Signal Integration by Lipid-Mediated Spatial Cross Talk between Ras

Lipid-anchored Ras GTPases form transient, spatially segregated nanoclusters on the plasmamembrane that are essential for high-fidelity signal transmission. The lipid composition of Ras nanoclusters, however, has not previously been investigated. High-resolution spatial mapping shows that different Ras nanoclusters have distinct lipid compositions, indicating that Ras pro-teins engage in isoform-selective lipid sorting and accounting for different signal outputs from different Ras isoforms. Phospha-tidylserine is a common constituent of all Ras nanoclusters but is only an obligate structural component of K-Ras nanoclusters. Segregation of K-Ras and H-Ras into spatially and compositionally distinct lipid assemblies is exquisitely sensitive to plasma membrane phosphatidylserine levels. Phosphatidylserine spatial organization is also modified by Ras nanocluster formation. In consequence, Ras nanoclusters engage in remote lipid-mediated communication, whereby activated H-Ras disrupts the assembly and operation of spatially segregated K-Ras nanoclusters. Computational modeling and experimentation reveal that complex effects of caveolin and cortical actin on Ras nanoclustering are similarly mediated through regulation of phosphatidylserine spa-tiotemporal dynamics. We conclude that phosphatidylserine maintains the lateral segregation of diverse lipid-based assemblies on the plasmamembrane and that lateral connectivity between spatially remote lipid assemblies offers important previously unexplored opportunities for signal integration and signal processing. H-, N-, and K-Ras are small GTPases that operate as molecularswitches to regulate cell growth, proliferation, and differen-tiation (1, 2). H-, N-, and K-Ras comprise nearly identical G do