The C-terminal polylysine region and methylation of K-Ras are critical for the interaction between K-Ras and microtubules

Connecting the dots: LncRNAs in the KRAS pathway and cancer

Long non-coding RNAs (lncRNAs) have been identified as important participants in several biological functions, particularly their complex interactions with the KRAS pathway, which provide insights into the significant roles lncRNAs play in cancer development. The KRAS pathway, a central signaling cascade crucial for cell proliferation, survival, and differentiation, stands out as a key therapeutic target due to its aberrant activation in many human cancers. Recent investigations have unveiled a myriad of lncRNAs, such as H19, ANRIL, and MEG3, intricately modulating the KRAS pathway, influencing both its activation and repression through various mechanisms, including epigenetic modifications, transcriptional regulation, and post-transcriptional control. These lncRNAs function as fine-tuners, delicately orchestrating the balance required for normal cellular function. Their dysregulation has been linked to the development and progression of multiple malignancies, including lung, pancreatic, and colorectal carcinomas, which frequently harbor KRAS mutations. This scrutiny delves into the functional diversity of specific lncRNAs within the KRAS pathway, elucidating their molecular mechanisms and downstream effects on cancer phenotypes. Additionally, it underscores the diagnostic and prognostic potential of these lncRNAs as indicators for cancer detection and assessment. The complex regulatory network that lncRNAs construct within the context of the KRAS pathway offers important insights for the creation of focused therapeutic approaches, opening new possibilities for precision medicine in oncology. However, challenges such as the dual roles of lncRNAs in different cancer types and the difficulty in therapeutically targeting these molecules highlight the ongoing debates and need for further research. As ongoing studies unveil the complexities of lncRNA-mediated KRAS pathway modulation, the potential for innovative cancer interventions becomes increasingly promising.

KRASG12C Inhibitors in Non-Small Cell Lung Cancer: A Review

Rat sarcoma virus (RAS) GTPase is one of the most important drivers of non-small cell lung cancer (NSCLC). RAS has three different isoforms (Harvey rat sarcoma viral oncogene homolog [HRAS], Kirsten rat sarcoma viral oncogene homolog [KRAS] and Neuroblastoma ras viral oncogene homolog [NRAS]), of which KRAS is most commonly mutated in NSCLC. The mutated KRAS protein was historically thought to be "undruggable" until the development of KRASG12C inhibitors. In this review, from the aspect of brain metastasis, we aim to provide an overview of the advances in therapies that target KRASG12C, the limitations of the current treatments, and future prospects in patients with KRAS p.G12C mutant NSCLC.

Circular RNAs in the KRAS pathway: Emerging players in cancer progression

Circular RNAs (circRNAs) have been recognized as key components in the intricate regulatory network of the KRAS pathway across various cancers. The KRAS pathway, a central signalling cascade crucial in tumorigenesis, has gained substantial emphasis as a possible therapeutic target. CircRNAs, a subgroup of non-coding RNAs known for their closed circular arrangement, play diverse roles in gene regulation, contributing to the intricate landscape of cancer biology. This review consolidates existing knowledge on circRNAs within the framework of the KRAS pathway, emphasizing their multifaceted functions in cancer progression. Notable circRNAs, such as Circ_GLG1 and circITGA7, have been identified as pivotal regulators in colorectal cancer (CRC), influencing KRAS expression and the Ras signaling pathway. Aside from their significance in gene regulation, circRNAs contribute to immune evasion, apoptosis, and drug tolerance within KRAS-driven cancers, adding complexity to the intricate interplay. While our comprehension of circRNAs in the KRAS pathway is evolving, challenges such as the diverse landscape of KRAS mutant tumors and the necessity for synergistic combination therapies persist. Integrating cutting-edge technologies, including deep learning-based prediction methods, holds the potential for unveiling disease-associated circRNAs and identifying novel therapeutic targets. Sustained research efforts are crucial to comprehensively unravel the molecular mechanisms governing the intricate interplay between circRNAs and the KRAS pathway, offering insights that could potentially revolutionize cancer diagnostics and treatment strategies.

Small molecular inhibitors for KRAS-mutant cancers

Three rat sarcoma (RAS) gene isoforms, KRAS, NRAS, and HRAS, constitute the most mutated family of small GTPases in cancer. While the development of targeted immunotherapies has led to a substantial improvement in the overall survival of patients with non-KRAS-mutant cancer, patients with RAS-mutant cancers have an overall poorer prognosis owing to the high aggressiveness of RAS-mutant tumors. KRAS mutations are strongly implicated in lung, pancreatic, and colorectal cancers. However, RAS mutations exhibit diverse patterns of isoforms, substitutions, and positions in different types of cancers. Despite being considered "undruggable", recent advances in the use of allele-specific covalent inhibitors against the most common mutant form of RAS in non-small-cell lung cancer have led to the development of effective pharmacological interventions against RAS-mutant cancer. Sotorasib (AMG510) has been approved by the FDA as a second-line treatment for patients with KRAS-G12C mutant NSCLC who have received at least one prior systemic therapy. Other KRAS inhibitors are on the way to block KRAS-mutant cancers. In this review, we summarize the progress and promise of small-molecule inhibitors in clinical trials, including direct inhibitors of KRAS, pan-RAS inhibitors, inhibitors of RAS effector signaling, and immune checkpoint inhibitors or combinations with RAS inhibitors, to improve the prognosis of tumors with RAS mutations.

Novel natural inhibitors targeting KRAS G12C by computational study

Ideal leading and nominee compounds with inhibiting effects on KRAS G12C were selected from the ZINC database, laying a cornerstone for the progress of anticancer drugs. A variety of computational virtual screening methods were utilized to screen possible inhibitors of KRAS G12C. LibDock was utilized to estimate 17 930 compounds and the top 20 were nominated for additional study, which was absorption, distribution, metabolism, and excretion and harmfulness prediction. Molecule docking was employed to prove the binding connection between certain ligands and KRAS G12C. Natural novel compounds ZINC000012494057 and ZINC000003789195 were selected to bind stably with KRAS G12C. In addition, they had lower scores in Ames mutagenicity, rodent carcinogenicity, cytochrome P450 2D6(CYP2D6) tolerance, and non-developmental toxicity potential. Molecular dynamic simulations demonstrate that the combination of ZINC000012494057 and ZINC000003789195 with KRAS G12C has more favorable potential energy, which provides conditions for their stable existence in the natural environment. Natural compounds ZINC000012494057 and ZINC000003789195 were identified as KRAS G12C potential inhibitors. These two compounds have been verified to have enormous importance for the progress of anticancer medicines.

