Abstract 1500: How K-Ras4B attaches to the membrane and forms a dimer: A new paradigm

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Abstract Ras is a small GTPase, controlling signal transduction pathways and promoting cell proliferation and survival. KRAS is frequently mutated in cancer. Ras consists of highly homologous catalytic domains (G-domains) and flexible C-terminal hypervariable regions (HVRs) that differ significantly across Ras isoforms. Recent nuclear magnetic resonance (NMR) and molecular dynamics (MD) simulations discovered that the HVR of K-Ras4B in the GDP-bound state extensively interacts with the catalytic domain. However, it weakly interacts with the catalytic domain in the GTP-bound state. Here, using MD simulations we modeled K-Ras4B membrane interaction and dimerization. Membrane binding of K-Ras4B through the anchoring of the positively charged HVR is thought to be critical to its function as an oncogene and initiates signaling events. At the membrane, the catalytic domain takes on multiple orientations, including perpendicular and parallel alignments of the allosteric helices with respect to the membrane normal. In the autoinhibited state, the HVR is sandwiched between the effector lobe and the membrane; in the active state, with the farnesyl anchored into the membrane and the HVR unrestrained, the catalytic domain fluctuates reinlessly, exposing its effector binding site. Notably, we also observe HVR-autoinhibited K-Ras4B-GTP states that display GDP-bound-like orientations of the helices. We thus propose that GDP/GTP nucleotide exchange may not be sufficient for K-Ras4B activation; composite mechanisms including HVR sequestration, farnesyl insertion, and orientation/localization of the catalytic domain on the membrane can determine the functional state of K-Ras4B. Remarkably, K-Ras4B-GTP, but not GDP-bound, is able to form stable homodimers with different dimer interfaces, suggesting that the nucleotide-dependent dimerization with various dimer interfaces can resolve nanoclustering and cluster reorganization accomplishment with Raf's activation. Ras was believed to function as a monomer; however, since Raf dimerizes, it has been suspected that Ras can also dimerize. Dimerization and clustering could rein the fluctuations producing more productive pre-organized conformations. Funded by Frederick National Laboratory for Cancer Research, National Institutes of Health, under contract HHSN261200800001E. Citation Format: Hyunbum Jang, Ruth Nussinov. How K-Ras4B attaches to the membrane and forms a dimer: A new paradigm. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1500.