RASopathy mutations provide functional insight into the BRAF cysteine-rich domain and reveal the importance of autoinhibition in BRAF regulation.

BRAF is frequently mutated in human cancer and the RASopathy syndromes, with RASopathy mutations often observed in the cysteine-rich domain (CRD). Although the CRD participates in phosphatidylserine (PS) binding, the RAS-RAF interaction, and RAF autoinhibition, the impact of these activities on RAF function in normal and disease states is not well characterized. Here, we analyze a panel of CRD mutations and show that they increase BRAF activity by relieving autoinhibition and/or enhancing PS binding, with relief of autoinhibition being the major factor determining mutation severity. Further, we show that CRD-mediated autoinhibition prevents the constitutive plasma membrane localization of BRAF that causes increased RAS-dependent and RAS-independent function. Comparison of the BRAF- and CRAF-CRDs also indicates that the BRAF-CRD is a stronger mediator of autoinhibition and PS binding, and given the increased catalytic activity of BRAF, our studies reveal a more critical role for CRD-mediated autoinhibition in BRAF regulation.

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.

Distinct Binding Preferences between Ras and Raf Family Members and the Impact on Oncogenic Ras Signaling.

The Ras GTPases are frequently mutated in human cancer, and, although the Raf kinases are essential effectors of Ras signaling, the tumorigenic properties of specific Ras-Raf complexes are not well characterized. Here, we examine the ability of individual Ras and Raf proteins to interact in live cells using bioluminescence resonance energy transfer (BRET) technology. We find that C-Raf binds all mutant Ras proteins with high affinity, whereas B-Raf exhibits a striking preference for mutant K-Ras. This selectivity is mediated by the acidic, N-terminal segment of B-Raf and requires the K-Ras polybasic region for high-affinity binding. In addition, we find that C-Raf is critical for mutant H-Ras-driven signaling and that events stabilizing B-Raf/C-Raf dimerization, such as Raf inhibitor treatment or certain B-Raf mutations, can allow mutant H-Ras to engage B-Raf with increased affinity to promote tumorigenesis, thus revealing a previously unappreciated role for C-Raf in potentiating B-Raf function.

M-Ras/Shoc2 signaling modulates E-cadherin turnover and cell-cell adhesion during collective cell migration.

Collective cell migration is required for normal embryonic development and contributes to various biological processes, including wound healing and cancer cell invasion. The M-Ras GTPase and its effector, the Shoc2 scaffold, are proteins mutated in the developmental RASopathy Noonan syndrome, and, here, we report that activated M-Ras recruits Shoc2 to cell surface junctions where M-Ras/Shoc2 signaling contributes to the dynamic regulation of cell-cell junction turnover required for collective cell migration. MCF10A cells expressing the dominant-inhibitory M-RasS27N variant or those lacking Shoc2 exhibited reduced junction turnover and were unable to migrate effectively as a group. Through further depletion/reconstitution studies, we found that M-Ras/Shoc2 signaling contributes to junction turnover by modulating the E-cadherin/p120-catenin interaction and, in turn, the junctional expression of E-cadherin. The regulatory effect of the M-Ras/Shoc2 complex was mediated at least in part through the phosphoregulation of p120-catenin and required downstream ERK cascade activation. Strikingly, cells rescued with the Noonan-associated, myristoylated-Shoc2 mutant (Myr-Shoc2) displayed a gain-of-function (GOF) phenotype, with the cells exhibiting increased junction turnover and reduced E-cadherin/p120-catenin binding and migrating as a faster but less cohesive group. Consistent with these results, Noonan-associated C-Raf mutants that bypass the need for M-Ras/Shoc2 signaling exhibited a similar GOF phenotype when expressed in Shoc2-depleted MCF10A cells. Finally, expression of the Noonan-associated Myr-Shoc2 or C-Raf mutants, but not their WT counterparts, induced gastrulation defects indicative of aberrant cell migration in zebrafish embryos, further demonstrating the function of the M-Ras/Shoc2/ERK cascade signaling axis in the dynamic control of coordinated cell movement.

Enhanced BRAF engagement by NRAS mutants capable of promoting melanoma initiation.

A distinct profile of NRAS mutants is observed in each tumor type. It is unclear whether these profiles are determined by mutagenic events or functional differences between NRAS oncoproteins. Here, we establish functional hallmarks of NRAS mutants enriched in human melanoma. We generate eight conditional, knock-in mouse models and show that rare melanoma mutants (NRAS G12D, G13D, G13R, Q61H, and Q61P) are poor drivers of spontaneous melanoma formation, whereas common melanoma mutants (NRAS Q61R, Q61K, or Q61L) induce rapid tumor onset with high penetrance. Molecular dynamics simulations, combined with cell-based protein-protein interaction studies, reveal that melanomagenic NRAS mutants form intramolecular contacts that enhance BRAF binding affinity, BRAF-CRAF heterodimer formation, and MAPK > ERK signaling. Along with the allelic series of conditional mouse models we describe, these results establish a mechanistic basis for the enrichment of specific NRAS mutants in human melanoma.

Ras-Mediated Activation of the Raf Family Kinases.

The extracellular signal-regulated kinase (ERK) cascade comprised of the Raf, MEK, and ERK protein kinases constitutes a key effector cascade used by the Ras GTPases to relay signals regulating cell growth, survival, proliferation, and differentiation. Of the ERK cascade components, the regulation of the Raf kinases is by far the most complex, involving changes in subcellular localization, protein and lipid interactions, as well as alterations in the Raf phosphorylation state. The Raf kinases interact directly with active, membrane-localized Ras, and this interaction is often the first step in the Raf activation process, which ultimately results in ERK activation and the downstream phosphorylation of cellular targets that will specify a particular biological response. Here, we will examine our current understanding of how Ras promotes Raf activation, focusing on the molecular mechanisms that contribute to the Raf activation/inactivation cycle.