The pre-hydrolysis state of p21(ras) in complex with GTP: new insights into the role of water molecules in the GTP hydrolysis reaction of ras-like proteins

High Pressure Promotes Binding of the Allosteric Inhibitor Zn2+-Cyclen in Crystals of Activated H-Ras

In this work, we experimentally investigate the potency of high pressure to drive a protein toward an excited state where an inhibitor targeted for this state can bind. Ras proteins are small GTPases cycling between active GTP-bound and inactive GDP-bound states. Various states of GTP-bound Ras in active conformation coexist in solution, amongst them, state 2 which binds to effectors, and state 1, weakly populated at ambient conditions, which has a low affinity for effectors. Zn2+-cyclen is an allosteric inhibitor of Ras protein, designed to bind specifically to the state 1. In H-Ras(wt).Mg2+.GppNHp crystals soaked with Zn2+-cyclen, no binding could be observed, as expected in the state 2 conformation which is the dominant state at ambient pressure. Interestingly, Zn2+-cyclen binding is observed at 500 MPa pressure, close to the nucleotide, in Ras protein that is driven by pressure to a state 1 conformer. The unknown binding mode of Zn2+-cyclen to H-Ras can thus be fully characterized in atomic details. As a more general conjunction from our study, high pressure x-ray crystallography turns out to be a powerful method to induce transitions allowing drug binding in proteins that are in low-populated conformations at ambient conditions, enabling the design of specific inhibitors.

Crystal structure of NRAS Q61K with a ligand-induced pocket near switch II

The RAS isoforms (KRAS, HRAS and NRAS) have distinct cancer type-specific profiles. NRAS mutations are the second most prevalent RAS mutations in skin and hematological malignancies. Although RAS proteins were considered undruggable for decades, isoform and mutation-specific investigations have produced successful RAS inhibitors that are either specific to certain mutants, isoforms (pan-KRAS) or target all RAS proteins (pan-RAS). While extensive structural and biochemical investigations have focused mainly on K- and H-RAS mutations, NRAS mutations have received less attention, and the most prevalent NRAS mutations in human cancers, Q61K and Q61R, are rare in K- and H-RAS. This manuscript presents a crystal structure of the NRAS Q61K mutant in the GTP-bound form. Our structure reveals a previously unseen pocket near switch II induced by the binding of a ligand to the active form of the protein. This observation reveals a binding site that can potentially be exploited for development of inhibitors against mutant NRAS. Furthermore, the well-resolved catalytic site of this GTPase bound to native GTP provides insight into the stalled GTP hydrolysis observed for NRAS-Q61K.

Unveiling the State Transition Mechanisms of Ras Proteins through Enhanced Sampling and QM/MM Simulations

In cells, wild-type RasGTP complexes exist in two distinct states: active State 2 and inactive State 1. These complexes regulate their functions by transitioning between the two states. However, the mechanisms underlying this state transition have not been clearly elucidated. To address this, we conducted a detailed simulation study to characterize the energetics of the stable states involved in the state transitions of the HRasGTP complex, specifically from State 2 to State 1. This was achieved by employing multiscale quantum mechanics/molecular mechanics and enhanced sampling molecular dynamics methods. Based on the simulation results, we constructed the two-dimensional free energy landscapes that provide crucial information about the conformational changes of the HRasGTP complex from State 2 to State 1. Furthermore, we also explored the conformational changes from the intermediate state to the product state during guanosine triphosphate hydrolysis. This study on the conformational changes involved in the HRas state transitions serves as a valuable reference for understanding the corresponding events of both KRas and NRas as well.

Far-reaching effects of tyrosine64 phosphorylation on Ras revealed with BeF3- complexes

Tyrosine phosphorylation on Ras by Src kinase is known to uncouple Ras from upstream regulation and downstream communication. However, the mechanisms by which phosphorylation modulates these interactions have not been detailed. Here, the major mono-phosphorylation level on tyrosine64 is quantified by 31P NMR and mutagenesis. Crystal structures of unphosphorylated and tyrosine64-phosphorylated Ras in complex with a BeF3- ground state analogue reveal "closed" Ras conformations very different from those of the "open" conformations previously observed for non-hydrolysable GTP analogue structures of Ras. They deliver new mechanistic and conformational insights into intrinsic GTP hydrolysis. Phosphorylation of tyrosine64 delivers conformational changes distant from the active site, showing why phosphorylated Ras has reduced affinity to its downstream effector Raf. 19F NMR provides evidence for changes in the intrinsic GTPase and nucleotide exchange rate and identifies the concurrent presence of a major "closed" conformation alongside a minor yet functionally important "open" conformation at the ground state of Ras. This study expands the application of metal fluoride complexes in revealing major and minor conformational changes of dynamic and modified Ras proteins.

Allosteric site variants affect GTP hydrolysis on Ras

RAS GTPases are proto-oncoproteins that regulate cell growth, proliferation, and differentiation in response to extracellular signals. The signaling functions of RAS, and other small GTPases, are dependent on their ability to cycle between GDP-bound and GTP-bound states. Structural analyses suggest that GTP hydrolysis catalyzed by HRAS can be regulated by an allosteric site located between helices 3, 4, and loop 7. Here we explore the relationship between intrinsic GTP hydrolysis on HRAS and the position of helix 3 and loop 7 through manipulation of the allosteric site, showing that the two sites are functionally connected. We generated several hydrophobic mutations in the allosteric site of HRAS to promote shifts in helix 3 relative to helix 4. By combining crystallography and enzymology to study these mutants, we show that closure of the allosteric site correlates with increased hydrolysis of GTP on HRAS in solution. Interestingly, binding to the RAS binding domain of RAF kinase (RAF-RBD) inhibits GTP hydrolysis in the mutants. This behavior may be representative of a cluster of mutations found in human tumors, which potentially cooperate with RAF complex formation to stabilize the GTP-bound state of RAS.

Modulation of the proteostasis network promotes tumor resistance to oncogenic KRAS inhibitors

Despite substantial advances in targeting mutant KRAS, tumor resistance to KRAS inhibitors (KRASi) remains a major barrier to progress. Here, we report proteostasis reprogramming as a key convergence point of multiple KRASi-resistance mechanisms. Inactivation of oncogenic KRAS down-regulated both the heat shock response and the inositol-requiring enzyme 1α (IRE1α) branch of the unfolded protein response, causing severe proteostasis disturbances. However, IRE1α was selectively reactivated in an ER stress-independent manner in acquired KRASi-resistant tumors, restoring proteostasis. Oncogenic KRAS promoted IRE1α protein stability through extracellular signal-regulated kinase (ERK)-dependent phosphorylation of IRE1α, leading to IRE1α disassociation from 3-hydroxy-3-methylglutaryl reductase degradation (HRD1) E3-ligase. In KRASi-resistant tumors, both reactivated ERK and hyperactivated AKT restored IRE1α phosphorylation and stability. Suppression of IRE1α overcame resistance to KRASi. This study reveals a druggable mechanism that leads to proteostasis reprogramming and facilitates KRASi resistance.

