Structural evidence for feedback activation by Ras.GTP of the Ras-specific nucleotide exchange factor SOS

Impact of Mutational Status on Intracellular Effects of Cell-Permeable CaaX Peptides in Pancreatic Cancer Cells

Prenyltransferases add a lipid group to the cysteine of a CaaX motif of proteins. This posttranslational modification enables proteins to attach to membranes where they are essential hubs for signaling, trafficking, and apoptosis. Recently, cell-permeable CaaX-peptides are developed as possible tools to interfere with the prenylation machinery. These peptides cause cytotoxic effects, particularly in KRas mutant pancreatic cancer cells (PANC-1) in which they also alter downstream signaling of Ras proteins. Herein, the aim is to get more clues about the relevance of the mutational status of KRas. Therefore, the activity of CaaX-peptides in KRas wildtype BxPC-3 and KRas mutated PANC-1 cells is compared. CaaX-peptides differently influence these two cell lines, although they internalize pretty much to the same extent. Indeed, an altered KRas plasma membrane localization in PANC-1 cells is observed, probably induced by disturbed KRas prenylation based on the presence of CaaX-peptides. The impact of CaaX-peptides on KRas signaling is likely dependent on the KRas mutation in PANC-1 cells in which they further trigger effects on KRas-dependent regulators, e.g., Neurofibromin -1 (NF1) and son of sevenless homolog 1 (SOS1). All in all, CaaX peptides are identified as promising tools for studying and manipulating the function of therapeutically important prenylated proteins.

In Silico Identification of Putative Allosteric Pockets and Inhibitors for the KRASG13D-SOS1 Complex in Cancer Therapy

RAS mutations occur in about 30% of human cancers, leading to enhanced RAS signaling and tumor growth. KRAS is the most commonly mutated oncogene in human tumors, especially lung, pancreatic, and colorectal cancers. Direct targeting of KRAS is difficult due to its highly conserved sequence; but, its complex with the guanine nucleotide exchange factor Son of Sevenless (SOS) 1 promises an attractive target for inhibiting RAS-mediated signaling. Here, we first revealed putative allosteric binding sites of the SOS1, KRASG12C-SOS1 complex, and the ternary KRASG13D-SOS1 complex structures using two network-based models, the essential site scanning analysis and the residue interaction network model. The results enabled us to identify two new putative allosteric pockets for the ternary KRASG13D-SOS1 complex. These were then screened together with the known ligand binding site against the natural compounds in the InterBioScreen (IBS) database using the Glide software package developed by Schrödinger, Inc. The docking poses of seven hit compounds were assessed using 400 ns long molecular dynamics (MD) simulations with two independent replicas using Desmond, coupled with thermal MM-GBSA calculations for the estimation of the binding free energy values. The structural skeleton of the seven proposed compounds consists of different functional groups and heterocyclic rings that possess anti-cancer activity and exhibit persistent interactions with key residues in binding pockets throughout the MD simulations. STOCK1N-09823 was determined as the most promising hit that promoted the disruption of the interactions R73 (chain A)/N879 and R73 (chain A)/Y884, which are key for SOS1-mediated KRAS activation.

Evaluation of information flows in the RAS-MAPK system using transfer entropy measurements

The RAS-MAPK system plays an important role in regulating various cellular processes, including growth, differentiation, apoptosis, and transformation. Dysregulation of this system has been implicated in genetic diseases and cancers affecting diverse tissues. To better understand the regulation of this system, we employed information flow analysis based on transfer entropy (TE) between the activation dynamics of two key elements in cells stimulated with EGF: SOS, a guanine nucleotide exchanger for the small GTPase RAS, and RAF, a RAS effector serine/threonine kinase. TE analysis allows for model-free assessment of the timing, direction, and strength of the information flow regulating the system response. We detected significant amounts of TE in both directions between SOS and RAF, indicating feedback regulation. Importantly, the amount of TE did not simply follow the input dose or the intensity of the causal reaction, demonstrating the uniqueness of TE. TE analysis proposed regulatory networks containing multiple tracks and feedback loops and revealed temporal switching in the reaction pathway primarily responsible for reaction control. This proposal was confirmed by the effects of an MEK inhibitor on TE. Furthermore, TE analysis identified the functional disorder of a SOS mutation associated with Noonan syndrome, a human genetic disease, of which the pathogenic mechanism has not been precisely known yet. TE assessment holds significant promise as a model-free analysis method of reaction networks in molecular pharmacology and pathology.

Allosteric modulation of protein-protein interactions in signal transduction with Nanobodies

Nanobodies (Nbs), the variable domains of heavy-chain only antibodies that naturally occur in camelids, are exquisite molecular tools to stabilize dynamic proteins in unique functional conformations. Recent developments in Nb discovery allow to select allosteric Nbs that perturb the distribution of conformational ensembles of protein complexes that mediate signaling, leading to the allosteric modulation of the signals they transmit. Evidence is also accumulating that such conformational specific Nbs do not stabilize new conformational states but rather change the distribution of existing states to allosterically induce transitions, imprinted by the natural ligands of the system.

Unraveling atomic-scale mechanisms of GDP extraction catalyzed by SOS1 in KRAS-G12 and KRAS-D12 oncogenes

The guanine exchange factor SOS1 plays a pivotal role in the positive feedback regulation of the KRAS signaling pathway. Recently, the regulation of KRAS-SOS1 interactions and KRAS downstream effector proteins has emerged as a key focus in the development of therapies targeting KRAS-driven cancers. However, the detailed dynamic mechanisms underlying SOS1-catalyzed GDP extraction and the impact of KRAS mutations remain largely unexplored. In this study, we unveil and describe in atomic detail the primary mechanisms by which SOS1 facilitates GDP extraction from KRAS oncogenes. For GDP-bound wild-type KRAS (KRAS-G12), four critical amino acids (Lys811, Glu812, Lys939, and Glu942) are identified as essential for the catalytic function of SOS1. Notably, the KRAS-G12D mutation (KRAS-D12) significantly accelerates the rate of GDP extraction. The molecular basis of this enhancement are attributed to hydrogen bonding interactions between the mutant residue Asp12 and a positively charged pocket in the intrinsically disordered region (residues 807-818), comprising Ser807, Trp809, Thr810, and Lys811. These findings provide novel insights into SOS1-KRAS interactions and offer a foundation for developing anti-cancer strategies aimed at disrupting these mechanisms.

Ionic control of small GTPase HRas using calmodulin

HRas is a small GTPase that plays physiologically important roles in various intracellular signal transduction processes, such as cell growth and proliferation. The structure and action mechanisms of HRas have been well characterized, leading to its widespread use as a molecular switch in bionanomachines. Calmodulin (CaM), a calcium ion-binding protein, acts as an ion-binding molecular switch and activates the target enzymes. We previously demonstrated that the fusion protein of HRas (M13-HRas) with the CaM target peptide M13 at the N-terminus of HRas exhibits reversible regulation of GTPase activity and the interaction between M13-HRas and the downstream signalling factor Raf by calcium ions with CaM. In this study, we prepared two new HRas fusion proteins with the M13 peptide at the C-terminus (HRas-M13) and both termini (M13-HRas-M13) of HRas and analysed the calcium-dependent regulation of HRas function. M13-HRas-M13 more efficiently controlled GTPase, interaction with Raf and the HRas regulator GEF by calcium ions with CaM.

