Interaction of KRas4b with anionic membranes: A special role for PIP.

KRas4b is a small G-protein whose constitutively active oncogenic mutants are present in 90% of pancreatic cancers. Using fully post-translationally modified KRAS4b, we investigated the role of lipid identity in the recruitment of KRas4b to a membrane surface of defined composition. Application of a newly developed single frequency fluorescence anisotropy decay experiment to this system revealed that KRas4b has a significant binding preference for Nanodisc bilayers containing PIP. We conducted molecular dynamics simulations to look for an origin of this specificity. In the case of membranes containing PIP the protein formed long-lived salt bridges with PIP head groups but not the monovalent DMPS, explaining the experimentally observed lipid specificity. Additionally, we report that PIP forms key contacts with Helix-4 on the catalytic domain of KRas4b that orient the protein in a manner expected to facilitate association with upstream and downstream signaling partners.