Inhibition of RAS function through targeting an allosteric regulatory site.

RAS GTPases are important mediators of oncogenesis in humans. However, pharmacological inhibition of RAS has proved challenging. Here we describe a functionally critical region, located outside the effector lobe of RAS, that can be targeted for inhibition. We developed NS1, a synthetic binding protein (monobody) that bound with high affinity to both GTP- and GDP-bound states of H-RAS and K-RAS but not N-RAS. NS1 potently inhibited growth factor signaling and oncogenic H-RAS- and K-RAS-mediated signaling and transformation but did not block oncogenic N-RAS, BRAF or MEK1. NS1 bound the α4-ß6-α5 region of RAS, which disrupted RAS dimerization and nanoclustering and led to blocking of CRAF-BRAF heterodimerization and activation. These results establish the importance of the α4-ß6-α5 interface in RAS-mediated signaling and define a previously unrecognized site in RAS for inhibiting RAS function.

A dimerization-dependent mechanism drives RAF catalytic activation.

The ERK (extracellular signal-regulated kinase) pathway is an evolutionarily conserved signal transduction module that controls cellular growth, differentiation and survival. Activation of receptor tyrosine kinases (RTKs) by the binding of growth factors initiates GTP loading of RAS, which triggers the initial steps in the activation of the ERK pathway by modulating RAF family kinase function. Once activated, RAF participates in a sequential cascade of phosphorylation events that activate MEK, and in turn ERK. Unbridled signalling through the ERK pathway caused by activating mutations in RTKs, RAS or RAF has been linked to several human cancers. Of note, one member of the RAF family, BRAF, is the most frequently mutated oncogene in the kinase superfamily. Not surprisingly, there has been a colossal effort to understand the underlying regulation of this family of kinases. In particular, the process by which the RAF kinase domain becomes activated towards its substrate MEK remains of topical interest. Here, using Drosophila Schneider S2 cells, we demonstrate that RAF catalytic function is regulated in response to a specific mode of dimerization of its kinase domain, which we term the side-to-side dimer. Moreover, we find that the RAF-related pseudo-kinase KSR (kinase suppressor of Ras) also participates in forming side-to-side heterodimers with RAF and can thereby trigger RAF activation. This mechanism provides an elegant explanation for the longstanding conundrum about RAF catalytic activation, and also provides an explanation for the capacity of KSR, despite lacking catalytic function, to directly mediate RAF activation. We also show that RAF side-to-side dimer formation is essential for aberrant signalling by oncogenic BRAF mutants, and identify an oncogenic mutation that acts specifically by promoting side-to-side dimerization. Together, our data identify the side-to-side dimer interface of RAF as a potential therapeutic target for intervention in BRAF-dependent tumorigenesis.

Mechanistic principles of RAF kinase signaling.

The RAF family of kinases are key components acting downstream of receptor tyrosine kinases and cells employ several distinct mechanisms to strictly control their activity. RAF transitions from an inactive state, where the N-terminal regulatory region binds intramolecularly to the C-terminal kinase domain, to an open state capable of executing the phosphoryl transfer reaction. This transition involves changes both within and between the protein domains in RAF. Many different proteins regulate the transition between inactive and active states of RAF, including RAS and KSR, which are arguably the two most prominent regulators of RAF function. Recent developments have added several new twists to our understanding of RAF regulation. Among others, dimerization of the RAF kinase domain is emerging as a crucial step in the RAF activation process. The multitude of regulatory protein-protein interactions involving RAF remains a largely untapped area for therapeutic applications.

CNK and HYP form a discrete dimer by their SAM domains to mediate RAF kinase signaling.

RAF kinase functions in the mitogen-activated protein kinase (MAPK) pathway to transmit growth signals to the downstream kinases MEK and ERK. Activation of RAF catalytic activity is facilitated by a regulatory complex comprising the proteins CNK (Connector enhancer of KSR), HYP (Hyphen), and KSR (Kinase Suppressor of Ras). The sterile alpha-motif (SAM) domain found in both CNK and HYP plays an essential role in complex formation. Here, we have determined the x-ray crystal structure of the SAM domain of CNK in complex with the SAM domain of HYP. The structure reveals a single-junction SAM domain dimer of 1:1 stoichiometry in which the binding mode is a variation of polymeric SAM domain interactions. Through in vitro and in vivo mutational analyses, we show that the specific mode of dimerization revealed by the crystal structure is essential for RAF signaling and facilitates the recruitment of KSR to form the CNK/HYP/KSR regulatory complex. We present two docking-site models to account for how SAM domain dimerization might influence the formation of a higher-order CNK/HYP/KSR complex.

Spotlight on pembrolizumab in the treatment of advanced melanoma.

Metastatic melanoma is an aggressive cancer with a poor prognosis. Many approved therapies often do not achieve durable responses in patients. This underscores the need for novel therapeutic strategies. Recruiting a robust immune response is an important antineoplastic treatment strategy. Immune checkpoints offer a molecular target for modulating the immune response and a promising therapeutic target in metastatic melanoma. Here we discuss the recent approval of pembrolizumab by the US Food and Drug Administration for the treatment of metastatic melanoma and its impact on future management of the disease.

Bench to bedside: mechanistic principles of targeting the RAF kinase in melanoma.

Melanoma, in its advanced form, is an aggressive cancer with a poor prognosis. To date, no therapeutic modality has afforded a high likelihood of curative outcome, with the exception of early surgical resection in patients diagnosed with local disease. However, recent advances in our understanding of the molecular mechanisms and pathophysiology of melanoma have paved the way towards the development of targeted therapeutics. A central player in melanomagenesis is the RAF family of kinases. Key mechanistic details regarding the regulation of RAF kinases have now begun to emerge. Already, vemurafenib, a tailored kinase inhibitor of aberrant RAF function in melanoma, has led to clinical benefit. Despite vemurafenib's success, acquired resistance to the drug warrants the need for further drug development. In this review, we discuss the critical role of RAF dimerization in both melanomagenesis and resistance to RAF inhibitors such as vemurafenib. We also highlight the potential for inhibitors of RAF dimerization to lead to improved outcomes in patients with advanced melanoma.

Allosteric protein kinase regulation by pseudokinases: insights from STRAD.

Protein kinases regulate a plethora of diverse cellular functions. Their highly controlled activation is subject to an equally diverse repertoire of regulatory mechanisms. Pseudokinases, a class of proteins that possess a structurally related protein kinase domain that lacks phospho-transfer function, are emerging as critical yet mysterious regulators of other protein kinases. A new structural and functional analysis of the pseudokinase STRAD provides insight into the mechanism by which it allosterically regulates the catalytic function of the protein kinase LKB1 and hints at an evolution from a classical kinase-substrate relationship.