Abstract 5229: High affinity interaction of K-Ras4B HVR with calmodulin
Authors: Jang Hyunbum , Banerjee Avik , Gaponenko Vadim , Nussinov Ruth 
Published: July 2017

Abstract
Ras proteins promote cell proliferation and survival, controlling signal transduction that involves two major pathways, such as Raf/MEK/ERK and PI3K/Akt. K-Ras4B is the most abundant oncogenic isoform. In calcium- and calmodulin (CaM)-rich environments, such as ductal tissues, it plays a critical role in adenocarcinomas promoting PI3K/Akt signaling, particularly in pancreatic, lung, and colorectal cancers. It was suggested that in adenocarcinomas with elevated calcium levels, Ca2+/CaM recruits PI3Kα through interaction with its n/cSH2 domains and sequesters K-Ras4B from the membrane, organizing a CaM/K-Ras4B/PI3Kα ternary complex. Ca2+/CaM can replace the missing receptor tyrosine kinase (RTK) signal, acting to fully activate PI3Kα. Structural data suggests that CaM uniquely binds GTP-bound K-Ras4B but not N-Ras or H-Ras isoforms. Experimental evidence points to the farnesylated hypervariable region (HVR) of K-Ras4B as a major binding domain for CaM. Here, using molecular dynamics (MD) simulations we modeled K-Ras4B HVR interaction with Ca2+/CaM. Initial contacts of K-Ras4B HVR with CaM were determined by the nuclear magnetic resonance (NMR) chemical shift perturbation(CSP) data, which guided major interacting residue pairs between the HVR and CaM. In the simulations, two different topologies of Ca2+/CaM were modeled: CaM with stretched and flexible linkers. CaM has N- and C-lobes connected by a linker, and each lobe has its own hydrophobic pocket. Thus, the hydrophobic farnesyl tail of the HVR can dock into either lobe of CaM. We observed that the HVR strongly interacts with the linker of CaM. The K-Ras4B HVR is highly polybasic with lysine-rich, while the linker region of CaM is negatively charged. The docking of the farnesyl group into the hydrophobic pocket additionally helps to stabilize the HVR/CaM interaction. This added stabilization by the farnesyl is significant in the K-Ras4B/CaM interaction even though the interaction with the Ras catalytic domain is involved. K-Ras4B has only farnesyl modification, while other isoforms bear farnesyl and palmitoyl groups. The additional lipid modification in the HVR would obstruct CaM binding, suggesting CaM's K-Ras4B-specific action. Our structural model of K-Ras4B/CaM association provides plausible clues to the PI3Kα activation involving the ternary complex. The abundant Ca2+/CaM can bind to the p85 domain of PI3Kα while binding to the K-Ras4B farnesylated HVR, releasing catalytic kinase domain autoinhibition and allosterically leading to full PI3Kα activation. Funded by Frederick National Laboratory for Cancer Research, National Institutes of Health, under contract HHSN261200800001E.
Citation Format: Hyunbum Jang, Avik Banerjee, Vadim Gaponenko, Ruth Nussinov. High affinity interaction of K-Ras4B HVR with calmodulin [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5229. doi:10.1158/1538-7445.AM2017-5229