Targeting protein-protein interactions in drug discovery: Modulators approved or in clinical trials for cancer treatment

Protein-protein interactions (PPIs) form complex cellular networks fundamental to many key biological processes, including signal transduction, cell proliferation and DNA repair. In consequence, their perturbation is often associated with many human diseases. Targeting PPIs offers a promising approach in drug discovery and ongoing advancements in this field hold the potential to provide highly specific therapies for a wide range of complex diseases. Despite the development of PPI modulators is challenging, advances in the genetic, proteomic and computational level have facilitated their discovery and optimization. Focusing on anticancer drugs, in the last years several PPI modulators have entered clinical trials and venetoclax, which targets Bcl-2 family proteins, has been approved for treating different types of leukemia. This review discusses the clinical development status of drugs modulating several PPIs, such as MDM2-4/p53, Hsp90/Hsp90, Hsp90/CDC37, c-Myc/Max, KRAS/SOS1, CCR5/CCL5, CCR2/CCL2 or Smac/XIAP, in cancer drug discovery.

Pharmacological restoration of GTP hydrolysis by mutant RAS

Approximately 3.4 million patients worldwide are diagnosed each year with cancers that have pathogenic mutations in one of three RAS proto-oncogenes (KRAS, NRAS and HRAS)1,2. These mutations impair the GTPase activity of RAS, leading to activation of downstream signalling and proliferation3-6. Long-standing efforts to restore the hydrolase activity of RAS mutants have been unsuccessful, extinguishing any consideration towards a viable therapeutic strategy7. Here we show that tri-complex inhibitors-that is, molecular glues with the ability to recruit cyclophilin A (CYPA) to the active state of RAS-have a dual mechanism of action: not only do they prevent activated RAS from binding to its effectors, but they also stimulate GTP hydrolysis. Drug-bound CYPA complexes modulate residues in the switch II motif of RAS to coordinate the nucleophilic attack on the γ-phosphate of GTP in a mutation-specific manner. RAS mutants that were most sensitive to stimulation of GTPase activity were more susceptible to treatment than mutants in which the hydrolysis could not be enhanced, suggesting that pharmacological stimulation of hydrolysis potentiates the therapeutic effects of tri-complex inhibitors for specific RAS mutants. This study lays the foundation for developing a class of therapeutics that inhibit cancer growth by stimulating mutant GTPase activity.

A New Dawn for Targeted Cancer Therapy: Small Molecule Covalent Binding Inhibitor Targeting K-Ras (G12C)

K-Ras is a frequently mutated oncogene in human malignancies, and the development of inhibitors targeting various oncogenic K-Ras mutant proteins is a major challenge in targeted cancer therapy, especially K-Ras(G12C) is the most common mutant, which occurs in pancreatic ductal adenocarcinoma (PDAC), non-small cell lung cancer (NSCLC), colorectal cancer (CRC) and other highly prevalent malignancies. In recent years, significant progress has been made in developing small molecule covalent inhibitors targeting K-Ras(G12C), thanks to the production of nucleophilic cysteine by the G12C mutant, breaking the "spell" that K-Ras protein cannot be used as a drug target. With the successful launch of sotorasib and adagrasib, the development of small molecule inhibitors targeting various K-Ras mutants has continued to gain momentum. In recent years, with the popularization of highly sensitive surface plasmon resonance (SPR) technology, fragment-based drug design strategies have shown great potential in the development of small molecule inhibitors targeting K-Ras(G12C), but with the increasing number of clinically reported acquired drug resistance, addressing inhibitor resistance has gradually become the focus of this field, indirectly indicating that such small molecule inhibitors still the potential for the development of these small molecule inhibitors are also indirectly indicated. This paper traces the development of small molecule covalent inhibitors targeting K-Ras(G12C), highlighting and analyzing the structural evolution and optimization process of each series of inhibitors and the previous inhibitor design methods and strategies, as well as their common problems and general solutions, in order to provide inspiration and help to the subsequent researchers.

Peptide inhibitors targeting Ras and Ras-associated protein-protein interactions

Peptides represent attractive molecules for targeting protein-protein interactions, and peptide drug development has made great progress during the last decades. Ras protein, the most promising target in cancer therapy, is one of the major growth drivers in various cancers. Although many small molecule inhibitors have been reported to effectively target Ras protein and some inhibitors (such as MRTX849 and AMG 510) have been translated into clinical application, just a few peptide inhibitors have been reported. Here we summarize different types of peptide inhibitors, including monocyclic peptides, bicyclic peptides, stapled peptides, and proteomimetic inhibitors, developed in recent years; emphasize the limits and achievements; and discuss the outlook and challenges associated with future research in peptide inhibitors. This review aims to provide a reference for the discovery of Ras peptide inhibitors.

Development and safety of investigational and approved drugs targeting the RAS function regulation in RAS mutant cancers

The RAS gene family holds a central position in controlling key cellular activities such as migration, survival, metabolism, and other vital biological processes. The activation of RAS signaling cascades is instrumental in the development of various cancers. Although several RAS inhibitors have gained approval from the US Food and Drug Administration for their substantial antitumor effects, their widespread and severe adverse reactions significantly curtail their practical usage in the clinic. Thus, there exists a pressing need for a comprehensive understanding of these adverse events, ensuring the clinical safety of RAS inhibitors through the establishment of precise management guidelines, suitable intermittent dosing schedules, and innovative combination regimens. This review centers on the evolution of RAS inhibitors in cancer therapy, delving into the common adverse effects associated with these inhibitors, their underlying mechanisms, and the potential strategies for mitigation.

Comprehensive structure-function analysis reveals gain- and loss-of-function mechanisms impacting oncogenic KRAS activity

To dissect variant-function relationships in the KRAS oncoprotein, we performed deep mutational scanning (DMS) screens for both wild-type and KRASG12D mutant alleles. We defined the spectrum of oncogenic potential for nearly all possible KRAS variants, identifying several novel transforming alleles and elucidating a model to describe the frequency of KRAS mutations in human cancer as a function of transforming potential, mutational probability, and tissue-specific mutational signatures. Biochemical and structural analyses of variants identified in a KRASG12D second-site suppressor DMS screen revealed that attenuation of oncogenic KRAS can be mediated by protein instability and conformational rigidity, resulting in reduced binding affinity to effector proteins, such as RAF and PI3-kinases, or reduced SOS-mediated nucleotide exchange activity. These studies define the landscape of single amino acid alterations that modulate the function of KRAS, providing a resource for the clinical interpretation of KRAS variants and elucidating mechanisms of oncogenic KRAS inactivation for therapeutic exploitation.

Positive feedback in Ras activation by full-length SOS arises from autoinhibition release mechanism

Signaling through the Ras-MAPK pathway can exhibit switch-like activation, which has been attributed to the underlying positive feedback and bimodality in the activation of RasGDP to RasGTP by SOS. SOS contains both catalytic and allosteric Ras binding sites, and a common assumption is that allosteric activation selectively by RasGTP provides the mechanism of positive feedback. However, recent single-molecule studies have revealed that SOS catalytic rates are independent of the nucleotide state of Ras in the allosteric binding site, raising doubt about this as a positive feedback mechanism. Here, we perform detailed kinetic analyses of receptor-mediated recruitment of full-length SOS to the membrane while simultaneously monitoring its catalytic activation of Ras. These results, along with kinetic modeling, expose the autoinhibition release step in SOS, rather than either recruitment or allosteric activation, as the underlying mechanism giving rise to positive feedback in Ras activation.