Combining temperature perturbations with X-ray crystallography to study dynamic macromolecules: A thorough discussion of experimental methods

Temperature is an important state variable that governs the behavior of microscopic systems, yet crystallographers rarely exploit temperature changes to study the structure and dynamics of biological macromolecules. In fact, approximately 90% of crystal structures in the Protein Data Bank were determined under cryogenic conditions, because sample cryocooling makes crystals robust to X-ray radiation damage and facilitates data collection. On the other hand, cryocooling can introduce artifacts into macromolecular structures, and can suppress conformational dynamics that are critical for function. Fortunately, recent advances in X-ray detector technology, X-ray sources, and computational data processing algorithms make non-cryogenic X-ray crystallography easier and more broadly applicable than ever before. Without the reliance on cryocooling, high-resolution crystallography can be combined with various temperature perturbations to gain deep insight into the conformational landscapes of macromolecules. This Chapter reviews the historical reasons for the prevalence of cryocooling in macromolecular crystallography, and discusses its potential drawbacks. Next, the Chapter summarizes technological developments and methodologies that facilitate non-cryogenic crystallography experiments. Finally, the chapter discusses the theoretical underpinnings and practical aspects of multi-temperature and temperature-jump crystallography experiments, which are powerful tools for understanding the relationship between the structure, dynamics, and function of proteins and other biological macromolecules.

Characterization of mutant versions of the R-RAS2/TC21 GTPase found in tumors

The R-RAS2 GTP hydrolase (GTPase) (also known as TC21) has been traditionally considered quite similar to classical RAS proteins at the regulatory and signaling levels. Recently, a long-tail hotspot mutation targeting the R-RAS2/TC21 Gln⁷² residue (Q72L) was identified as a potent oncogenic driver. Additional point mutations were also found in other tumors at low frequencies. Despite this, little information is available regarding the transforming role of these mutant versions and their relevance for the tumorigenic properties of already-transformed cancer cells. Here, we report that many of the RRAS2 mutations found in human cancers are highly transforming when expressed in immortalized cell lines. Moreover, the expression of endogenous R-RAS2^Q72L is important for maintaining optimal levels of PI3K and ERK activities as well as for the adhesion, invasiveness, proliferation, and mitochondrial respiration of ovarian and breast cancer cell lines. Endogenous R-RAS2^Q72L also regulates gene expression programs linked to both cell adhesion and inflammatory/immune-related responses. Endogenous R-RAS2^Q72L is also quite relevant for the in vivo tumorigenic activity of these cells. This dependency is observed even though these cancer cell lines bear concurrent gain-of-function mutations in genes encoding RAS signaling elements. Finally, we show that endogenous R-RAS2, unlike the case of classical RAS proteins, specifically localizes in focal adhesions. Collectively, these results indicate that gain-of-function mutations of R-RAS2/TC21 play roles in tumor initiation and maintenance that are not fully redundant with those regulated by classical RAS oncoproteins.

Structures and mechanisms of actin ATP hydrolysis

A variety of cellular functions are driven by actin, which undergoes cyclic transitions between the monomeric G-form and the filamentous F-form. To gain insights into actin dynamics, the mechanism by which the energy is supplied by the ATP hydrolysis reaction in the F-form actin must be elucidated. This has been hampered by the lack of actin filament structures at atomic resolutions. Here, we have crystallized actin molecules trapped in the F-form without forming filaments, and based upon these structures we determined the reaction path by quantum mechanics calculations. The results are consistent with previous biochemical data. Remarkably, the hydrolysis reaction mechanism is essentially identical to those of motor proteins, while the process of Pi release is distinct.

The major cytoskeleton protein actin undergoes cyclic transitions between the monomeric G-form and the filamentous F-form, which drive organelle transport and cell motility. This mechanical work is driven by the ATPase activity at the catalytic site in the F-form. For deeper understanding of the actin cellular functions, the reaction mechanism must be elucidated. Here, we show that a single actin molecule is trapped in the F-form by fragmin domain-1 binding and present their crystal structures in the ATP analog-, ADP-Pi-, and ADP-bound forms, at 1.15-Å resolutions. The G-to-F conformational transition shifts the side chains of Gln137 and His161, which relocate four water molecules including W1 (attacking water) and W2 (helping water) to facilitate the hydrolysis. By applying quantum mechanics/molecular mechanics calculations to the structures, we have revealed a consistent and comprehensive reaction path of ATP hydrolysis by the F-form actin. The reaction path consists of four steps: 1) W1 and W2 rotations; 2) P^G–O^3B bond cleavage; 3) four concomitant events: W1–PO₃⁻ formation, OH⁻ and proton cleavage, nucleophilic attack by the OH⁻ against P^G, and the abstracted proton transfer; and 4) proton relocation that stabilizes the ADP-Pi–bound F-form actin. The mechanism explains the slow rate of ATP hydrolysis by actin and the irreversibility of the hydrolysis reaction. While the catalytic strategy of actin ATP hydrolysis is essentially the same as those of motor proteins like myosin, the process after the hydrolysis is distinct and discussed in terms of Pi release, F-form destabilization, and global conformational changes.

Mechanisms of isoform-specific residue influence on GTP-bound HRas, KRas, and NRas

HRas, KRas, and NRas are GTPases with a common set of effectors that control many cell-signaling pathways, including proliferation through Raf kinase. Their G-domains are nearly identical in sequence, with a few isoform-specific residues that have an effect on dynamics and biochemical properties. Here, we use accelerated molecular dynamics (aMD) simulations consistent with solution x-ray scattering experiments to elucidate mechanisms through which isoform-specific residues associated with each Ras isoform affects functionally important regions connected to the active site. HRas-specific residues cluster in loop 8 to stabilize the nucleotide-binding pocket, while NRas-specific residues on helix 3 directly affect the conformations of switch I and switch II. KRas, the most globally flexible of the isoforms, shows greatest fluctuations in the switch regions enhanced by a KRas-specific residue in loop 7 and a highly dynamic loop 8 region. The analysis of isoform-specific residue effects on Ras proteins is supported by NMR experiments and is consistent with previously published biochemical data.

Functional and biological heterogeneity of KRASQ61 mutations

Missense mutations at the three hotspots in the guanosine triphosphatase (GTPase) RAS-Gly12, Gly13, and Gln61 (commonly known as G12, G13, and Q61, respectively)-occur differentially among the three RAS isoforms. Q61 mutations in KRAS are infrequent and differ markedly in occurrence. Q61H is the predominant mutant (at 57%), followed by Q61R/L/K (collectively 40%), and Q61P and Q61E are the rarest (2 and 1%, respectively). Probability analysis suggested that mutational susceptibility to different DNA base changes cannot account for this distribution. Therefore, we investigated whether these frequencies might be explained by differences in the biochemical, structural, and biological properties of KRASQ61 mutants. Expression of KRASQ61 mutants in NIH 3T3 fibroblasts and RIE-1 epithelial cells caused various alterations in morphology, growth transformation, effector signaling, and metabolism. The relatively rare KRASQ61E mutant stimulated actin stress fiber formation, a phenotype distinct from that of KRASQ61H/R/L/P, which disrupted actin cytoskeletal organization. The crystal structure of KRASQ61E was unexpectedly similar to that of wild-type KRAS, a potential basis for its weak oncogenicity. KRASQ61H/L/R-mutant pancreatic ductal adenocarcinoma (PDAC) cell lines exhibited KRAS-dependent growth and, as observed with KRASG12-mutant PDAC, were susceptible to concurrent inhibition of ERK-MAPK signaling and of autophagy. Our results uncover phenotypic heterogeneity among KRASQ61 mutants and support the potential utility of therapeutic strategies that target KRASQ61 mutant-specific signaling and cellular output.