Role of oncogenic KRAS in the prognosis, diagnosis and treatment of colorectal cancer

Colorectal cancer (CRC) is a heterogeneous disease at the cellular and molecular levels. Kirsten rat sarcoma (KRAS) is a commonly mutated oncogene in CRC, with mutations in approximately 40% of all CRC cases; its mutations result in constitutive activation of the KRAS protein, which acts as a molecular switch to persistently stimulate downstream signaling pathways, including cell proliferation and survival, thereby leading to tumorigenesis. Patients whose CRC harbors KRAS mutations have a dismal prognosis. Currently, KRAS mutation testing is a routine clinical practice before treating metastatic cases, and the approaches developed to detect KRAS mutations have exhibited favorable sensitivity and accuracy. Due to the presence of KRAS mutations, this group of CRC patients requires more precise therapies. However, KRAS was historically thought to be an undruggable target until the development of KRAS^G12C allele-specific inhibitors. These promising inhibitors may provide novel strategies to treat KRAS-mutant CRC. Here, we provide an overview of the role of KRAS in the prognosis, diagnosis and treatment of CRC.

Post-translational Modifications of the Protein Termini

Post-translational modifications (PTM) involve enzyme-mediated covalent addition of functional groups to proteins during or after synthesis. These modifications greatly increase biological complexity and are responsible for orders of magnitude change between the variety of proteins encoded in the genome and the variety of their biological functions. Many of these modifications occur at the protein termini, which contain reactive amino- and carboxy-groups of the polypeptide chain and often are pre-primed through the actions of cellular machinery to expose highly reactive residues. Such modifications have been known for decades, but only a few of them have been functionally characterized. The vast majority of eukaryotic proteins are N- and C-terminally modified by acetylation, arginylation, tyrosination, lipidation, and many others. Post-translational modifications of the protein termini have been linked to different normal and disease-related processes and constitute a rapidly emerging area of biological regulation. Here we highlight recent progress in our understanding of post-translational modifications of the protein termini and outline the role that these modifications play in vivo.

The Hypervariable Region of K-Ras4B Governs Molecular Recognition and Function

The flexible C-terminal hypervariable region distinguishes K-Ras4B, an important proto-oncogenic GTPase, from other Ras GTPases. This unique lysine-rich portion of the protein harbors sites for post-translational modification, including cysteine prenylation, carboxymethylation, phosphorylation, and likely many others. The functions of the hypervariable region are diverse, ranging from anchoring K-Ras4B at the plasma membrane to sampling potentially auto-inhibitory binding sites in its GTPase domain and participating in isoform-specific protein-protein interactions and signaling. Despite much research, there are still many questions about the hypervariable region of K-Ras4B. For example, mechanistic details of its interaction with plasma membrane lipids and with the GTPase domain require further clarification. The roles of the hypervariable region in K-Ras4B-specific protein-protein interactions and signaling are incompletely defined. It is also unclear why post-translational modifications frequently found in protein polylysine domains, such as acetylation, glycation, and carbamoylation, have not been observed in K-Ras4B. Expanding knowledge of the hypervariable region will likely drive the development of novel highly-efficient and selective inhibitors of K-Ras4B that are urgently needed by cancer patients.

Depletion of phosphatidylinositol 4-phosphate at the Golgi translocates K-Ras to mitochondria

Ras proteins are small GTPases localized to the plasma membrane (PM), which regulate cellular proliferation, apoptosis and differentiation. After a series of post-translational modifications, H-Ras and N-Ras traffic to the PM from the Golgi via the classical exocytic pathway, but the exact mechanism of K-Ras trafficking to the PM from the ER is not fully characterized. ATP5G1 (also known as ATP5MC1) is one of the three proteins that comprise subunit c of the F ₀ complex of the mitochondrial ATP synthase. In this study, we show that overexpression of the mitochondrial targeting sequence of ATP5G1 perturbs glucose metabolism, inhibits oncogenic K-Ras signaling, and redistributes phosphatidylserine (PtdSer) to mitochondria and other endomembranes, resulting in K-Ras translocation to mitochondria. Also, it depletes phosphatidylinositol 4-phosphate (PI4P) at the Golgi. Glucose supplementation restores PtdSer and K-Ras PM localization and PI4P at the Golgi. We further show that inhibition of the Golgi-localized PI4-kinases (PI4Ks) translocates K-Ras, and PtdSer to mitochondria and endomembranes, respectively. We conclude that PI4P at the Golgi regulates the PM localization of PtdSer and K-Ras.This article has an associated First Person interview with the first author of the paper.

On the interaction of ribosomal protein RPL22e with microtubules

Microtubule (MT) protein preparations often contain components of the translation machinery, including ribosome proteins. To understand the biological meaning of it we studied the interaction of ribosomal protein RPL22e with the MT. We found that bacteria expressed purified RPL22e-GFP-6His did co-sediment with brain tubulin MTs with 1.3 µM dissociation coefficient. Such a K_D is comparable to some specific MT-associated proteins. Distinct in vitro interaction of RPL22e-GFP with MTs was also observed by TIRF microscopy. In real-time assay, RPL22e-GFP molecules stayed bound to MTs for several seconds, and 15% of them demonstrated random-walk along MTs with diffusion coefficient 0.03 µ² /s. Deletion of basic areas of RPL22e did not have an impact on K_D , and deletion of acidic tail slightly increased association with MTs. Interestingly, the deletion of acidic tail increased diffusion coefficient as well. The interaction of RPL22e with MTs is hardly noticeable in vivo in cultured cells, probably since a significant part of the protein is incorporated into the ribosomes. The mobility of ribosomal protein on the MTs probably prevents its interfering with MT-dependent transport and could ameliorate its transport to the nucleus.

Perspectives of RAS and RHEB GTPase Signaling Pathways in Regenerating Brain Neurons

Cellular activation of RAS GTPases into the GTP-binding “ON” state is a key switch for regulating brain functions. Molecular protein structural elements of rat sarcoma (RAS) and RAS homolog protein enriched in brain (RHEB) GTPases involved in this switch are discussed including their subcellular membrane localization for triggering specific signaling pathways resulting in regulation of synaptic connectivity, axonal growth, differentiation, migration, cytoskeletal dynamics, neural protection, and apoptosis. A beneficial role of neuronal H-RAS activity is suggested from cellular and animal models of neurodegenerative diseases. Recent experiments on optogenetic regulation offer insights into the spatiotemporal aspects controlling RAS/mitogen activated protein kinase (MAPK) or phosphoinositide-3 kinase (PI3K) pathways. As optogenetic manipulation of cellular signaling in deep brain regions critically requires penetration of light through large distances of absorbing tissue, we discuss magnetic guidance of re-growing axons as a complementary approach. In Parkinson’s disease, dopaminergic neuronal cell bodies degenerate in the substantia nigra. Current human trials of stem cell-derived dopaminergic neurons must take into account the inability of neuronal axons navigating over a large distance from the grafted site into striatal target regions. Grafting dopaminergic precursor neurons directly into the degenerating substantia nigra is discussed as a novel concept aiming to guide axonal growth by activating GTPase signaling through protein-functionalized intracellular magnetic nanoparticles responding to external magnets.