Capturing RAS oligomerization on a membrane

RAS GTPases associate with the biological membrane where they function as molecular switches to regulate cell growth. Recent studies indicate that RAS proteins oligomerize on membranes, and disrupting these assemblies represents an alternative therapeutic strategy. However, conflicting reports on RAS assemblies, ranging in size from dimers to nanoclusters, have brought to the fore key questions regarding the stoichiometry and parameters that influence oligomerization. Here, we probe three isoforms of RAS [Kirsten Rat Sarcoma viral oncogene (KRAS), Harvey Rat Sarcoma viral oncogene (HRAS), and Neuroblastoma oncogene (NRAS)] directly from membranes using mass spectrometry. We show that KRAS on membranes in the inactive state (GDP-bound) is monomeric but forms dimers in the active state (GTP-bound). We demonstrate that the small molecule BI2852 can induce dimerization of KRAS, whereas the binding of effector proteins disrupts dimerization. We also show that RAS dimerization is dependent on lipid composition and reveal that oligomerization of NRAS is regulated by palmitoylation. By monitoring the intrinsic GTPase activity of RAS, we capture the emergence of a dimer containing either mixed nucleotides or GDP on membranes. We find that the interaction of RAS with the catalytic domain of Son of Sevenless (SOScat) is influenced by membrane composition. We also capture the activation and monomer to dimer conversion of KRAS by SOScat. These results not only reveal the stoichiometry of RAS assemblies on membranes but also uncover the impact of critical factors on oligomerization, encompassing regulation by nucleotides, lipids, and palmitoylation.

Targeting guanine nucleotide exchange factors for novel cancer drug discovery

INTRODUCTION

Guanine nucleotide exchange factors (GEFs) regulate the activation of small GTPases (G proteins) of the Ras superfamily proteins controlling cellular functions. Ras superfamily proteins act as 'molecular switches' that are turned 'ON' by guanine exchange. There are five major groups of Ras family GTPases: Ras, Ran, Rho, Rab and Arf, with a variety of different GEFs regulating their GTP loading. GEFs have been implicated in various diseases including cancer. This makes GEFs attractive targets to modulate signaling networks controlled by small GTPases.

AREAS COVERED

In this review, the roles and mechanisms of GEFs in malignancy are outlined. The mechanism of guanine exchange activity by GEFs on a small GTPase is illustrated. Then, some examples of GEFs that are significant in cancer are presented with a discussion on recent progress in therapeutic targeting efforts using a variety of approaches.

EXPERT OPINION

Recently, GEFs have emerged as potential therapeutic targets for novel cancer drug development. Targeting small GTPases is challenging; thus, targeting their activation by GEFs is a promising strategy. Most GEF-targeted drugs are still in preclinical development. A deeper biological understanding of the underlying mechanisms of GEF activity and utilizing advanced technology are necessary to enhance drug discovery for GEFs in cancer.

Allosteric nanobodies to study the interactions between SOS1 and RAS

Protein-protein interactions (PPIs) are central in cell metabolism but research tools for the structural and functional characterization of these PPIs are often missing. Here we introduce broadly applicable immunization (Cross-link PPIs and immunize llamas, ChILL) and selection strategies (Display and co-selection, DisCO) for the discovery of diverse nanobodies that either stabilize or disrupt PPIs in a single experiment. We apply ChILL and DisCO to identify competitive, connective, or fully allosteric nanobodies that inhibit or facilitate the formation of the SOS1•RAS complex and modulate the nucleotide exchange rate on this pivotal GTPase in vitro as well as RAS signalling in cellulo. One of these connective nanobodies fills a cavity that was previously identified as the binding pocket for a series of therapeutic lead compounds. The long complementarity-determining region (CDR3) that penetrates this binding pocket serves as pharmacophore for extending the repertoire of potential leads.

Design, synthesis, and bioevaluation of SOS1 PROTACs derived from pyrido[2,3-d]pyrimidin-7-one-based SOS1 inhibitor

Oncogenic KRAS mutations drive an approximately 25 % of all human cancers. Son of Sevenless 1 (SOS1), a critical guanine nucleotide exchange factor, catalyzes the activation of KRAS. Targeting SOS1 degradation has engaged as a promising therapeutic strategy for KRAS-mutant cancers. Herein, we designed and synthesized a series of novel CRBN-recruiting SOS1 PROTACs using the pyrido[2,3-d]pyrimidin-7-one-based SOS1 inhibitor as the warhead. One representative compound 11o effectively induced the degradation of SOS1 in three different KRAS-mutant cancer cell lines with DC50 values ranging from 1.85 to 7.53 nM. Mechanism studies demonstrated that 11o-induced SOS1 degradation was dependent on CRBN and proteasome. Moreover, 11o inhibited the phosphorylation of ERK and displayed potent anti-proliferative activities against SW620, A549 and DLD-1 cells. Further optimization of 11o may provide us promising SOS1 degraders with favorable drug-like properties for developing new chemotherapies targeting KRAS-driven cancers.

Bimodality in Ras signaling originates from processivity of the Ras activator SOS without deterministic bistability

Ras is a small GTPase that is central to important functional decisions in diverse cell types. An important aspect of Ras signaling is its ability to exhibit bimodal or switch-like activity. We describe the total reconstitution of a receptor-mediated Ras activation-deactivation reaction catalyzed by SOS and p120-RasGAP on supported lipid membrane microarrays. The results reveal a bimodal Ras activation response, which is not a result of deterministic bistability but is rather driven by the distinct processivity of the Ras activator, SOS. Furthermore, the bimodal response is controlled by the condensation state of the scaffold protein, LAT, to which SOS is recruited. Processivity-driven bimodality leads to stochastic bursts of Ras activation even under strongly deactivating conditions. This behavior contrasts deterministic bistability and may be more resistant to pharmacological inhibition.

Identifying Potential SOS1 Inhibitors via Virtual Screening of Multiple Small Molecule Libraries against KRAS-SOS1 Interaction

The RAS-MAPK signaling pathway, crucial for cell proliferation and differentiation, involves key proteins KRAS and SOS1. Mutations in the KRAS and SOS1 genes are implicated in various cancer types, including pancreatic, lung, and juvenile myelomonocytic leukemia. There is considerable interest in identifying inhibitors targeting KRAS and SOS1 to explore potential therapeutic strategies for cancer treatment. In this study, advanced in silico techniques were employed to screen small molecule libraries at this interface, leading to the identification of promising lead compounds as potential SOS1 inhibitors. Comparative analysis of the average binding free energies of these predicted potent compounds with known SOS1 small molecule inhibitors revealed that the identified compounds display similar or even superior predicted binding affinities compared to the known inhibitors. These findings offer valuable insights into the potential of these compounds as candidates for further development as effective anti-cancer agents.

Cereblon-based Bifunctional Degrader of SOS1, BTX-6654, Targets Multiple KRAS Mutations and Inhibits Tumor Growth

Mutations within the oncogene KRAS drive an estimated 25% of all cancers. Only allele-specific KRAS G12C inhibitors are currently available and are associated with the emergence of acquired resistance, partly due to upstream pathway reactivation. Given its upstream role in the activation of KRAS, son of sevenless homolog 1 (SOS1), has emerged as an attractive therapeutic target. Agents that target SOS1 for degradation could represent a potential pan-KRAS modality that may be capable of circumventing certain acquired resistance mechanisms. Here, we report the development of two SOS1 cereblon-based bifunctional degraders, BTX-6654 and BTX-7312, cereblon-based bifunctional SOS1 degraders. Both compounds exhibited potent target-dependent and -specific SOS1 degradation. BTX-6654 and BTX-7312 reduced downstream signaling markers, pERK and pS6, and displayed antiproliferative activity in cells harboring various KRAS mutations. In two KRAS G12C xenograft models, BTX-6654 degraded SOS1 in a dose-dependent manner correlating with tumor growth inhibition, additionally exhibiting synergy with KRAS and MEK inhibitors. Altogether, BTX-6654 provided preclinical proof of concept for single-agent and combination use of bifunctional SOS1 degraders in KRAS-driven cancers.

