Analysis of Raft Affinity of Membrane Proteins by Detergent-Insolubility

Isolation of detergent-resistant membranes (DRMs; also known as detergent-insoluble glycolipid-enriched membranes [DIGs] or glycolipid-enriched membranes [GEMs]) that are enriched in proteins and lipids with a high affinity for rafts is one of the simplest and most widely used methods for studying rafts. However, it is essential to understand the limitations as well as the advantages of this method. DRMs do not correspond precisely to rafts in living cells. For this reason, finding a protein enriched in DRMs does not prove that it was in rafts in the living cell. Furthermore, the fraction of a protein found in DRMs provides no quantitative information about the fraction of the protein originally in rafts. In fact, DRMs may be isolated from membranes that did not even contain rafts before detergent extraction. DRM-association is useful because it reflects a high-inherent affinity of a protein for the ordered membrane state found in rafts. Treatments that affect the DRM-association of a protein can thus be inferred to affect its raft affinity. Current models suggest that rafts may form in a regulated manner, often associated with clustering of membrane proteins or lipids, during processes such as signal transduction. DRM-association is a read-out of whether a protein is likely to associate with rafts that form under these conditions.