Ras Isoform-Specific Membrane Interactions: A Molecular Mechanism for Isoform-Specific Ras Signaling

Abstract

The existence of membrane subdomains with different lipid composition and the relationship between lipid-domain formation and the conformation and functional properties of membrane-associated proteins is one of the central questions in the fields of membrane biochemistry and biophysics. Raft domains are suggested to play a role in signal transduction processes by acting as “signaling platforms”.In the present study, a well established zwitterionic and anionic model raft membrane system has been used to analyze the lateral segregation in and conformational orientation at heterogeneous membranes of two different, fully lipidated Ras isoforms, namely N-Ras and K-Ras4B. The results show that membrane microdomain localization of Ras is mediated by the isoform-specific, posttranslationally modified C-terminal sequence. Whereas a time-dependent diffusion and subsequent clustering in the lo/ld phase boundary region of phase separated membranes was observed for N-Ras [1], formation of new fluid, protein-enriched domains was detected for K-Ras4B by time-lapse atomic force microscopy [2]. Corresponding IRRA and ATR-FTIR spectroscopy experiments revealed an isoform-specific orientation of the Ras G-domain at the membrane interface that is also dependent on GDP/GTP-loading. Altogether, the results of this study lead to the postulation of a molecular mechanism for isoform-specific Ras signaling from separate membrane microdomains that could potentially operate as an effective, high fidelity signaling platform with distinct signal outputs for the Ras isoforms: minimization of the line energy (N-Ras) versus lipid sorting through electrostatic interactions (K-Ras4B) [2].References1) Weise K et al. (2009) J. Am. Chem. Soc. 131: 1557-1564.2) Weise K et al. (2011) J. Am. Chem. Soc. 133: 880-887.