Current advances and development strategies of targeting son of sevenless 1 (SOS1) in drug discovery

The Son of Sevenless 1 (SOS1) guanine nucleotide exchange factor, prevalent across eukaryotic species, plays a pivotal role in facilitating the attachment of RAS protein to GTP, thereby regulating the activation of intracellular RAS proteins. This regulation is part of a feedback mechanism involving SOS1, which allows both activators and inhibitors of SOS1 to exert control over downstream signaling pathways, demonstrating potential anti-tumor effects. Predominantly, small molecule modulators that target SOS1 focus on a hydrophobic pocket within the CDC25 protein domain. The effectiveness of these modulators largely depends on their ability to interact with specific amino acids, notably Phe890 and Tyr884. This interaction is crucial for influencing the protein-protein interaction (PPI) between RAS and the catalytic domain of SOS1. Currently, most small molecule modulators targeting SOS1 are in the preclinical research phase, with a few advancing to clinical trials. This progression raises safety concerns, making the assurance of drug safety a primary consideration alongside the enhancement of efficacy in the development of SOS1 modulators. This review encapsulates recent advancements in the chemical categorization of SOS1 inhibitors and activators. It delves into the evolution of small molecule modulation targeting SOS1 and offers perspectives on the design of future generations of selective SOS1 small molecule modulators.

SOS2 modulates the threshold of EGFR signaling to regulate osimertinib efficacy and resistance in lung adenocarcinoma

Son of sevenless 1 and 2 (SOS1 and SOS2) are RAS guanine nucleotide exchange factors (RasGEFs) that mediate physiologic and pathologic receptor tyrosine kinase (RTK)-dependent RAS activation. Here, we show that SOS2 modulates the threshold of epidermal growth factor receptor (EGFR) signaling to regulate the efficacy of and resistance to the EGFR tyrosine kinase inhibitor (EGFR-TKI) osimertinib in lung adenocarcinoma (LUAD). SOS2 deletion (SOS2KO ) sensitized EGFR-mutated cells to perturbations in EGFR signaling caused by reduced serum and/or osimertinib treatment to inhibit phosphatidylinositol 3-kinase (PI3K)/AKT pathway activation, oncogenic transformation, and survival. Bypassing RTK reactivation of PI3K/AKT signaling represents a common resistance mechanism to EGFR-TKIs; SOS2KO reduced PI3K/AKT reactivation to limit osimertinib resistance. In a forced HGF/MET-driven bypass model, SOS2KO inhibited hepatocyte growth factor (HGF)-stimulated PI3K signaling to block HGF-driven osimertinib resistance. Using a long-term in situ resistance assay, most osimertinib-resistant cultures exhibited a hybrid epithelial/mesenchymal phenotype associated with reactivated RTK/AKT signaling. In contrast, RTK/AKT-dependent osimertinib resistance was markedly reduced by SOS2 deletion; the few SOS2KO cultures that became osimertinib resistant primarily underwent non-RTK-dependent epithelial-mesenchymal transition (EMT). Since bypassing RTK reactivation and/or tertiary EGFR mutations represent most osimertinib-resistant cancers, these data suggest that targeting proximal RTK signaling, here exemplified by SOS2 deletion, has the potential to delay the development osimertinib resistance and enhance overall clinical responses for patients with EGFR-mutated LUAD.

The energetic and allosteric landscape for KRAS inhibition

Thousands of proteins have been validated genetically as therapeutic targets for human diseases1. However, very few have been successfully targeted, and many are considered 'undruggable'. This is particularly true for proteins that function via protein-protein interactions-direct inhibition of binding interfaces is difficult and requires the identification of allosteric sites. However, most proteins have no known allosteric sites, and a comprehensive allosteric map does not exist for any protein. Here we address this shortcoming by charting multiple global atlases of inhibitory allosteric communication in KRAS. We quantified the effects of more than 26,000 mutations on the folding of KRAS and its binding to six interaction partners. Genetic interactions in double mutants enabled us to perform biophysical measurements at scale, inferring more than 22,000 causal free energy changes. These energy landscapes quantify how mutations tune the binding specificity of a signalling protein and map the inhibitory allosteric sites for an important therapeutic target. Allosteric propagation is particularly effective across the central β-sheet of KRAS, and multiple surface pockets are genetically validated as allosterically active, including a distal pocket in the C-terminal lobe of the protein. Allosteric mutations typically inhibit binding to all tested effectors, but they can also change the binding specificity, revealing the regulatory, evolutionary and therapeutic potential to tune pathway activation. Using the approach described here, it should be possible to rapidly and comprehensively identify allosteric target sites in many proteins.

Biophysical Characterization of RAS-SOS Complexes by Native Mass Spectrometry

RAS is regulated by specific guanine nucleotide exchange factors, such as Son of Sevenless (SOS), that activates RAS by facilitating the exchange of inactive, GDP-bound RAS with GTP. The catalytic activity of SOS is known to be allosterically modulated by an active, GTP-bound RAS. However, it remains poorly understood how oncogenic RAS mutants interact with SOS and modulate its activity. In this chapter, we describe the application of native mass spectrometry (MS) to monitor the assembly of the catalytic domain of SOS (SOScat) with RAS and cancer-associated mutants. Results from this approach have led to the discovery of different molecular assemblies and distinct conformers of SOScat engaging KRAS. It was also found that KRASG13D exhibits high affinity for SOScat and is a potent allosteric modulator of its SOScat activity. KRASG13D-GTP can allosterically increase the nucleotide exchange rate of KRAS at the active site by more than twofold compared to the wild-type protein. Furthermore, small-molecule RAS•SOS disruptors fail to dissociate KRASG13D•SOScat complexes, underscoring the need for more potent disruptors targeting oncogenic RAS mutants. Taken together, native MS will be instrumental in better understanding the interaction between oncogenic RAS mutants and SOS, which is of crucial importance for development of improved therapeutics.

Diacylglycerol kinases: A look into the future of immunotherapy

Cancer still represents the second leading cause of death right after cardiovascular diseases. According to the World Health Organization (WHO), cancer provoked around 10 million deaths in 2020, with lung and colon tumors accounting for the deadliest forms of cancer. As tumor cells become resistant to traditional therapeutic approaches, immunotherapy has emerged as a novel strategy for tumor control. T lymphocytes are key players in immune responses against tumors. Immunosurveillance allows identification, targeting and later killing of cancerous cells. Nevertheless, tumors evolve through different strategies to evade the immune response and spread in a process called metastasis. The ineffectiveness of traditional strategies to control tumor growth and expansion has led to novel approaches considering modulation of T cell activation and effector functions. Program death receptor 1 (PD-1) and cytotoxic T-lymphocyte antigen 4 (CTLA-4) showed promising results in the early 90s and nowadays are still being exploited together with other drugs for several cancer types. Other negative regulators of T cell activation are diacylglycerol kinases (DGKs) a family of enzymes that catalyze the conversion of diacylglycerol (DAG) into phosphatidic acid (PA). In T cells, DGKα and DGKζ limit the PLCγ/Ras/ERK axis thus attenuating DAG mediated signaling and T cell effector functions. Upregulation of either of both isoforms results in impaired Ras activation and anergy induction, whereas germline knockdown mice showed enhanced antitumor properties and more effective immune responses against pathogens. Here we review the mechanisms used by DGKs to ameliorate T cell activation and how inhibition could be used to reinvigorate T cell functions in cancer context. A better knowledge of the molecular mechanisms involved upon T cell activation will help to improve current therapies with DAG promoting agents.

