Structure of KRas in Complex with the CRaf Effector Protein at the Lipid Membrane as Determined by Neutron Reflectometry
Ras genes are the most frequently mutated oncogenes in human cancer. Ras exists as three isoforms (H, N and K-Ras). These Ras isoforms have different mutational frequencies in human cancers, with KRas being the most frequent. The Ras proteins function as a molecular switch that transmit mitogenic signals across the cell membrane. When Ras is active in the GTP state and binds to a series of effector proteins, with CRaf kinase being one of the best characterized. The hydrolysis of GTP to GDP results in a conformational change such that Ras no longer interacts with effector proteins. KRas has proven to be an elusive drug target, in part because the membrane-bound state of the protein, in particular in complex with effector proteins, is unknown. We present structural studies of farnesylated KRas by itself and in complex with partial constructs of CRaf at the membrane using neutron reflectometry. In contrast to structural models based on NMR and MD simulations, the G-domain of KRas is found to be located on average about 30 Å away from the membrane while the hypervariable region serves as a flexible membrane tether. Binding to CRaf-RBD (Ras Binding Domain) does not affect this conformation, whereas preliminary data indicates that binding to CRaf-RBD-CRD (Cysteine Rich Domain) initiates a closer engagement of the G-domain with the membrane. This work constitutes first steps toward a structural characterization of the entire membrane-bound KRas/CRaf complex and is validated against NMR-PRE data. The development of new methods in neutron reflectometry that enable this research are discussed in detail.
Cm-Bio 4-19 Heinrich flyer