine-rich clusters from the rat K(B)-Ras appear not essential for activity to transform.

Characterization of RasGRP2, a plasma membrane-targeted, dual specificity Ras/Rap exchange factor

Ras proteins operate as molecular switches in signal transduction pathways downstream of tyrosine kinases and G-protein-coupled receptors. Ras is switched from the inactive GDP-bound state to the active GTP-bound state by guanine nucleotide exchange factors (GEFs). We report here the cloning and characterization of RasGRP2, a longer alternatively spliced form of the recently cloned RapGEF, CalDAG-GEFI. A unique feature of RasGRP2 is that it is targeted to the plasma membrane by a combination of N-terminal myristoylation and palmitoylation. In vivo, RasGRP2 selectively catalyzes nucleotide exchange on N- and Ki-Ras, but not Ha-Ras. RasGRP2 also catalyzes nucleotide exchange on Rap1, but this RapGEF activity is less potent than that associated with CalDAG-GEFI. The nucleotide exchange activity of RasGRP2 toward N-Ras is stimulated by diacylglycerol and inhibited by calcium. The effects of diacylglycerol and calcium are additive but are not accompanied by any detectable change in the subcellular localization of RasGRP2. In contrast, CalDAG-GEFI is localized predominantly to the cytosol and lacks Ras exchange activity in vivo. However, prolonged exposure to phorbol esters, or growth in serum, results in localization of CalDAG-GEFI to the cell membrane and restoration of Ras exchange activity. Expression of RasGRP2 or CalDAG-GEFI in NIH3T3 cells transfected with wild type N-Ras results in an accelerated growth rate but not morphologic transformation. Thus, under appropriate growth conditions, CalDAG-GEFI and RasGRP2 are dual specificity Ras and Rap exchange factors.

Functional implications of protein isoprenylation in plants

Plant protein isoprenylation has received considerable attention in the past decade. Since the initial discovery of isoprenylated plant proteins and their respective protein isoprenyltransferases, several research groups have endeavored to understand the physiological significance of this process in plants. Various experimental approaches, including inhibitor studies, systematic methods of protein identification, and mutant analyses in Arabidopsis thaliana, have enabled these groups to elucidate important roles for isoprenylated proteins in cell cycle control, signal transduction, cytoskeletal organization, and intracellular vesicle transport. This article reviews recent progress in understanding the functional implications of protein isoprenylation in plants.

Cellular compartmentalization in insulin action: altered signaling by a lipid-modified IRS-1

While most receptor tyrosine kinases signal by recruiting SH2 proteins directly to phosphorylation sites on their plasma membrane receptor, the insulin receptor phosphorylates intermediary IRS proteins that are distributed between the cytoplasm and a state of loose association with intracellular membranes. To determine the importance of this distribution to IRS-1-mediated signaling, we constructed a prenylated, constitutively membrane-bound IRS-1 by adding the COOH-terminal 9 amino acids from p21(ras), including the CAAX motif, to IRS-1 (IRS-CAAX) and analyzed its function in 32D cells expressing the insulin receptor. IRS-CAAX migrated more slowly on sodium dodecyl sulfate-polyacrylamide gel electrophoresis than did IRS-1 and demonstrated increased levels of serine/threonine phosphorylation. Insulin-stimulated tyrosyl phosphorylation of IRS-CAAX was slightly decreased, while IRS-CAAX-mediated phosphatidylinositol 3'-kinase (PI3'-kinase) binding and activation were decreased by approximately 75% compared to those for wild-type IRS-1. Similarly, expression of IRS-CAAX desensitized insulin-stimulated [(3)H]thymidine incorporation into DNA by about an order of magnitude compared to IRS-1. By contrast, IRS-CAAX-expressing cells demonstrated increased signaling by mitogen-activated protein kinase, Akt, and p70(S6) kinase in response to insulin. Hence, tight association with the membrane increased IRS-1 serine phosphorylation and reduced coupling between the insulin receptor, PI3'-kinase, and proliferative signaling while enhancing other signaling pathways. Thus, the correct distribution of IRS-1 between the cytoplasm and membrane compartments is critical to the normal balance in the network of insulin signaling.