A guide to ERK dynamics, part 1: mechanisms and models

Extracellular signal-regulated kinase (ERK) has long been studied as a key driver of both essential cellular processes and disease. A persistent question has been how this single pathway is able to direct multiple cell behaviors, including growth, proliferation, and death. Modern biosensor studies have revealed that the temporal pattern of ERK activity is highly variable and heterogeneous, and critically, that these dynamic differences modulate cell fate. This two-part review discusses the current understanding of dynamic activity in the ERK pathway, how it regulates cellular decisions, and how these cell fates lead to tissue regulation and pathology. In part 1, we cover the optogenetic and live-cell imaging technologies that first revealed the dynamic nature of ERK, as well as current challenges in biosensor data analysis. We also discuss advances in mathematical models for the mechanisms of ERK dynamics, including receptor-level regulation, negative feedback, cooperativity, and paracrine signaling. While hurdles still remain, it is clear that higher temporal and spatial resolution provide mechanistic insights into pathway circuitry. Exciting new algorithms and advanced computational tools enable quantitative measurements of single-cell ERK activation, which in turn inform better models of pathway behavior. However, the fact that current models still cannot fully recapitulate the diversity of ERK responses calls for a deeper understanding of network structure and signal transduction in general.

Studying early structural changes in SOS1 mediated KRAS activation mechanism

KRAS activation is known to be modulated by a guanine nucleotide exchange factor (GEF), namely, Son of Sevenless1 (SOS1). SOS1 facilitates the exchange of GDP to GTP thereby leading to activation of KRAS. The binding of GDP/GTP to KRAS at the REM/allosteric site of SOS1 regulates the activation of KRAS at CDC25/catalytic site by facilitating its exchange. Different aspects of the allosteric activation of KRAS through SOS1 are still being explored. To understand the SOS1 mediated activation of KRAS, molecular dynamics simulations for a total of nine SOS1 complexes (KRAS-SOS1-KRAS) were performed. These nine systems comprised different combinations of KRAS-bound nucleotides (GTP/GDP) at REM and CDC25 sites of SOS1. Various conformational and thermodynamic parameters were analyzed for these simulation systems. MMPBSA free energy analysis revealed that binding at CDC25 site of SOS1 was significantly low for GDP-bound KRAS as compared to that of GTP-bound KRAS. It was observed that presence of either GDP/GTP bound KRAS at the REM site of SOS1 affected the activation related changes in the KRAS present at CDC25 site. The conformational changes at the catalytic site of SOS1 resulting from GDP/GTP-bound KRAS at the allosteric changes may hint at KRAS activation through different pathways (slow/fast/rare). The allosteric effect on activation of KRAS at CDC25 site may be due to conformations adopted by switch-I, switch-II, beta2 regions of KRAS at REM site. The effect of structural rearrangements occurring at allosteric KRAS may have led to increased interactions between SOS1 and KRAS at both the sites. The SOS1 residues involved in these important interactions with KRAS at the REM site were R694, S732 and K735. Whereas the ones interacting with KRAS at CDC25 site were S807, W809 and K814. This may suggest the crucial role of these residues in guiding the allosteric activation of KRAS at CDC25 site. The conformational shifts observed in the switch-I, switch-II and alpha3 regions of KRAS at CDC25 site may be attributed to be a part of allosteric activation. The binding affinities, interacting residues and conformational dynamics may provide an insight into development of inhibitors targeting the SOS1 mediated KRAS activation.

Inhibition of Son of Sevenless Homologue 1 (SOS1): Promising therapeutic treatment for KRAS-mutant cancers

Kristen rat sarcoma (KRAS) is one of the most common oncogenes in human cancers. As a guanine nucleotide exchange factor, Son of Sevenless Homologue 1 (SOS1) represents a potential therapeutic concept for the treatment of KRAS-mutant cancers because of its activation on KRAS and downstream signaling pathways. In this review, we provide a comprehensive overview of the structure, biological function, and regulation of SOS1. We also focus on the recent advances in SOS1 inhibitors and emphasize their binding modes, structure-activity relationships and pharmacological activities. We hope that this publication can provide a comprehensive compendium on the rational design of SOS1 inhibitors.

Redox-Mediated Rewiring of Signalling Pathways: The Role of a Cellular Clock in Brain Health and Disease

Metazoan signalling pathways can be rewired to dampen or amplify the rate of events, such as those that occur in development and aging. Given that a linear network topology restricts the capacity to rewire signalling pathways, such scalability of the pace of biological events suggests the existence of programmable non-linear elements in the underlying signalling pathways. Here, we review the network topology of key signalling pathways with a focus on redox-sensitive proteins, including PTEN and Ras GTPase, that reshape the connectivity profile of signalling pathways in response to an altered redox state. While this network-level impact of redox is achieved by the modulation of individual redox-sensitive proteins, it is the population by these proteins of critical nodes in a network topology of signal transduction pathways that amplifies the impact of redox-mediated reprogramming. We propose that redox-mediated rewiring is essential to regulate the rate of transmission of biological signals, giving rise to a programmable cellular clock that orchestrates the pace of biological phenomena such as development and aging. We further review the evidence that an aberrant redox-mediated modulation of output of the cellular clock contributes to the emergence of pathological conditions affecting the human brain.

Pathogen-specific structural features of Candida albicans Ras1 activation complex: uncovering new antifungal drug targets

An important feature associated with Candida albicans pathogenicity is its ability to switch between yeast and hyphal forms, a process in which CaRas1 plays a key role. CaRas1 is activated by the guanine nucleotide exchange factor (GEF) CaCdc25, triggering hyphal growth-related signaling pathways through its conserved GTP-binding (G)-domain. An important function in hyphal growth has also been proposed for the long hypervariable region downstream the G-domain, whose unusual content of polyglutamine stretches and Q/N repeats make CaRas1 unique within Ras proteins. Despite its biological importance, both the structure of CaRas1 and the molecular basis of its activation by CaCdc25 remain unexplored. Here, we show that CaRas1 has an elongated shape and limited conformational flexibility and that its hypervariable region contains helical structural elements, likely forming an intramolecular coiled-coil. Functional assays disclosed that CaRas1-activation by CaCdc25 is highly efficient, with activities up to 2,000-fold higher than reported for human GEFs. The crystal structure of the CaCdc25 catalytic region revealed an active conformation for the α-helical hairpin, critical for CaRas1-activation, unveiling a specific region exclusive to CTG-clade species. Structural studies on CaRas1/CaCdc25 complexes also revealed an interaction surface clearly distinct from that of homologous human complexes. Furthermore, we identified an inhibitory synthetic peptide, prompting the proposal of a key regulatory mechanism for CaCdc25. To our knowledge, this is the first report of specific inhibition of the CaRas1-activation via targeting its GEF. This, together with their unique pathogen-structural features, disclose a set of novel strategies to specifically block this important virulence-related mechanism. IMPORTANCE Candida albicans is the main causative agent of candidiasis, the commonest fungal infection in humans. The eukaryotic nature of C. albicans and the rapid emergence of antifungal resistance raise the challenge of identifying novel drug targets to battle this prevalent and life-threatening disease. CaRas1 and CaCdc25 are key players in the activation of signaling pathways triggering multiple virulence traits, including the yeast-to-hypha interconversion. The structural similarity of the conserved G-domain of CaRas1 to those of human homologs and the lack of structural information on CaCdc25 has impeded progress in targeting these proteins. The unique structural and functional features for CaRas1 and CaCdc25 presented here, together with the identification of a synthetic peptide capable of specifically inhibiting the GEF activity of CaCdc25, open new possibilities to uncover new antifungal drug targets against C. albicans virulence.