Oncogenic Ras Isoforms Signaling Specificity at the Membrane

How do Ras isoforms attain oncogenic specificity at the membrane? Oncogenic KRas, HRas, and NRas (K-Ras, H-Ras, and N-Ras) differentially populate distinct cancers. How they selectively activate effectors and why is KRas4B the most prevalent are highly significant questions. Here, we consider determinants that may bias isoform-specific effector activation and signaling at the membrane. We merge functional data with a conformational view to provide mechanistic insight. Cell-specific expression levels, pathway cross-talk, and distinct interactions are the key, but conformational trends can modulate selectivity. There are two major pathways in oncogenic Ras-driven proliferation: MAPK (Raf/MEK/ERK) and PI3Kα/Akt/mTOR. All membrane-anchored, proximally located, oncogenic Ras isoforms can promote Raf dimerization and fully activate MAPK signaling. So why the differential statistics of oncogenic isoforms in distinct cancers and what makes KRas so highly oncogenic? Many cell-specific factors may be at play, including higher KRAS mRNA levels. As a key factor, we suggest that because only KRas4B binds calmodulin, only KRas can fully activate PI3Kα/Akt signaling. We propose that full activation of both MAPK and PI3Kα/Akt proliferative pathways by oncogenic KRas4B-but not by HRas or NRas-may help explain why the KRas4B isoform is especially highly populated in certain cancers. We further discuss pharmacologic implications. Cancer Res; 78(3); 593-602. ©2017 AACR.

Oncogenic Ras Isoforms Signaling Specificity at the Membrane

How do Ras isoforms attain oncogenic specificity at the membrane? Oncogenic KRas, HRas, and NRas (K-Ras, H-Ras, and N-Ras) differentially populate distinct cancers. How they selectively activate effectors and why is KRas4B the most prevalent are highly significant questions. Here, we consider determinants that may bias isoform-specific effector activation and signaling at the membrane. We merge functional data with a conformational view to provide mechanistic insight. Cell-specific expression levels, pathway cross-talk, and distinct interactions are the key, but conformational trends can modulate selectivity. There are two major pathways in oncogenic Ras-driven proliferation: MAPK (Raf/MEK/ERK) and PI3Kα/Akt/mTOR. All membrane-anchored, proximally located, oncogenic Ras isoforms can promote Raf dimerization and fully activate MAPK signaling. So why the differential statistics of oncogenic isoforms in distinct cancers and what makes KRas so highly oncogenic? Many cell-specific factors may be at play, including higher KRAS mRNA levels. As a key factor, we suggest that because only KRas4B binds calmodulin, only KRas can fully activate PI3Kα/Akt signaling. We propose that full activation of both MAPK and PI3Kα/Akt proliferative pathways by oncogenic KRas4B-but not by HRas or NRas-may help explain why the KRas4B isoform is especially highly populated in certain cancers. We further discuss pharmacologic implications. Cancer Res; 78(3); 593-602. ©2017 AACR.

Efficient farnesylation of an extended C-terminal C(x)3X sequence motif expands the scope of the prenylated proteome

Protein prenylation is a post-translational modification that has been most commonly associated with enabling protein trafficking to and interaction with cellular membranes. In this process, an isoprenoid group is attached to a cysteine near the C terminus of a substrate protein by protein farnesyltransferase (FTase) or protein geranylgeranyltransferase type I or II (GGTase-I and GGTase-II). FTase and GGTase-I have long been proposed to specifically recognize a four-amino acid CAAX C-terminal sequence within their substrates. Surprisingly, genetic screening reveals that yeast FTase can modify sequences longer than the canonical CAAX sequence, specifically C(x)₃X sequences with four amino acids downstream of the cysteine. Biochemical and cell-based studies using both peptide and protein substrates reveal that mammalian FTase orthologs can also prenylate C(x)₃X sequences. As the search to identify physiologically relevant C(x)₃X proteins begins, this new prenylation motif nearly doubles the number of proteins within the yeast and human proteomes that can be explored as potential FTase substrates. This work expands our understanding of prenylation's impact within the proteome, establishes the biologically relevant reactivity possible with this new motif, and opens new frontiers in determining the impact of non-canonically prenylated proteins on cell function.

The disordered hypervariable region and the folded catalytic domain of oncogenic K-Ras4B partner in phospholipid binding

The C-terminal hypervariable region (HVR) of the splice variant KRAS4B is disordered. Classically, the role of the post-translationally-modified HVR is to navigate Ras in the cell and to anchor it in localized plasma membrane regions. Here, we propose additional regulatory roles, including auto-inhibition by shielding the effector binding site in the GDP-bound state and release upon GTP binding and in the presence of certain oncogenic mutations. The released HVR can interact with calmodulin. We show that oncogenic mutations (G12V/G12D) modulate the HVR-phospholipid binding specificity, resulting in preferential interactions with phosphatidic acid. The shifts in the conformational preferences and binding specificity in the disordered state exemplify the critical role of the unstructured tail of K-Ras4B in cancer.

Divergent roles of CAAX motif-signaled posttranslational modifications in the regulation and subcellular localization of Ral GTPases

The Ras-like small GTPases RalA and RalB are well validated effectors of RAS oncogene-driven human cancer growth, and pharmacologic inhibitors of Ral function may provide an effective anti-Ras therapeutic strategy. Intriguingly, although RalA and RalB share strong overall amino acid sequence identity, exhibit essentially identical structural and biochemical properties, and can utilize the same downstream effectors, they also exhibit divergent and sometimes opposing roles in the tumorigenic and metastatic growth of different cancer types. These distinct biological functions have been attributed largely to sequence divergence in their carboxyl-terminal hypervariable regions. However, the role of posttranslational modifications signaled by the hypervariable region carboxyl-terminal tetrapeptide CAAX motif (C = cysteine, A = aliphatic amino acid, X = terminal residue) in Ral isoform-selective functions has not been addressed. We determined that these modifications have distinct roles and consequences. Both RalA and RalB require Ras converting CAAX endopeptidase 1 (RCE1) for association with the plasma membrane, albeit not with endomembranes, and loss of RCE1 caused mislocalization as well as sustained activation of both RalA and RalB. In contrast, isoprenylcysteine carboxylmethyltransferase (ICMT) deficiency disrupted plasma membrane localization only of RalB, whereas RalA depended on ICMT for efficient endosomal localization. Furthermore, the absence of ICMT increased stability of RalB but not RalA protein. Finally, palmitoylation was critical for subcellular localization of RalB but not RalA. In summary, we have identified striking isoform-specific consequences of distinct CAAX-signaled posttranslational modifications that contribute to the divergent subcellular localization and activity of RalA and RalB.

Cell cycle restriction is more important than apoptosis induction for RASSF1A protein tumor suppression

The Ras association domain family protein 1A (RASSF1A) is arguably one of the most frequently inactivated tumor suppressors in human cancer. RASSF1A modulates apoptosis via the Hippo and Bax pathways but also modulates the cell cycle. In part, cell cycle regulation appears to be dependent upon the ability of RASSF1A to complex with microtubules and regulate their dynamics. Which property of RASSF1A, apoptosis induction or microtubule regulation, is responsible for its tumor suppressor function is not known. We have identified a short conserved motif that is essential for the binding of RASSF family proteins with microtubule-associated proteins. By making a single point mutation in the motif, we were able to generate a RASSF1A variant that retains wild-type apoptotic properties but completely loses the ability to bind microtubule-associated proteins and complex with microtubules. Comparison of this mutant to wild-type RASSF1A showed that, despite retaining its proapoptotic properties, the mutant was completely unable to induce cell cycle arrest or suppress the tumorigenic phenotype. Therefore, it appears that the cell cycle/microtubule effects of RASSF1A are key to its tumor suppressor function rather than its apoptotic effects.

