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Molecular and cellular biology

Raft protein clustering alters N-Ras membrane interactions and activation pattern.


PMID 21807892

Abstract

The trafficking, membrane localization, and lipid raft association of Ras proteins, which are crucial oncogenic mediators, dictate their isoform-specific biological responses. Accordingly, their spatiotemporal dynamics are tightly regulated. While extensively studied for H- and K-Ras, such information on N-Ras, an etiological oncogenic factor, is limited. Here, we report a novel mechanism regulating the activation-dependent spatiotemporal organization of N-Ras, its modulation by biologically relevant stimuli, and isoform-specific effects on signaling. We combined patching/immobilization of another membrane protein with fluorescence recovery after photobleaching (patch-FRAP) and FRAP beam size analysis to investigate N-Ras membrane interactions. Clustering of raft-associated proteins, either glycosylphosphatidylinositol-anchored influenza virus hemagglutinin (HA-GPI) or fibronectin receptors, selectively enhanced the plasma membrane-cytoplasm exchange of N-Ras-GTP (preferentially associated with raft domains) in a cholesterol-dependent manner. Electron microscopy (EM) analysis showed N-Ras-GTP localization in cholesterol-sensitive clusters, from which it preferentially detached upon HA-GPI cross-linking. HA-GPI clustering enhanced the Golgi compartment (GC) accumulation and signaling of epidermal growth factor (EGF)-stimulated N-Ras-GTP. Notably, the cross-linking-mediated enhancement of N-Ras-GTP exchange and GC accumulation depended strictly on depalmitoylation. We propose that the N-Ras activation pattern (e.g., by EGF) is altered by raft protein clustering, which enhances N-Ras-GTP raft localization and depalmitoylation, entailing its exchange and GC accumulation following repalmitoylation. This mechanism demonstrates a functional signaling role for the activation-dependent differential association of Ras isoforms with raft nanodomains.