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 hotspot mutation targeting the R-RAS2 GTPase acts as a potent oncogenic driver in a wide spectrum of tumors

A missense change in RRAS2 (Gln⁷² to Leu), analogous to the Gln⁶¹-to-Leu mutation of RAS oncoproteins, has been identified as a long-tail hotspot mutation in cancer and Noonan syndrome. However, the relevance of this mutation for in vivo tumorigenesis remains understudied. Here we show, using an inducible knockin mouse model, that R-Ras2^Q72L triggers rapid development of a wide spectrum of tumors when somatically expressed in adult tissues. These tumors show limited overlap with those originated by classical Ras oncogenes. R-Ras2^Q72L-driven tumors can be classified into different subtypes according to therapeutic susceptibility. Importantly, the most relevant R-Ras2^Q72L-driven tumors are dependent on mTORC1 but independent of phosphatidylinositol 3-kinase-, MEK-, and Ral guanosine diphosphate (GDP) dissociation stimulator. This pharmacological vulnerability is due to the extensive rewiring by R-Ras2^Q72L of pathways that orthogonally stimulate mTORC1 signaling. These findings demonstrate that RRAS2^Q72L is a bona fide oncogenic driver and unveil therapeutic strategies for patients with cancer and Noonan syndrome bearing RRAS2 mutations.

Detection of RAS gene mutation and its clinical significance in children with acute lymphoblastic leukemia

OBJECTIVES

To investigate the mutation rate of the RAS gene and its clinical significance in children with acute lymphoblastic leukemia.

METHODS

A retrospective analysis was performed on the medical data of 120 children with newly diagnosed acute lymphoblastic leukemia, who were admitted to the Third Affiliated Hospital of Zhengzhou University from January 2015 to January 2020 and underwent next-generation sequencing. The clinical and molecular features were analyzed. The impact of RAS gene mutation on the overall survival rate was evaluated in these children.

RESULTS

Among the 120 children, 35 (29.2%) had RAS gene mutation, 30 (25.0%) had KRAS gene mutation, and 5 (4.2%) had both NRAS and KRAS gene mutations. All NRAS mutations and 71% (25/35) of KRAS mutations were located at the 12th and 13th codons. RAS gene mutation was detected in 35 (33.3%) out of 105 children with B-lineage acute lymphoblastic leukemia, but it was not detected in those with acute T lymphocyte leukemia. Of all the children, 11 (9.2%) were lost to follow-up, and among the 109 children followed up, 16 (14.7%) died. The children with RAS gene mutation had a significantly lower 2-year overall survival rate than those without RAS gene mutation (P<0.05). The prognosis of children with RAS gene mutation combined with WT1 overexpression and WBC>50×109/L at diagnosis was worse (P<0.05).

CONCLUSIONS

RAS gene mutation is commonly observed in children with B-lineage acute lymphoblastic leukemia and may have an adverse effect on prognosis.

Regulation of GTPase function by autophosphorylation

A unifying feature of the RAS superfamily is a conserved GTPase cycle by which these proteins transition between active and inactive states. We demonstrate that autophosphorylation of some GTPases is an intrinsic regulatory mechanism that reduces nucleotide hydrolysis and enhances nucleotide exchange, altering the on/off switch that forms the basis for their signaling functions. Using X-ray crystallography, nuclear magnetic resonance spectroscopy, binding assays, and molecular dynamics on autophosphorylated mutants of H-RAS and K-RAS, we show that phosphoryl transfer from GTP requires dynamic movement of the switch II region and that autophosphorylation promotes nucleotide exchange by opening the active site and extracting the stabilizing Mg2+. Finally, we demonstrate that autophosphorylated K-RAS exhibits altered effector interactions, including a reduced affinity for RAF proteins in mammalian cells. Thus, autophosphorylation leads to altered active site dynamics and effector interaction properties, creating a pool of GTPases that are functionally distinct from their non-phosphorylated counterparts.

Equilibria between conformational states of the Ras oncogene protein revealed by high pressure crystallography

In this work, we experimentally investigate the allosteric transitions between conformational states on the Ras oncogene protein using high pressure crystallography. Ras protein is a small GTPase involved in central regulatory processes occurring in multiple conformational states. Ras acts as a molecular switch between active GTP-bound, and inactive GDP-bound states, controlling essential signal transduction pathways. An allosteric network of interactions between the effector binding regions and the membrane interacting regions is involved in Ras cycling. The conformational states which coexist simultaneously in solution possess higher Gibbs free energy than the ground state. Equilibria between these states can be shifted by applying pressure favouring conformations with lower partial molar volume, and has been previously analyzed by high-pressure NMR spectroscopy. High-pressure macromolecular crystallography (HPMX) is a powerful tool perfectly complementary to high-pressure NMR, allowing characterization at the molecular level with a high resolution the different allosteric states involved in the Ras cycling. We observe a transition above 300 MPa in the crystal leading to more stable conformers. Thus, we compare the crystallographic structures of Ras(wt)·Mg²⁺·GppNHp and Ras(D33K)·Mg²⁺·GppNHp at various high hydrostatic pressures. This gives insight into per-residue descriptions of the structural plasticity involved in allosteric equilibria between conformers. We have mapped out at atomic resolution the different segments of Ras protein which remain in the ground-state conformation or undergo structural changes, adopting excited-energy conformations corresponding to transient intermediate states. Such in crystallo phase transitions induced by pressure open the possibility to finely explore the structural determinants related to switching between Ras allosteric sub-states without any mutations nor exogenous partners.

The equilibria between structural states induced by pressure within the crystal structure of Ras are illustrated with different colors corresponding to different Ras substates.

HPLC method to resolve, identify and quantify guanine nucleotides bound to recombinant ras GTPase

The Ras superfamily of small G proteins play central roles in diverse signaling pathways. Superfamily members act as molecular on-off switches defined by their occupancy with GTP or GDP, respectively. In vitro functional studies require loading with a hydrolysis-resistant GTP analogue to increase the on-state lifetime, as well as knowledge of fractional loading with activating and inactivating nucleotides. The present study describes a method combining elements of previous approaches with new, optimized features to analyze the bound nucleotide composition of a G protein loaded with activating (GMPPNP) or inactivating (GDP) nucleotide. After nucleotide loading, the complex is washed to remove unbound nucleotides then bound nucleotides are heat-extracted and subjected to ion-paired, reverse-phase HPLC-UV to resolve, identify and quantify the individual nucleotide components. These data enable back-calculation to the nucleotide composition and fractional activation of the original, washed G protein population prior to heat extraction. The method is highly reproducible. Application to multiple HRas preparations and mutants confirms its ability to fully extract and analyze bound nucleotides, and to resolve the fractional on- and off-state populations. Furthermore, the findings yield a novel hypothesis for the molecular disease mechanism of Ras mutations at the E63 and Y64 positions.