Analysis of the role of the hypervariable region of yeast Ras2p and its farnesylation in the interaction with exchange factors and adenylyl cyclase

Ras proteins from Saccharomyces cerevisiae differ from mammalian Ha-Ras in their extended C-terminal hypervariable region. We have analyzed the function of this region and the effect of its farnesylation with respect to the action of the GDP/GTP exchange factors (GEFs) Cdc25p and Sdc25p and the target adenylyl cyclase. Whereas Ras2p farnesylation had no effect on the interaction with purified GEFs from the Cdc25 family, this modification became a strict requirement for stimulation of the nucleotide exchange on Ras using reconstituted cell-free systems with GEFs bound to the cell membrane. Determination of GEF effects showed that in cell membrane the Cdc25p dependent activity on Ras2p was predominant over that of Sdc25p. In contrast to full-length GEFs, a membrane-bound C-terminal region containing the catalytic domain of Cdc25p was still able to react productively with unfarnesylated Ras2p. These results indicate that in membrane-bound full-length GEF the N-terminal moiety regulates the interaction between catalytic domain and farnesylated Ras2p.GDP. Differently from GEF, full activation of adenylyl cyclase did not require farnesylation of Ras2p.GTP, even if this step of maturation was found to facilitate the interaction. The use of Ha-Ras/Ras2p chimaeras of different length emphasized the key role of the hypervariable region of Ras2p in inducing maximum activation of adenylyl cyclase and for a productive interaction with membrane-bound GEF.

Class II phosphoinositide 3-kinases are downstream targets of activated polypeptide growth factor receptors

The class II phosphoinositide 3-kinases (PI3K) PI3K-C2alpha and PI3K-C2beta are two recently identified members of the large PI3K family. Both enzymes are characterized by the presence of a C2 domain at the carboxy terminus and, in vitro, preferentially utilize phosphatidylinositol and phosphatidylinositol 4-monophosphate as lipid substrates. Little is understood about how the catalytic activity of either enzyme is regulated in vivo. In this study, we demonstrate that PI3K-C2alpha and PI3K-C2beta represent two downstream targets of the activated epidermal growth factor (EGF) receptor in human carcinoma-derived A431 cells. Stimulation of quiescent cultures with EGF resulted in the rapid recruitment of both enzymes to a phosphotyrosine signaling complex that contained the EGF receptor and Erb-B2. Ligand addition also induced the appearance of a second, more slowly migrating band of PI3K-C2alpha and PI3K-C2beta immunoreactivity on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Since both PI3K enzymes can utilize Ca(2+) as an essential divalent cation in lipid kinase assays and since the catalytic activity of PI3K-C2alpha is refractory to the inhibitor wortmannin, these properties were used to confirm the recruitment of each PI3K isozyme to the activated EGF receptor complex. To examine this interaction in greater detail, PI3K-C2beta was chosen for further investigation. EGF and platelet-derived growth factor also stimulated the association of PI3K-C2beta with their respective receptors in other cells, including epithelial cells and fibroblasts. The use of EGF receptor mutants and phosphopeptides derived from the EGF receptor and Erb-B2 demonstrated that the interaction with recombinant PI3K-C2beta occurs through E(p)YL/I phosphotyrosine motifs. The N-terminal region of PI3K-C2beta was found to selectively interact with the EGF receptor in vitro, suggesting that it mediates the association of this PI3K with the receptor. However, the mechanism of this interaction remains unclear. We conclude that class II PI3K enzymes may contribute to the generation of 3' phosphoinositides following the activation of polypeptide growth factor receptors in vivo and thus mediate certain aspects of their biological activity.

Recent advances in the study of prenylated proteins

Post-translational modification of proteins with isoprenoids was first recognized as a general phenomenon in 1984. In recent years, our understanding, including mechanistic studies, of the enzymatic reactions associated with these modifications and their physiological functions has increased dramatically. Of particular functional interest is the role of prenylation in facilitating protein-protein interactions and membrane-associated protein trafficking. The loss of proper localization of Ras proteins when their farnesylation is inhibited has also permitted a new target for anti-malignancy pharmaceuticals. Recent advances in the enzymology and function of protein prenylation are reviewed in this article.