Summary The mechanisms that underlie and dictate the different biological outcomes of E2F-1 activity have yet to be elucidated. We describe the residue-specific methylation of E2F-1 by the asymmetric dimethylating protein arginine methyltransferase (PRMT) 1 and symmetric dimethylating PRMT5, and relate the marks to different functional consequences of E2F-1 activity. Methylation by PRMT1 hinders methylation by PRMT5, which augments E2F-1-dependent apoptosis, whereas PRMT5-dependent methylation favours proliferation by antagonising methylation by PRMT1. The ability of E2F-1 to prompt apoptosis in DNA damaged cells coincides with enhanced PRMT1 methylation. In contrast, cyclin A binding to E2F-1 impedes PRMT1 methylation and augments PRMT5 methylation, thus ensuring that E2F-1 is locked into its cell cycle progression mode. The Tudor domain protein p100-TSN reads the symmetric methylation mark, and binding of p100-TSN down-regulates E2F-1 apoptotic activity. Our results define an exquisite level of precision in the reader-writer interplay that governs the biological outcome of E2F-1 activity.

Ribosome biogenesis is a highly complex process requiring many assisting factors. Studies in yeast have yielded comprehensive knowledge of the cellular machinery involved in this process. However, many aspects of ribosome synthesis are different in higher eukaryotes, and the global set of mammalian ribosome biogenesis factors remains unexplored. We used an imaging-based, genome-wide RNAi screen to find human proteins involved in 40S ribosomal subunit biogenesis. Our analysis identified ∼ 300 factors, many part of essential protein modules such as the small subunit (SSU) processome, the eIF3 and chaperonin complexes, and the ubiquitin-proteasome system. We demonstrate a role for the vertebrate-specific factor RBIS in ribosome synthesis, uncover a requirement for the CRL4 E3 ubiquitin ligase in nucleolar ribosome biogenesis, and reveal that intracellular glutamine synthesis supports 40S subunit production.

Purpose: Treatment of BRAF V600E -mutant melanomas with MAPK inhibitors (MAPKi) results in significant tumor regression, but acquired resistance is pervasive. To understand nonmutational mechanisms underlying the adaptation to MAPKi and to identify novel vulnerabilities of melanomas treated with MAPKi, we focused on the initial response phase during treatment with MAPKi.Experimental Design: By screening proteins expressed on the cell surface of melanoma cells, we identified the fatty acid transporter CD36 as the most consistently upregulated protein upon short-term treatment with MAPKi. We further investigated the effects of MAPKi on fatty acid metabolism using in vitro and in vivo models and analyzing patients' pre-and ontreatment tumor specimens.Results: Melanoma cells treated with MAPKi displayed increased levels of CD36 and of PPARa-mediated and carnitine palmitoyltransferase 1A (CPT1A)-dependent fatty acid oxidation (FAO). While CD36 is a useful marker of melanoma cells during adaptation and drug-tolerant phases, the upregulation of CD36 is not functionally involved in FAO changes that characterize MAPKi-treated cells. Increased FAO is required for BRAF V600E -mutant melanoma cells to survive under the MAPKi-induced metabolic stress prior to acquiring drug resistance. The upfront and concomitant inhibition of FAO, glycolysis, and MAPK synergistically inhibits tumor cell growth in vitro and in vivo.Conclusions: Thus, we identified a clinically relevant therapeutic approach that has the potential to improve initial responses and to delay acquired drug resistance of BRAF V600E -mutant melanoma.

Kinases can switch between active and inactive conformations of the ATP/Mg(2+) binding motif DFG, which has been explored for the development of type I or type II inhibitors. However, factors modulating DFG conformations remain poorly understood. We chose CDK2 as a model system to study the DFG in-out transition on a target that was thought to have an inaccessible DFG-out conformation. We used site-directed mutagenesis of key residues identified in structural comparisons in conjunction with biochemical and biophysical characterization of the generated mutants. As a result, we identified key residues that facilitate the DFG-out movement, facilitating binding of type II inhibitors. However, surprisingly, we also found that wild type CDK2 is able to bind type II inhibitors. Using protein crystallography structural analysis of the CDK2 complex with an aminopyrimidine-phenyl urea inhibitor (K03861) revealed a canonical type II binding mode and the first available type II inhibitor CDK2 cocrystal structure. We found that the identified type II inhibitors compete with binding of activating cyclins. In addition, analysis of the binding kinetics of the identified inhibitors revealed slow off-rates. The study highlights the importance of residues that may be distant to the ATP binding pocket in modulating the energetics of the DFG-out transition and hence inhibitor binding. The presented data also provide the foundation for a new class of slow off-rate cyclin-competitive CDK2 inhibitors targeting the inactive DFG-out state of this important kinase target.