2'-Deoxy Guanosine Nucleotides Alter the Biochemical Properties of Ras

Ras proteins in the mitogen-activated protein kinase (MAPK) signaling pathway represent one of the most frequently mutated oncogenes in cancer. Ras binds guanosine nucleotides and cycles between active (GTP) and inactive (GDP) conformations to regulate the MAPK signaling pathway. Guanosine and other nucleotides exist in cells as either 2'-hydroxy or 2'-deoxy forms, and imbalances in the deoxyribonucleotide triphosphate pool have been associated with different diseases, such as diabetes, obesity, and cancer. However, the biochemical properties of Ras bound to dGNP are not well understood. Herein, we use native mass spectrometry to monitor the intrinsic GTPase activity of H-Ras and N-Ras oncogenic mutants, revealing that the rate of 2'-deoxy guanosine triphosphate (dGTP) hydrolysis differs compared to the hydroxylated form, in some cases by seven-fold. Moreover, K-Ras expressed from HEK293 cells exhibited a higher than anticipated abundance of dGNP, despite the low abundance of dGNP in cells. Additionally, the GTPase and dGTPase activity of K-RasG12C was found to be accelerated by 10.2- and 3.8-fold in the presence of small molecule covalent inhibitors, which may open opportunities for the development of Pan-Ras inhibitors. The molecular assemblies formed between H-Ras and N-Ras, including mutant forms, with the catalytic domain of SOS (SOScat) were also investigated. The results show that the different mutants of H-Ras and N-Ras not only engage SOScat differently, but these assemblies are also dependent on the form of guanosine triphosphate bound to Ras. These findings bring to the forefront a new perspective on the nucleotide-dependent biochemical properties of Ras that may have implications for the activation of the MAPK signaling pathway and Ras-driven cancers.

The membrane surface as a platform that organizes cellular and biochemical processes

Membranes are essential for life. They act as semi-permeable boundaries that define cells and organelles. In addition, their surfaces actively participate in biochemical reaction networks, where they confine proteins, align reaction partners, and directly control enzymatic activities. Membrane-localized reactions shape cellular membranes, define the identity of organelles, compartmentalize biochemical processes, and can even be the source of signaling gradients that originate at the plasma membrane and reach into the cytoplasm and nucleus. The membrane surface is, therefore, an essential platform upon which myriad cellular processes are scaffolded. In this review, we summarize our current understanding of the biophysics and biochemistry of membrane-localized reactions with particular focus on insights derived from reconstituted and cellular systems. We discuss how the interplay of cellular factors results in their self-organization, condensation, assembly, and activity, and the emergent properties derived from them.

Bimodality in Ras signaling originates from processivity of the Ras activator SOS without classic kinetic bistability

Ras is a small GTPase that is central to important functional decisions in diverse cell types. An important aspect of Ras signaling is its ability to exhibit bimodal, or switch-like activity. We describe the total reconstitution of a receptor-mediated Ras activation-deactivation reaction catalyzed by SOS and p120-RasGAP on supported lipid membrane microarrays. The results reveal a bimodal Ras activation response, which is not a result of classic kinetic bistability, but is rather driven by the distinct processivity of the Ras activator, SOS. Furthermore, the bimodal response is controlled by the condensation state of the scaffold protein, LAT, to which SOS is recruited. Processivity-driven bimodality leads to stochastic bursts of Ras activation even under strongly deactivating conditions. This behavior contrasts classic kinetic bistability and is distinctly more resistant to pharmacological inhibition.

Pan-KRAS inhibitor disables oncogenic signalling and tumour growth

KRAS is one of the most commonly mutated proteins in cancer, and efforts to directly inhibit its function have been continuing for decades. The most successful of these has been the development of covalent allele-specific inhibitors that trap KRAS G12C in its inactive conformation and suppress tumour growth in patients1-7. Whether inactive-state selective inhibition can be used to therapeutically target non-G12C KRAS mutants remains under investigation. Here we report the discovery and characterization of a non-covalent inhibitor that binds preferentially and with high affinity to the inactive state of KRAS while sparing NRAS and HRAS. Although limited to only a few amino acids, the evolutionary divergence in the GTPase domain of RAS isoforms was sufficient to impart orthosteric and allosteric constraints for KRAS selectivity. The inhibitor blocked nucleotide exchange to prevent the activation of wild-type KRAS and a broad range of KRAS mutants, including G12A/C/D/F/V/S, G13C/D, V14I, L19F, Q22K, D33E, Q61H, K117N and A146V/T. Inhibition of downstream signalling and proliferation was restricted to cancer cells harbouring mutant KRAS, and drug treatment suppressed KRAS mutant tumour growth in mice, without having a detrimental effect on animal weight. Our study suggests that most KRAS oncoproteins cycle between an active state and an inactive state in cancer cells and are dependent on nucleotide exchange for activation. Pan-KRAS inhibitors, such as the one described here, have broad therapeutic implications and merit clinical investigation in patients with KRAS-driven cancers.

SOS1-inspired hydrocarbon-stapled peptide as a pan-Ras inhibitor

Blocking the interaction between Ras and Son of Sevenless homolog 1 (SOS1) has been an attractive therapeutic strategy for treating cancers involving oncogenic Ras mutations. K-Ras mutation is the most common in Ras-driven cancers, accounting for 86%, with N-Ras mutation and H-Ras mutation accounting for 11% and 3%, respectively. Here, we report the design and synthesis of a series of hydrocarbon-stapled peptides to mimic the alpha-helix of SOS1 as pan-Ras inhibitors. Among these stapled peptides, SSOSH-5 was identified to maintain a well-constrained alpha-helical structure and bind to H-Ras with high affinity. SSOSH-5 was furthermore validated to bind with Ras similarly to the parent linear peptide through structural modeling analysis. This optimized stapled peptide was proven to be capable of effectively inhibiting the proliferation of pan-Ras-mutated cancer cells and inducing apoptosis in a dose-dependent manner by modulating downstream kinase signaling. Of note, SSOSH-5 exhibited a high capability of crossing cell membranes and strong proteolytic resistance. We demonstrated that the peptide stapling strategy is a feasible approach for developing peptide-based pan-Ras inhibitors. Furthermore, we expect that SSOSH-5 can be further characterized and optimized for the treatment of Ras-driven cancers.

