Lipid-dependent Targeting of G Proteins into Rafts

Domains rich in sphingolipids and cholesterol, or rafts, may organize signal transduction complexes at the plasma membrane. Raft lipids are believed to exist in a state similar to the liquid-ordered phase. It has been proposed that proteins with a high affinity for an ordered lipid environment will preferentially partition into rafts (Melkonian, K. A., Ostermeyer, A. G., Chen, J. Z., Roth, M. G., and Brown, D. A. (1999) J. Biol. Chem. 274, 3910-3917). We investigated the possibility that lipid-lipid interactions between lipid-modified proteins and raft lipids mediate targeting of proteins to these domains. G protein monomers or trimers were reconstituted in liposomes, engineered to mimic raft domains. Assay for partitioning of G proteins into rafts was based on Triton X-100 insolubility. Myristoylation and palmitoylation of Galpha(i) were necessary and sufficient for association with liposomes and partitioning into rafts. Strikingly, the amount of fatty-acylated Galpha(i) in rafts was significantly reduced when myristoylated Galpha(i) was thioacylated with cis-unsaturated fatty acids instead of saturated fatty acids such as palmitate. Prenylated betagamma subunits were excluded from rafts, whether reconstituted alone or with fatty-acylated alpha subunits. These results suggest that the structural difference between lipids that modify proteins is one basis for the selectivity of protein targeting to rafts.