Conformational Features of Ras: Key Hydrogen-Bonding Interactions of Gln61 in the Intermediate State during GTP Hydrolysis

The Ras protein is one of the most important drug targets for battling cancers. To effectively design novel drugs of Ras, we characterize here its conformational ensembles for the hydrolysis intermediate state RasGDP·Pi and the product state RasGDP by extensive replica-exchange molecular dynamics simulations. Several substates for RasGDP·Pi have been identified, while structural analyses have revealed an unrecognized hydrogen-bonding network that stabilizes the hydrolysis intermediate state. More interestingly, Gln61, which is involved in numerous oncogenic mutations, was found to be engaged in this hydrogen-bonding network, adopting a specific conformation that always points to Pi in contrast to that in the RasGTP state. The simulations also reveal that RasGDP has more than one substate, suggesting a conformational selection mechanism for the interaction between Ras and the guanine nucleotide exchange factors (GEFs). These findings offer new opportunities for the drug design of Ras by stabilizing the hydrolysis intermediate or disrupting its interaction with the GEFs.

Inhibitory activities of 20(R, S)-protopanaxatriol against epidermal growth factor receptor tyrosine kinase

As major metabolites of protopanaxatriol-type ginsenosides, 20(R, S)-protopanaxatriol [20(R, S)-PPT] display multiple bioactivities. This work aimed to investigate the inhibitory activities of 20(R, S)-PPT against epidermal growth factor receptor tyrosine kinase and the potential mechanism. 20(R, S)-PPT inhibited the proliferation of HepG2 cells in a dose-dependent manner and blocked cell cycle progression at G1/G0 phase. Then 20(R, S)-PPT were found to influence the protein expressions involved in epidermal growth factor receptor (EGFR)-mitogen-activated protein kinase (MAPK) signaling pathway. Molecular docking suggested that 20(R, S)-PPT could bind to the active sites of all target proteins in EGFR-MAPK pathway. It is worth noting that 20(R, S)-PPT showed stronger binding capacities with EGFR, compared with other proteins. Hence, this work further investigated the binding interactions and binding stabilities between 20(R, S)-PPT and EGFR. Both hydrophobic interactions and hydrogen bonds contributed to the 20(R, S)-PPT-EGFR binding. In addition, the in vitro inhibitory activities of 20(R, S)-PPT against EGFR tyrosine kinase were observed in a homogeneous time-resolved fluorescence assay, with the IC₅₀ values of 24.10 ± 0.17 and 33.19 ± 0.19 μM respectively. Taken together with the above results, both of 20(R)-PPT and 20(S)-PPT might serve as potential EGFR tyrosine kinase inhibitors.

Residue interaction networks of K-Ras protein with water molecules identifies the potential role of switch II and P-loop

The mutant K-Ras with aberrant signaling is the primary cause of several cancers. The proposed study investigated the influence of water molecules in K-Ras crystal structure, where they have a significant function by understanding their residue interaction networks (RINs). We analyzed the RINs of K-Ras with and without water molecules and determined their interaction properties. RINs were developed with the help of StructureViz2 and RINspector; further, the changes in K-Ras backbone flexibility were predicted with the DynaMine. We found that the residues K42, I142, and L159 are the hotspots from water, including the K-Ras-GTP complex with the highest residue centrality analysis (RCA) Z-score. The DynaMine prediction calculated the NMR S² value for the frequently mutated positions G12, G13, and Q61 showing a minor shift in flexibility, which make up the P-Loop and switch II of the K-Ras protein. This flexibility shift can account for changes in conformational activity and the protein's GTPase activity, making it difficult to recognize by the effectors and exchange factors. Taken together, our study helps in understanding the functional importance of the water molecules in K-Ras protein and the impact of mutation that modulate the conformational state of the protein.

GLI1: A Therapeutic Target for Cancer

GLI1 is a transcriptional effector at the terminal end of the Hedgehog signaling (Hh) pathway and is tightly regulated during embryonic development and tissue patterning/differentiation. GLI1 has low-level expression in differentiated tissues, however, in certain cancers, aberrant activation of GLI1 has been linked to the promotion of numerous hallmarks of cancer, such as proliferation, survival, angiogenesis, metastasis, metabolic rewiring, and chemotherapeutic resistance. All of these are driven, in part, by GLI1’s role in regulating cell cycle, DNA replication and DNA damage repair processes. The consequences of GLI1 oncogenic activity, specifically the activity surrounding DNA damage repair proteins, such as NBS1, and cell cycle proteins, such as CDK1, can be linked to tumorigenesis and chemoresistance. Therefore, understanding the underlying mechanisms driving GLI1 dysregulation can provide prognostic and diagnostic biomarkers to identify a patient population that would derive therapeutic benefit from either direct inhibition of GLI1 or targeted therapy towards proteins downstream of GLI1 regulation.

Identification of LRRK2 missense variants in the accelerating medicines partnership Parkinson's disease cohort

Pathogenic missense variants in the leucine-rich repeat kinase 2 (LRRK2) gene have been identified through linkage analysis in familial Parkinson disease (PD). Subsequently, other missense variants with lower effect sizes on PD risk have emerged, as well as non-coding polymorphisms (e.g. rs76904798) enriched in PD cases in genome-wide association studies. Here we leverage recent whole-genome sequences from the Accelerating Medicines Partnership-Parkinson's Disease (AMP-PD) and the Genome Aggregation (gnomAD) databases to characterize novel missense variants in LRRK2 and explore their relationships with known pathogenic and PD-linked missense variants. Using a computational prediction tool that successfully classifies known pathogenic LRRK2 missense variants, we describe an online web-based resource that catalogs characteristics of over 1200 LRRK2 missense variants of unknown significance. Novel high-pathogenicity scoring variants, some identified exclusively in PD cases, tightly cluster within the ROC-COR-Kinase domains. Structure-function predictions support that some of these variants exert gain-of-function effects with respect to LRRK2 kinase activity. In AMP-PD participants, all p.R1441G carriers (N = 89) are also carriers of the more common PD-linked variant p.M1646T. In addition, nearly all carriers of the PD-linked p.N2081D missense variant are also carriers of the LRRK2 PD-risk variant rs76904798. These results provide a compendium of LRRK2 missense variants and how they associate with one another. While the pathogenic p.G2019S variant is by far the most frequent high-pathogenicity scoring variant, our results suggest that ultra-rare missense variants may have an important cumulative impact in increasing the number of individuals with LRRK2-linked PD.