Quantitative determination of cellular farnesyltransferase activity: towards defining the minimum substrate reactivity for biologically relevant protein farnesylation

Prenylation is a post-translational modification wherein an isoprenoid group is attached to a protein substrate by a protein prenyltransferase. Hundreds of peptide sequences are in vitro substrates for protein farnesyltransferase (FTase), but it remains unknown which of these sequences can successfully compete for in vivo prenylation. Translating in vitro studies to predict in vivo protein farnesylation requires determining the minimum reactivity needed for modification by FTase within the cell. Towards this goal, we developed a reporter protein series spanning several orders of magnitude in FTase reactivity as a calibrated sensor for endogenous FTase activity. Our approach provides a minimally invasive method to monitor changes in cellular FTase activity in response to environmental or genetic factors. Determining the reactivity "threshold" for in vivo prenylation will help define the prenylated proteome and identify prenylation-dependent pathways for therapeutic targeting.

Inhibitors of K-Ras plasma membrane localization

Oncogenic mutant K-Ras is highly prevalent in multiple human tumors. Despite significant efforts to directly target Ras activity, no K-Ras-specific inhibitors have been developed and taken into the clinic. Since Ras proteins must be anchored to the inner leaflet of the plasma membrane (PM) for full biological activity, we devised a high-content screen to identify molecules with ability to displace K-Ras from the PM. Here we summarize the biochemistry and biology of three classes of compound identified by this screening method that inhibit K-Ras PM targeting: staurosporine and analogs, fendiline, and metformin. All three classes of compound significantly abrogate cell proliferation and Ras signaling in K-Ras-transformed cancer cells. Taken together, these studies provide an important proof of concept that blocking PM localization of K-Ras is a tractable therapeutic target.

Staurosporines disrupt phosphatidylserine trafficking and mislocalize Ras proteins

Oncogenic mutant Ras is frequently expressed in human cancers, but no anti-Ras drugs have been developed. Since membrane association is essential for Ras biological activity, we developed a high content assay for inhibitors of Ras plasma membrane localization. We discovered that staurosporine and analogs potently inhibit Ras plasma membrane binding by blocking endosomal recycling of phosphatidylserine, resulting in redistribution of phosphatidylserine from plasma membrane to endomembrane. Staurosporines are more active against K-Ras than H-Ras. K-Ras is displaced to endosomes and undergoes proteasomal-independent degradation, whereas H-Ras redistributes to the Golgi and is not degraded. K-Ras nanoclustering on the plasma membrane is also inhibited. Ras mislocalization does not correlate with protein kinase C inhibition or induction of apoptosis. Staurosporines selectively abrogate K-Ras signaling and proliferation of K-Ras-transformed cells. These results identify staurosporines as novel inhibitors of phosphatidylserine trafficking, yield new insights into the role of phosphatidylserine and electrostatics in Ras plasma membrane targeting, and validate a new target for anti-Ras therapeutics.

Expansion of protein farnesyltransferase specificity using "tunable" active site interactions: development of bioengineered prenylation pathways

Post-translational modifications play essential roles in regulating protein structure and function. Protein farnesyltransferase (FTase) catalyzes the biologically relevant lipidation of up to several hundred cellular proteins. Site-directed mutagenesis of FTase coupled with peptide selectivity measurements demonstrates that molecular recognition is determined by a combination of multiple interactions. Targeted randomization of these interactions yields FTase variants with altered and, in some cases, bio-orthogonal selectivity. We demonstrate that FTase specificity can be "tuned" using a small number of active site contacts that play essential roles in discriminating against non-substrates in the wild-type enzyme. This tunable selectivity extends in vivo, with FTase variants enabling the creation of bioengineered parallel prenylation pathways with altered substrate selectivity within a cell. Engineered FTase variants provide a novel avenue for probing both the selectivity of prenylation pathway enzymes and the effects of prenylation pathway modifications on the cellular function of a protein.

Protein farnesylation and disease

Prenylation consists of the addition of an isoprenoid group to a cysteine residue located near the carboxyl terminal of a protein. This enzymatic posttranslational modification is important for the maturation and processing of proteins. Both processes are necessary to mediate protein-protein and membrane-protein associations, in addition to regulating the localisation and function of proteins. The severe phenotype of animals deficient in enzymes involved in both prenylation and maturation highlights the significance of these processes. Moreover, alterations in the genes coding for isoprenylated proteins or enzymes that are involved in both prenylation and maturation processes have been found to be the basis of severe human diseases, such as cancer, neurodegenerative disorders, retinitis pigmentosa, and premature ageing syndromes. Recent studies on isoprenylation and postprenylation processing in pathological conditions have unveiled surprising aspects of these modifications and their roles in different cellular pathways. The identification of these enzymes as therapeutic targets has led researchers to validate their effects in vitro and in vivo as antitumour or antiageing agents. This review attempts to summarise the basic aspects of protein isoprenylation and postprenylation, integrating our data with that observed in other studies to provide a comprehensive scenario of progeroid syndromes and the therapeutic avenues.

TRPV1-tubulin complex: involvement of membrane tubulin in the regulation of chemotherapy-induced peripheral neuropathy

Existence of microtubule cytoskeleton at the membrane and submembranous regions, referred as 'membrane tubulin' has remained controversial for a long time. Since we reported physical and functional interaction of Transient Receptor Potential Vanilloid Sub Type 1 (TRPV1) with microtubules and linked the importance of TRPV1-tubulin complex in the context of chemotherapy-induced peripheral neuropathy, a few more reports have characterized this interaction in in vitro and in in vivo condition. However, the cross-talk between TRPs with microtubule cytoskeleton, and the complex feedback regulations are not well understood. Sequence analysis suggests that other than TRPV1, few TRPs can potentially interact with microtubules. The microtubule interaction with TRPs has evolutionary origin and has a functional significance. Biochemical evidence, Fluorescence Resonance Energy Transfer analysis along with correlation spectroscopy and fluorescence anisotropy measurements have confirmed that TRPV1 interacts with microtubules in live cell and this interaction has regulatory roles. Apart from the transport of TRPs and maintaining the cellular structure, microtubules regulate signaling and functionality of TRPs at the single channel level. Thus, TRPV1-tubulin interaction sets a stage where concept and parameters of 'membrane tubulin' can be tested in more details. In this review, I critically analyze the advancements made in biochemical, pharmacological, behavioral as well as cell-biological observations and summarize the limitations that need to be overcome in the future.

