Now, Kenworthy et al. have tested this idea directly (page 735). They used fluorescence recovery after photobleaching (FRAP) to follow green fluorescent protein (GFP)-tagged raft and nonraft proteins. The team then calculated the diffusion coefficients of the tested molecules. They found that raft-associated proteins and those not bound to rafts move similarly—neither faster nor slower.
This conflicts with some studies using single-particle tracking. There are several possible explanations for the new data. Meaningful rafts may not exist at all, at least in resting cells that are not actively signaling. The number of proteins in rafts and their mobility variation may be so small that the differences escape detection by the averaging methods used during FRAP. Or, say the authors, the proteins may not sit statically in rafts but rather move rapidly in and out. ▪