Mechanistic insights into the effect of phosphorylation on Ras conformational dynamics and its interactions with cell signaling proteins

Ras undergoes interconversion between the active GTP-bound state and the inactive GDP-bound state. This GTPase cycle, which controls the activities of Ras, is accelerated by Ras GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (SOS). Oncogenic Ras mutations could affect the GTPase cycle and impair Ras functions. Additionally, Src-induced K-Ras Y32/64 dual phosphorylation has been reported to disrupt GTPase cycle and hinder Ras downstream signaling. However, the underlying mechanisms remain unclear. To address this, we performed molecular dynamics simulations (~30 μs in total) on unphosphorylated and phosphorylated K-Ras4B in GTP- and GDP-bound states, and on their complexes with GTPase cycle regulators (GAP and SOS) and the effector protein Raf. We found that K-Ras4B dual phosphorylation mainly alters the conformation at the nucleotide binding site and creates disorder at the catalytic site, resulting in the enlargement of GDP binding pocket and the retard of Ras-GTP intrinsic hydrolysis. We observed phosphorylation-induced shift in the distribution of Ras-GTP inactive-active sub-states and recognized potential druggable pockets in the phosphorylated Ras-GTP. Moreover, decreased catalytic competence or signal delivery abilities due to reduced binding affinities and/or distorted catalytic conformations of GAP, SOS and Raf were observed. In addition, the allosteric pathway from Ras/Raf interface to the distal Raf L4 loop was compromised by Ras phosphorylation. These results reveal the mechanisms by which phosphorylation influences the intrinsic or GAP/SOS catalyzed transformations between GTP- and GDP-bound states of Ras and its signal transduction to Raf. Our findings project Ras phosphorylation as a target for cancer drug discovery.

Structure of an inactive conformation of GTP-bound RhoA GTPase

By using ³¹P NMR, we present evidence that the Rho family GTPase RhoA, similar to Ras GTPases, exists in an equilibrium of conformations when bound to GTP. High-resolution crystal structures of RhoA bound to the GTP analog GMPPNP and to GDP show that they display a similar overall inactive conformation. In contrast to the previously reported crystal structures of GTP analog-bound forms of two RhoA dominantly active mutants (G14V and Q63L), GMPPNP-bound RhoA assumes an open conformation in the Switch I loop with a previously unseen interaction between the γ-phosphate and Pro36, instead of the canonical Thr37. Molecular dynamics simulations found that the oncogenic RhoA^G14V mutant displays a reduced flexibility in the Switch regions, consistent with a crystal structure of GDP-bound RhoA^G14V. Thus, GDP- and GTP-bound RhoA can present similar inactive conformations, and the molecular dynamics in the Switch regions are likely to have a role in RhoA activation.

Ras-guanine nucleotide complexes: A UV spectral deconvolution method to analyze protein concentration, nucleotide stoichiometry, and purity

The many members of the Ras superfamily are small GTPases that serve as molecular switches. These proteins bind the guanine nucleotides GTP and GDP with picomolar affinities, thereby stabilizing on- and off-signaling states, respectively. Quantitative in vitro Ras studies require accurate determination of total protein, its fractional occupancy with guanine nucleotide, and spectroscopic purity. Yet the high nucleotide affinity of Ras and the overlapping UV spectra of the protein and bound nucleotide make such determinations challenging. Here we describe a generalizable UV spectral deconvolution method to analyze the total protein concentration, total nucleotide stoichiometry, and purity of Ras complexes.

Macromolecular room temperature crystallography

X-ray crystallography enables detailed structural studies of proteins to understand and modulate their function. Conducting crystallographic experiments at cryogenic temperatures has practical benefits but potentially limits the identification of functionally important alternative protein conformations that can be revealed only at room temperature (RT). This review discusses practical aspects of preparing, acquiring, and analyzing X-ray crystallography data at RT to demystify preconceived impracticalities that freeze progress of routine RT data collection at synchrotron sources. Examples are presented as conceptual and experimental templates to enable the design of RT-inspired studies; they illustrate the diversity and utility of gaining novel insights into protein conformational landscapes. An integrative view of protein conformational dynamics enables opportunities to advance basic and biomedical research.

Proteolysis of Rab32 by Salmonella GtgE induces an inactive GTPase conformation

Rab GTPases are central regulators of intracellular vesicular trafficking. They are frequently targeted by bacterial pathogens through post-translational modifications. Salmonella typhimurium secretes the cysteine protease GtgE during infection, leading to a regioselective proteolytic cleavage of the regulatory switch I loop in the small GTPases of the Rab32 subfamily. Here, using a combination of biochemical methods, molecular dynamics simulations, NMR spectroscopy, and single-pair Förster resonance energy transfer, we demonstrate that the cleavage of Rab32 causes a local increase of conformational flexibility in both switch regions. Cleaved Rab32 maintains its ability to interact with the GDP dissociation inhibitor (GDI). Interestingly, the Rab32 cleavage enables GDI binding also with an active GTP-bound Rab32 in vitro. Furthermore, the Rab32 proteolysis provokes disturbance in the interaction with its downstream effector VARP. Thus, the proteolysis of Rab32 is not a globally degradative mechanism but affects various biochemical and structural properties of the GTPase in a diverse manner.

Targeting KRAS(G12C): From Inhibitory Mechanism to Modulation of Antitumor Effects in Patients

KRAS mutations are among the most common genetic alterations in lung, colorectal, and pancreatic cancers. Direct inhibition of KRAS oncoproteins has been a long-standing pursuit in precision oncology, one established shortly after the discovery of RAS mutations in human cancer cells nearly 40 years ago. Recent advances in medicinal chemistry have established inhibitors targeting KRAS(G12C), a mutation found in ∼13% of lung adenocarcinomas and, at a lower frequency, in other cancers. Preclinical studies describing their discovery and mechanism of action, coupled with emerging clinical data from patients treated with these drugs, have sparked a renewed enthusiasm in the study of KRAS and its therapeutic potential. Here, we discuss how these advances are reshaping the fundamental aspects of KRAS oncoprotein biology and the strides being made toward improving patient outcomes in the clinic.

Comparative MD simulations and advanced analytics based studies on wild-type and hot-spot mutant A59G HRas

The Ras family of proteins is known to play an important role in cellular signal transduction. The oncoprotein Ras is also found to be mutated in ~90% of the pancreatic cancers, of which G12V, G13V, A59G and Q61L are the known hot-spot mutants. These ubiquitous proteins fall in the family of G-proteins, and hence switches between active GTP bound and inactive GDP bound states, which is hindered in most of its oncogenic mutant counterparts. Moreover, Ras being a GTPase has an intrinsic property to hydrolyze GTP to GDP, which is obstructed due to mutations and lends the mutants stuck in constitutively active state leading to oncogenic behavior. In this regard, the present study aims to understand the dynamics involved in the hot-spot mutant A59G-Ras using long 10μs classical MD simulations (5μs for each of the wild-type and mutant systems) and comparing the same with its wild-type counterpart. Advanced analytics using Markov State Model (MSM) based approach has been deployed to comparatively understand the transition path for the wild-type and mutant systems. Roles of crucial residues like Tyr32, Gln61 and Tyr64 have also been established using multivariate PCA analyses. Furthermore, this multivariate PCA analysis also provides crucial features which may be used as reaction coordinates for biased simulations for further studies. The absence of formation of pre-hydrolysis network is also reported for the mutant conformation, using the distance-based analyses (between crucial residues) of the conserved regions. The implications of this study strengthen the hypothesis that the disruption of the pre-hydrolysis network in the mutant A59G ensemble might lead to permanently active oncogenic conformation in the mutant conformers.