Mechanism of isoprenylcysteine carboxyl methylation from the crystal structure of the integral membrane methyltransferase ICMT

The posttranslational modification of C-terminal CAAX motifs in proteins such as Ras, most Rho GTPases, and G protein γ subunits, plays an essential role in determining their subcellular localization and correct biological function. An integral membrane methyltransferase, isoprenylcysteine carboxyl methyltransferase (ICMT), catalyzes the final step of CAAX processing after prenylation of the cysteine residue and endoproteolysis of the -AAX motif. We have determined the crystal structure of a prokaryotic ICMT ortholog, revealing a markedly different architecture from conventional methyltransferases that utilize S-adenosyl-L-methionine (SAM) as a cofactor. ICMT comprises a core of five transmembrane α helices and a cofactor-binding pocket enclosed within a highly conserved C-terminal catalytic subdomain. A tunnel linking the reactive methyl group of SAM to the inner membrane provides access for the prenyl lipid substrate. This study explains how an integral membrane methyltransferase achieves recognition of both a hydrophilic cofactor and a lipophilic prenyl group attached to a polar protein substrate.

Functional specificity of ras isoforms: so similar but so different

H-ras, N-ras, and K-ras are canonical ras gene family members frequently activated by point mutation in human cancers and coding for 4 different, highly related protein isoforms (H-Ras, N-Ras, K-Ras4A, and K-Ras4B). Their expression is nearly ubiquitous and broadly conserved across eukaryotic species, although there are quantitative and qualitative differences of expression depending on the tissue and/or developmental stage under consideration. Extensive functional studies have determined during the last quarter century that these Ras gene products are critical components of signaling pathways that control eukaryotic cell proliferation, survival, and differentiation. However, because of their homology and frequent coexpression in various cellular contexts, it remained unclear whether the different Ras proteins play specific or overlapping functional roles in physiological and pathological processes. Initially, their high degree of sequence homology and the observation that all Ras isoforms share common sets of downstream effectors and upstream activators suggested that they were mostly redundant functionally. In contrast, the notion of functional specificity for each of the different Ras isoforms is supported at present by an increasing body of experimental observations, including 1) the fact that different ras isoforms are preferentially mutated in specific types of tumors or developmental disorders; 2) the different transforming potential of transfected ras genes in different cell contexts; 3) the distinct sensitivities exhibited by the various Ras family members for modulation by different GAPs or GEFs; 4) the demonstration that different Ras isoforms follow distinct intracellular processing pathways and localize to different membrane microdomains or subcellular compartments; 5) the different phenotypes displayed by genetically modified animal strains for each of the 3 ras loci; and 6) the specific transcriptional networks controlled by each isoform in different cellular settings.

Ubiquitination of K-Ras enhances activation and facilitates binding to select downstream effectors

The guanosine triphosphate (GTP)--loaded form of the guanosine triphosphatase (GTPase) Ras initiates multiple signaling pathways by binding to various effectors, such as the kinase Raf and phosphatidylinositol 3-kinase (PI3K). Ras activity is increased by guanine nucleotide exchange factors that stimulate guanosine diphosphate release and GTP loading and is inhibited by GTPase-activating proteins that stimulate GTP hydrolysis. KRAS is the most frequently mutated RAS gene in cancer. Here, we report that monoubiquitination of lysine-147 in the guanine nucleotide-binding motif of wild-type K-Ras could lead to enhanced GTP loading. Furthermore, ubiquitination increased the binding of the oncogenic Gly12Val mutant of K-Ras to the downstream effectors PI3K and Raf. Thus, monoubiquitination could enhance GTP loading on K-Ras and increase its affinity for specific downstream effectors, providing a previously unidentified mechanism for Ras activation.

Membrane-mediated induction and sorting of K-Ras microdomain signaling platforms

The K-Ras4B GTPase is a major oncoprotein whose signaling activity depends on its correct localization to negatively charged subcellular membranes and nanoclustering in membrane microdomains. Selective localization and clustering are mediated by the polybasic farnesylated C-terminus of K-Ras4B, but the mechanisms and molecular determinants involved are largely unknown. In a combined chemical biological and biophysical approach we investigated the partitioning of semisynthetic fully functional lipidated K-Ras4B proteins into heterogeneous anionic model membranes and membranes composed of viral lipid extracts. Independent of GDP/GTP-loading, K-Ras4B is preferentially localized in liquid-disordered (l(d)) lipid domains and forms new protein-containing fluid domains that are recruiting multivalent acidic lipids by an effective, electrostatic lipid sorting mechanism. In addition, GDP-GTP exchange and, thereby, Ras activation results in a higher concentration of activated K-Ras4B in the nanoscale signaling platforms. Conversely, palmitoylated and farnesylated N-Ras proteins partition into the l(d) phase and concentrate at the l(d)/l(o) phase boundary of heterogeneous membranes. Next to the lipid anchor system, the results reveal an involvement of the G-domain in the membrane interaction process by determining minor but yet significant structural reorientations of the GDP/GTP-K-Ras4B proteins at lipid interfaces. A molecular mechanism for isoform-specific Ras signaling from separate membrane microdomains is postulated from the results of this study.

Biochemical and docking analysis of substrate interactions with polyisoprenylated methylated protein methyl esterase

Polyisoprenylated proteins (PPs) methylation by polyisoprenylated protein methyl transferase (PPMTase) is counteracted by polyisoprenylated methylated protein methyl esterase (PMPMEase). This is the only reversible step of the polyisoprenylation pathway as the relative amounts of the acid and ester forms are determined by the two competing reactions. Since PMPMEase and PPMTase may influence both the structural/functional conformations of PPs, a thorough study of these enzymes is essential to our understanding of the structural/functional features of PPs. PMPMEase has been reported under such pseudonyms as human carboxylesterase 1 (hCE1) because of its apparent broad substrate spectrum. The current study aimed to show the complementarity between its active site and the polyisoprenylated substrates that it metabolizes. Kinetics analysis was conducted with N-, S- and O-substituted substrates using porcine liver PMPMEase and docking analysis using Arguslab. Consistent with the biochemical analysis, the S-ethyl analog yielded an AScore binding energy of -11.32 compared to -13.48, -14.88, -16.15, and -16.81 kcal/mol for S-prenyl (C-5), S-trans-geranyl (C-10), S-trans,trans-farnesyl (C-15) and S-all trans-geranylgeranyl (C-20), respectively. The all trans-geranylgeranyl moiety provides the optimal size for active site interactions. The data reveal that the trans,trans-farnesyl and all trans-geranylgeranyl groups, which are reminiscent of endogenous PPs modifications, have the highest affinity for PMPMEase. Since PPs such as monomeric G-proteins are oncogenic, PMPMEase may be viewed as a viable target for anticancer drug development. The analyses reveal the important structural elements for the design of specific PMPMEase inhibitors to serve in the modulation of oncogenic PPs activities. The results also show that hCE1's repertoire of substrates extends beyond xenobiotics to include PPs as its endogenous substrates.

