Bleaching experiments indicate that raft and nonraft proteins move at the same speeds.

Ever since the introduction of the fluid mosaic model, proteins have been viewed as rather hyperactive ships in a sea of lipids. Cholesterol-rich rafts were thought to be at least a partial exception to this trend. Although raft proteins move, their movement was predicted to be dampened by their inclusion in rafts, which act as stages for signaling.

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. ▪