The Ras switch in structural and historical perspective

Since its discovery as an oncogene more than 40 years ago, Ras has been and still is in the focus of many academic and pharmaceutical labs around the world. A huge amount of work has accumulated on its biology. However, many questions about the role of the different Ras isoforms in health and disease still exist and a full understanding will require more intensive work in the future. Here we try to survey some of the structural findings in a historical perspective and how it has influenced our understanding of structure-function and mechanistic relationships of Ras and its interactions. The structures show that Ras is a stable molecular machine that uses the dynamics of its switch regions for the interaction with all regulators and effectors. This conformational flexibility has been used to create small molecule drug candidates against this important oncoprotein.

Structural impact of GTP binding on downstream KRAS signaling

Oncogenic RAS proteins, involved in ∼30% of human tumors, are molecular switches of various signal transduction pathways. Here we apply a new protocol for the NMR study of KRAS in its (inactive) GDP- and (activated) GTP-bound form, allowing a comprehensive analysis of the backbone dynamics of its WT-, G12C- and G12D variants. We found that Tyr32 shows opposite mobility with respect to the backbone of its surroundings: it is more flexible in the GDP-bound form while more rigid in GTP-complexes (especially in WT- and G12D-GTP). Using the G12C/Y32F double mutant, we showed that the presence of the hydroxyl group of Tyr32 has a marked effect on the G12C-KRAS-GTP system as well. Molecular dynamics simulations indicate that Tyr32 is linked to the γ-phosphate of GTP in the activated states – an arrangement shown, using QM/MM calculations, to support catalysis. Anchoring Tyr32 to the γ-phosphate contributes to the capture of the catalytic waters participating in the intrinsic hydrolysis of GTP and supports a simultaneous triple proton transfer step (catalytic water → assisting water → Tyr32 → O1G of the γ-phosphate) leading to straightforward product formation. The coupled flip of negatively charged residues of switch I toward the inside of the effector binding pocket potentiates ligand recognition, while positioning of Thr35 to enter the coordination sphere of the Mg²⁺ widens the pocket. Position 12 mutations do not disturb the capture of Tyr32 by the γ-phosphate, but (partially) displace Gln61, which opens up the catalytic pocket and destabilizes catalytic water molecules thus impairing intrinsic hydrolysis.

Nucleotide exchange to the physiological, activated, GTP-bound form of KRAS results in the anchoring of Tyr32 within the active site.

RAS Function in cancer cells: translating membrane biology and biochemistry into new therapeutics

The three human RAS proteins are mutated and constitutively activated in ∼20% of cancers leading to cell growth and proliferation. For the past three decades, many attempts have been made to inhibit these proteins with little success. Recently; however, multiple methods have emerged to inhibit KRAS, the most prevalently mutated isoform. These methods and the underlying biology will be discussed in this review with a special focus on KRAS-plasma membrane interactions.

Crystal structure of the FYCO1 RUN domain suggests possible interfaces with small GTPases

FYCO1 is a multidomain adaptor protein that plays an important role in autophagy by mediating the kinesin-dependent microtubule plus-end-directed transport of autophagosomes. FYCO1 contains a RUN domain, which is hypothesized to function as a specific effector for members of the Ras superfamily of small GTPases, but its role has not been well characterized and its interaction partner(s) have not been identified. Here, the crystal structure of the FYCO1 RUN domain was determined at 1.3 Å resolution. The overall structure of the FYCO1 RUN domain was similar to those of previously reported RUN domains. Detailed structural comparisons with other RUN domains and docking studies suggested a possible interaction interface of the FYCO1 RUN domain with small GTPases of the Ras superfamily.

NMR-Derived Conformational Ensemble of State 1 of Activated Ras Reveals Insights into a Druggable Pocket

The lack of apparent pockets in the ground conformation of Ras has long challenged the rational design of inhibitors against this oncogenic protein. The sparsely populated, transiently formed state 1 of activated Ras, on the other hand, shows appreciable surface roughness and is increasingly recognized as a potential target for drug discovery. State 1, however, is extremely flexible, and a static structure cannot fully unveil its conformational space that can be exploited for drug design. Here, we present a conformational ensemble of state 1 that was derived using chemical shift-based modeling. The ensemble reveals the intrinsic plasticity of a druggable pocket in state 1 and demonstrates the mechanism of conformational selection for inhibitor recognition. The large set of structural templates in the ensemble, providing a comprehensive description of thermally accessible pocket conformations, is expected to significantly aid the rational design of anti-Ras drugs.

Comprehensive characterization of RAS mutations in colon and rectal cancers in old and young patients

Colorectal cancer (CRC) is increasingly appreciated as a heterogeneous disease, with factors such as microsatellite instability (MSI), cancer subsite within the colon versus rectum, and age of diagnosis associated with specific disease course and therapeutic response. Activating oncogenic mutations in KRAS and NRAS are common in CRC, driving tumor progression and influencing efficacy of both cytotoxic and targeted therapies. The RAS mutational spectrum differs substantially between tumors arising from distinct tissues. Structure-function analysis of relatively common somatic RAS mutations in G12, Q61, and other codons is characterized by differing potency and modes of action. Here we show the mutational profile of KRAS, NRAS, and the less common HRAS in 13,336 CRC tumors, comparing the frequency of specific mutations based on age of diagnosis, MSI status, and colon versus rectum subsite. We identify mutation hotspots, and unexpected differences in mutation spectrum, based on these clinical parameters.

Activating oncogenic mutations in KRAS and NRAS are common in colorectal cancer, which is a heterogenous disease. Here, the authors show that the RAS mutation spectrum is markedly different between colon and rectal cancer, and also different based on age of diagnosis and microsatellite instability.

GTP Hydrolysis Without an Active Site Base: A Unifying Mechanism for Ras and Related GTPases

GTP hydrolysis is a biologically crucial reaction, being involved in regulating almost all cellular processes. As a result, the enzymes that catalyze this reaction are among the most important drug targets. Despite their vital importance and decades of substantial research effort, the fundamental mechanism of enzyme-catalyzed GTP hydrolysis by GTPases remains highly controversial. Specifically, how do these regulatory proteins hydrolyze GTP without an obvious general base in the active site to activate the water molecule for nucleophilic attack? To answer this question, we perform empirical valence bond simulations of GTPase-catalyzed GTP hydrolysis, comparing solvent- and substrate-assisted pathways in three distinct GTPases, Ras, Rab, and the G_αi subunit of a heterotrimeric G-protein, both in the presence and in the absence of the corresponding GTPase activating proteins. Our results demonstrate that a general base is not needed in the active site, as the preferred mechanism for GTP hydrolysis is a conserved solvent-assisted pathway. This pathway involves the rate-limiting nucleophilic attack of a water molecule, leading to a short-lived intermediate that tautomerizes to form H₂PO₄^- and GDP as the final products. Our fundamental biochemical insight into the enzymatic regulation of GTP hydrolysis not only resolves a decades-old mechanistic controversy but also has high relevance for drug discovery efforts. That is, revisiting the role of oncogenic mutants with respect to our mechanistic findings would pave the way for a new starting point to discover drugs for (so far) "undruggable" GTPases like Ras.