Reactive oxygen species, Ki-Ras, and mitochondrial superoxide dismutase cooperate in nerve growth factor-induced differentiation of PC12 cells

Nerve growth factor (NGF) induces terminal differentiation in PC12, a pheochromocytoma-derived cell line. NGF binds a specific receptor on the membrane and triggers the ERK1/2 cascade, which stimulates the transcription of neural genes. We report that NGF significantly affects mitochondrial metabolism by reducing mitochondrial-produced reactive oxygen species and stabilizing the electrochemical gradient. This is accomplished by stimulation of mitochondrial manganese superoxide dismutase (MnSOD) both transcriptionally and post-transcriptionally via Ki-Ras and ERK1/2. Activation of MnSOD is essential for completion of neuronal differentiation because 1) expression of MnSOD induces the transcription of a neuronal specific promoter and neurite outgrowth, 2) silencing of endogenous MnSOD by small interfering RNA significantly reduces transcription induced by NGF, and 3) a Ki-Ras mutant in the polylysine stretch at the COOH terminus, unable to stimulate MnSOD, fails to induce complete differentiation. Overexpression of MnSOD restores differentiation in cells expressing this mutant. ERK1/2 is also downstream of MnSOD, as a SOD mimetic drug stimulates ERK1/2 with the same kinetics of NGF and silencing of MnSOD reduces NGF-induced late ERK1/2. Long term activation of ERK1/2 by NGF requires SOD activation, low levels of hydrogen peroxide, and the integrity of the microtubular cytoskeleton. Confocal immunofluorescence shows that NGF stimulates the formation of a complex containing membrane-bound Ki-Ras, microtubules, and mitochondria. We propose that active NGF receptor induces association of mitochondria with plasma membrane. Local activation of ERK1/2 by Ki-Ras stimulates mitochondrial SOD, which reduces reactive oxygen species and produces H(2)O(2). Low and spatially restricted levels of H(2)O(2) induce and maintain long term ERK1/2 activity and ultimately differentiation of PC12 cells.

Plasma membrane tubulin

The association of tubulin with the plasma membrane comprises multiple levels of penetration into the bilayer: from integral membrane protein, to attachment via palmitoylation, to surface binding, and to microtubules attached by linker proteins to proteins in the membrane. Here we discuss the soundness and weaknesses of the chemical and biochemical evidence marshaled to support these associations, as well as the mechanisms by which tubulin or microtubules may regulate functions at the plasma membrane.

Genetic analyses of the role of RCE1 in RAS membrane association and transformation

Proteins terminating with a CAAX motif, such as the nuclear lamins and the RAS family of proteins, undergo post-translational modification of a carboxyl-terminal cysteine with an isoprenyl lipid--a process called protein prenylation. After prenylation, the last three residues of CAAX proteins are clipped off by an endoprotease of the endoplasmic reticulum. RCE1 is responsible for the endoproteolytic processing of the RAS proteins and is likely responsible for endoproteolytic processing of the vast majority of CAAX proteins. Prenylation has been shown to be essential for the proper intracellular targeting and function of several CAAX proteins, but the physiologic importance of the endoprotease step has remained less certain. Here, we will review methods that have been used to define the physiologic importance of the endoproteolytic processing step of CAAX protein processing.

Isoprenylcysteine carboxy methylation is essential for development in Dictyostelium discoideum

Members of the Ras superfamily of small GTPases and the heterotrimeric G protein gamma subunit are methylated on their carboxy-terminal cysteine residues by isoprenylcysteine methyltransferase. In Dictyostelium discoideum, small GTPase methylation occurs seconds after stimulation of starving cells by cAMP and returns quickly to basal levels, suggesting an important role in cAMP-dependent signaling. Deleting the isoprenylcysteine methyltransferase-encoding gene causes dramatic defects. Starving mutant cells do not propagate cAMP waves in a sustained manner, and they do not aggregate. Motility is rescued when cells are pulsed with exogenous cAMP, or coplated with wild-type cells, but the rescued cells exhibit altered polarity. cAMP-pulsed methyltransferase-deficient cells that have aggregated fail to differentiate, but mutant cells plated in a wild-type background are able to do so. Localization of and signaling by RasG is altered in the mutant. Localization of the heterotrimeric Ggamma protein subunit was normal, but signaling was altered in mutant cells. These data indicate that isoprenylcysteine methylation is required for intercellular signaling and development in Dictyostelium.

10 Genetic approaches to understanding the physiologic importance of the carboxyl methylation of isoprenylated proteins

This chapter examines recent studies on the physiologic importance of the carboxyl methylation of isoprenylated proteins, focusing largely on what has been learned from cells lacking the Icmt methyltransferase. Proteins terminating with a CaaX motif (e.g., the nuclear lamins, the Ras family of proteins) undergo posttranslational modification of a carboxyl-terminal cysteine with an isoprenyl lipid (a process generally called protein isoprenylation or protein prenylation). Following this lipidation step, CaaX proteins generally undergo two additional processing steps: endoproteolytic release of the last three residues of the protein (i.e., the -aaX of the CaaX motif) and methylesterification of the newly exposed isoprenylcysteine a-carboxyl group. The CaaX proteins are not, however, the only prenylated proteins that undergo carboxyl methylation. A subset of the Rab family of proteins, those terminating with a CXC motif, undergo methylesterification of a carboxyl-terminal geranylgeranylcysteine. The methylation of CaaX proteins and the CXC Rab proteins is carried out by a single membrane methyltransferase of the endoplasmic reticulum, Icmt (for isoprenylcysteine carboxyl methyltransferase). Many studies have shown that protein prenylation is essential for the proper intracellular targeting and function of numerous intracellular proteins, but the physiologic importance of the carboxyl methylation step has remained less certain. Here, we review recent studies that have shed light on the importance of carboxyl methylation of prenylated proteins.

Identification and characterisation of novel tubulin-binding motifs located within the C-terminus of TRPV1

Previously, we reported that TRPV1, the vanilloid receptor, interacts with soluble alphabeta-tubulin dimers as well as microtubules via its C-terminal cytoplasmic domain. The interacting region of TRPV1, however, has not been defined. We found that the TRPV1 C-terminus preferably interacts with beta-tubulin and less with alpha-tubulin. Using a systematic deletion approach and biotinylated-peptides we identified two tubulin-binding sites present in TRPV1. These two sequence stretches are highly conserved in all known mammalian TRPV1 orthologues and partially conserved in some of the TRPV1 homologues. As these sequence stretches are not similar to any known tubulin-binding sequences, we conclude that TRPV1 interacts with tubulin and microtubule through two novel tubulin-binding motifs.

Ras oncogenes and their downstream targets

RAS proteins are small GTPases, which serve as master regulators of a myriad of signaling cascades involved in highly diverse cellular processes. RAS oncogenes have been originally discovered as retroviral oncogenes, and ever since constitutively activating RAS mutations have been identified in human tumors, they are in the focus of intense research. In this review, we summarize the biochemical properties of RAS proteins, trace down the evolution of RAS signaling and present an overview of the spatio-temporal activation of major RAS isoforms. We further discuss RAS effector pathways, their role in normal and transformed cell physiology and summarize ongoing attempts to interfere with aberrant RAS signaling. Finally, we comment on the role of micro RNAs in modulating RAS expression, contribution of RAS to stem cell function and on high-throughput analyses of RAS signaling networks.

