PIP2 Influences the Conformational Dynamics of Membrane-Bound KRAS4b

KRAS4b is a small GTPase involved in cellular signaling through receptor tyrosine kinases. The activation of KRAS4b occurs only after recruitment of the regulatory proteins to the plasma membrane, thus making the role of the phospholipid bilayer an integral part of the signaling mechanism. Phospholipids, primarily with anionic headgroups, interact with both the membrane-anchoring hypervariable (HVR) region and the G-domain (catalytic domain) and influence the orientation of KRAS4b on the membrane surface, potentially playing a key role in the regulation of activation. Although there has been significant research focused on the role of the anionic phosphatidylserine, less effort has been spent on the role of the important signaling lipid phosphatidylinositol 4,5-bisphosphate (PIP2). Using instrumentation to measure the fluorescence anisotropy decay of site specifically labeled 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) Nanodiscs over a wide frequency range, we quantitate the binding of KRAS4b to Nanodiscs containing either 30% phosphatidylserine (PS) or 10% L-alpha-phosphatidylinositol 4,5-bisphosphate by measuring the rotational correlation time of the Nanodisc-KRAS4b complex. We find that KRAS4b binds significantly tighter to Nanodiscs containing PIP2 but that at any level of binding saturation of KRAS4b, both 30% PS and 10% PIP2 containing Nanodiscs display similar rotational correlation times. This shows that the overall hydrodynamic radii of the KRAS4b-Nanodisc complexes are similar regardless of the incorporated anionic lipid. Atomic force microscopy is used to visualize KRAS4b when bound to individual Nanodiscs. Clean images are observed with the PIP2-doped Nanodiscs, but significantly blurred images are obtained when the anionic lipid is PS. This suggests that KRAS4b is not only more tightly bound overall with PIP2 as the anionic lipid but also less mobile on the bilayer surface. Microsecond molecular dynamics simulations of KRAS4b on PS- and PIP2-containing membranes show that the dynamics of the G-domain at the bilayer surface are significantly altered in the presence of PIP2, due to the formation of long-lived salt bridges with basic residues on the G-domain. The orientation and dynamics of KRAS4b on the membrane are critical to understanding the mechanisms of oncoprotein signaling, and our results with the GDP-bound form show subtle differences from that published for GTP-KRAS4b.