Harmine suppresses hyper-activated Ras-MAPK pathway by selectively targeting oncogenic mutated Ras/Raf in Caenorhabditis elegans

Background

Mutationally activated Ras proteins are closely linked to a wide variety of human cancers. Hence, there has been an intensive search for anti-Ras therapies for cancer treatment. The sole Ras gene, which encodes LET-60, in Caenorhabditis elegans regulates vulval development. While the loss of let-60 function leads to failure of vulva formation, the let-60(n1046gf) allele, which contains a missense mutation mimicking a Ras codon 13 mutation found in human cancers, results in extra vulval tissue, a phenotype named Muv (multiple vulvas).

Methods

By taking advantage of the easy-to-score Muv phenotype of let-60(n1046gf), we used a step-by-step screening approach (from crude extract to active fraction to active natural compound) to search for inhibitors of oncogenic Ras. Mutants of other key components in the Ras-mitogen-activated protein kinase (MAPK) pathway were used to identify other candidate targets.

Results

The natural compound harmine, isolated from the plant Peganum harmala, was found to suppress the Muv phenotype of let-60(n1046gf). In addition, harmine targets the hyper-activation of the Ras/MAPK pathway specifically caused by overexpression or mutated forms of LET-60/Ras and its immediate downstream molecule LIN-45/Raf. Finally, harmine can be absorbed into the worm body and probably functions in its native form, rather than requiring metabolic activation.

Conclusion

In sum, we have revealed for the first time the anti-Ras activity of harmine in a C. elegans model system. Our results revealed the potential anti-cancer mechanism of harmine, which may be useful for the treatment of specific human cancers that are associated with oncogenic Ras mutations.

Hybrid QM/MM vs Pure MM Molecular Dynamics for Evaluating Water Distribution within p21N-ras and the Resulting GTP Electronic Density

p21^ras protein activity, regulated by GTP hydrolysis, constitutes an active field of research for the development of cancer targeted therapies that would concern ∼30% of human tumors to which specific mutations have been associated. Indeed, the catalyzing mechanisms provided by the protein environment during GTP hydrolysis and how they are impaired by specific mutations remain to be fully elucidated. In this article, we present results from molecular mechanics (MM) molecular dynamics (MD) simulations and density functional theory (DFT) calculations carried out for wild-type p21 ^N-ras and six Gln 61 mutants. In the first part, we present the water distribution within the active site of the wild-type protein according to MM MD. Significant differences are observed when comparing the results to the previous distribution assessed through quantum mechanics/molecular mechanics (QM/MM) MD. Such method-dependent results highlight the importance of accounting for the electrostatic coupling between the protein complex and the solvent molecules in identifying hydration sites. In the second part, we present the results from DFT calculations performed to determine the electronic distribution of the GTP ligand, considering the wild-type active site arrangement according to both classical and hybrid approaches. Only in the QM/MM-based configuration is the ligand electronic density similar to that of a GDP-like state observed experimentally. For this reason, in the last set of calculations carried out for p21 ^N-ras Gln 61 mutants, only the active site structural conformations obtained through hybrid MD are considered. Through the analysis of the GTP electronic density, we conclude that the wild-type active site arrangement according to QM/MM MD is closer to a catalytically efficient conformation of the protein than the arrangement according to MM MD. Hence, water distribution according to the hybrid approach must correspond to the optimal placement of solvent in the active site. Within all of the studied Gln 61 substituted proteins, p21^ras major catalyzing effect, which consists of stabilizing a more GDP-like state, is lost.

Targeting Alterations in the RAF-MEK Pathway

The MAPK pathway is one of the most commonly mutated oncogenic pathways in cancer. Although RAS mutations are the most frequent MAPK alterations, less frequent alterations in downstream components of the pathway, including the RAF and MEK genes, offer promising therapeutic opportunities. In addition to BRAFV600 mutations, for which several approved therapeutic regimens exist, other alterations in the RAF and MEK genes may provide more rare, but tractable, targets. However, recent studies have illustrated the complexity of MAPK signaling and highlighted that distinct alterations in these genes may have strikingly different properties. Understanding the unique functional characteristics of specific RAF and MEK alterations, reviewed herein, will be critical for developing effective therapeutic approaches for these targets. SIGNIFICANCE: Alterations in the RAF and MEK genes represent promising therapeutic targets in multiple cancer types. However, given the unique and complex signaling biology of the MAPK pathway, the diverse array of RAF and MEK alterations observed in cancer can possess distinct functional characteristics. As outlined in this review, understanding the key functional properties of different RAF and MEK alterations is fundamental to selecting the optimal therapeutic approach.

Water Distribution within Wild-Type NRas Protein and Q61 Mutants during Unrestrained QM/MM Dynamics

Point mutations in p21^ras are associated with ∼30% of human tumors by disrupting its GTP hydrolysis cycle, which is critical to its molecular switch function in cellular signaling pathways. In this work, we investigate the impact of Gln 61 substitutions in the structure of the p21^N-ras active site and particularly focus on water reorganization around GTP, which appears to be crucial to evaluate favorable and unfavorable hydration sites for hydrolysis. The NRas-GTP complex is analyzed using a hybrid quantum mechanics/molecular mechanics approach, treating for the first time to our knowledge transient water molecules at the ab initio level and leading to results that account for the electrostatic coupling between the protein complex and the solvent. We show that for the wild-type protein, water molecules are found around the GTP γ-phosphate group, forming an arch extended from residues 12 to 35. Two density peaks are observed, supporting previous results that suggest the presence of two water molecules in the active site, one in the vicinity of residue 35 and a second one stabilized by hydrogen bonds formed with nitrogen backbone atoms of residues 12 and 60. The structural changes observed in NRas Gln 61 mutants result in the drastic delocalization of water molecules that we discuss. In mutants Q61H and Q61K, for which water distribution is overlocalized next to residue 60, the second density peak supports the hypothesis of a second water molecule. We also conclude that Gly 60 indirectly participates in GTP hydrolysis by correctly positioning transient water molecules in the protein complex and that Gln 61 has an indirect steric effect in stabilizing the preorganized catalytic site.

From Optimization to Mapping: An Evolutionary Algorithm for Protein Energy Landscapes

Stochastic search is often the only viable option to address complex optimization problems. Recently, evolutionary algorithms have been shown to handle challenging continuous optimization problems related to protein structure modeling. Building on recent work in our laboratories, we propose an evolutionary algorithm for efficiently mapping the multi-basin energy landscapes of dynamic proteins that switch between thermodynamically stable or semi-stable structural states to regulate their biological activity in the cell. The proposed algorithm balances computational resources between exploration and exploitation of the nonlinear, multimodal landscapes that characterize multi-state proteins via a novel combination of global and local search to generate a dynamically-updated, information-rich map of a protein's energy landscape. This new mapping-oriented EA is applied to several dynamic proteins and their disease-implicated variants to illustrate its ability to map complex energy landscapes in a computationally feasible manner. We further show that, given the availability of such maps, comparison between the maps of wildtype and variants of a protein allows for the formulation of a structural and thermodynamic basis for the impact of sequence mutations on dysfunction that may prove useful in guiding further wet-laboratory investigations of dysfunction and molecular interventions.