A novel protein RASON encoded by a lncRNA controls oncogenic RAS signaling in KRAS mutant cancers

Mutations of the RAS oncogene are found in around 30% of all human cancers yet direct targeting of RAS is still considered clinically impractical except for the KRAS^G12C mutant. Here we report that RAS-ON (RASON), a novel protein encoded by the long intergenic non-protein coding RNA 00673 (LINC00673), is a positive regulator of oncogenic RAS signaling. RASON is aberrantly overexpressed in pancreatic ductal adenocarcinoma (PDAC) patients, and it promotes proliferation of human PDAC cell lines in vitro and tumor growth in vivo. CRISPR/Cas9-mediated knockout of Rason in mouse embryonic fibroblasts inhibits KRAS-mediated tumor transformation. Genetic deletion of Rason abolishes oncogenic KRAS-driven pancreatic and lung cancer tumorigenesis in LSL-Kras^G12D; Trp53^R172H/+ mice. Mechanistically, RASON directly binds to KRAS^G12D/V and inhibits both intrinsic and GTPase activating protein (GAP)-mediated GTP hydrolysis, thus sustaining KRAS^G12D/V in the GTP-bound hyperactive state. Therapeutically, deprivation of RASON sensitizes KRAS mutant pancreatic cancer cells and patient-derived organoids to EGFR inhibitors. Our findings identify RASON as a critical regulator of oncogenic KRAS signaling and a promising therapeutic target for KRAS mutant cancers.

PPDPF Promotes the Development of Mutant KRAS-Driven Pancreatic Ductal Adenocarcinoma by Regulating the GEF Activity of SOS1

The guanine nucleotide exchange factor (GEF) SOS1 catalyzes the exchange of GDP for GTP on RAS. However, regulation of the GEF activity remains elusive. Here, the authors report that PPDPF functions as an important regulator of SOS1. The expression of PPDPF is significantly increased in pancreatic ductal adenocarcinoma (PDAC), associated with poor prognosis and recurrence of PDAC patients. Overexpression of PPDPF promotes PDAC cell growth in vitro and in vivo, while PPDPF knockout exerts opposite effects. Pancreatic-specific deletion of PPDPF profoundly inhibits tumor development in KRASG12D -driven genetic mouse models of PDAC. PPDPF can bind GTP and transfer GTP to SOS1. Mutations of the GTP-binding sites severely impair the tumor-promoting effect of PPDPF. Consistently, mutations of the critical amino acids mediating SOS1-PPDPF interaction significantly impair the GEF activity of SOS1. Therefore, this study demonstrates a novel model of KRAS activation via PPDPF-SOS1 axis, and provides a promising therapeutic target for PDAC.

Design and Discovery of MRTX0902, a Potent, Selective, Brain-Penetrant, and Orally Bioavailable Inhibitor of the SOS1:KRAS Protein-Protein Interaction

SOS1 is one of the major guanine nucleotide exchange factors that regulates the ability of KRAS to cycle through its “on” and “off” states. Disrupting the SOS1:KRAS^G12C protein–protein interaction (PPI) can increase the proportion of GDP-loaded KRAS^G12C, providing a strong mechanistic rationale for combining inhibitors of the SOS1:KRAS complex with inhibitors like MRTX849 that target GDP-loaded KRAS^G12C. In this report, we detail the design and discovery of MRTX0902—a potent, selective, brain-penetrant, and orally bioavailable SOS1 binder that disrupts the SOS1:KRAS^G12C PPI. Oral administration of MRTX0902 in combination with MRTX849 results in a significant increase in antitumor activity relative to that of either single agent, including tumor regressions in a subset of animals in the MIA PaCa-2 tumor mouse xenograft model.

Controlling oncogenic KRAS signaling pathways with a Palladium-responsive peptide

RAS oncoproteins are molecular switches associated with critical signaling pathways that regulate cell proliferation and differentiation. Mutations in the RAS family, mainly in the KRAS isoform, are responsible for some of the deadliest cancers, which has made this protein a major target in biomedical research. Here we demonstrate that a designed bis-histidine peptide derived from the αH helix of the cofactor SOS1 binds to KRAS with high affinity upon coordination to Pd(II). NMR spectroscopy and MD studies demonstrate that Pd(II) has a nucleating effect that facilitates the access to the bioactive α-helical conformation. The binding can be suppressed by an external metal chelator and recovered again by the addition of more Pd(II), making this system the first switchable KRAS binder, and demonstrates that folding-upon-binding mechanisms can operate in metal-nucleated peptides. In vitro experiments show that the metallopeptide can efficiently internalize into living cells and inhibit the MAPK kinase cascade.

AIMP2-DX2 provides therapeutic interface to control KRAS-driven tumorigenesis

Recent development of the chemical inhibitors specific to oncogenic KRAS (Kirsten Rat Sarcoma 2 Viral Oncogene Homolog) mutants revives much interest to control KRAS-driven cancers. Here, we report that AIMP2-DX2, a variant of the tumor suppressor AIMP2 (aminoacyl-tRNA synthetase-interacting multi-functional protein 2), acts as a cancer-specific regulator of KRAS stability, augmenting KRAS-driven tumorigenesis. AIMP2-DX2 specifically binds to the hypervariable region and G-domain of KRAS in the cytosol prior to farnesylation. Then, AIMP2-DX2 competitively blocks the access of Smurf2 (SMAD Ubiquitination Regulatory Factor 2) to KRAS, thus preventing ubiquitin-mediated degradation. Moreover, AIMP2-DX2 levels are positively correlated with KRAS levels in colon and lung cancer cell lines and tissues. We also identified a small molecule that specifically bound to the KRAS-binding region of AIMP2-DX2 and inhibited the interaction between these two factors. Treatment with this compound reduces the cellular levels of KRAS, leading to the suppression of KRAS-dependent cancer cell growth in vitro and in vivo. These results suggest the interface of AIMP2-DX2 and KRAS as a route to control KRAS-driven cancers.

Direct targeting of oncogenic KRAS activity is a challenge. Here the authors report that a splice variant of AIMP2, AIMP2-DX2, enhances KRAS stability by blocking ubiquitin-mediated degradation of KRAS via the E3 ligase, Smurf2, and identify a chemical that can hinder AIMP2-DX2 from interacting with KRAS.

Delineating The RAS Conformational Landscape

Mutations in RAS isoforms (KRAS, NRAS, and HRAS) are among the most frequent oncogenic alterations in many cancers, making these proteins high priority therapeutic targets. Effectively targeting RAS isoforms requires an exact understanding of their active, inactive, and druggable conformations. However, there is no structural catalog of RAS conformations to guide therapeutic targeting or examining the structural impact of RAS mutations. Here we present an expanded classification of RAS conformations based on analyses of the catalytic switch 1 (SW1) and switch 2 (SW2) loops. From 721 human KRAS, NRAS, and HRAS structures available in the Protein Data Bank (206 RAS-protein co-complexes, 190 inhibitor-bound, and 325 unbound, including 204 WT and 517 mutated structures), we created a broad conformational classification based on the spatial positions of Y32 in SW1 and Y71 in SW2. Clustering all well-modeled SW1 and SW2 loops using a density-based machine learning algorithm defined additional conformational subsets, some previously undescribed. Three SW1 conformations and nine SW2 conformations were identified, each associated with different nucleotide states (GTP-bound, nucleotide-free, and GDP-bound) and specific bound proteins or inhibitor sites. The GTP-bound SW1 conformation could be further subdivided based on the hydrogen bond type made between Y32 and the GTP γ-phosphate. Further analysis clarified the catalytic impact of G12D and G12V mutations and the inhibitor chemistries that bind to each druggable RAS conformation. Overall, this study has expanded our understanding of RAS structural biology, which could facilitate future RAS drug discovery.

