Role of lipid modifications in targeting proteins to detergent-resistant membrane rafts - Many raft proteins are acylated, while few are prenylated

Sphingolipid and cholesterol-rich Triton X-100-insoluble membrane fragments (detergent-resistant membranes, DRMs) containing lipids in a state similar to the liquid-ordered phase can be isolated from mammalian cells, and prob ably exist as discrete domains or rafts in intact membranes. We postulated that proteins with a high affinity for such an ordered lipid environment mi ght be targeted to rafts. Saturated acyl chains should prefer an extended c onformation that would fit well in rafts. In contrast, prenyl groups, which are as hydrophobic as acyl chains but have a branched and bulky structure, should be excluded from rafts. Here, we showed that at least half of the p roteins in Madin-Darby canine kidney cell DRMs (other than cytoskeletal con taminants) could be labeled with [H-3]palmitate. Association of influenza h emagglutinin with DRMs required all three of its palmitoylated Cys residues . Prenylated proteins, detected by [H-3]mevalonate labeling or by blotting for Rap1, Rab5, G(beta), or Ras, were excluded from DRMs. Rab5 and H-Ras ea ch contain more than one lipid group, showing that hydrophobicity alone doe s not target multiply lipid-modified proteins to DRMs. Partitioning of cova lently linked saturated acyl chains into liquid-ordered phase domains is li kely to be an important mechanism for targeting proteins to DRMs.