Specific Substates of Ras To Interact with GAPs and Effectors: Revealed by Theoretical Simulations and FTIR Experiments

The oncogenic Ras protein adopts various specific conformational states to execute its function in signal transduction. The large number of Ras structures obtained from X-ray and NMR experiments illustrates the diverse conformations that Ras adopts. It is difficult, however, to connect specific structural features with Ras functions. We report the free-energy landscape of Ras·GTP based on extensive explicit solvent simulations. The free-energy map clearly shows that the functional state 2 of Ras·GTP in fact has two distinct substates, denoted here as "Tyr32_in" and "Tyr32_out". Unbiased MD simulations show that the two substrates interconvert on the submicrosecond scale in solution, pointing to a novel mechanism for Ras·GTP to selectively interact with GAPs and effectors. This proposal is further supported by time-resolved FTIR experiments, which demonstrate that Tyr32 destabilizes the Ras·GAP complex and facilitates an efficient termination of Ras signaling.

Challenges and advances in the computational modeling of biological phosphate hydrolysis

Phosphate ester hydrolysis is fundamental to many life processes, and has been the topic of substantial experimental and computational research effort. However, even the simplest of phosphate esters can be hydrolyzed through multiple possible pathways that can be difficult to distinguish between, either experimentally, or computationally. Therefore, the mechanisms of both the enzymatic and non-enzymatic reactions have been historically controversial. In the present contribution, we highlight a number of technical issues involved in reliably modeling these computationally challenging reactions, as well as proposing potential solutions. We also showcase examples of our own work in this area, discussing both the non-enzymatic reaction in aqueous solution, as well as insights obtained from the computational modeling of organophosphate hydrolysis and catalytic promiscuity amongst enzymes that catalyze phosphoryl transfer.

The protonation states of GTP and GppNHp in Ras proteins

The small GTPase Ras transmits signals in a variety of cellular signaling pathways, most prominently in cell proliferation. GTP hydrolysis in the active center of Ras acts as a prototype for many GTPases and is the key to the understanding of several diseases, including cancer. Therefore, Ras has been the focus of intense research over the last decades. A recent neutron diffraction crystal structure of Ras indicated a protonated γ-guanylyl imidodiphosphate (γ-GppNHp) group, which has put the protonation state of GTP in question. A possible protonation of GTP was not considered in previously published mechanistic studies. To determine the detailed prehydrolysis state of Ras, we calculated infrared and NMR spectra from quantum mechanics/molecular mechanics (QM/MM) simulations and compared them with those from previous studies. Furthermore, we measured infrared spectra of GTP and several GTP analogs bound to lipidated Ras on a membrane system under near-native conditions. Our findings unify results from previous studies and indicate a structural model confirming the hypothesis that γ-GTP is fully deprotonated in the prehydrolysis state of Ras.

Mechanistic Insights into the Differential Catalysis by RheB and Its Mutants: Y35A and Y35A-D65A

RheB GTPase is a Ras-related molecular switch, which regulates the mTOR signaling pathway by cycling between the active [guanosine triphosphate (GTP)] state and inactive [guanine diphosphate (GDP)] state. Impairment of GTPase activity because of mutations in several small GTPases is known to be associated with several cancers. The conventional GTPase mechanism such as in H-Ras requires a conserved glutamine (Q64) in the switch-II region of RheB to align the catalytic water molecule for efficient GTP hydrolysis. The conformation of this conserved glutamine is different in RheB, resulting in an altered conformation of the entire switch-II region. Studies on the atypical switch-II conformation in RheB revealed a distinct, noncanonical mode of GTP hydrolysis. An RheB mutant Y35A was previously shown to exclusively enhance the intrinsic GTPase activity of RheB, whereas the Y35A-D65A double mutant was shown to reduce the elevated GTPase activity. Here, we have used all-atom molecular dynamics (MD) simulations for comprehensive understanding of the conformational dynamics associated with the fast (Y35A) and slow (Y35A-D65A) hydrolyzing mutants of RheB. Using a combination of starting models from PDB structures and in-silico generated mutant structures, we discuss the observed conformational deviations in wild type (WT) versus mutants. Our results show that a number of interactions of RheB with phosphates of GTP as well as Mg²⁺ are destabilized in Y35A mutant in the switch-I region. We report distinct water dynamics at the active site of WT and mutants. Furthermore, principal component analysis showed significant differences in the conformational space sampled by the WT and mutants. Our observations provide improved understanding of the noncanonical GTP hydrolysis mechanism adopted by RheB and its modulation by Y35A and Y35A-D65A mutants.

The small GTPases K-Ras, N-Ras, and H-Ras have distinct biochemical properties determined by allosteric effects

H-Ras, K-Ras, and N-Ras are small GTPases that are important in the control of cell proliferation, differentiation, and survival, and their mutants occur frequently in human cancers. The G-domain, which catalyzes GTP hydrolysis and mediates downstream signaling, is 95% conserved between the Ras isoforms. Because of their very high sequence identity, biochemical studies done on H-Ras have been considered representative of all three Ras proteins. We show here that this is not a valid assumption. Using enzyme kinetic assays under identical conditions, we observed clear differences between the three isoforms in intrinsic catalysis of GTP by Ras in the absence and presence of the Ras-binding domain (RBD) of the c-Raf kinase protein (Raf-RBD). Given their identical active sites, isoform G-domain differences must be allosteric in origin, due to remote isoform-specific residues that affect conformational states. We present the crystal structure of N-Ras bound to a GTP analogue and interpret the kinetic data in terms of structural features specific for H-, K-, and N-Ras.

Interface analysis of small GTP binding protein complexes suggests preferred membrane orientations

Crystal structures of small GTP binding protein complexes with their effectors and regulators reveal that one particularly flat side of the G domain that contains helix α4 and the C-terminal helix α5 is practically devoid of contacts. Although this observation seems trivial as the main binding targets are the switch I and II regions opposite of this side, the fact that all interacting proteins, even the largest ones, seem to avoid occupying this area (except for Ran, that does not localize to membranes) is very striking. An orientation with this 'flat' side parallel to the membrane was proposed before and would allow simultaneous interaction of the lipidated C-terminus and positive charges in the α4 helix with the membrane while being bound to effector or regulator molecules. Furthermore, this 'flat' side might be involved in regulatory mechanisms: a Ras dimer that is found in different crystal forms interacts exactly at this side. Additional interface analysis of GTPase complexes nicely confirms the effect of different flexibilities of the GTP and GDP forms. Besides Ran proteins, guanine nucleotide exchange factors (GEFs) bury the largest surface areas to provide the binding energy to open up the switch regions for nucleotide exchange.