Mitogen-regulated cell functions, such as growth, division, proliferation and migration, correlate with distinct cell morphologies. Cancer cell transformation and metastasis also change cell morphology. How cell surface architecture communicates with mitogenic signaling is poorly understood. Lipid-anchored Ras Small GTPases (isoforms H-, N- and splice variants K-Ras4A and K-Ras4B) are key upstream regulators of mitogenic cascades and play pivotal roles in human cancer. Ras signaling is mostly compartmentalized to the plasma membrane (PM) through complex interactions between their C-terminal lipid anchors and PM lipids. Because cell morphology is defined by the curvature of the plasma membrane (PM), where lipids distribute heterogeneously, Ras function/pathology may be sensitive to changes in PM curvature. This mechanism may contribute to the correlation between cell morphology and mitogenic signaling. Indeed, Ras isoforms form nanometer-sized domains, termed as nanoclusters, on the PM. These proteolipid nano-assemblies are essential to Ras function/pathology because most of their effectors contain specific lipid-binding domains. Ras effector activation requires binding of distinct lipid types in the PM. We, thus, propose that Ras function/pathology, such as lipid sorting, nanoclustering, effector binding and signaling, senses PM curvature via specific lipid sorting.<br/>            We integrated quantitative super resolution imaging with nanofabrication and biophysical assays to examine Ras membrane curvature sensing in synthetic model bilayers and intact/live cells. We show that Ras proteins sense membrane curvature in an isoform-specific manner: increasing PM curvature enhances the nanoclustering and signaling of H-Ras, but disrupts those of K-Ras4B. Screening of various anionic phospholipids reveals that phosphatidylserine (PS) favors less curved membranes, whereas phosphoinositol 4,5-bisphosphate (PIP2) favors more curved membranes. Depleting endogenous PS abolishes K-Ras4B curvature sensing, while supplementation of only mixed-chain PS rescued K-Ras4B PM curvature sensing. Taken together, Ras nanoclusters act as relay stations to convert mechanical stimulations to intracellular mitogenic signaling.

Membrane curvature sensing by lipid anchored K-Ras small GTPase

2019 Spring National Meeting

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