Spatio-temporal segregation of Ras signals: one ship, three anchors, many harbors

Dynamic assembly of spatially separated signaling platforms enables a cell to tune cellular outputs in response to different input stimuli. Understanding how a vast diversity in signaling responses can be generated from a limited protein repertoire requires knowledge of how cells maintain the segregation of proteins and thereby orchestrate their local activities. Ras proteins are subject to this type of precise regulation of localization, and thus activity, in space and time. A model emerges where different lipid anchors dynamically shuttle Ras between specific membrane compartments, where differences in the accessibility of signaling environments and in the residence time of Ras therein account for isoform-specific signaling responses.

Genetic and pharmacologic analyses of the role of Icmt in Ras membrane association and function

After isoprenylation, the Ras proteins and other proteins terminating with a so-called CAAX motif undergo two additional modifications: (1) endoproteolytic cleavage of the -AAX by Ras converting enzyme 1 (Rce1) and (2) carboxyl methylation of the isoprenylated cysteine residue by isoprenylcysteine carboxyl methyltransferase (Icmt). Although CAAX protein isoprenylation has been studied in great detail, until recently, very little was known about the biological role and functional importance of Icmt in mammalian cells. Studies over the past few years, however, have begun to fill in the blanks. Genetic experiments showed that Icmt-deficient embryos die at mid-gestation, whereas conditional inactivation of Icmt in the liver, spleen, and bone marrow is not associated with obvious pathology. One potential explanation for the embryonic lethality is that Icmt is the only enzyme in mouse cells capable of methylating isoprenylated CAAX proteins--including the Ras proteins. Furthermore, in addition to the CAAX proteins, Icmt methylates the CXC class of isoprenylated Rab proteins. In the absence of carboxyl methylation, the Ras proteins are mislocalized away from the plasma membrane and exhibit a shift in electrophoretic mobility. Given the important role of oncogenic Ras proteins in human tumorigenesis and the mislocalization of Ras proteins in Icmt-deficient cells, it has been hypothesized that inhibition of Icmt could be a strategy to block Ras-induced oncogenic transformation. Recent data provide strong support to that hypothesis: conditional inactivation of Icmt in mouse embryonic fibroblasts and treatment of cells with a novel selective inhibitor of Icmt, termed cysmethynil, results in a striking inhibition of Ras-induced oncogenic transformation.

Molecular targets for altering radiosensitivity: lessons from Ras as a pre-clinical and clinical model

Ras activation has been correlated with malignant and metastatic cancer phenotypes and poor prognosis for cancer patients. In the preclinical setting, Ras activation by mutation or EGFR amplification results in increased clonogenic cell survival and decreased tumor growth delay following irradiation. Activation of the Ras pathway has also been associated with increased risk of local failure and decreased overall survival in patients receiving radiotherapy. Prenyltransferase inhibitors target the post-translational processing of Ras and have been shown to increase the radiosensitivity of human cancer cell lines. In the clinical setting, these inhibitors have been used with concurrent radiotherapy in a small number of phase I clinical trials with acceptable toxicity. Therefore, inhibiting Ras activation represents a promising molecular approach for radiosensitization in cancer therapy.

Direct visualization of mRNA colocalization with mitochondria in living cells using molecular beacons

The intracellular localization and specific organelle association of mRNA may reflect essential functions, stages, and stability of mRNA. We report the direct visualization of subcellular localization of K-ras and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) mRNAs in live HDF cells using molecular beacons together with membrane-permeabilization and peptide-based delivery. Unexpectedly, we found that both K-ras and GAPDH mRNAs colocalize with mitochondria. Extensive control studies are performed, including the use of fluorescence in-situ hybridization (FISH), negative-control beacons, and the detection of colocalization of 28S ribosomal RNA with the rough endoplasmic reticulum (ER), suggesting that the mRNA localization and colocalization patterns observed in our study are true and specific. Our observation reveals intriguing subcellular associations of mRNA with organelles such as mitochondria, which may provide new insight into the transport, dynamics, and functions of mRNA and mRNA-protein interactions.

Farnesyltransferase inhibitors and human malignant pleural mesothelioma: a first-step comparative translational study

It is known that the potential clinical use of farnesyltransferase inhibitors (FTI) could be expanded to include cancers harboring activated receptor tyrosine kinases. Approximately 70% of malignant pleural mesotheliomas (MPM) overexpress epidermal growth factor receptors (EGFR) and a subset express both EGFR and transforming growth factor alpha (TGF-alpha), suggesting an autocrine role for EGFR in MPM. We checked on MPM cells (10 human cell lines, 11 primary cultures obtained by human biopsies, and 7 short-term normal mesothelial cell cultures) concerning the following: (a) the relative overexpression of EGFR (Western blotting, flow cytometry, immunohistochemistry), (b) the relative expression of EGFR ligands (EGF, amphiregulin, TGF-alpha, ELISA), (c) the relative increase of the activated form of Ras (Ras-bound GTP) after EGF stimulation (Ras activation assay), (d) the efficacy of five different FTIs (HDJ2 prenylation, cell cytotoxicity, and apoptosis using ApopTag and gel ladder). EGFR was overexpressed in MPM cells compared with normal pleural mesothelial cells in equivalent levels as in non-small cell lung cancer cells A459. MPM cells constitutively expressed EGFR ligands; however, Ras activation was attenuated at high EGF concentrations (100 ng/mL). Growth of MPM cells was substantially not affected by treatment with different FTIs (SCH66336, BMS-214662, R115777, RPR-115135, and Manumycin). Among these, BMS-214662 was the only one moderately active. BMS-214662 triggered apoptosis in a small fraction of cells (not higher than 30%) that was paralleled by a slight decrease in the levels of TGF-alpha secreted by treated MPM cells. Our data highlighted the concept that the same signaling pathway can be regulated in different ways and these regulations can differ between different cells of different origin.

Proteomic analysis reveals hyperactivation of the mammalian target of rapamycin pathway in neurofibromatosis 1-associated human and mouse brain tumors

Individuals with the tumor predisposition syndrome, neurofibromatosis 1 (NF1), are prone to development of nervous system tumors, including neurofibromas and pilocytic astrocytomas. Based on the ability of the NF1 gene product (neurofibromin) to function as a GTPase activating protein for RAS, initial biologically based therapies for NF1-associated tumors focused on the use of RAS inhibitors, but with limited clinical success. In an effort to identify additional targets for therapeutic drug design in NF1, we used an unbiased proteomic approach to uncover unanticipated intracellular signaling pathways dysregulated in Nf1-deficient astrocytes. We found that the expression of proteins involved in promoting ribosome biogenesis was increased in the absence of neurofibromin. In addition, Nf1-deficient astrocytes exhibit high levels of mammalian target of rapamycin (mTOR) pathway activation, which was inhibited by blocking K-RAS or phosphatidylinositol 3-kinase activation. This mTOR pathway hyperactivation was reflected by high levels of ribosomal S6 activation in both Nf1 mutant mouse optic nerve gliomas and in human NF1-associated pilocytic astrocytoma tumors. Moreover, inhibition of mTOR signaling in Nf1-/- astrocytes abrogated their growth advantage in culture, restoring normal proliferative rates. These results suggest that mTOR pathway inhibition may represent a logical and tractable biologically based therapy for brain tumors in NF1.

