Membrane curvature sensing of the lipid-anchored K-Ras small GTPases

Abstract

Cell morphologies, defined by plasma membrane (PM) local curvature, change during mitogen-dependent function and pathology, such as growth, division and proliferation. The lipid-anchored Ras small GTPases are essential upstream regulators of the mitogen-activated protein kinases (MAPKs) cascades and play key roles in many pathological conditions, especially cancer. Ras signaling is mostly compartmentalized to the cell PM through the formation of nanometer-sized domains, termed as nanoclusters, and undergo selective lipid sorting for efficient effector recruitment and activation. Thus, Ras function might be sensitive to changing PM curvature, potentially regulating mechanosensing of mitogen signaling. We employed nanofabrication and super-resolution imaging and found that Ras functions respond to PM curvature modulations in an isoform specific manner: nanoclustering and signaling of the most oncogenically prevalent isoform K-Ras favor less curved PM, while those of another isoform H-Ras favor more curved PM. We then examined whether Ras membrane curvature sensing is mediated by lipid sorting. We found that anionic phospholipids sense changing PM curvature in distinct manners: phosphatidylserine (PS) localization shows preference for less curved membrane but phosphoinositol 4,5-bisphosphate (PIP2) localization favors more curved PM. Depletion of endogenous PS abolishes K-Ras PM curvature sensing. Exogenous PS addback and synthetic bilayer binding assays further show that only mixed-chain PS species, but not other PS species, mediate K-Ras curvature sensing. Taken together, the Ras proteolipid nano-assemblies on the PM act as relay stations to convert mechanical stimulations to mitogenic signaling circuits, thus a novel mechanism for cancer cell mechanotransduction