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.

KRAS is the most frequently mutated oncogene. The incidence of specifi c KRAS alleles varies between cancers from different sites, but it is unclear whether allelic selection results from biological selection for specifi c mutant KRAS proteins. We used a crossdisciplinary approach to compare KRAS G12D , a common mutant form, and KRAS A146T , a mutant that occurs only in selected cancers. Biochemical and structural studies demonstrated that KRAS A146T exhibits a marked extension of switch 1 away from the protein body and nucleotide binding site, which activates KRAS by promoting a high rate of intrinsic and guanine nucleotide exchange factorinduced nucleotide exchange. Using mice genetically engineered to express either allele, we found that KRAS G12D and KRAS A146T exhibit distinct tissue-specifi c effects on homeostasis that mirror mutational frequencies in human cancers. These tissue-specifi c phenotypes result from allele-specifi c signaling properties, demonstrating that context-dependent variations in signaling downstream of different KRAS mutants drive the KRAS mutational pattern seen in cancer. SIGNIFICANCE: Although epidemiologic and clinical studies have suggested allele-specifi c behaviors for KRAS , experimental evidence for allele-specifi c biological properties is limited. We combined structural biology, mass spectrometry, and mouse modeling to demonstrate that the selection for specifi c KRAS mutants in human cancers from different tissues is due to their distinct signaling properties.

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Highlights d Ras G13D proteins have open active sites with disconnected switches I and II d KRas G13D shows unique destabilization of the nucleotidebinding pocket d KRas G13D has attenuated oncogenic phenotype relative to KRas G12D d KRas G13D and KRas G12D are more sensitive to Erk than to Akt inhibition

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.

RAS proteins are signal transduction gatekeepers that mediate cell growth, survival, and differentiation through interactions with multiple effector proteins. The RAS effector RAS- and RAB-interacting protein 1 (RIN1) activates its own downstream effectors, the small GTPase RAB5 and the tyrosine kinase Abelson tyrosine-protein kinase (ABL), to modulate endocytosis and cytoskeleton remodeling. To identify ABL substrates downstream of RAS-to-RIN1 signaling, we examined human HEK293T cells overexpressing components of this pathway. Proteomic analysis revealed several novel phosphotyrosine peptides, including Harvey rat sarcoma oncogene (HRAS)-pTyr(137). Here we report that ABL phosphorylates tyrosine 137 of H-, K-, and NRAS. Increased RIN1 levels enhanced HRAS-Tyr(137) phosphorylation by nearly 5-fold, suggesting that RAS-stimulated RIN1 can drive ABL-mediated RAS modification in a feedback circuit. Tyr(137) is well conserved among RAS orthologs and is part of a transprotein H-bond network. Crystal structures of HRAS(Y137F) and HRAS(Y137E) revealed conformation changes radiating from the mutated residue. Although consistent with Tyr(137) participation in allosteric control of HRAS function, the mutations did not alter intrinsic GTP hydrolysis rates in vitro. HRAS-Tyr(137) phosphorylation enhanced HRAS signaling capacity in cells, however, as reflected by a 4-fold increase in the association of phosphorylated HRAS(G12V) with its effector protein RAF proto-oncogene serine/threonine protein kinase 1 (RAF1). These data suggest that RAS phosphorylation at Tyr(137) allosterically alters protein conformation and effector binding, providing a mechanism for effector-initiated modulation of RAS signaling.

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This article expands on crystal structure data for human H-RAS with mutations at position Y137, briefly described in a paper on the effects of phosphorylation of Y137 by ABL kinases (Tyrosine phosphorylation of RAS by ABL allosterically enhances effector binding, published in the FASEB Journal [1]). The crystal structures of the Y137E mutant (phosphorylation mimic) and of the Y137F mutant (without the hydroxyl group where phosphorylation occurs) were deposited in the Protein Data Bank with PDB codes 4XVQ (H-RASY137E) and 4XVR (H-RASY137F). This article includes details for expression and purification of RAS and its mutants with no affinity tags, in vitro exchange of guanine nucleotides, protein crystallization, X-ray data collection and structure refinement.