Post-prenylation-processing enzymes as new targets in oncogenesis

RAS and many other oncogenic proteins undergo a complex series of post-translational modifications that are initiated by the addition of an isoprenoid lipid through a process known as prenylation. Following prenylation, these proteins usually undergo endoproteolytic processing by the RCE1 protease and then carboxyl methylation by a unique methyltransferase known as isoprenylcysteine carboxyl methyltransferase (ICMT). Although inhibitors that have been designed to target the prenylation step are now in advanced-stage clinical trials, their utility and efficacy seem to be limited. Recent findings, however, indicate that the inhibition of these post-prenylation-processing steps--particularly that of ICMT-catalysed methylation--might provide a better approach to the control of cancer-cell proliferation.

Glioma Formation in Neurofibromatosis 1 Reflects Preferential Activation of K-RAS in Astrocytes

Children with the tumor predisposition syndrome, neurofibromatosis 1 (NF1), develop optic pathway gliomas. The NF1 gene product, neurofibromin, functions as a negative regulator of RAS, such that NF1 inactivation results in RAS hyperactivation. Recent studies have highlighted the divergent biological and biochemical properties of the various RAS isoforms, which prompted us to examine the consequence of Nf1 inactivation in astrocytes on RAS isoform activation in vitro and in vivo. In this report, we show that only K-RAS is activated in Nf1−/− astrocytes and that activation of K-RAS, but not H-RAS, accounts for the proliferative advantage and abnormal actin cytoskeleton–mediated processes observed in Nf1−/− astrocytes in vitro. Moreover, dominant inhibitory K-RAS corrects these abnormalities in Nf1−/− astrocytes invitro. Lastly, we show that Nf1+/− mice with astrocyte-specific activated K-RAS expression in vivo develop optic pathway gliomas, similar to our previously reported Nf1+/− mice with astrocyte Nf1 inactivation. Collectively, our results show that K-RAS is the primary target for neurofibromin GTPase-activating protein activity in vitro and in vivo and that K-RAS activation in astrocytes recapitulates the biochemical, biological, and tumorigenic properties of neurofibromin loss.

Analysis of the kinetic mechanism of recombinant human isoprenylcysteine carboxylmethyltransferase (Icmt)

Background

Isoprenylcysteine carboxyl methyltransferase (Icmt) is the third of three enzymes that posttranslationally modify proteins that contain C-terminal CaaX motifs. The processing of CaaX proteins through this so-called prenylation pathway via a route initiated by addition of an isoprenoid lipid is required for both membrane targeting and function of the proteins. The involvement of many CaaX proteins such as Ras GTPases in oncogenesis and other aberrant proliferative disorders has led to the targeting of the enzymes involved in their processing for therapeutic development, necessitating a detailed understanding of the mechanisms of the enzymes.

Results

In this study, we have investigated the kinetic mechanism of recombinant human Icmt. In the reaction catalyzed by Icmt, S-adenosyl-L-methionine (AdoMet) provides the methyl group that is transferred to the second substrate, the C-terminal isoprenylated cysteine residue of a CaaX protein, thereby generating a C-terminal prenylcysteine methyl ester on the protein. To facilitate the kinetic analysis of Icmt, we synthesized a new small molecule substrate of the enzyme, biotin-S-farnesyl-L-cysteine (BFC). Initial kinetic analysis of Icmt suggested a sequential mechanism for the enzyme that was further analyzed using a dead end competitive inhibitor, S-farnesylthioacetic acid (FTA). Inhibition by FTA was competitive with respect to BFC and uncompetitive with respect to AdoMet, indicating an ordered mechanism with SAM binding first. To investigate the order of product dissociation, product inhibition studies were undertaken with S-adenosyl-L-homocysteine (AdoHcy) and the N-acetyl-S-farnesyl-L-cysteine methylester (AFCME). This analysis indicated that AdoHcy is a competitive inhibitor with respect to AdoMet, while AFCME shows a noncompetitive inhibition with respect to BFC and a mixed-type inhibition with respect to AdoMet. These studies established that AdoHcy is the final product released, and that BFC and AFCME bind to different forms of the enzyme.

Conclusions

These studies establish that catalysis by human Icmt proceeds through an ordered sequential mechanism and provide a kinetic framework for analysis of specific inhibitors of this key enzyme.

Analysis of estrogen receptor α signaling complex at the plasma membrane

There is accumulating evidence that the estrogen receptor (ER) can transduce specific signals at the plasma membrane. We tried to clarify the biological function of ER as a signaling molecule by identifying proteins that interact with the membrane-localized ER. The activation function 1 and 2 (AF-1 and AF-2) domains of ERalpha with or without the membrane-targeting sequence were stably expressed in the breast cancer cell line, MCF-7. The level of tyrosine phosphorylation of AF-2 was significantly elevated by the membrane localization. By mass-spectrometry analysis, alpha- and beta-tubulins and heat shock protein 70 were identified as the AF-1-associated proteins. Of these, tubulins are associated only with membrane-targeted AF-1.

A designed probe for acidic phospholipids reveals the unique enriched anionic character of the cytosolic face of the mammalian plasma membrane

It is generally accepted that the cytosolic face of the plasma membrane of mammalian cells is enriched in acidic phospholipids due to an asymmetric distribution of neutral and anionic phospholipids in the two bilayer leaflets. However, the phospholipid asymmetry across intracellular membranes is not known. Two models have been proposed for the selective targeting of K-Ras4B, which contains a C-terminal farnesyl cysteine methyl ester adjacent to a polybasic peptide segment, to the cytosolic face of the plasma membrane. One involves electrostatic interaction of the lipidated polybasic domain with anionic phospholipids in the plasma membrane, and the other involves binding of K-Ras4B to a specific protein receptor. To address this issue, we prepared by semi-synthesis a green fluorescent protein variant that is linked to a farnesylated, polybasic peptide corresponding to the K-Ras4B C terminus as well as a variant that contains an all-d amino acid version of the K-Ras4B peptide. As expected based on electrostatics, both constructs showed preferential in vitro binding to anionic phospholipid vesicles versus those composed only of zwitterionic phospholipid. Both constructs fully targeted to the plasma membrane when microinjected into live Chinese hamster ovary and Madin-Darby canine kidney cells. Because the all-d amino acid peptide should be devoid of binding affinity to a putative highly specific K-Ras membrane receptor, these results support an electrostatic basis for the targeting of K-Ras4B to the plasma membrane, and they support an intracellular landscape of phospholipids in which the cytosolic face of the plasma membrane is the most enriched in acidic phospholipids.