A two-component protein condensate of the EGFR cytoplasmic tail and Grb2 regulates Ras activation by SOS at the membrane

We reconstitute a phosphotyrosine-mediated protein condensation phase transition of the ∼200 residue cytoplasmic tail of the epidermal growth factor receptor (EGFR) and the adaptor protein, Grb2, on a membrane surface. The phase transition depends on phosphorylation of the EGFR tail, which recruits Grb2, and crosslinking through a Grb2-Grb2 binding interface. The Grb2 Y160 residue plays a structurally critical role in the Grb2-Grb2 interaction, and phosphorylation or mutation of Y160 prevents EGFR:Grb2 condensation. By extending the reconstitution experiment to include the guanine nucleotide exchange factor, SOS, and its substrate Ras, we further find that the condensation state of the EGFR tail controls the ability of SOS, recruited via Grb2, to activate Ras. These results identify an EGFR:Grb2 protein condensation phase transition as a regulator of signal propagation from EGFR to the MAPK pathway.

Autopromotion of K-Ras4B Feedback Activation Through an SOS-Mediated Long-Range Allosteric Effect

The Ras-specific guanine nucleotide exchange factors Son of Sevenless (SOS) regulates Ras activation by converting inactive GDP-bound to active GTP-bound states. The catalytic activity of Ras is further allosterically regulated by GTP-Ras bound to a distal site through a positive feedback loop. To address the mechanism underlying the long-range allosteric activation of the catalytic K-Ras4B by an additional allosteric GTP-Ras through SOS, we employed molecular dynamics simulation of the K-Ras4BG13D•SOScat complex with and without an allosteric GTP-bound K-Ras4BG13D. We found that the binding of an allosteric GTP-K-Ras4BG13D enhanced the affinity between the catalytic K-Ras4BG13D and SOScat, forming a more stable conformational state. The peeling away of the switch I from the nucleotide binding site facilitated the dissociation of GDP, thereby contributing to the increased nucleotide exchange rate. The community networks further showed stronger edge connection upon allosteric GTP-K-Ras4BG13D binding, which represented an increased interaction between catalytic K-Ras4BG13D and SOScat. Moreover, GTP-K-Ras4BG13D binding transmitted allosteric signaling pathways though the Cdc25 domain of SOS that enhanced the allosteric regulatory from the K-Ras4BG13D allosteric site to the catalytic site. This study may provide an in-depth mechanism for abnormal activation and allosteric regulation of K-Ras4BG13D.

Classification of KRAS-Activating Mutations and the Implications for Therapeutic Intervention

Members of the family of RAS proto-oncogenes, discovered just over 40 years ago, were among the first cancer-initiating genes to be discovered. Of the three RAS family members, KRAS is the most frequently mutated in human cancers. Despite intensive biological and biochemical study of RAS proteins over the past four decades, we are only now starting to devise therapeutic strategies to target their oncogenic properties. Here, we highlight the distinct biochemical properties of common and rare KRAS alleles, enabling their classification into functional subtypes. We also discuss the implications of this functional classification for potential therapeutic avenues targeting mutant subtypes.

SIGNIFICANCE

Efforts in the recent past to inhibit KRAS oncogenicity have focused on kinases that function in downstream signal transduction cascades, although preclinical successes have not translated to patients with KRAS-mutant cancer. Recently, clinically effective covalent inhibitors of KRASG12C have been developed, establishing two principles that form a foundation for future efforts. First, KRAS is druggable. Second, each mutant form of KRAS is likely to have properties that make it uniquely druggable.

Conformations and binding pockets of HRas and its guanine nucleotide exchange factors complexes in the guanosine triphosphate exchange process

The human Son of Sevenless (SOS) activates the signal-transduction protein Ras by forming the complex SOS·Ras and accelerating the guanosine triphosphate (GTP) exchange in Ras. Inhibition of SOS·Ras could regulate the function of Ras in cells and has emerged as an effective strategy for battling Ras related cancers. A key factor to the success of this approach is to understand the conformational change of Ras during the GTP exchange process. In this study, we perform an extensive molecular dynamics simulation to characterize the specific conformations of Ras without and with guanine nucleotide exchange factors (GEFs) of SOS, especially for the substates of State 1 of HRasGTP∙Mg²⁺ . The potent binding pockets on the surfaces of the RasGDP∙Mg²⁺ , the S1.1 and S1.2 substates in State 1 of RasGTP∙Mg²⁺ and the ternary complexes with SOS are predicted, including the binding sites of other domains of SOS. These findings help to obtain a more thorough understanding of Ras functions in the GTP cycling process and provide a structural foundation for future drug design.

A saturation-mutagenesis analysis of the interplay between stability and activation in Ras

Cancer mutations in Ras occur predominantly at three hotspots: Gly 12, Gly 13, and Gln 61. Previously, we reported that deep mutagenesis of H-Ras using a bacterial assay identified many other activating mutations (Bandaru et al., 2017). We now show that the results of saturation mutagenesis of H-Ras in mammalian Ba/F3 cells correlate well with the results of bacterial experiments in which H-Ras or K-Ras are co-expressed with a GTPase-activating protein (GAP). The prominent cancer hotspots are not dominant in the Ba/F3 data. We used the bacterial system to mutagenize Ras constructs of different stabilities and discovered a feature that distinguishes the cancer hotspots. While mutations at the cancer hotspots activate Ras regardless of construct stability, mutations at lower-frequency sites (e.g. at Val 14 or Asp 119) can be activating or deleterious, depending on the stability of the Ras construct. We characterized the dynamics of three non-hotspot activating Ras mutants by using NMR to monitor hydrogen-deuterium exchange (HDX). These mutations result in global increases in HDX rates, consistent with destabilization of Ras. An explanation for these observations is that mutations that destabilize Ras increase nucleotide dissociation rates, enabling activation by spontaneous nucleotide exchange. A further stability decrease can lead to insufficient levels of folded Ras - and subsequent loss of function. In contrast, the cancer hotspot mutations are mechanism-based activators of Ras that interfere directly with the action of GAPs. Our results demonstrate the importance of GAP surveillance and protein stability in determining the sensitivity of Ras to mutational activation.

Targeting RAS oncogenesis with SOS1 inhibitors

RAS proteins play major roles in many human cancers, but programs to develop direct RAS inhibitors so far have only been successful for the oncogenic KRAS mutant G12C. As an alternative approach, inhibitors for the RAS guanine nucleotide exchange factor SOS1 have been investigated by several academic groups and companies, and major progress has been achieved in recent years in the optimization of small molecule activators and inhibitors of SOS1. Here, we review the discovery and development of small molecule modulators of SOS1 and their molecular binding modes and modes of action. As targeting the RAS pathway is expected to result in the development of resistance mechanisms, SOS1 inhibitors will most likely be best applied in vertical combination approaches where two nodes of the RAS signaling pathway are hit simultaneously. We summarize the current understanding of which combination partners may be most beneficial for patients with RAS driven tumors.

KRASG12R-Independent Macropinocytosis in Pancreatic Cancer

Macropinocytosis is a critical route of nutrient acquisition in pancreatic cancer cells. Constitutive macropinocytosis is promoted by mutant KRAS, which activates the PI3Kα lipid kinase and RAC1, to drive membrane ruffling, macropinosome uptake and processing. However, our recent study on the KRAS^G12R mutant indicated the presence of a KRAS-independent mode of macropinocytosis in pancreatic cancer cell lines, thereby increasing the complexity of this process. We found that KRAS^G12R-mutant cell lines promote macropinocytosis independent of KRAS activity using PI3Kγ and RAC1, highlighting the convergence of regulation on RAC signaling. While macropinocytosis has been proposed to be a therapeutic target for the treatment of pancreatic cancer, our studies have underscored how little we understand about the activation and regulation of this metabolic process. Therefore, this review seeks to highlight the differences in macropinocytosis regulation in the two cellular subtypes while also highlighting the features that make the KRAS^G12R mutant atypical.